Considerations for Measurement of Sodium Intake

Abstract

Current salt consumption is the major risk factor for hypertension and consequently cardiovascular disease (CVD). Accurate measurement of Na intake is an important component of developing dietary interventions to treat hypertension and lower CVD risk. Given that existing methods have a large subject burden, quick and practical ways to assess Na intake in individuals, particularly in hypertensive subjects, are needed. Such tools may be used for motivation to quantify salt intake and to set targets for lifestyle changes for prevention of CVD within a clinic setting. Patients at high risk of development of CVD may be identified and targeted for motivational interviewing. They may also be used as part of cardiac rehabilitation programs and will allow individuals to measure their own intake and to see the results of their individual action.

There is strong evidence that current salt consumption is the major risk factor for hypertension and consequently CVD.

Cardiovascular disease (CVD) is the leading cause of death in Hawaii, the United States, and worldwide.^1-3 It has been shown that 80% of major cardiovascular events can be accounted for by 3 risk factors: hypertension, hyperlipidemia, and cigarette smoking. ⁴ In 2017, nearly half a million deaths in the United States included hypertension as a primary or contributing cause. ⁵ There is strong evidence that current salt consumption is the major risk factor for hypertension and consequently CVD. ⁶ In addition to contributing to hypertension, salt-loading has also been linked to left ventricular hypertrophy, as well as cardiac, renal, and arterial fibrosis. ⁷ Given the adverse effects of excess Na (sodium) intake, the American Heart Association has put forth a call to action for population-wide Na reduction, with special attention to subpopulations at risk including persons with hypertension, as a key component of public health efforts to prevent CVD, stroke, and kidney disease. ⁸ Many countries, including Canada, the United States, and Brazil, have used their dietary guidelines to provide recommendations regarding Na consumption and to advise the population on the risks related to high Na intake.^9-11

The Canadian Dietary Guidelines recommend people aged 14 and over should not consume more than 2300 mg of Na per day as the upper limit (UL), because a “intake above 2300 mg Na per day is likely to pose a health risk.” ⁹ Despite the governmental efforts to decrease Na consumption, ¹² results from the 2017 Canadian Health Survey show that over 58% of Canadians aged 1 year and older had Na intakes exceeding the UL. ¹³ In 2015-2016, the average Na intake of the US population was 3535 mg per day, ¹⁴ well above the recommendations established by the 2010 Dietary Guidelines for Americans of 2300 mg per day for the general population and 1500 mg per day for subpopulations at risk. ¹⁰ The Brazilian food guide from 2015 focuses on overall diet quality, not mentioning specific Na intake values, and affirming “sodium is an essential nutrient, but it is beneficial only in small amounts. In large amounts, it increases the risk of heart diseases.” ¹¹

Over the last decades, the consumption of fresh food items has been steadily replaced in many countries by the consumption of processed and ultra-processed food products. The high intake of such foods was demonstrated in a prospective study performed with patients undergoing Roux-en-Y gastric bypass surgery in Brazil, which found the consumption of processed and ultra-processed food exceeded 50% of the total calorie intake of the diet. ¹⁵ Potential harmful effects on health of these foods are due to higher contents of added sugar, saturated fat, and sodium.^16-19 Although the food industry is engaged in Na reduction activities, there are still necessary changes to be made on the Na content of many processed food items. ¹² The efforts toward reducing Na consumption worldwide must be coordinated action between individuals, health agencies, and food companies.

In addition, it is important to address the needs of those particularly at risk of developing CVD. To address the racial/ethnic disparities that exist in CVD, programs have been developed to address risk factors that disproportionately affect specific populations, which include dietary recommendations such as decreased salt intake. ²⁰ Importantly, such efforts need to be supported by effective and validated tools for Na intake assessment.

