Association between dietary folate intake and severe abdominal aorta calcification in adults: A cross-sectional analysis of the national health and nutrition examination survey

Abstract

Background

Prior studies have established a connection between folate intake and cardiovascular disease (CVD). Abdominal aortic calcification (AAC) has been introduced as a good predictor of CVD events, but no previous study has investigated the relationship between dietary folate intake and severe AAC. Therefore, the study aims to explore the association between dietary folate intake and severe AAC in the United States (US) middle-aged and elderly population.

Methods

This study employed cross-sectional data from the 2013-2014 National Health and Nutrition Examination Survey (NHANES) to examine the relationship between dietary folate intake and severe AAC. Two 24-h dietary recall interviews were conducted to assess dietary folate intake and its sources, while a DXA scan was used to determine the AAC score. To analyze the association between dietary folate intake and severe AAC, a multivariable logistic regression model was applied, and a subgroup analysis was performed.

Results

Our analysis utilized data from 2640 participants aged 40 years and above, including 288 individuals diagnosed with severe AAC. After adjusting for confounding factors, we observed an inverted L-shaped association between folate intake and severe AAC. Upon further adjustment for specific confounding factors and covariates, the multivariable-adjusted odds ratios (ORs) and corresponding 95% confidence intervals (CIs) for the second, third, and fourth quartiles of folate intake, using the first quartile as the reference, were as follows: 1.24 (0.86-1.79), 0.86 (0.58-1.27), and 0.63 (0.41-0.97), respectively. Subgroup analysis results were consistent with the logistic regression models, indicating concordant findings. Moreover, no significant interaction was observed in the subgroup analyses.

Conclusions

The study findings suggest an inverted L-shaped association between dietary folate intake and severe AAC. However, additional prospective investigations are necessary to explore the impact of dietary folate intake on severe AAC in patients.

Keywords

Abdominal aortic calcification epidemiology national health and nutrition examination survey dietary folate intake associated

Introduction

Globally, the number of patients with cardiovascular disease (CVD) increases year by year, from 271 million in 1990 to 523 million in 2019.¹ With the characteristics of poor prognosis and high disability and death rates, cardiovascular disease seriously affects people’s quality of life.^2–4 Recent studies have highlighted vascular calcification is a major risk factor for cardiovascular morbidity and mortality.^5,6 The development of vascular calcification is partly attributed to mineral metabolism disorders, such as hyperphosphatemia, which triggers the transformation of vascular smooth muscle cells (VSMCs) into collagen-secreting osteoblasts.⁷ While coronary artery calcification has been the primary focus of vascular calcification research in the past, there has been a growing emphasis on abdominal aortic calcification (AAC) in recent years.⁸ AAC has emerged as a subclinical indicator of atherosclerosis, with comparable predictive capabilities for cardiovascular disease as coronary artery calcification.^9,10 Multiple studies have identified vascular calcification as a prognostic indicator for cardiovascular mortality in patients.¹¹ Consequently, comprehending the factors influencing aortic calcification could significantly impact the progression and prognosis of CVD. In recent years, the significance of dietary intake factors in relation to aortic calcification has gained increasing recognition.¹²

Folate, also referred to as vitamin B9, is a water-soluble B vitamin with a crucial role in amino acid metabolism.¹³ Its metabolic function involves providing the necessary building blocks, known as one-carbon units, for the synthesis of purines and pyrimidines. These compounds are essential for the synthesis of DNA and RNA.¹⁴ Since the human body cannot synthesize folate endogenously, it must be obtained from dietary sources or supplements.¹⁵ Rich dietary sources encompass green leafy vegetables, legumes, liver, eggs, and fortified grain products.¹⁶ Folic acid actively engages in the homocysteine metabolic pathway, directly or indirectly reducing plasma homocysteine levels, a recognized factor in vascular dysfunction and heightened cardiovascular disease risk.^17–19 Multiple randomized controlled trials (RCTs) substantiate that folate supplementation significantly lowers intima-media thickness (IMT) in cardiovascular disease (CVD) patients or high-risk individuals,^20–22 although one RCT reported nonsignificant outcomes.²² A preceding NHANES study unveiled elevated risk of severe abdominal aortic calcification (AAC) in individuals with either low or high red blood cell (RBC) folate levels.²³ However, research has pointed out that the increased blood/serum folate levels may not be strictly related to dietary folate intake, but be a result of metabolic specificity.²⁴ Consequently, evaluating the connection between dietary folate intake and AAC is imperative.

