Why bother to take vitamins?

Abstract

Introduction

The first clear description of ‘fibrocystic disease of the pancreas’ or Cystic Fibrosis (CF) came in the late 1930s¹ and evidence of vitamin A deficiency was reported in 10 of 49 patients as early as 1939.² Indeed, this paper recommended ‘generous dosages’ of vitamin A for these patients.

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Contributorship

AM is the sole contributor

Biochemical evidence of fat-soluble vitamin deficiency occurs early in infants diagnosed with CF by newborn screening.^3–6 Sokol et al. reported the fat-soluble vitamin status of 36 infants diagnosed by newborn screening between 1984 and 1987 with the initial evaluation being undertaken at around seven weeks of age. There was an inverse correlation between three-day faecal fat excretion and serum α-tocopherol levels (vitamin E). Twenty-one percent of patients had low vitamin A levels, 35% low vitamin D, 38% low vitamin E and vitamin E:lipid ratio though none had an elevated PIVKA-II (protein induced by vitamin K absence or antagonism) – one indicator of vitamin K deficiency. Introduction of pancreatic enzyme replacement therapy, and standard fat-soluble vitamin supplementation corrected vitamin A and D status at 6 and 12 months of age. However, 10% of patients remained vitamin E deficient.³

Feranchak et al. reported that 44 of 96 (45.8%) infants diagnosed by newborn screening between 1984 and 1997 had a deficiency of one or more of the fat-soluble vitamins by 4–8 weeks of age. Vitamin A deficiency occurred in 29% of patients, vitamin D deficiency in 22.5% and vitamin E deficiency in 22.8%.⁴ Bines et al. reported low serum retinol in 20 out of 39 (51%) infants and low α-tocopherol in nine of 38 (24%) infants diagnosed by newborn screening between 1991 and 1994.⁵

These studies are supported by more contemporaneous data. Neville and Ranganathan reported 58 infants diagnosed by newborn screening between 2001 and 2006. Median age of diagnosis was 1 month (range 0–3 months). Vitamin D deficiency was described in 11 of 30 infants (37%), vitamin E deficiency in seven of 45 (16%) and vitamin A deficiency 27 of 45 (60%). Vitamin A and E levels were significantly lower in pancreatic insufficient patients. Vitamin A and E status was significantly correlated with pancreatic status though there was no significant correlation with pancreatic status and vitamin D deficiency.⁶

Why bother to take vitamins? Firstly, because at the time of diagnosis of CF there is clear biochemical evidence of vitamin A deficiency in up to 60% of infants⁶ and vitamin D and E deficiency in up to 37%⁶ and 38%,³ respectively. Untreated, the incidence of biochemical and later clinical vitamin deficiencies will increase.

Most people with CF are at risk of fat-soluble vitamin deficiencies due to fat malabsorption and maldigestion as a consequence of pancreatic insufficiency and bile salt deficiency. Patients treated with pancreatic enzyme replacement therapy may still experience refractory steator-rhoea due to enzyme inactivation as a consequence of high intestinal pH, small bowel bacterial overgrowth,⁷ the presence of a mucus barrier lining the intestine, short gut syndrome due to previous bowel resection and CF-related liver disease. Adherence rates are difficult to assess⁸ but non-adherence to pancreatic enzyme replacement is commonly reported^9,10 and contributes to malabsorption. We know that some patients do not bother to take their prescribed vitamins as adherence rates in people with CF of 47% of the recommended vitamin prescription have been reported.^9,11 Modi et al. compared different methods of assessing adherence to vitamin therapy. Adherence was variable and ranged from 22% when recorded in a diet diary to 94% for self-report by children.¹⁰ Non-adherence to therapy, poor dietary intake either due to anorexia or poor dietary sources of vitamins can also contribute to fat-soluble vitamin deficiency in CF. All of these factors put people with CF at high risk of vitamin deficiencies and in addition there are vitamin-specific risk factors detailed below.

Advances in the clinical and medical management of people with CF have led to improved life expectancy and new and emerging co-morbidities. The introduction of newborn screening allowing for earlier and more aggressive intervention together with improved pancreatic enzyme replacement therapy, the early introduction of appropriate fat soluble vitamin supplementation and an overall improvement in nutritional status together with advances in our understanding of fat-soluble vitamin metab-olism¹² mean we need to consider fat-soluble vitamin status in the context of modern disease. The emphasis of how vitamin status is defined has shifted. Traditionally vitamin status was discussed in terms of overt deficiency resulting in the classically recognized deficiency symptoms such as night blindness and xeropthalmia in vitamin A deficiency. Now vitamin status is also considered in terms of subclinical deficiencies which are defined as low serum, tissue or airway surface liquid vitamin levels with no visible signs or symptoms of deficiency. It is increasingly recognized that subclinical deficiencies in CF may play a significant role. Vitamin status may now be considered in the context of health outcomes rather than overt deficiency¹² with levels being described as deficient, insufficient, adequate, optimal and toxic.

Vitamin A

The term vitamin A refers to a group of fat-soluble compounds known as the retinoids. Preformed vitamin A refers to retinol or the fatty acid ester derivative. This is the active form of the vitamin and in health it is well absorbed from the gut. Preformed vitamin A is found in liver, fortified margarines, dairy products, oily fish and fish oils. The carotenoids are precursors of vitamin A and the most common is beta carotene. Beta carotene is found in green leafy vegetables such as spinach and broccoli, carrots, red peppers, tomatoes and yellow fruits such as peaches and mangoes. Beta carotene is less efficiently absorbed from the gut and the conversion rate decreases as oral intake increases therefore there is less risk of toxicity.

Why bother to take vitamin A? Firstly in addition to maldigestion and malabsorption of fat there are a number of risk factors for vitamin A deficiency in CF. Ahmed et al. reported a significant increase in faecal losses of retinol in CF unrelated to the degree of fat loss in the stools. They concluded that there may be a specific retinol handling defect in the gastrointestinal tract in people with CF possibly unrelated to digestion and absorption of dietary fat.¹³ Also beta carotene is an antioxidant and oxidative stress and increased free radical formation may contribute to a conditional deficiency due to increased needs.