Measurement of Na Intake

The accurate measurement of Na intake, both to set goals for intake and to evaluate the impact of low Na diets, has become an important component of developing dietary interventions to treat hypertension and lower CVD risk. A recent article focused on intake of electrolytes and CVD risk. In this article, Bennett et al comment on the limitations of current methods to assess Na intake. ²¹ The gold standard for estimating salt intake is repeated 24-hour measurement of urinary Na excretion, ²² although this is not practical for large studies, is inconvenient for individuals, and is potentially prone to measurement errors in the outpatient setting. Importantly, 24-hour urinary Na measurement also cannot identify specific sources of salt, and cannot account for the high day-to-day variability in Na intake in the typical diet. ²³ Spot urine collection may also be used to assess intake, and while cheaper and less cumbersome than 24-hour collection, individual measurements are prone to measurement error due to intra-individual variability in Na intake. ²⁴ Another existing method of assessing Na intake is the 24-hour dietary recall. ²² This self-report method has less subject burden than collection of urine samples, but relies on memory and may not accurately capture the amount of salt added to foods in preparation or at the table.

Struggles with finding the best dietary measurement method for Na intake started many years ago, especially for studies with large populations. ²² In addition to measuring intake of Na naturally present in food, it is also relevant to assess and quantify the salt added to the food while cooking or while eating, which is known as “table salt.” ²⁵ Depending on individual socioeconomic and cultural dietary habits, the amounts of added salt can represent a significant portion of the total daily Na intake. ²⁶ Recently, methods involving the collection of data in real time using technology have also been developed and may be used to quantify Na intake, such as capturing images of food using camera-enabled cell or smartphones. ²⁷ While these methods do not require recall, training of participants on use of the technology is required, and accuracy is unknown. ²⁸ In studies examining Na intake, 2 or more methods may be used to enhance accuracy; for example, several dietary recalls may be performed along with 24-hour urine collection. ²⁹

A dietary measurement tool frequently used to capture usual dietary intake is the food frequency questionnaire (FFQ). Considered the most practical and cost-effective method of collecting comprehensive dietary data in large epidemiological studies, FFQs are relatively brief, convenient, and a reliable method to estimate dietary Na intake over time. ³⁰ These tools list specific foods and require respondents to report if they eat them and, if so, how often and how much they eat. ³¹ The number of food items listed on this type of questionnaire tends to vary widely, and those with a more specific purpose, such as assessing consumption of foods high in Na, may be shorter than those examining the whole diet. ³² Among its advantages, the FFQ can measure the duration of high Na content food consumption and can account for a variety of dietary sources of salt intake, which is very useful due to regional, cultural, and ethnic differences in dietary behaviors. ²² Such tools are generally developed using previous dietary survey information in the population of interest; thus, a tool developed in one population may not apply to others with very different dietary habits. The validity and reliability of the FFQ must be assessed prior to use to determine whether it provides consistent results and measures the dietary variables of interest. These tools are used commonly in a wide range of situations and types of dietary studies. In clinical settings, brief FFQs are generally used to discriminate between high and low consumers of certain foods or nutrients. ³²

Assessment of validity and reliability of a newly developed tool to assess dietary intake is important to ensure that the tool will measure the construct it purports to measure and will yield consistent results. ³³ Validation with tools using biomarkers is recommended because this provides an objective measure of intake, in contrast with self-report measures, which are subject to a host of biases. Two previous studies have assessed validity of FFQs designed to assess Na intake using urinary Na as one of the validation measures, and these tools demonstrated reasonable evidence of validity and reliability for use in the target populations in Brazil and South Africa.^26,34 While these tools were found to be valid and reliable in the populations in which they were tested, they do not reflect the dietary patterns of the US population, as the sources of dietary Na may substantially differ. Therefore, the use of these FFQs in the general US population may not be accurate. Indeed, given that the US population is composed of many racial/ethnic groups with distinct dietary habits, the ideal FFQ tool to assess intake should contain foods consumed in various segments of the US population to ensure the generalizability of the tool and to allow for use in multiple groups. Asians, the fastest growing minority population in the United States, for example, have high Na intake and merit inclusion in the development of such as tool.

Conclusions

There is a need for quick and practical ways to assess Na intake in individuals, particularly in hypertensive subjects, to ensure that individual counseling would be based on the best available evidence. Currently, there are no existing tools to assess Na intake that have been developed and validated in a multiethnic population in the United States that include Asians, who exhibit high mortality rates from CVD. In US states with large Asian populations, such as Hawaii, this is of particular importance.