Nonetheless, the precise correlation between folate intake and AAC remains unexamined in a nationally representative U.S. population. Thus, our hypothesis postulates that higher dietary folate intake may correspond with a reduced prevalence of severe AAC. To probe this conjecture, we conducted an observational study to assess the correlation between dietary folate intake and severe AAC in a representative sample of middle-aged and older adults utilizing data from the 2013-2014 National Health and Nutrition Examination Survey (NHANES). NHANES serves as a comprehensive survey, monitoring the nutritional status of the U.S. civilian populace.

Methods

Data sources and study population

This cross-sectional study utilized population data from the 2013-2014 cycle of the National Health and Nutrition Examination Survey (NHANES), which is conducted by the National Center for Health Statistics (NCHS) to obtain a representative sample of the private, noninstitutionalized population in the United States.²⁵ Dietary assessments were conducted in private settings within an NHANES Mobile Examination Center (MEC).²⁶ Our analysis focused on individuals over 40 years (n = 3815) who underwent evaluation for abdominal aortic calcification (AAC) among the 10,175 participants who completed the interviews. After excluding 675 subjects with missing data on abdominal aortic calcium and 500 subjects with missing information on folate intake, a final sample of 2640 participants were used for analysis. The selection process is depicted in Figure 1. Written informed consent was obtained from all participants, and each NHANES cycle was approved by the NCHS institutional review board. For a more detailed explanation of the NHANES study design and procedures, please refer to the reference 27. Throughout the study reporting, we followed the Strengthening Reporting of Observational Studies in Epidemiology (STROBE) guidelines.

Figure 1.

Flowchart of patient selection.

Dietary folate intake

All NHANES study participants were eligible to complete a 24-h dietary recall questionnaire, which collected comprehensive information on the types and quantities of food consumed within the previous 24 h.²⁸ To evaluate the participants’ daily folate intake, NHANES investigators employed two 24-h food recall interviews.²⁹ The initial interview took place in-person at the mobile examination center (day 1 recall), while the second interview was conducted over the phone 3-10 days later (day 2 recall).³⁰ Participants were requested to report their daily folate intake, including supplements, for the past 30 days and were encouraged to provide additional reference bottles. To calculate the total folate intake, encompassing folic acid from foods, fortified foods, and supplements, we converted it into dietary folate equivalents. The subjects were then categorized into four groups based on their quartile of folate consumption.

Abdominal aortic calcification

As previously detailed in the literature,^23,31,32 abdominal aortic calcification (AAC) assessment involved the precise use of dual-energy X-ray absorptiometry (DXA) for lateral scanning of the lumbar spine, focusing on vertebrae L1-L4. DXA machines can capture single-energy lateral spine images, effectively detecting AAC.^33–35 During the 2013-2014 NHANES Mobile Examination Center (MEC) evaluation, transverse DXA scans of the thoracolumbar spine facilitated immediate vertebral assessment (IVA) through lateral spine scans. This approach measured as AAC at vertebrae T4-L4 and L1-L4.³⁶ The AAC score was generated using the AAC-24 semiquantitative scoring technique (SQ), dividing the anterior and posterior aortic walls into four segments corresponding to the anterior regions of lumbar spine L1-L4. Each vertebral level received a score ranging from “0” to “6,” culminating in an overall score on the AAC-24 scale.³³ A higher AAC score signifies a more extensive degree of calcification. The AAC score ranges from 0 to 24, with a severity threshold set at greater than 6 (>6) as established in previous studies^23,37–40 Based on the AAC score, calcification severity was categorized into two groups: no severe calcification (0 < AAC ≤6) and severe calcification.

Covariates

The current study employed covariates selected through clinical expertise and references from published literature.^37,41,42 These covariates encompassed a range of factors including sex, age, race, poverty-income ratio, education, body mass index, heart failure, coronary heart disease (CHD), angina, stroke, hypertension, diabetes, kidney stone, alcohol consumption, smoking, albumin, total cholesterol, creatinine, calcium, phosphorus, aspartate aminotransferase (AST), and 25-Hydroxy Vitamin D2 and D3 (25OHD2+25OHD3). Participants were categorized into two groups based on sex, namely female and male. Racial or ethnic categories included Mexican Americans, other Hispanics, non-Hispanic whites, non-Hispanic blacks, non-Hispanic Asians, and other (multiethnic) groups. Education level was classified as less than high school, high school graduate, and beyond high school. The poverty-income ratio (PIR), which calculates household income relative to the federal poverty line for household size, was categorized as at or above the poverty level (PIR ≥1.3) and below the poverty level (PIR <1.3).⁴³ Alcohol usage and current cigarette smoking were classified as present or absent. History of adverse cardiovascular events, such as heart failure, angina, stroke, hypertension, and coronary heart disease (CHD), was recorded.⁴⁴ Biochemical indicators included albumin, total cholesterol, creatinine, calcium, phosphorus, AST, and 25-Hydroxy Vitamin D2 and D3 (25OHD2+25OHD3).