Assessment of vitamin A status in CF is not without some inherent difficulties. Serum retinol is one indicator of vitamin A status but it is an insensitive indicator and remains normal until hepatic stores are almost depleted.¹⁴ In addition, serum retinol concentrations decrease transiently during the acute phase response to infection^15-17 which can make interpretation of levels in CF difficult. Retinol binding protein (RBP) is important for the transfer of retinyl esters to the tissues and synthesis of RBP is responsible for the homeostatic control of plasma retinol concentrations. Synthesis of RBP decreases during the acute phase response¹⁸ so assessment of RBP and retinol may be helpful in CF. Plasma zinc deficiency depresses the synthesis of RBP and reduces mobilization from hepatic stores. Therefore assessment of vitamin A in CF would be aided by serum retinol levels, retinol binding protein, plasma zinc and an assessment of a positive acute phase protein such as C-reactive protein (CRP).¹⁹

These difficulties in assessing vitamin status and interpreting plasma levels in people with CF have been highlighted in three studies. Duggan et al. reported that vitamin A levels were significantly lower on admission for an acute respiratory exacerbation (1.14 ±0.5 μmol/L) than at discharge (1.70 ± 0.6 μmol/L) (paired t-test: P <0.0001) with 23% of patients being considered deficient on admission. The mean value for retinol binding protein increased significantly during treatment for an acute respiratory exacerbation from 1.46 to 2.24μmol/L (P = 0.0003) and CRP concentrations fell from 25.7 to 9.8 mg/L (P = 0.002). Univariate-regression analysis found retinol concentration on admission correlated best with RBP on admission (r =0.87). There was a negative correlation between plasma retinol concentration and both peak body temperature on admission (r = –0.41) and CRP concentration (r = –0.44).²⁰

Lindblad et al. investigated 15 patients with CF (aged 8–34 years). Five (33%) had serum concentrations of retinol below the reference range and seven (47%) had decreased serum levels of retinol binding protein. There was a strong correlation between serum levels of retinol and retinol binding protein (r_s = 0.90, P = 0.01).²¹

Greer et al. used multiple regression analysis for predictors of vitamin A status and found CRP was negatively correlated with vitamin A in patients with CF (r_s = –0.37, p <0.0001).²²

Biologically, vitamin A is important for dark adaptation and normal vision, epithelial cell proliferation and differentiation, maintenance of mucus-secreting epithelia and maintaining integrity of epithelial cells, apoptosis, immune regulation and immunocompetence; and its precursor beta carotene has antioxidant properties.

The major consequence of vitamin A deficiency is ocular with abnormal dark adaptation (night blindness), conjunctival and corneal xerosis that can lead to blindness. Historically, xerophthalmia and night blindness have been reported in patients with CF²³ and have been reported as presenting features leading to a diagnosis of CF.^24,25 Asymptomatic conjunctival xerosis with associated night blindness or abnormal dark adaptation despite vitamin A supplementation has also been reported.^26–29

More recently, Morkeberg et al. investigated 35 adult patients with CF who were receiving the recommended daily supplemental dose of vitamin A. None of the patients had serum vitamin A levels indicative of deficiency and none of the patients had clinical (ocular) signs of vitamin A deficiency. They did however report a high incidence (26%) of keratoconjunctivitis sicca (dry eye).³⁰ Ansari et al. compared 28 adults with CF to 25 age- and sex-matched controls. With appropriate supplementation none of the patients had biochemical vitamin A deficiency, dark adaptation was normal compared to controls and only two patients (7%) had clinical evidence of dry eye.³¹ Whatham et al. compared electroretinograms in 29 pancreatic insufficient patients with CF supplemented with vitamin A and compared them with 12 pancreatic sufficient patients with CF aged 4–17 years. They reported no significant difference in vitamin A or retinol binding protein and reported similar levels of retinal function (electroretinogram amplitudes and implicit times).³² Taking appropriate vitamin A supplements can prevent the development of the ocular signs of deficiency.

In people with CF the roles of vitamin A in the maintenance of mucus secreting epithelia and immunocompetence together with the role of beta carotene as an antioxidant and how this may impact on respiratory health is important. Vitamin A is important in the maintenance of mucus-secreting epithelia including the respiratory epithelia. Mild or subclinical vitamin A deficiency may impair respiratory epithelium integrity.³³ It leads to a loss of ciliated cells and an increase in mucus-secreting goblet-cells which impairs muco-ciliary clearance and favours bacterial adherence.^34–36 In addition, bacterial adhesion to nasopharyngeal epithelial cells is increased in vitamin A deficiency³⁷ and may permit increased colonization. In severe vitamin A deficiency squamous epithelium replaces the mucus-secreting ciliated epithelium resulting in squamous metaplasia. Vitamin A also plays a role in immunity and suggested mechanisms include T-cell subset depression, cell differentiation, cytokine modulation and phagocyte stimulation.^38–40 Beta carotene also has antioxidant properties, it acts as a ‘free-radical’ scavenger and may help to prevent or slow oxidative damage in the lung. This suggests subclinical vitamin A deficiency may be of importance in CF.

Linking subclinical vitamin A deficiency and lung function is difficult due to the impact of the acute phase response on plasma vitamin A levels and the multifactorial basis of respiratory exacerbations. However, in a retrospective review of 102 patients with CF, mean age of 11.1±6.4 years (range 1.5–27 years) experiencing 597 pulmonary exacerbations there was a direct correlation between increased number of exacerbations and lower vitamin A levels. This correlation was sustained within normal serum levels, across all lung function and in both pancreatic sufficient and insufficient patients.⁴¹ Aird et al. reported a significant correlation between serum vitamin A concentration and forced expiratory volume in one second (% predicted) (r = 0.37, P =0.02), forced vital capacity (r = 0.39, P = 0.02) and peak expiratory flow (r = 0.41, P = 0.01) in 38 patients with CF (mean age 15.3 years [range 6.1-25.2 years]).⁴²

More recently interest has turned to increased levels of vitamins in patients with CF. Hypervitaminosis A has been reported in patients with CF following lung transplantation.⁴³ Two retrospective studies highlighted elevated serum retinol concentrations in children and young adults with CF. Graham-Maar et al. compared intakes of 73 pancreatic insufficient children (aged 8.0-11.9 years) with CF to data from the 1999-2000 National Health and Nutrition Examination Survey (NHANES). Mean serum retinol was 52 ± 13 μg/dl (range 26-98 μg/dl) in patients with CF with no patients being in the deficient range. Mean serum retinol concentrations were significantly higher in patients with CF 52 ± 13 μg/dl (range 26-98 μg/dl) compared to NHANES 37 ± 10 μg/dl (range17-63 μg/dl) (p < 0.001).⁴⁴ In a second study from Maqbool et al. reported 78 pancreatic insufficient patients with CF (aged 8-25 years) median serum retinol was 80 μg/dl (range 33-208 μg/dl) with 58% of patients having serum retinol levels above the NHANES reference range.⁴⁵ These studies highlight the importance of regular assessment and adjustment of supplemental doses.

