Methods of dietary sodium estimation

P Aparna; Harshal Ramesh Salve; Anand Krishnan; Ramakrishnan Lakshmy; Sanjeev Kumar Gupta; Baridalyne Nongkynrih

doi:10.4103/injms.injms_97_21

REVIEW ARTICLE

Year : 2021 | Volume : 12 | Issue : 4 | Page : 183-187

Methods of dietary sodium estimation

P Aparna¹, Harshal Ramesh Salve¹, Anand Krishnan¹, Ramakrishnan Lakshmy², Sanjeev Kumar Gupta¹, Baridalyne Nongkynrih¹
¹ Centre for Community Medicine, All India Institute of Medical Sciences, New Delhi, India
² Department of Cardiac Biochemistry, All India Institute of Medical Sciences, New Delhi, India

Date of Submission	24-Aug-2021
Date of Acceptance	21-Sep-2021
Date of Web Publication	20-Oct-2021

Correspondence Address:
Dr. P Aparna
232, 20^th Main, B block, 3^rd Stage, Vijaynagar, Mysore - 570 030, Karnataka
India

Source of Support: None, Conflict of Interest: None

DOI: 10.4103/injms.injms_97_21

Abstract

Excessive dietary sodium intake is found to be associated with high blood pressure and its consequences such as cardiovascular disease and stroke. Daily sodium intake is found to vary from place to place as it is dependent on geography, ethnicity, socioeconomic factors, etc. In this review, we summarize the various methods of assessment of daily sodium intake such as dietary estimation methods using 24-h dietary recall, food frequency questionnaire and diet record, and biochemical methods using 24-h urine sodium or spot urine sodium estimation. Daily sodium intake estimated with spot urine sodium is dependent on the equation used to convert spot urine sodium to 24-h urine sodium. To identify the appropriate equation, validation studies need to be conducted. Population level sodium intake and it's monitoring is important for reduction of noncommunicable diseases.

Keywords: Diet, estimation, method, salt, sodium, urine

How to cite this article:
Aparna P, Salve HR, Krishnan A, Lakshmy R, Gupta SK, Nongkynrih B. Methods of dietary sodium estimation. Indian J Med Spec 2021;12:183-7

How to cite this URL:
Aparna P, Salve HR, Krishnan A, Lakshmy R, Gupta SK, Nongkynrih B. Methods of dietary sodium estimation. Indian J Med Spec [serial online] 2021 [cited 2023 Jun 9];12:183-7. Available from: http://www.ijms.in/text.asp?2021/12/4/183/328644

Introduction

Salt or table salt is a chemical compound usually made of approximately 40% sodium and 60% chloride. Sodium and chloride are important electrolytes needed for normal physiological functioning of the human body. Approximately 90% of sodium we eat is derived from the table salt. The terms, salt and sodium, are often used interchangeably by health-care providers as the sodium portion is the one relevant for health and the estimation of one gives the estimate of the other. Sodium is involved in maintaining fluid-electrolyte balance, cell integrity, generating nerve impulses, and various other important functions.^[1] Metabolic studies have shown that sodium balance can be maintained with long term intake of 100–375 mg sodium (i.e., 0.25 g–0.9 g salt) per day.^[2] Over the ages, population-level salt intake has showed upward trend. The global mean salt intake was estimated to be 10.1 g/day (95% confidence interval [CI]: 9.9–10.2) in 2010.^[3] A systematic review done in India which included studies from 1986 to 2014 showed overall mean weighted salt intake as 10.9 g/day (95% CI: 8.6–13.4).^[4]

Scientific and medical evidence associates excessive sodium intake with high blood pressure and secondary consequences such as cardiovascular disease and stroke.^[5],[6] The mechanisms by which dietary salt increases arterial pressure are not fully understood, but they seem related to the inability of the kidneys to excrete large amounts of salt.^[7] High salt intake is also known to cause osteoporosis, obesity, gastric cancer, and chronic kidney disease.^[8]

The World Health Organization (WHO) recommends a reduction in sodium intake to <2 g/day in adults (≥16 years). This recommendation is to reduce blood pressure and risk of cardiovascular disease and stroke.^[9]

The estimates of daily salt intake all over the world are more than the recommendation given by the WHO. Some of the countries consume more than twice the recommendation for daily salt intake.^{[10],[11],[12],[13]} As a part of the “25 by 25” initiative, the WHO set a target of 30% reduction in mean population salt consumption by 2025, which has been also endorsed by all the member nations. This initiative also includes a 25% reduction in premature mortality from cardiovascular disease and a 25% reduction in raised blood pressure under the Global Action Plan for the Prevention and Control of NCDs.^[14] Reducing salt intake is one of the 16 “best buys,” which are considered to be the most cost-effective and feasible for implementation in low-and lower middle-income countries, advocated by the WHO to tackle noncommunicable diseases (NCDs).^[15]

To bring about reduction in salt intake, we need to identify the sources of dietary sodium and estimate the sodium/salt intake and regularly monitor the same. Reducing salt intake is inevitable to bring about a reduction in morbidity and mortality related to excessive salt intake.