Statistical analysis

For the analysis of baseline characteristics, we employed the chi-square test and one-way analysis of variance (ANOVA) to compare the participants’ characteristics. Continuous variables were presented as mean and standard deviation (SD) or median and interquartile range (IQR), while categorical variables were expressed as population proportions and percentages. We used one-way ANOVA and chi-square tests to examine the statistical differences in dietary calcium intake across baseline characteristics. To handle missing baseline data, we performed multiple imputation by filling in the missing values based on the dietary folate intake of our population. In this study, men were categorized into four groups based on their daily dietary folate intake in milligrams (Q1: ≤254.4 mg/day, Q2: 254.4 mg/day-349.2 mg/day, Q3: 349.2 mg/day-471.5 mg/day, Q4: ≥471.5 mg/day).

To assess the impact of dietary folate intake on severe abdominal aortic calcification (AAC), we used binary logistic regression models, reporting odds ratios (OR) and 95% confidence intervals (CI), while adjusting for major covariates. The following models were used: Model 1: No adjustment. Model 2: Adjusted for demographic variables, including sex, age, race, poverty-income ratio, and education. Model 3: Adjusted for demographic variables, body mass index (BMI), and underlying diseases such as heart failure, coronary heart disease (CHD), angina, stroke, hypertension, diabetes, and kidney stones. Model 4: Adjusted for demographic variables, BMI, underlying diseases, and lifestyle factors such as smoking and drinking. Model 5: Adjusted for demographic variables, BMI, lifestyle factors, underlying diseases, and biochemical indicators including albumin, total cholesterol, creatinine, calcium, phosphorus, aspartate aminotransferase (AST), and 25-Hydroxy Vitamin D2 and D3 (25OHD2+25OHD3).

To account for the non-linear relationship between dietary folate intake and severe AAC, we used generalized additive models and smooth curve fitting. Additionally, we employed two-piecewise binary logistic regression models or linear regression models to examine non-linearity and provide further explanation.

For subgroup analyses, we used hierarchical binary logistic regression models. Continuous variables were categorized based on clinical cut-offs or quantiles, and interaction tests were conducted. Effect adjustment tests were performed on subgroup measures, followed by likelihood ratio tests. To handle missing covariate data, sensitivity analysis was conducted using multiple imputation. Data analysis was performed using R-4.1.1 (R Foundation for Statistical Computing, Vienna, Austria) and FREE Statistics (version 1.7). Statistical significance was defined as p-values less than 0.05 using a two-tailed test.

Results

Selected baseline characteristics of the study population

Following a rigorous screening process, a final analysis was conducted on a cohort of 2640 participants. Table 1 provides an overview of the descriptive characteristics of the study population. The baseline characteristics of the enrolled participants, comprising 1243 men and 1397 women, were categorized into quartiles based on their folate intake: Q1 (≤254.4 mg/day), Q2 (254.4 mg/day-349.2 mg/day), Q3 (349.2 mg/day-471.5 mg/day), and Q4 (≥471.5 mg/day). Among the participants, 288 individuals (10.9%) exhibited severe abdominal aortic calcification. An association was observed between higher folate intake and male sex, non-Hispanic white ethnicity, age below 60 ages, poverty income ratio of 1.3 or higher, higher body mass index (BMI), lower smoking rates, absence of hypertension, lower prevalence of diabetes, higher levels of AST, and a decreased incidence of severe calcification.

Table 1.

Characteristics of the study population, national health and nutrition examination survey (NHANES) 2013–2014 (N = 2640).