Current recommendations for vitamin A supplementation in pancreatic insufficient patients are based on historical data and vary between countries.^46-49 In the UK the recommended starting doses are less than 1 year old 4000 IU (1200 μg), older than 1 year 4000-10,000 IU (1200-3000 μg/ day).⁴⁷ Doses should be guided by individuals’ serum levels accepting that some people will need more and some less.

Vitamin E

Vitamin E is a generic term for a group of eight lipid soluble compounds synthesized by plants. There are two classes; tocopherols (α, β, γ, δ) and tocotrienols (α, β, γ, δ) which exhibit differing levels of activity. Alpha-tocopherol has the highest biological potency and is the standard against which other forms are compared. Most vitamin E in the diet is derived from vegetable oils particularly corn, soya and sunflower seed oil, margarines and fat spreads depending on the base oil used and fortification, some nuts and foods such as crisps and savoury snacks in which fat is a major ingredient.

Why bother to take vitamin E? In addition to maldigestion and malabsorption which increases the risk of vitamin E deficiency. Vitamin E requirements may be increased due to the role of vitamin E as an antioxidant.

Serum or plasma vitamin E levels are most commonly used to assess vitamin E status. Vitamin E circulates in the blood bound to lipoproteins and as a consequence more accurate assessment of status may be assessed using the vitamin E to total lipid ratio. Normal ratios of α-tocopherol to total lipid are >0.6 mg and >0.8 mg α-tocopherol/gram total lipid in children and adults, respectively.⁵⁰ In a study carried out in people without CF 47% of low vitamin E levels were normal when re-evaluated using vitamin E to lipid ratio and 58% with elevated plasma vitamin E were normal or low when re-evaluated with lipid ratio. Elevated triglyceride levels in non-fasting specimens were the most common reason for abnormal results when lipid was not considered.⁵¹ In people with CF, low plasma vitamin E levels may be a reflection of low total lipid levels and the vitamin E to total lipid ratio may actually be within the normal range.

Vitamin E is present in all cell membranes and is important in maintaining neurological function. It is an important antioxidant. Antioxidants protect cells from the damaging effects of free radicals. Free radicals are highly reactive molecules and react with oxygen to form reactive oxygen species. The body forms reactive oxygen species when it converts food to energy and the body is also exposed to environmental sources of free radicals, e.g. air pollution. In CF, bacterial infection and chronic inflammation may also contribute to increased free radical production. Vitamin E stops the production of reactive oxygen species formed when fat undergoes oxidation. The antioxidant properties of vitamin E may therefore help to reduce the effects of free radicals and help to protect cell membranes from oxidative damage.

Severe vitamin E deficiency is rare but leads to neurological degeneration, muscle weakness, hyporeflexia, ataxia and abnormal brainstem evoked auditory potential. Vitamin E deficiency also causes haemolytic anaemia due to decreased erythrocyte survival time and increased red blood cell fragility with susceptibility to haemolysis. Neurological symptoms of vitamin E deficiency have been described in children and adults with CF including peripheral neuropathy, areflexia, ataxia, and pigmentary retinopathy, abnormal somatosensory and visual evoked potentials.^52–54 Haemolytic anaemia has been reported as occurring as early as 6 weeks in three infants with CF⁵⁵ and also as a presenting feature of CF.⁵⁶

More recently, there has been interest in the role of vitamin E in cognitive function. Koscik et al. reported that patients with prolonged vitamin E deficiency due to delayed diagnosis had significantly lower Cognitive Skills Index and cognitive factor scores. This suggests that early diagnosis, prevention of prolonged periods of malnutrition and minimizing the duration of vitamin E deficiency, as seen in newborn screening programmes, is associated with better cognitive function in children with CF.^57,58

Why bother to take vitamin E? Taking vitamin E in people with CF is important to prevent overt deficiency symptoms but also for its potentially protective roles in cognitive function and for its antioxidant properties.

However, as with vitamin A high plasma levels of vitamin E have been reported in patients with CF following lung transplantation.⁴³ In a cross-sectional study vitamin E levels of 69 children with CF (aged 6.0–10.0 years) were compared to levels from 222 children from the NHANES Survey III sample (aged 6.0–11.9 years). Forty-eight percent of patients with CF had serum vitamin E levels greater than the NHANES 95th percentile (high levels) and 83% had high vitamin E to cholesterol ratios.⁵⁹ While vitamin E deficiency is our primary concern in patients with CF, care givers should be aware that with early diagnosis and improved treatments high levels can occur. The consequence of exposure to high vitamin E levels in people with CF is unknown. Plasma levels should be monitored regularly and the dose should be adjusted accordingly.

Current recommendations for vitamin E supplementation in pancreatic insufficient patients are based on historical data and vary between countries.^46–49 In the UK the recommended starting doses are less than 1 year 10–50 mg, older than one year 50–100 mg, adults 100–200 mg.⁴⁷ Doses should be guided by serum levels accepting that some people will need variable doses.

Vitamin D

There are several compounds with vitamin D activity. Two nutritionally significant compounds are ergocalciferol (vitamin D₂) and colecalciferol (vitamin D₃). Dietary intake of ergocalciferol is very low as few foods are naturally rich in vitamin D₂, e.g. fatty fish and fish oils, liver. In many countries foods may be fortified with vitamin D, e.g. margarine, fat spreads and breakfast cereals, and this may make a significant contribution to dietary intake. Both colecalciferol and ergocalciferol are used to fortify foods. Most of the vitamin D required by humans is produced photochemically by exposure of the 7-dehydrocholesterol present in the skin to sunlight or ultraviolet light irradiation which causes the production of vitamin D₃ (colecalciferol). Specific wavelengths of light are required for this process and as a consequence seasonal variation is reported in both people with CF⁶⁰ and the general population with those living at higher latitudes being most at risk of low levels. In addition, the prescription of supplemental vitamin D as either ergocalciferol or colecalciferol is significant in CF.

Why bother to take vitamin D? In addition to malabsorption and maldigestion people with CF are at increased risk of vitamin D deficiency due to suboptimal levels of vitamin D binding protein,^61,62 reduced sunlight exposure because of illness and photosensitivity related to prescribed medications, and reduced stores in some due to depleted fat mass.

The conventional marker of vitamin D status is plasma 25-hydroxyvitamin D as it is most readily available and it reflects the main storage form of the precursor substrate for 1, 25 dihydroxyvitamin D. Measurement of 1, 25 dihydroxyvitamin D is possible but less commonly performed. It is important to take into account seasonal variation in vitamin D levels. In addition, it is essential to ensure that the assay includes both ergocalciferol and colecalciferol and when interpreting results to recognize that inter-assay and inter-laboratory variability have been reported.⁶³ Vitamin D status is not thought of solely in terms of deficiency but is considered in terms of sufficiency and insufficiency in relation to overall health out-comes⁶⁴ with levels greater than 75 nmol/L being considered by some optimal for health out-comes^64,65 though this remains controversial.