This paper presents a review of the methods of dietary sodium estimation under the following headings:

Sources of dietary sodium
Methods for estimation of dietary sodium intake

Dietary estimation

24 h dietary recall
Food frequency questionnaire (FFQ)
Diet record
Limitations of dietary sodium estimation methods.

Biochemical estimation

24 h urine collection
Overnight urine collection
Spot urine collection
Limitations of biochemical estimation.

Important studies estimating dietary sodium intake

INTERSALT (International Study of Salt and Blood Pressure).
The INTERMAP (International Study of Macro-and Micro-Nutrients and Blood Pressure).

Conversion of spot urine sodium into estimates of 24 h sodium excretion.

Sources of dietary sodium

Sodium is used in our diet for various reasons. Adding flavor is probably its most well-known function. It can also be used as a preservative to keep food safe, enhance a food's color or give it a firmer texture. For example, sodium in the form of baking soda (sodium bicarbonate) is used to help bread and other baked goods rise. Even though sodium plays a key role in many foods, more sodium in the form of salt is often added than is necessary.

The sources of dietary sodium can be found using 24-h dietary recall, FFQ, or diet record. Majority of the sodium in our diet comes from the salt that is added while preparing or processing the food, or at the table while consuming the food. Unprocessed foods such as fruits, vegetables, whole grains, nuts, meat, and dairy foods are low in sodium.

Sources of dietary sodium are known to vary between countries. In developed countries, the predominant source of sodium in diet is processed foods. In Mexico, Turkey, and Brazil, bread followed by processed food was found to be the main source of dietary sodium.^{[10],[16],[17]} In the Asian countries, the main source of dietary salt was the salt added while cooking, i.e., discretionary salt.^{[18],[19],[20],[21],[22],[23]} Identification of the major sources of sodium helps us in controlling the daily sodium intake.

Methods for estimation of dietary sodium intake

Dietary sodium intake can be estimated by either dietary estimation or biochemical estimation.^[24] In biochemical estimation method, sodium excretion of urine is used, as most of the consumed sodium gets excreted in the urine. Sodium excretion gives an estimation of salt intake.

Dietary estimation

24 h dietary recall

This method is designed to capture all food and drinks consumed in a period of 24 h. The distribution of single daily intake may show a strong variation. Interviewing in a subsample on a different day (ideally on a weekday and one weekend) to calculate the usual intake will give an estimation of sodium intake.

Food frequency questionnaire

The FFQ includes a limited list of foods and beverages and responses to indicate how often each item was consumed over a specified period of time. FFQ can capture typical intakes rather than a single day of intake. In general, FFQ does not include questions about salt added in cooking or at the table.

Diet record

It is a detailed description of the types and amounts of foods, beverages, and supplements consumed over a specified time period. It is more reliable but requires a high level of knowledge and motivation. It may require training about accurate record intakes.

Limitations of dietary sodium estimation methods

Dietary estimation methods are considered to be more practical for conducting field surveys. However, there are certain limitations which include: Quantifying accurately the amount of sodium chloride added during cooking (including at restaurants) and at the table; variation in the proportion of salt added during cooking that is retained by the food (i.e., salt left behind on the plate); and variation in the sodium content of manufactured foods and in the sodium concentration of local water supplies.

Dietary assessment also allows the linking of sodium intake with dietary patterns or intake of other nutrients (such as potassium) associated with disease-related outcomes in order to inform public health interventions. Sodium intake is highly correlated with energy intake so that if the dietary assessment tool is significantly underestimating total food intake (and therefore total energy intake) then there will be a comparable underestimation of dietary sodium from food sources.

Dietary recall, weighed diet records, food diary, and FFQ are the various methods used for assessing nutrient intakes and are labor intensive for both participants and researchers. While the validity of dietary assessment tools is variable, they are essential for informing public health interventions for dietary sodium reduction, as they enable identification of sources of sodium intake. Identification of foods associated with high intake in different populations and cultural groups is essential to inform public health interventions based on reformulation of processed foods, and consumer education, and changing dietary practices.^[25]

Biochemical estimation

24 h urine collection

Here, 24-h urine sodium is estimated. This method is often used to compare and validate other methods.^[24] Multiple nonconsecutive 24-h urine sodium is considered the gold standard method.