Variables	Total (n = 2640)	Q1 (≤254.4)	Q2 (254.4-349.2)	Q3 (349.2-471.5)	Q4 (≥471.5)	p value^a
Gender, n (%)						<0.001
Male	1243 (47.1)	217 (32.9)	280 (42.4)	329 (49.9)	417 (63.1)
Female	1397 (52.9)	443 (67.1)	380 (57.6)	330 (50.1)	244 (36.9)
Race, n (%)						<0.001
Mexican American	337 (12.8)	75 (11.4)	80 (12.1)	85 (12.9)	97 (14.7)
Other hispanic	244 (9.2)	57 (8.6)	64 (9.7)	59 (9)	64 (9.7)
Non-hispanic white	1221 (46.2)	300 (45.5)	303 (45.9)	312 (47.3)	306 (46.3)
Non-hispanic black	523 (19.8)	176 (26.7)	142 (21.5)	119 (18.1)	86 (13)
Non-hispanic Asian	263 (10.0)	34 (5.2)	62 (9.4)	74 (11.2)	93 (14.1)
Other	52 (2.0)	18 (2.7)	9 (1.4)	10 (1.5)	15 (2.3)
Year, n (%)						<0.001
<60	1398 (53.0)	317 (48)	332 (50.3)	360 (54.6)	389 (58.9)
>60	1242 (47.0)	343 (52)	328 (49.7)	299 (45.4)	272 (41.1)
Education, n (%)						<0.001
Below high school	544 (20.6)	164 (24.9)	124 (18.8)	122 (18.5)	134 (20.3)
High school	598 (22.7)	175 (26.6)	145 (22)	145 (22)	133 (20.1)
Above high school	1497 (56.7)	320 (48.6)	391 (59.2)	392 (59.5)	394 (59.6)
Poverty-income ratio, n (%)						<0.001
<1.3	763 (28.9)	236 (35.8)	176 (26.7)	161 (24.4)	190 (28.7)
≥1.3	1877 (71.1)	424 (64.2)	484 (73.3)	498 (75.6)	471 (71.3)
Body mass index, n (%)						0.003
<25	716 (27.1)	165 (25)	176 (26.7)	168 (25.5)	207 (31.3)
25-30	958 (36.3)	226 (34.2)	230 (34.8)	247 (37.5)	255 (38.6)
>30	966 (36.6)	269 (40.8)	254 (38.5)	244 (37)	199 (30.1)
Heart failure, n (%)						0.407
Yes	91 (3.4)	25 (3.8)	29 (4.4)	18 (2.7)	19 (2.9)
No	2545 (96.4)	633 (95.9)	630 (95.5)	640 (97.1)	642 (97.1)
Boundary	4 (0.2)	2 (0.3)	1 (0.2)	1 (0.2)	0 (0)
CHD, n (%)						0.074
Yes	145 (5.5)	36 (5.5)	47 (7.1)	24 (3.6)	38 (5.7)
No	2490 (94.3)	622 (94.2)	612 (92.7)	633 (96.1)	623 (94.3)
Boundary	5 (0.2)	2 (0.3)	1 (0.2)	2 (0.3)	0 (0)
Angina, n (%)						0.888
Yes	88 (3.3)	26 (3.9)	23 (3.5)	20 (3)	19 (2.9)
No	2549 (96.6)	633 (95.9)	636 (96.4)	638 (96.8)	642 (97.1)
Boundary	3 (0.1)	1 (0.2)	1 (0.2)	1 (0.2)	0 (0)
Stroke, n (%)						0.004
Yes	114 (4.3)	47 (7.1)	22 (3.3)	22 (3.3)	23 (3.5)
No	2523 (95.6)	612 (92.7)	637 (96.5)	637 (96.7)	637 (96.4)
Boundary	3 (0.1)	1 (0.2)	1 (0.2)	0 (0)	1 (0.2)
Smoking, n (%)						0.122
Yes	1204 (45.6)	303 (45.9)	278 (42.1)	315 (47.8)	308 (46.6)
No	1434 (54.3)	355 (53.8)	382 (57.9)	344 (52.2)	353 (53.4)
Boundary	2 (0.1)	2 (0.3)	0 (0)	0 (0)	0 (0)
Hypertension, n (%)						<0.001
Yes	1260 (47.7)	358 (54.2)	322 (48.8)	315 (47.8)	265 (40.1)
No	1378 (52.2)	301 (45.6)	338 (51.2)	343 (52)	396 (59.9)
Boundary	2 (0.1)	1 (0.2)	0 (0)	1 (0.2)	0 (0)
Diabetes, n (%)						0.004
Yes	438 (16.6)	143 (21.7)	112 (17)	94 (14.3)	89 (13.5)
No	2091 (79.2)	492 (74.5)	518 (78.5)	535 (81.2)	546 (82.6)
Boundary	111 (4.2)	25 (3.8)	30 (4.6)	30 (4.6)	26 (3.9)
Alcohol, n (%)						0.005
Yes	1841 (69.7)	429 (65)	455 (68.9)	473 (71.8)	484 (73.2)
No	713 (27.0)	210 (31.8)	188 (28.5)	166 (25.2)	149 (22.5)
Boundary	86 (3.3)	21 (3.2)	17 (2.6)	20 (3)	28 (4.2)
Kidney stone, n (%)						0.226
Yes	295 (11.2)	85 (12.9)	78 (11.8)	65 (9.9)	67 (10.1)
No	2342 (88.7)	573 (86.8)	582 (88.2)	594 (90.1)	593 (89.7)
Boundary	3 (0.1)	2 (0.3)	0 (0)	0 (0)	1 (0.2)
25OHD2+25OHD3, mean ± SD	71.0 ± 29.3	70.4 ± 31.9	70.5 ± 30.3	70.5 ± 27.0	72.6 ± 27.8	0.436
AST, mean ± SD	42.2 ± 3.1	41.9 ± 3.2	42.0 ± 3.1	42.4 ± 3.0	42.6 ± 3.0	<0.001
Albumin, mean ± SD	25.6 ± 15.6	26.0 ± 20.7	25.4 ± 12.8	25.2 ± 16.4	25.7 ± 10.5	0.797
Calcium, mean ± SD	9.5 ± 0.4	9.5 ± 0.4	9.4 ± 0.3	9.4 ± 0.4	9.4 ± 0.4	0.446
Total cholesterol, mean ± SD	195.8 ± 44.1	197.6 ± 43.3	195.5 ± 48.4	197.8 ± 44.6	192.3 ± 39.6	0.085
Creatinine, mean ± SD	0.9 ± 0.5	1.0 ± 0.8	0.9 ± 0.3	0.9 ± 0.5	0.9 ± 0.2	0.083
Phosphorus, mean ± SD	3.8 ± 0.6	3.8 ± 0.6	3.8 ± 0.6	3.8 ± 0.6	3.8 ± 0.6	0.995
Severe AAC, n (%)						<0.001
No	2352 (89.1)	574 (87)	569 (86.2)	595 (90.3)	614 (92.9)
Yes	288 (10.9)	86 (13)	91 (13.8)	64 (9.7)	47 (7.1)

