Revised Reference Values for the Intake of Sodium and Chloride

The D-A-CH ‘reference values for nutrient intake’ [1] are jointly issued by the nutrition societies of Germany, Austria and Switzerland (the abbreviation D-A-CH arises from the initial letters of the common country identification for the countries Germany [D], Austria [A] and -Switzerland [CH]). They elaborated the revised reference values for sodium and chloride, which were published in German in January 2017. This paper gives a summary of this work.

Reference value is a collective term for recommended intake values, estimated values and guiding values. A recommended intake value, according to its definition, meets the requirement of nearly any person (approximately 98%) of a defined group of healthy people. Estimated values are given when human requirements cannot be determined with desirable accuracy. Guiding values are stated in terms of aids for orientation [1]. Different scientific bodies derived dietary reference values for sodium and chloride: For adults, intake values range from 920–2,300 mg/day for sodium [2-4] and an intake value is set at 2,300 mg/day for chloride [2].

Quantitatively, sodium (Na⁺) and chloride are the most abundant electrolytes in the extracellular space [5, 6]. Both electrolytes occur in almost all foods, where they are found naturally or are added during processing, cooking or at the table.

Sodium and chloride are osmotically active in the extracellular fluid. They serve to maintain osmotic pressure as well as water, electrolyte and acid-base balance and affect extracellular volume [7] and blood pressure [8, 9]. At cellular level, sodium is involved in maintaining membrane potential and in facilitating the active transport of molecules across cell membranes. For instance, glucose is transported together with sodium through a coupled mechanism (symport) in small intestinal enterocytes and in renal tubular epithelial cells [5, 7, 10]. Chloride is essential for the production of hydrochloric acid in the parietal cells of the stomach. As a component of gastric juice, hydrochloric acid is involved in digestion and in the defense activities against unspecific pathogens [5].

It is difficult to precisely assess sodium and chloride supply using dietary assessment methods due to the variable extent of salt addition. In fact, it has been demonstrated that dietary assessment methods typically underestimate salt intake [11, 12]. Due to homoeostasis, plasma concentration of sodium and chloride is also not suitable to assess the supply with sodium and chloride. A more accurate measurement of supply is the assessment of 24-h urinary excretion. Based on sodium supply, chloride supply can be determined.

Measurement of 24-h urinary sodium excretion is considered the gold standard method for the assessment of sodium intake [13]. However, 24-h urine collections are complex and require a high level of compliance. Therefore, this method is of limited suitability for large population studies. Thus, in order to assess the sodium intake in the general population, 24-h sodium excretion is often estimated based on spot urinary sodium concentrations. However, due to, for example, large within-person variation, spot urine samples provide relatively inaccurate estimates of sodium intake in individuals. To determine sodium intake in an individual using renal sodium excretion, several 24-h urine samples would be required [14]. It is also possible to extrapolate from spot urine concentration to daily sodium excretion by using a set of published equations [15].

Sodium requirement is dependent on individual and environmental factors and is thus subject to high individual variability. It is therefore not possible to set average requirements for certain population groups. Thus, the reference values for sodium intake are set as estimated values.

Intake of chloride usually occurs together with sodium, due to the addition of salt in food processing and in preparation or intake of foods [5, 16]. Thus, except for infants, the reference values for chloride intake were derived based on the estimated values for sodium intake on the molar level considering the molecular weight of chloride. One mmol sodium is equivalent to 23.0 mg sodium and 1 mmol chloride is equivalent to 35.5 mg chloride, which means that 1 mg sodium (0.04 mmol) corresponds to 1.54 mg chloride.

In extreme conditions, it is possible to survive on very low sodium intake of about 23 mg/day (1 mmol/day) due to adaptive mechanisms regarding sodium excretion through sweat, urine and faeces [17, 18]. However, such a low sodium intake cannot be compared to the physiological requirement in population groups under different circumstances, which can only be determined individually. In most population groups, sodium intake exceeds sodium requirement by a multiple. Only few studies determined the sodium intake that is required for health in persons adapted to low sodium intake over a longer period [19].

In the balance study by Allsopp et al. [20, 21], sodium balance in male subjects was determined at different sodium intake levels (1,500, 4,000, and 8,000 mg/day) over a period of 8 days based on urinary sodium excretion. At a sodium intake of 1,500 mg/day, sodium balance was negative during the first 4 days with increased sodium loss through sweat due to moderate physical activity and heat exposure (from day 4 onwards for 12 h/day at 40°C) in non-acclimatised subjects. However, on the 8th day, mean sodium balance was positive (36.8 ± 117.3 mg/day) at a sodium intake of 1,500 mg/day under the conditions mentioned above. In addition, with a diet providing 1,500 mg sodium/day, the other reference values for nutrient intake except for iodine and fluoride intake are reached [22, 23]. Therefore, the estimated value for adequate sodium intake is set to 1,500 mg/day (Table 1).

Corresponding to the estimated value for sodium intake, the equimolar estimated value for chloride intake for adults from the age of 19 years is set to 2,300 mg/day (Table 1).