Absorbed vitamin D is hydroxylated in the liver to 25-hydroxyvitamin D further hydroxylation occurs in the kidney to produce the active metabolite 1, 25-dihydroxyvitamin D (calcitriol). The active metabolite 1, 25-dihydroxyvitamin D is a steroid hormone which regulates calcium and phosphate metabolism. In the kidney, 1, 25-dihydroxyvitamin D regulates calcium transport in the proximal tubule and it regulates calcium and phosphate absorption from the small intestine. 1, 25-dihydroxyvitamin D is also involved in the maintenance of plasma calcium levels via bone resorption and accrual under the regulation of parathyroid hormone which is secreted in response to low calcium levels.¹⁴

Vitamin D is classically thought of for its role in calcium absorption, metabolism and bone mineralization. However, there is increasing recognition and interest in the non-skeletal roles of vitamin D in muscle function, innate immunity, diabetes, cardiovascular disease and some forms of cancer.^66,67

Vitamin D deficiency leads to reduced calcification of bones and causes rickets in children and osteomalacia in adults. Overt vitamin D deficiency in CF is rare but both rickets and osteomalacia have been reported.^68–70

Reduced bone mineral density is common in patients with CF with pooled prevalence rates for osteoporosis being reported as 23.5% and for vertebral and non-vertebral fractures being 14% and 19.7%, respectively.^71,72 Reduced bone mineral density is of consequence in people with CF as it is associated with higher fracture rates,^73–75 kyphosis,⁷⁴ and pain which may limit physiotherapy; it may also be a contraindication to transplantation.⁷⁶

Reduced bone mineral density in patients with CF was first described in 1979^77,78 and since these initial reports more than 40 published studies have reported low vitamin D level in patients with CF and reduced bone mineral density. There are numerous recently published studies showing that levels remain suboptimal in most paediatric and adult patients with CF despite routine and high dose supplementation.^60,79–82 Despite the evidence of extensive vitamin D insufficiency in people with CF most studies do not report a direct link between low vitamin D levels and low bone density because CF-related low bone mineral density is complex and multifactorial. However, it appears highly likely that low vitamin D levels do play a role in low bone mineral density in CF⁸³ as they do in the general population.⁸⁴

An emerging consideration is the relationship between vitamin D status and lung function. In the third National Health and Nutritional Examination Survey a strong relationship was found between serum 25-hydroxyvitamin D levels and lung function.⁸⁵ Green and colleagues reported that a higher FEV₁ (implying improved lung function) was associated with higher levels of 25-hydroxyvitamin D in children. For each 10% increase in FEV₁, the 25-hydroxyvitamin D level increased by 1.0 ng/mL.⁸⁶ Wolfenden et al. reported a small but significant positive association between serum 25- hydroxyvitamin D levels and FEV₁ percent predicted in adults with CF.⁶⁰ Stephenson et al. reported a trend towards increasing FEV₁ percent predicted with higher vitamin D levels and a significant difference in FEV₁ percent predicted between patients with a plasma vitamin D level less than 25 nmol/L and those with a plasma vitamin D level greater than 50 nmol/L.⁷⁹

Why bother to take vitamin D? Optimizing vitamin D status and minimizing the risk factors for CF-related low bone mineral density would seem prudent in view of the prevalence data and potential consequences. In addition, the increasing recognition and interest in the non-skeletal roles of vitamin D in respiratory function, muscle function, innate immunity, diabetes, cardiovascular disease and some forms of cancer may also have relevance in people with CF.

Current recommendations for vitamin D supplementation are based on historical data and vary between countries.^46–49 It is recognized that this level of supplementation is not sufficient to achieve adequate levels in many patients with CF.^60,79–82 In the UK the recommended starting doses are less than 1 year 400 IU (10 μg), older than 1 year 400–800 IU (10–20 μg/day), adults 800–2000 IU (20–50 μg) per day.^47,87 Doses should be guided by serum levels accepting that many people will need a higher dose.

Vitamin K

Vitamin K is a fat-soluble vitamin that exists naturally in multiple dietary forms. It is not a single compound but a group of homologous fat-soluble compounds. Vitamin K₁ (phylloquinone) is synthesized by plants and in the diet is present in dark green leafy vegetables such as spinach, broccoli, cabbage and kale. Some vegetable oils such as rapeseed, soy bean and olive oil are rich sources. Dairy products, meat and eggs also contain vitamin K though in lesser amounts. Vitamin K₂ (menaquinone) is synthesized by Gram-positive bacteria present in the jejunum and ileum. It is unclear how much the bacterial flora contributes to overall vitamin K status.^88,89 Vitamin K₃ (mena-dione) and vitamin K₄ (menadiol) are synthetic forms of the vitamin but it is recommended that K₃ is best avoided in humans as it is linked to haemolysis and liver damage in the newborn and has the potential for mutagenicity.^14,90

Vitamin K is an essential nutrient but even in the general population recommendations on adequate intake have not been precisely established due to the uncertainty of the contribution of Vitamin K₂. Absorption of vitamin K from the gut is dependent on bile salts and pancreatic lipase secretion stimulated by dietary fat. In addition to fat malabsorption and maldigestion risk factors for vitamin K deficiency in CF include bile salt deficiency, liver disease, chronic antibiotic use, short gut due to bowel resection and inadequate dietary intake.⁹¹

Vitamin K is an essential cofactor in the post-translational conversion of glutamyl (Glu) residues to γ carboxyglutamyl (Gla) residues. Under-carboxylated Gla-dependent proteins are functionally inactive. Major Gla-proteins (active) include prothrombin, osteocalcin and other bone metabolism-related proteins. Hence vitamin K is important for blood coagulation, bone health and mineralization and more recently interest has been shown in its role in energy metabolism and inflammation⁹² which may be of importance in CF.

No single biomarker of vitamin K status is a robust measure of vitamin K sufficiency or insufficiency.^93,94 The presence of circulating Gla proteins in their under-carboxylated forms is the most sensitive indicator of vitamin K deficiency but it appears that different vitamin K proteins have different vitamin K requirements. Prothrombin time is not a very sensitive marker of vitamin K status as just 50% of the normal pro-thrombin concentration produces a normal pro-thrombin time. Therefore a prolonged prothrombin time is a marker of advanced subclinical vitamin K deficiency. It also reflects only vitamin K deficiency of the liver. Protein or pro-thrombin induced by vitamin K absence or antagonism (PIVKA II) is the under-carboxylated form of prothrombin and is a more sensitive indicator of vitamin K deficiency of the liver. Under-carboxylated osteocalcin is the most sensitive indicator of vitamin K status of the bone and is the first Gla protein to occur in the undercarboxylated form. Under-carboxylated osteocalcin is the most sensitive indicator of vitamin K status.⁹⁵ PIVKA II and under-carboxylated osteocalcin are not readily available in many CF centres and currently tend to be used mainly as research tools.