The unit of folate is mg/day.

Abbreviations: %, weighted proportion.; AAC: Abdominal aortic calcification.

Severe AAC: AAC score>6,no severe AAC: AAC score ≤6.

^ap values of multiple comparisons were corrected by the False Discovery Rate method.

Q1-Q4: Quartile according to folate absorption.

Association of folate intake with severe AAC

The association between folate intake and severe abdominal aortic calcification (AAC) was examined using multivariate logistic regression analysis. Table 2 presents the results of this analysis. The findings indicate a significant inverse relationship between higher folate intake and the risk of severe AAC. After adjusting for confounding variables, the odds ratios (OR) for quartiles Q2, Q3, and Q4, in comparison to the lowest quartile (Q1) of folate intake, were 1.24 (95% CI = 0.86-1.79) (p = .255), 0.86 (95% CI = 0.58-1.27) (p = .445), and 0.63 (95% CI = 0.41-0.97) (p = .035), respectively. The observed trend in each quartile exhibited statistical significance (p < .05).

Table 2.

Multivariable logistic regression to assess the association of Folate Intake with severe AAC.

	Model 1		Model 2		Model 3		Model 4		Model 5
Variable	OR_95CI	p value	OR_95CI	p value	OR_95CI	p value	OR_95CI	p value	OR_95CI	p value
Folate quartile
Q1 (≤254.4)	1(Ref)		1(Ref)		1(Ref)		1(Ref)		1(Ref)
Q2 (254.4-349.2)	1.07 (0.78∼1.47)	0.686	1.12 (0.79∼1.57)	0.527	1.18 (0.82∼1.7)	0.367	1.21 (0.84∼1.74)	0.308	1.24 (0.86∼1.79)	0.255
Q3 (349.2-471.5)	0.72 (0.51∼1.01)	0.058	0.76 (0.53∼1.1)	0.145	0.87 (0.59∼1.28)	0.484	0.86 (0.58∼1.27)	0.443	0.86 (0.58∼1.27)	0.445
Q4 (≥471.5)	0.51 (0.35∼0.74)	<0.001	0.57 (0.38∼0.85)	0.006	0.63 (0.41∼0.96)	0.032	0.63 (0.41∼0.97)	0.034	0.63 (0.41∼0.97)	0.035
p for trend		<0.001		0.002		0.017		0.015		0.014

Abbreviations: The unit of folate is mg/day. %, weighted proportion.; AAC: Abdominal aortic calcification.