It is not considered useful to derive gender-specific estimated values for the intake of sodium and chloride for adults.

No data are available regarding the sodium requirement for children and adolescents. Therefore, the reference values for children and adolescents are based on the values compiled for adults and taking into account differences in body weight and growth factors to consider the requirements for growth (Table 2). Growth factors at the different ages were calculated as the proportional increase in protein requirement for growth relative to the maintenance requirement according to WHO [1, 24]. When -using the age groups and reference body weights, the -D-A-CH reference values are based upon [1] the resulting estimated values for sodium intake range from 400 mg/day (for 1 to under 4-year-olds) to 1,500 mg/day (for 15 to under 19-year-olds; Tables 1, 2).

The estimated values for chloride intake for children and adolescents are set in equimolar amounts corresponding to the estimated values for sodium. The resulting estimated values for chloride intake range from 600 mg/day (for 1 to under 4-year-olds) to 2 300 mg/day (for 15 to under 19-year-olds; Table 1).

Also, for children and adolescents, it is not considered useful to derive gender-specific estimated values for the intake of sodium and chloride.

The reference values for the intake of sodium and chloride for infants aged 0 to under 4 months were derived based on the sodium and chloride content of breast milk, which is considered to be the optimal diet for infants [26, 27]. The reference values for infants are therefore estimated values.

The average breast milk intake of an exclusively breastfed infant is 750 mL/day [28]. Based on a mean sodium and chloride content of breast milk of 17 mg/100 mL [29-35] and 40 mg/100 mL [31-34, 36] within the first 4 months, the estimated value for the intake of sodium and chloride for breastfed infants aged 0 to under 4 months is set at 130 and 300 mg/day respectively (Table 1).

The consumption of breast milk declines along with the introduction of solid foods. Since no data is available from Germany with regard to sodium and chloride intake via solid foods, the estimated value for infants aged 0 to under 4 months is used to derive the reference value for infants over 4 months of age (Table 3). Taking into account the differences of the average body weight, an estimated value of 200 mg sodium/day and of 450 mg chloride/day for infants aged 4 to under 12 months was derived (Tables 1, 3).

In order to maintain the increase in plasma and interstitial fluid volume during pregnancy, additional 70 mg sodium/day is necessary [38]. The organism can provide this quantity due to homoeostatic mechanisms [18, 39]. Thus, the estimated value for sodium intake for pregnant women does not differ from that for non-pregnant women and is set to 1,500 mg/day. Accordingly, the estimated value for chloride intake for pregnant women is set to 2,300 mg/day.

A total of 130 mg sodium/day is secreted with breast milk [29-35], assuming an average breast milk intake of 750 mL/day of an exclusively breastfed infant [28]. The lactating organism can compensate these losses through homoeostatic mechanisms [18]. Thus, the estimated value for sodium intake for lactating women is also set to 1,500 mg/day. Accordingly, the estimated value for chloride intake for lactating women does not differ from that of non-lactating women and is set to 2,300 mg/day.

Sodium and chloride are usually consumed together in the form of sodium chloride or salt. The role of sodium, chloride or salt in the prevention of nutrition-related diseases has recently been summarized by the DGE in a scientific statement on salt intake [40].

There is convincing evidence for an association between salt intake and blood pressure: High salt intake is associated with increased or suboptimal blood pressure and low salt intake with normotensive or optimal blood pressure¹. High blood pressure is a main cardiovascular disease risk factor and thus there is convincing evidence for an indirect effect of high salt intake on the risk of cardiovascular diseases. However, there is currently insufficient evidence for a direct association between high salt intake and risk of cardiovascular diseases due to different and partly contradictory results [40, 41].

There is a high individual variability in sodium requirement, as it is dependent on individual and environmental factors. It is therefore not possible to set an average sodium requirement for a certain population group and the reference values for sodium intake are set as estimated values. Measurement of 24-h urinary sodium excretion is the most accurate measurement of sodium supply.

For adults, the derivation of the reference values is based on one study investigating sodium balance by sodium excretion in male subjects at different sodium intake levels over a period of 8 days. Based on this balance study and the fact that with a diet providing 1,500 mg sodium/day, the other reference values for nutrient intake except for iodine and fluoride intake are reached, the estimated value for sodium intake for adults is set to 1,500 mg/day. Except for infants, the reference values for chloride intake were derived based upon the estimated values for sodium intake on the molar level taking into account the molecular weight of chloride. The estimated value for chloride intake for adults is set to 2,300 mg/day.

Considering dietary recommendations for sodium and chloride, it must be taken into account that high intake of sodium and chloride as salt (sodium chloride) is associated with adverse health effects. For the prevention of hypertension and cardiovascular diseases, it is therefore necessary to lower salt intake in the general population [40].

The authors thank Anna M. Amini, Professor Dr. Stefan Lorkowski, Friederike Maretzke, Birte Peterson-Sperlich, Professor Dr. Bernhard Watzl and Professor Dr. Günther Wolfram for their valuable suggestions and contribution to the preparation of the revised reference values for sodium and chloride intake.

The authors have no conflicts of interest to declare.