Fifty years ago Shwachman, and Di Sant'Agnese and Vidaurreta reported that bleeding and vitamin K deficiency may be an early sign of CF.^96,97 Since that time numerous authors have reported coagulopathies including haematomas, intracerebral haemorrhage and severe life-threatening bleeding in people with CF and vitamin K deficiency.^98–100

Subclinical deficiency of vitamin K as assessed by PIVKA II is common in CF. Rashid et al. published a prospective study assessing vitamin K status using PIVKA II in 98 patients (83 pancreatic insufficient) and 62 healthy controls. None of the controls but 78% of pancreatic insufficient patients, 33% of pancreatic sufficient patients and all those with CF-related liver disease had elevated PIVKA II levels.¹⁰¹ Conway et al. reported elevated PIVKA II in 42% of 93 children with CF.¹⁰²

A number of studies have also been published which indicate subclinical vitamin K deficiency despite supplementation with varying doses of vitamin K suggesting the appropriate dose of vitamin K has yet to be established. De Montalembert et al. reported 14 (33%) of 43 patients supplemented with 5–10 mg vitamin K daily had elevated PIVKA II levels.¹⁰³ Wilson et al. reported 58 (81%) of 72 patients had abnormal PIVKA II levels. After supplementation with a mean vitamin K intake of 0.18 mg/day for a minimum of four months 20 (40%) still had abnormal PIVKA II levels.¹⁰⁴ Van Hoorn et al. compared 39 healthy subjects with 20 patients with CF receiving either no vitamin K supplementation (n = 10), low dose vitamin K supplementation (<0.25 mg/day) (n = 6) and ‘high’ dose supplementation (>1 mg/day) (n = 4). Only patients in the high dose supplementation group had normal PIVKA II levels.¹⁰⁵

Similar findings have recently been reported by Dougherty et al. who compared patients supplemented with what they defined as low (<150 μg/day), middle (150-999 μg/day) and high (1000 μg/day) dose supplementation. Despite supplementation 50% of patients (n = 60) had PIVKA II in the deficient range. In addition to PIVKA II this study determined vitamin K status as defined by under-carboxylated osteocalcin, the most sensitive indicator of vitamin K status of the bone. Seventy-four percent of the patients had under-carboxylated osteocalcin levels in the insufficient or deficient range. Vitamin K supplementation was negatively associated with the percentage of under-carboxylated osteocalcin.¹⁰⁶

Increased levels of under-carboxylated osteocalcin have been reported in other studies and have been associated with reduced lumbar spine bone mineral content,¹⁰⁷ reduced levels of bone markers for bone mineral accrual^80,102,107 and increased levels of markers for bone turnover.¹⁰²

These studies suggest subclinical vitamin K deficiency is common in CF. In the general population vitamin K intervention studies have reported increased bone mineral density, reduced fracture rates and improved bone strength. However, as with vitamin D because of the multifactorial nature of low bone mineral density in CF cause and effect will be difficult to establish.

A recent observational study in the non-CF population has shown that low vitamin K status is inversely associated with circulating measures of inflammation.¹⁰⁸ The mechanism underlying the potential influence of vitamin K on inflammatory cytokine production is unclear but and it has been suggested that vitamin K may suppress inflammation by decreasing expression of genes for individual cytokines. This may be of importance in CF where inflammation and infection contribute to lung damage.

Why bother to take vitamin K? Overt deficiency can lead to life-threatening bleeding. Subclinical deficiency is common and may contribute to cystic fibrosis low bone mineral density. In addition, vitamin K and vitamin K dependent proteins may play a role in energy metabolism and inflammation which may be of relevance in CF.

Supplementation with vitamin K is not currently universal. Current recommendations for vitamin K supplementation are based on historical data and vary between countries.^46–49 In the UK the recommended doses are less than 2 years 300 ?g/kg/day rounded to the nearest mg, 2–7 years 5 mg/day and older than 7 years 10 mg/ day.⁸⁷ As vitamin K is metabolized within 24 hours a daily dose appears most appropriate.

Conclusion

Why bother to take vitamins? Correcting fat-soluble vitamin deficiencies is an early management goal. Prevention of overt deficiency is the primary goal but optimizing fat-soluble vitamin status in people will influence health outcomes. Our understanding of these vital nutrients continues to expand. Further research is needed to determine optimal supplemental doses in the context of modern disease. High levels are reported so continuous monitoring and adjustment of doses is essential.

Footnotes

Acknowledgements

None

References

Andersen

DH.

Cystic fibrosis of the pancreas and its relation to celiac disease: a clinical and pathological study.

Am J Dis Child 1938; 56: 344–99.

Andersen

DH.

Cystic fibrosis of the pancreas, vitamin A deficiency and bronchiectasis.

J Paediatr 1939; 15: 63–71.

Sokol

, Reardon

, Accurso

FJ.

Fat-soluble-vitamin status during the first year of life in infants with cystic fibrosis identified by screening of newborns.

Am J Clin Nutr 1989; 50: 1064–71.

Feranchak

, Sontag

, Wagener

, Hammond

, Accurso

, Sokol

RJ.

Prospective, long-term study of fat-soluble vitamin status in children with cystic fibrosis identified by newborn screen.

J Pediatr 1999; 136: 601–10.

Bines

, Truby

, Armstrong

, Carzino

, Grimwood

Vitamin A and E deficiency and lung disease in infants with cystic fibrosis.

J Pediatr Child Health 2005; 41: 663–8.

Neville

, Ranganathan

SC.

Vitamin D in infants with cystic fibrosis diagnosed by newborn screening.

J Pediatr Child Health 2009; 45: 36–41.

Fridge

, Conrad

, Gerson

, Castillo

, Cox

Risk factors for small bowel bacterial overgrowth in cystic fibrosis.

J Pediatr Gastroenterol Nutr 2007; 44: 212–18.

Abbott

, Havermans

, Hart

Adherence to the medical regimen: clinical implications of new findings.

Curr Opin Pulm Med 2009 Aug 6 [Epub ahead of print]

Abbott

, Dodd

, Bilton

, Webb

AK.

Treatment compliance in adults with cystic fibrosis.

Thorax 1994; 49: 15–20.

10.