Severe AAC: AAC score≥6,no severe AAC: AAC score <6.

CI: Confidence interval; OR: Odds ratios, ref: reference.

Model 1: No adjustment.

Model 2: Adjusted for demographic variables (sex, age, race, poverty-income ratio, education).

Model 3: Adjusted for demographic variables, body mass index, basic diseases (heartfailure, CHD, angina, stroke,Hypertension, diabetes,kidney stone).

Model 4: Adjusted for demographic variables, body mass index, basic diseases, life style (alcohol, smoking).

Model 5: Adjusted for demographic variables, body mass index, life style,basic diseases and biochemical index (albumin, total cholesterol, creatinine, calcium, phosphorus, AST,25OHD2+25OHD3).

Dose–response relationships

After accounting for confounding variables, the dose-response relationship between dietary folate intake and severe abdominal aortic calcification (AAC) was analyzed. Figure 2 displays an inverted L-shaped curve, indicating a non-linear association (p for non-linearity = 0.021). To assess this relationship, a threshold effect analysis was conducted and the inflection point of the curve was determined to be 280.126 mg/day, as presented in Table 3. Our analysis demonstrates a significant decrease in the incidence of severe AAC when dietary folate intake exceeds 280.126 mg/day (OR = 0.997, 95% CI: 0.995-0.999, p-value = .0444). Specifically, our findings suggest that with each 1 mg increase in dietary folate intake, there is a relative decrease of 0.3% in the incidence of severe AAC. Conversely, when dietary folate intake is below 280.126 mg/day, there is no significant correlation with the incidence of severe AAC (OR = 1.002, 95% CI: 0.997-1.006, p-value = .529).

Figure 2.

Association between dietary folate intake and severe AAC odds ratio. Solid and dashed lines represent the predicted value and 95% confidence intervals.Adjusted for Gender, age, race, financial status, education level, body mass index, heart failure, coronary artery disease, angina, stroke, smoking, high blood pressure, diabetes, alcohol, kidney stones, albumin, total cholesterol, creatinine, calcium, phosphorus, AST, 25OHD2+ 25OHD3. Only 95% of the data is shown.; Abbreviations: AAC: Abdominal aortic calcification: severe AAC:AAC score≥6, no Severe AAC: AAC score <6 CI: confidence interval; OR: odds ratios, Ref: reference.

Table 3.

Threshold effect analysis of relationship of dietary folate intake and severe AAC.

	Adjusted OR 95CI	p value
Two model
folate intake ≤ 280.126 mg/day	1.002 (0.997∼1.006)	0.529
folate intake ≥ 280.126 mg/day	0.997 (0.995∼0.999)	<0.01
Likelihood ratio test	-	0.03
Non-linear test 1		0.009
Non-linear test 2		0.021

Adjusted for sex, age, race, financial status, education level, body mass index, heart failure, coronary artery disease, angina, stroke, smoking, high blood pressure, diabetes, alcohol, kidney stones, albumin, total cholesterol, creatinine, calcium, phosphorus, AST, 25OHD2+ 25OHD3.

Abbreviations: %, weighted proportion.; AAC: Abdominal aortic calcification.

Severe AAC: AAC score>6,no severe AAC: AAC score ≤6.

CI: Confidence interval;OR: Odds ratios, ref: reference.

Subgroup analysis

In order to further explore the potential association between folate intake and the risk of severe calcification in the abdominal aorta, subgroup analyses were conducted considering various baseline characteristics of the participants. Stratified analyses were undertaken to validate the findings and examine potential bidirectional interactions among factors including sex, age, education level, poverty-income ratio (PIR), and BMI. The results of these subgroup analyses are depicted in the Supplemental Figure. Upon examination, no statistically significant interactions were observed in the subgroup analyses (p for interaction >0.05).