Overnight urine collection

Low-burden alternative to 24-h collection as fewer voidings are required, and the participant does not have to continue the collection during daily activities. However, it may be biased because of the marked diurnal variation in sodium excretion.^[26]

Spot urine collection

This method is a convenient and affordable alternative to 24-h urine collection. This can be used when a calibration study for use of spot urine has been done in the specific population of interest.^[27]

Limitations of biochemical estimation

Biochemical estimation, though considered a better and more objective method also, has its limitations. For example, factors that can affect sodium absorption, metabolism, and excretion, can alter the sodium intake estimation using urinary sodium excretion. In homeostasis, kidneys handle most of the sodium consumed in a day, and approximately 90%–95% of sodium is excreted in the urine within 24 h. To capture the marked diurnal variation in sodium, chloride, and water excretion, 24 h period is necessary. Electrolyte excretion in healthy individuals reaches a maximum at or before midday, and a minimum at night toward the end of sleep.

The 24-h urinary excretion method takes no account of electrolyte loss other than through the kidney and therefore will tend to underestimate true intake. Losses of sodium in the feces are small; losses through sweat are minimal in temperate climates but can be considerable in certain conditions.^[28]

Unlike most dietary methods, the 24-h urine collection is not prone to reporting biases. However, 24-h urine sodium estimation has its own share of disadvantages, such as:

Due to high burden on the participant, rates of attrition may be high
Estimates are dependent on sample collection volume
There is no absolute check on completeness of sample collection (though the sodium/creatinine ratio and para-aminobenzoic acid marker have been used)
The collection must be accurately timed to avoid over as well as undercollection and so that minor deviations from a 24-h collection period can be corrected.

Important studies estimating dietary sodium intake

Twenty-four hour urine collection has been used to assess population sodium intake in the landmark INTERSALT (International Study of Salt and Blood Pressure) study, which estimated dietary sodium intake.^[29] INTERSALT collected standardized data on 24-h urinary excretion of sodium among 10,079 men and women (aged 20–59 years) from 52 population sample clusters in 32 countries, this was by far the most extensive set of standardized data on 24-h urinary sodium excretion patterns around the world. Across the centers, median sodium levels ranged from 4.6 to 5.6 g/24 h and was significantly related to blood pressure in individuals. On multiple regression, a difference of 100 mmol/day in average population sodium intake corresponded to 2.2 mmHg lower systolic pressure.

The INTERMAP (International Study of Macro-and Micro-Nutrients and Blood Pressure) included measurements of dietary sodium intake and urinary sodium excretion from men and women, aged 40–59 years, from 17 population samples in four countries.^[30] Mean sodium excretion of the total 4680 participants was 4165 mg. The participants were divided into quartiles based on the excretion of sodium. With control for age, sex, and sample only, systolic blood pressure and diastolic blood pressure were consistently higher for those in the highest, compared with those in the lowest quartile of sodium excretion.

Conversion of spot urine sodium into estimates of 24 h sodium excretion

Spot urine sodium has been considered to be an affordable alternative to 24 h urine sodium. There have been few published formulae or equations to convert spot urinary sodium into estimates of 24 h excretion [Table 1]. Two have been derived in Japanese populations by Tanaka et al. and Kawasaki et al.^[31],[32] One equation has been proposed by the Pan American Health Organisation (PAHO), the regional office of the WHO (the PAHO formula), and one equation derived from analysis of data from the INTERSALT study in North American and European populations.^[27],[33] Toft equation was developed for the Danish population.^[34] The various variables used in these equations are age, weight, height, sex, spot urine sodium, spot urine potassium, and spot urine creatinine. The choice of formula depends on the population being studied. The sodium intake calculated using these equations only gives us the estimate of population-level sodium intake and not of the individual.

Table 1: Equations used for the conversion of spot urine sodium to 24 h urine sodium

Click here to view

The studies which compared the mean dietary salt intake based on spot urine sodium and 24 h urine sodium showed that the estimate of daily salt intake calculated with spot urine sodium is dependent on the equation used. Kawasaki equation was found to overestimate the daily intake, and the INTERSALT equation gave an estimate comparable with the 24 h urine sodium in some studies.^{[35],[36],[37],[38],[39]} More calibration or validation studies are required to find the appropriate equation for any particular region of the world.

Conclusion

Salt reduction at the population level is one important component in reducing cardiovascular diseases.^[40] Dietary sodium intake can be estimated by either dietary estimation or biochemical estimation. Estimates based on the food diary, weighed records, food-frequency questionnaire, or 24-h dietary recall tend to underestimate sodium intakes as compared with 24-h urine collections. Hence, 24-h urinary sodium excretion has become the preferred method of obtaining data on sodium intakes in population surveys. However, from a practical point of view, the spot urine or casual urine sample collection is more feasible for participants and is the preferred method for large population and surveys.

Salt reduction at population level has been identified as one of the top five priority interventions to prevent NCDs based on certain parameters such as health effects, cost-effectiveness, low implementation costs, and financial and political feasibility.^[41] In order to reduce the growing burden caused by the NCDs, the countries all over the world should take measures for reduction of salt consumption at the population level.

Financial support and sponsorship

None.

Conflicts of interest

There are no conflicts of interest.

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