Modi

, Lim

, Yu

, Geller

, Wagner

, Quittner

AL.

A multi-method assessment of treatment adherence for children with cystic fibrosis.

J Cyst Fibros 2006; 5: 177–85.

11.

Borowitz

, Wegman

, Harris

Preventive care for patients with chronic illness. Multivitamin use in patients with cystic fibrosis.

Clin Pediatr 1994; 33: 720–5.

12.

Maqbool

, Stallings

VA.

Update on fat-soluble vitamins in cystic fibrosis.

Curr Opin Pulm Med 2008; 14: 574–81.

13.

Ahmed

, Ellis

, Murphy

, Wooton

, Jackson

AA.

Excessive faecal losses of vitamin A (retinol) in cystic fibrosis.

Arch Dis Child 1990; 65: 589–93.

14.

Expert Group on Vitamin and Minerals (EVM). Safe Upper Levels for Vitamins and Minerals. London: FSA, 2003

15.

Thurnham

DI.

Impact of disease on markers of micronutrient status.

Proc Nutr Soc 1997; 56: 421–31.

16.

Beisel

WR.

Infection-induced depression of serum retinol-a component of the acute phase response or a consequence?

Am J Clin Nutr 1998; 68: 993–4.

17.

Thurnham

, McCabe

, Northrop-Clewes

, Nestel

Effects of subclilical infection on plasma retinol concentrations and assessment of prevalence of vitamin A deficiency: meta-analysis.

Lancet 2003; 362: 2052–8.

18.

Louw

, Werbeck

, Louw

, Kotze

, Cooper

, Labadarios

Blood vitamin concentrations during the acute-phase response.

Crit Care Med 1992; 20: 934–41.

19.

Stephenson

, Gildengorin

Serum retinol, the acute phase response, and the apparent misclassification of vitamin A status in the third National Health and Nutrition Examination Survey.

Am J Clin Nutr 2000; 72: 1170–8.

20.

Duggan

, Colin

, Agil

, Higgins

, Rifai

Vitamin A status in acute exacerbations of cystic fibrosis.

Am J Clin Nutr 1996; 64: 635–9.

21.

Lindblad

, Diczfalusy

, Hultcrantz

, Thorell

, Strandvik

Vitamin A concentration in the liver decreases with age in patients with cystic fibrosis.

J Pediatr Gastroenterol Nutr 1997; 24: 264–70.

22.

Greer

, Buntain

, Lewindon

PJ.

vitamin A levels in patients with CF are influenced by the inflammatory response.

J Cyst Fibros 2004; 3: 143–9.

23.

Petersen

, Petersen

, Robb

RM.

Vitamin A deficiency with xerophthalmia and night blindness in cystic fibrosis.

Am J Dis Child 1968; 116: 662–5.

24.

Lindenmuth

, Del Monte

, Marino

LR.

Advanced xerophthalmia as a presenting sign in cystic fibrosis.

Ann Ophthalmol 1989; 21: 189–91.

25.

Joshi

, Dhawan

, Baker

, Heneghan

MA.

An atypical presentation of cystic fibrosis: a case report.

J Med Case Reports 2008; 2: 201–2.

26.

Vernon

, Neugebauer

MAZ

, Brimlow

, Tyrell

, Hiller

EJ.

Conjunctival xerosis in cystic fibrosis.

J Royal Soc Med 1989; 82: 46–7.

27.

Rayner

, Tyrrell

, Hiller

EJ.

Night blindness and conjunctival xerosis caused by vitamin A deficiency in patients with cystic fibrosis.

Arch Dis Child 1989; 64: 1151–6.

28.

Neugebauer

, Vernon

, Brimlow

, Tyrrell

, Hiller

, Marenah

Nyctalopia and conjunctival xerosis indicating vitamin A deficiency in cystic fibrosis.

Eye 1989; 3: 360–4.

29.

Huet

, Semama

, Maingueneau

, Charavel

, Nivelon

JL.

Vitamin A deficiency and nocturnal vision in teenagers with cystic fibrosis.

Eur J Pediatr 1997; 156: 949–51.

30.

Morkeberg

, Edmund

, Prause

, Lanng

, Koch

, Michaelson

KF.

Ocular findings in cystic fibrosis patients receiving vitamin A supplementation.

Graefes Arch Clin Exp Ophthalmol 1995; 233: 709–13.

31.

Ansari

, Sahni

, Etherington

Ocular signs and symptoms and vitamin A status in patients with cystic fibrosis treated with daily vitamin A supplements.

Br J Ophthalmol 1999; 83: 688–91.

32.

Whatham

, Suttle

, Blumenthal

, Allen

, Gaskin

ERGs in children with pancreatic enzyme insufficient and pancreatic enzyme sufficient cystic fibrosis.

Doc Ophthalmol 2009; 119: 43–50.

33.

McCullough

FSW

, Northrop-Clewes

, Thurnham

DI.

The effect of vitamin A on epithelial integrity.

Proc Nutr Soc 1999; 58: 289–93.

34.

Biesalski

, Stofft

Biochemical, morphological and functional aspects of systemic and local vitamin A deficiency in the respiratory tract.

Ann NY Acad Sci 1992; 669: 325–31.

35.

Biesalski

, Nohr

Importance of vitamin-A for lung function and development.

Mol Aspects Med 2003; 24: 431–40.

36.

Biesalski

, Nohr

New aspects in vitamin A metabolism: the role of retinyl esters as systemic and local sources for retinol in mucous epithelia.

J Nutr 2004; 134: 3453S–3457S.

37.

Chandra

RK.

Increased bacterial binding to respiratory epithelial cells in vitamin A deficiency.

BMJ 1988; 297: 834–5.

38.

Semba

, West

, Sommer

Abnormal T-cell subset proportions in vitamin-A- deficient children.

Lancet 1993; 8836: 5–8.

39.

Ross

AC.

Vitamin A status and relationship to immunity and the antibody response.

Proc Soc Exp Biol Med 1992; 200: 303–20.

40.

Stephenson

CB.

Vitamin A, infection, and immune function.

Annu Rev Nutr 2001; 21: 167–92.

41.

Hakim

, Kerem

, Rivlin

Vitamin A and E and pulmonary exacerbations in patients with cystic fibrosis.

J Pediatr Gastroenterol Nutr 2007; 45: 347–53.

42.

Aird

, Greene

, Ogston

, Macdonald

, Mukhopadhyay

Vitamin A and lung functionin CF.

J Cyst Fibros 2006; 5: 129–31.

43.

Stephenson

, Brotherwood

, Robert

Increased vitamin A and E levels in adult cystic fibrosis patients after lung transplantation.