Sensitivity analysis

A sensitivity analysis was conducted to assess the impact of imputation on our findings, as documented in the Supplemental Table. The analysis involved performing a multivariable regression on a subset of 1307 participants, excluding those with missing values for any variables. The objective of the sensitivity analysis was to evaluate the stability and reliability of our results. The findings from the sensitivity analysis confirmed the robustness of our results, consistently demonstrating a significant reduction in the risk of severe AAC associated with higher folate intake, even after adjusting for potential confounding factors. Compared to the lowest quartile (Q1), the adjusted odds ratios (ORs) for Q2, Q3, and Q4 were 1.26 (95% CI = 0.85-1.87) (p = .241), 0.89 (95% CI = 0.58-1.35) (p = .573), and 0.58 (95% CI = 0.36-0.92) (p = .021), respectively.

Discussion

This cross-sectional study aimed to assess the correlation between dietary folate intake and severe AAC in a representative sample of middle-aged and older adults. The study utilized data from the NHANES database, which provided access to a large and diverse population. Our analysis revealed significant associations between dietary folate intake and the risk of severe AAC. Notably, we observed an inverted L-shaped relationship between dietary folate intake and severe AAC, even after accounting for various confounding factors. The inflection point of this relationship was determined as 280.126 mg/day. Moreover, the reliability of our findings was confirmed through sensitivity analysis and subgroup analysis, further bolstering the robustness of the study’s results.

While the relationship between folate and AAC has received limited attention, the association between folate and carotid IMT has been explored in several studies. Two observational studies have identified a connection between low serum folate levels and carotid IMT.^45,46 Furthermore, multiple randomized controlled trials (RCTs) have demonstrated that folate supplementation significantly reduces IMT among patients with cardiovascular disease (CVD) or those at high risk,^20–22 although one RCT reported non-significant results.²² Nonetheless, these RCTs have limitations such as small sample sizes and short intervention durations. The conflicting outcomes emphasize the necessity for further research to investigate the role of folate status in the development of arterial calcification.

Our study introduces a novel approach to investigating the association between dietary folic acid intake and the risk of severe AAC within the general population. We have identified a distinctive inverted L-shaped pattern in the relationship between dietary folate intake and severe AAC. Specifically, there exists a range where increasing dietary folate intake demonstrates notable benefits, with the most significant effects seen in individuals with adequate folate intake, particularly those surpassing 280.126 mg/d. However, upon adjustment for confounding variables using a multifactorial model, our analysis revealed that the odds ratio (OR) for severe AAC in the second quartile (Q2) compared to the first quartile (Q1) was not statistically significant. This outcome may be attributed to limitations such as a small sample size or the presence of an insufficient effect size related to severe AAC. In forthcoming research, we plan to address these limitations by conducting prospective controlled trials with larger sample sizes. It is worth noting that folic acid levels are primarily influenced by dietary sources, including fruits, green leafy vegetables, liver, and fortified grain products.¹⁶ Our findings suggest that elevating dietary folate intake to a specific threshold may hold promise in reducing the risk of vascular calcification and AAC. This novel perspective adds valuable insights to this area of study.

Our research results are incongruent with a prior NHANES study, which identified a U-shaped dose-response relationship between RBC folate levels and the likelihood of severe AAC, even after accounting for various potential confounding factors.²³ In a representative sample of the U.S. adult population, it was observed that individuals with both low and high RBC folate levels faced elevated risks of severe AAC. However, our study has revealed a contrasting pattern: an inverted L-shaped curve between dietary folate intake and abdominal aortic calcification. These discrepant findings may be attributed to metabolic specificity,²⁴ a phenomenon previously observed in other diseases by scholars. Notably, two additional investigations examining dietary and serum folate levels yielded conflicting outcomes.^47,48 In forthcoming research endeavors, we plan to employ prospective cohort studies and more intricate experimental designs, potentially involving larger sample sizes, to further explore this matter.