Transplantation 2005; 79: 613–15.

44.

Graham-Maar

, Schall

, Stettler

, Zemel

, Stallings

VA.

Elevated vitamin A intake and serum retinol in preadolescent children with cystic fibrosis.

Am J Clin Nutr 2006; 84: 174–82.

45.

Maqbool

, Graham-Marr

, Schall

, Zemel

, Stallings

VA.

Vitamin A intake and elevated serum retinol levels in children and young adults with cystic fibrosis.

J Cyst Fibros 2008; 7: 137–41.

46.

Borowitz

, Baker

, Stallings

Consensus report on nutrition for pediatric patients with cystic fibrosis.

J Pediatr Gastroenterol Nutr 2002; 35: 246–59.

47.

Cystic Fibrosis Trust. Nutritional management of cystic fibrosis. UK Cystic Fibrosis Trust Nutrition Working Group. Bromley: Cystic Fibrosis Trust, 2002

48.

Sinaasappel

, Stern

, Littlewood

Nutrition in patients with cystic fibrosis: a European Consensus.

J Cyst Fibros 2002; 1: 51–75.

49.

Borowitz

, Robinson

, Rosenfeld

Cystic Fibrosis Foundation evidence-based guidelines for management of infants with cystic fibrosis.

J Pediatr 2009; 155: S73–93

50.

Tietz

ed. Clinical Guide to Laboratory Tests. 3rd Edition. Philadelphia, PA: WB Saunders Company, 1995

51.

Winbauer

, Pingree

, Nuttall

KL.

Evaluating serum alpha-tocopherol (vitamin E) in terms of lipid ratio.

Ann Clin Lab Sci 1999; 29: 185–91.

52.

Willison

, Muller

DPR

, Matthews

A study of the relationship between neurological function and serum vitamin E concentrations in patients with cystic fibrosis.

J Neurol Neurosurg Psychiatry 1985; 48: 1097–102.

53.

Bye

AME

, Muller

DPR

, Wilson

, Wright

, Mearns

MB.

Symptomatic vitamin E deficiency in cystic fibrosis.

Arch Dis Child 1985; 60: 162–4.

54.

Sitrin

, Lieberman

, Jensen

, Noronha

, Milburn

, Addington

Vitamin E deficiency and neurologic disease in adults with cystic fibrosis.

Ann Intern Med 1987; 107: 51–4.

55.

Wilfond

, Farrell

, Laxova

, Mischler

Severe haemolytic anemia associated with vitamin E deficiency in infants with cystic fibrosis: implications for neonatal screening.

Clin Pediatr 1994; 33: 2–7.

56.

Swann

, Kendra

JR.

Anaemia, vitamin E deficiency and failure to thrive in an infant.

Clin Lab Haematol 1998; 20: 61–3.

57.

Koscik

, Farrell

, Kosorok

MR.

Cognitive function of children with cystic fibrosis: deleterious effect of early malnutrition.

Pediatrics 2004; 113: 1549–58.

58.

Koscik

, Lai

, Laxova

Preventing early, prolonged vitamin E deficiency: an opportunity for better cognitive outcomes via early diagnosis through neonatal screening.

J Pediatr 2005; 147: S51–6

59.

Huang

, Schall

, Zemel

, Stallings

VA.

Vitamin E status in children with cystic fibrosis and pancreatic insufficiency.

J Pediatr 2006; 148: 556–9.

60.

Wolfenden

, Judd

, Shah

, Sanyal

, Ziegler

, Tangpricha

Vitamin D and bone health in adults with cystic fibrosis.

Clin Endocrinol 2008; 69: 374–81.

61.

Coppenhaver

, Kueppers

, Schidlow

Serum concentrations of vitamin D-binding protein (group-specific component) in cystic fibrosis.

Hum Genet 1981; 57: 399–403.

62.

Speeckaert

, Wehlou

, Vandewalle

, Taes

, Robberecht

, Delanghe

JR.

Vitamin D binding protein, a new nutritional marker in cystic fibrosis patients.

Clin Chem Lab Med 2008; 46: 365–70.

63.

Binkley

, Krueger

, Lensmeyer

25-hydroxyvitamin D measurement, 2009: a review for clinicians.

J Clin Densitom 2009; 12: 417–27.

64.

Bischoff-Ferrari

, Giovannucci

, Willett

, Dietrich

, Dawson-Hughes

Estimation of optimal serum concentrations of 25-hydroxyvitamin D for multiple health outcomes.

Am J Clin Nutr 2006; 84: 18–28.

65.

Vieth

, Bischoff-Ferrari

, Boucher

BJ.

The urgent need to recommend an intake of vitamin D that is effective.

Am J Clin Nutr 2007; 85: 649–50.

66.

Holick

MF.

Medical Progress: Vitamin D deficiency.

N Engl J Med 2007; 357: 266–81.

67.

Holick

, Chen

TC.

Vitamin D deficiency: a worldwide problem with health consequences.

Am J Clin Nutr 2008; 87: 1080S–1086S.

68.

Scott

, Elias

, Moult

, Barnes

, Wills

MR.

Rickets in adult cystic fibrosis with myopathy, pancreatic insufficiency and proximal tubular dysfunction.

Am J Med 1977; 63: 488–92.

69.

Friedman

, Ingman

, Favus

MJ.

Vitamin D metabolism and osteomalacia in cystic fibrosis.

Gastroenterology 1985; 88: 808–13.

70.

Elkin

, Vedi

, Bord

, Garrahan

, Hodson

, Compston

JE.

Histomorphometric analysis of bone biopsies from the iliac crest of adults with cystic fibrosis.

Am J Respir Crit Care Med 2002; 166: 1470–4.

71.

Haworth

CS.

Impact of cystic fibrosis on bone health.

Curr Opin Pulm Med 2010; 16: 616–22.

72.

Paccou

, Zeboulon

, Combescure

, Gossec

, Cortet

The prevalence of osteoporosis, osteopenia and fractures among adults with cystic fibrosis: a systematic literature review with meta-analysis.

Calcif Tissue Int 2010; 86: 1–7.

73.

Stephenson

, Jamal

, Dowdell

, Pearce

, Corey

, Tullis

Prevalence of vertebral fractures in adults with cystic fibrosis and their relationship to bone mineral density.

Chest 2006; 130: 539–44.

74.

Aris

, Renner

, Winders

AD.

Increased rates of fractures and severe kyphosis: sequelae of living into adulthood with cystic fibrosis.

Ann Intern Med 1998; 128: 186–93.

75.

Papaioannou

, Kennedy

, Freitag

Longitudinal analysis of vertebral fracture and BMD in a Canadian cohort of adult cystic fibrosis patients.

BMC Musculoskelet Disord 2008; 9: 125

76.

Orens

, Estenne

, Arcasoy

International guidelines for the selection of lung transplant candidates: 2006 update - a consensus report from the Pulmonary Scientific Council of the International Society for Heart and Lung Transplantation.

J Heart Lung Transplant 2006; 25: 745–55.

77.

Hahn

, Squires

, Halstead

, Strominger

DB.

Reduced serum 25-hydroxyvitamin D concentration and disordered mineral metabolism in patients with cystic fibrosis.

J Pediatr 1979; 94: 38–42.

78.

Mischler

, Chesney

, Mazess

RB.

Demineralization in cystic fibrosis detected by direct photon absorptiometry.

Am J Dis Child 1979; 133: 632–5.

79.

Stephenson

, Brotherwood

, Robert

, Atenafu

, Corey

, Tullis

Cholecalciferol significantly increases 25-hydroxyvitamin D concentrations in adults with cystic fibrosis.

Am J Clin Nutr 2007; 85: 1307–11.

80.

Grey

, Atkinson

, Dury

Prevalence of low bone mass and deficiencies of vitamin D and K in pediatric patients with cystic fibrosis from 3 Canadian centers.

Pediatrics 2008; 122: 1014–20.

81.

Green

, Leonard

, Paranjape

, Rosenstein

, Zeitlin

, Mogayzel

Jr . Transient effectiveness of vitamin D2 therapy in pediatric cystic fibrosis patients. J Cyst Fibros 2010; 9: 143–9.

82.

Conway

, Oldroyd

, Brownlee

, Wolfe

, Truscott

JG.

A cross-sectional study of bone mineral density in children and adolescents attending a Cystic Fibrosis Centre.

J Cyst Fibros 2008; 7: 469–76.

83.

Hall

, Sparks

, Aris

RM.

Vitamin D deficiency in cystic fibrosis.

Int J Endocrinol 2010; 2010: 218691. Epub 2010 Jan 28

84.

Cranney

, Horsley

, O'Donnell

Effectiveness and safety of vitamin D in relation to bone health.

Evid Rep Technol Assess 2007; 158: 1–235.

85.

Black

, Scragg

Relationship between serum 25-hydroxyvitamin D and pulmonary function in the third National Health and Nutritional Examination Survey.

Chest 2005; 128: 3792–8.

86.

Green

, Carson

, Leonard

Current treatment recommendations for correcting vitamin D deficiency in pediatric patients with CF are inadequate.

J Pediatr 2008; 153: 554–9.

87.

Cystic Fibrosis Trust. Bone mineralisation in cystic fibrosis. UK Cystic Fibrosis Trust Bone Mineralisation Working Group. Bromley: Cystic Fibrosis Trust, 2007

88.

Conly

, Stein

, Worobetz

, Rutlege-Harding

The contribution of vitamin K2 (menaquinones) produced by the intestinal flora to human nutritional requirements for vitamin K.

Am J Gastroenterol 1994; 86: 915–23.

89.

Paiva

SAR

, Sepe

, Booth

SL.

Interaction between vitamin K nutriture and bacterial overgrowth in hypochlorhydria induced by omeprazole.

Am J Clin Nutr 1998; 68: 699–704.

90.

Dietary Reference Values for Food Energy and Nutrients for the United Kingdom. Report of the Panel on Dietary Reference Values, Committee on Medical Aspects of Food and Nutrition Policy. London: HMSO, 1991

91.

Durie

PR.

Vitamin K and the management of the patient with cystic fibrosis.

Can Med Assoc J 1994; 151: 933–6.

92.

Booth

Roles for vitamin K beyond coagulation.

Annu Rev Nutr 2009; 29: 89–110.

93.

Mosler

, von Kries

, Vermeer

, Saupe

, Schmitz

, Schuster

Assessment of vitamin K deficiency in CF-how much sophistication is useful?

J Cyst Fibros 2003; 2: 91–6.

94.

Booth

, Al Rajabi

Determinants of vitamin K status in humans.

Vitam Horm 2008; 78: 1–22.

95.

Conway

SP.

Vitamin K in cystic fibrosis.

J R Soc Med 2004; 97: 48–51.

96.

Shwachman

Therapy of cystic fibrosis of the pancreas.

Pediatrics 1960; 25: 155–63.

97.

Di Sant'Agnese

, Vidaurreta

AM.

Cystic Fibrosis of the pancreas.

JAMA 1960; 172: 2065–72.

98.

Corrigan

, Taussig

, Beckerman

, Wagener

JS.

Factor II (prothrombin) coagulant activity in immunoreactive protein: detection of vitamin K deficiency and liver disease in patients with CF.

J Pediatr 1981; 99: 254–57.

99.

Hamid

, Khan

Cerebral hemorrhage as the initial manifestation of cystic fibrosis.

J Child Neurol 2007; 22: 114–15.

100.

McPhail

GL.

Coagulation disorder as a presentation of cystic fibrosis.

J Emerg Med 2010; 38: 320–2.

101.

Rahid

, Durie

, Andrew

Prevalence of vitamin K deficiency in cystic fibrosis.

Am J Clin Nutr 1999; 70: 378–82.

102.

Conway

, Wolfe

, Brownlee

KG.

Vitamin K status among children with cystic fibrosis and its relationship to bone mineral density and bone turnover.

Pediatrics 2005; 115: 1325–31.

103.

De Montalembert

, Lenoir

, Saint-Raymond

, Rey

, Lefrère

JJ.

Increased PIVKA-II concentrations in patients with cystic fibrosis.

J Clin Pathol 1992; 45: 180–1.

104.

Wilson

, Rashid

, Durie

Treatment of vitamin K deficiency in cystic fibrosis: Effectiveness of a daily fat-soluble vitamin combination.

J Pediatr 2001; 138: 851–5.

105.

Van Hoorn

JHL

, Hendricks

JJE

, Vermeer

, Forget

PPh.

Vitamin K supplementation in cystic fibrosis.

Arch Dis Child 2003; 88: 974–5.

106.

Dougherty

, Schall

, Stallings

VA.

Suboptimal vitamin K status despite supplementation in children and young adults with cystic fibrosis.

Am J Clin Nutr 2010; 92: 660–7.

107.

Fewtrell

, Benden

, Williams

JE.

Undercarboxylated osteocalcin and bone mass in 8-12 year old children with cystic fibrosis.

J Cyst Fibros 2008: 7:307–12

108.

Shea

, Dallal

, Dawson-Hughes

Vitamin K, circulating cytokines, and bone mineral density in older men and women.

Am J Clin Nutr 2008; 88: 356–63.