While the exact mechanism underlying the inverse relationship between dietary folate intake and AAC remains to be fully elucidated, our findings are consistent with existing evidence and offer plausible biological explanations. One potential contributing factor to this association is the modulation of homocysteine levels. Research has demonstrated that folic acid supplementation effectively reduces hyperhomocysteinemia, a recognized contributor to vascular dysfunction and elevated cardiovascular disease risk.^18,19 Additionally, oxidative stress may also play a role, as indicated by a study conducted in Japan, which proposed that uremia-induced oxidative stress could contribute to vascular calcification in CKD.⁴⁹ Animal studies have further revealed that folate deficiency intensifies oxidative stress and various aspects of the metabolic syndrome, both of which are associated with an increased risk of diabetes and cardiovascular disease.⁵⁰ Furthermore, white blood cell telomere length may serve as another potential factor. Studies have shown that longer telomeres are nominally linked to a reduced likelihood of AAC and an increased ABI, indicating a lower prevalence of subclinical atherosclerosis and peripheral arterial disease.⁵¹ The Framingham Offspring Study has identified a potential interference of high plasma folate status, possibly resulting from elevated folic acid intake, with the protective role of folate in maintaining telomere integrity.⁵² In summary, although further investigation is warranted to unravel the precise mechanism, the potential factors contributing to the inverse association between dietary folate intake and AAC include the reduction of homocysteine levels, attenuation of oxidative stress, and preservation of telomere integrity.

Our research offers unique advantages compared to prior related studies. To our knowledge, it represents the inaugural investigation into the correlations between dietary folate intake and severe abdominal aortic calcification in US adults utilizing the NHANES database. Additionally, we employed a Generalized Additive Model (GAM) and smooth curve fitting to assess the non-linear relationship, identifying the optimal inflection point for the impact of dietary folate intake on severe abdominal aortic calcification. Furthermore, to enhance the robustness of our findings, dietary folate intake was examined as both categorical and continuous variables.

Nevertheless, we recognize several limitations within our study. Firstly, the cross-sectional design impedes the establishment of causal relationships between dietary folate intake and severe abdominal aortic calcification. Hence, large-scale prospective cohort studies are imperative to affirm the consistency and robustness of our findings across various models in the future. Secondly, dietary data suffered from recall bias as participants self-reported their diets through 24-h dietary recall interviews, although this method has been utilized in multiple NHANES studies.^53,54 Finally, our study drew data from NHANES, which primarily focuses on the US population and dietary guidelines tailored for Americans. Given the age-related nature of vascular calcification and the heightened susceptibility of middle-aged and elderly individuals to such calcification, our study concentrated on the general adult population aged 40 years and older.⁵⁵ This is significant for vascular calcification prevention. However, further research is necessary to determine whether these associations are applicable in other countries or among younger subjects.

Conclusion

The study’s findings indicate an independent association between dietary folate intake and severe AAC. A dose-response analysis demonstrated an intriguing inverted L-shaped relationship between dietary calcium intake and severe AAC. However, in order to comprehensively comprehend the direct influence of folic acid on the development of severe AAC, further investigation is warranted. Additional research is required to delve into the precise effects of folic acid and its role in the progression of severe AAC.

Supplemental Material

Supplemental Material - Association between dietary folate intake and severe abdominal aorta calcification in adults: A cross-sectional analysis of the national health and nutrition examination survey

Supplemental Material for Association between dietary folate intake and severe abdominal aorta calcification in adults: A cross-sectional analysis of the national health and nutrition examination survey by Kai Zhang, Jianguo Chen, Bowen Chen, Yu Han, Tianyi Cai, JiaYu Zhao, ZhaoXuan Gu, Min Gao, Zhengyan Hou, Xiaoqi Yu, FangMing Gu, Yafang Gao, Rui Hu, Jinyu Xie, Tianzhou Liu, Dan Cui and Bo Li in Diabetes & Vascular Disease Research.

Footnotes

Acknowledgements

Thanks to Dr Liu Jie for his statistics, research design consultation and manuscript editing for the Department of Vascular Surgery of PLA General Hospital.

Author Contributions

KZ contributed as First authors of this manuscript. JGC,BWC, YH,FMG, and ZXG were responsible for the concept and design of the study. JGC, BWC and MG explain the analysis. ZYH,XYQ, TYC, YFG, JYX and TLZ are responsible for data recovery. BL is the primary corresponding author. All authors critically revised the important intellectual content of the paper and approved the final draft.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Jilin Province Health Technology Capability Enhancement Project (2022GL013).

Ethical statement

Compliance with ethical standards

All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.

ORCID iD

Kai Zhang

Data Availability Statement

Our research is based on public data from the NHANES, all details are from the official website ().To obtain the application executable files, please contact the author Kai Zhang by email zhangkai7018@jlu.edu.cn

Supplemental Material

Supplemental material for this article is available online.

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