Sodium: physiological function, recommendations, intakes and sources

1.6.1 Physiological function of sodium

Sodium is the key cation in extracellular fluid and is an essential nutrient required for acid base balance, maintenance of extracellular volume and serum,

transmission of nerve impulse and normal cell function (Adrogue and Madias 2007). Approximately 98% of sodium is absorbed in the small intestine during digestion and sodium balance is tightly maintained via the renal system (Adrogue and Madias 2007). There are a number of complex mechanisms (which are beyond the scope of this thesis) whereby increased dietary sodium increases cardiovascular risk, the primary effect being to increase blood pressure, increasing risk of stroke and myocardial infarction (Oberleithner et al. 2009; Versari et al. 2009). Put simply, a high salt diet alters sodium balance, which over time causes a reduction in kidney function and water retention, leading to elevated BP (Adrogue and Madias 2007).

1.6.2 Sodium recommendations for adults

In Australia, as in other countries, a system of nutrient reference values (NRVs) has been established by the National Health and Medical Research Council (NH&MRC) to specify the requirements needed for individuals over the life stages (National Health and Medical Research Council 2006) (Table 1.1). Regarding the NRVs for dietary sodium, due to the lack of data from dose- response trials, an Estimated Average Requirement (EAR) could not be determined and therefore Recommended Dietary Intake (RDI) could not be

11 derived (National Health and Medical Research Council 2006). As such, the NH&MRC has set an Adequate Intake (AI) for male and female adults for sodium of 460‒920 mg/d (1.15‒2.3 g salt), to ensure key nutritional requirements are met. Importantly, based on the adverse effects of higher sodium intake on BP, an Upper Level of Intake (UL) for adult males and females for sodium of 2300 mg/d (~6 g/d salt) has also been set (National Health and Medical Research Council 2006). In addition to the recommendations described and in recognition of the health benefits of maintaining low BP throughout life, a Suggested Dietary Target (SDT) of 1600 mg/d (~4 g/d salt) for sodium for both males and females aged 14 years and over is also recommended by the NH&MRC (National Health and Medical Research Council 2006). This SDT (intake per day on average) is also consistent with the WHO sodium guidelines, which recommend that adults consume no more than 2000 mg/d (5 g/d salt) (World Health Organization 2012a).

12 Table 1.1: National Health and Medical Research Council Nutrient

Reference Values definitions (National Health and Medical Research Council 2006) Nutrient Reference Values Definitions Estimated Average Requirement (EAR)

A daily nutrient level estimated to meet the requirements of half the healthy individuals in a particular life stage and gender group.

Recommended Dietary Intake (RDI)

The average daily dietary intake level that is sufficient to meet the nutrient requirements of nearly all (97–98 per cent)

healthy individuals in a particular life stage and gender group.

Adequate Intake (AI)

Used when an RDI cannot be determined. The average daily nutrient intake level based on observed or

experimentally- determined approximations or estimates of nutrient intake by a group (or groups) of apparently healthy people that are

assumed to be adequate. Upper Level of

Intake (UL)

The highest average daily nutrient intake level likely to pose no adverse health effects to almost all individuals in the general population. As intake increases above the UL, the

potential risk of adverse effects increases. Suggested

Dietary Target (SDT)

A daily average intake from food and beverages for certain nutrients that may help in prevention of chronic disease.

Applicable to those aged 14 years and over. 1.6.3 Dietary sodium intake in adults

The estimated physiological need of sodium for adults is ~230‒460 mg/d (~0.6‒ 1.2 g/d salt) (Brown et al. 2009). However, most adult populations from

developed countries have a mean sodium intake between 3600–4800 mg/d (9‒12 g/d salt) with many Asian countries consuming on average more than 4800 mg/d (12 g/d salt) (Anderson et al. 2010; Xu et al. 2014). According to dietary data from the recent 2011–13 Australian Health

13 Survey (AHS), the average sodium intake of Australian adults was 2430

mg/d (6 g/d salt) (Australian Bureau of Statistics 2014) which exceeded the UL of 2300 mg/d (~6 g/d salt) for adults aged 19 years and over and was 1.5 times the Suggested Dietary Target (SDT) of 1600 mg/d (4 g/d salt) (National Health and Medical Research Council 2006). It should be noted however that this reported average sodium amount was underestimated due to misreporting, limited accuracy of nutrient composition of foods in food databases and also as explained earlier in section 1.5, because dietary recall methods fail to capture discretionary salt use. Observational Australian studies that have used the objective measure of 24-hr urine collections to assess daily sodium intake in adults, have therefore reported higher intakes of between 2800–3040 mg/d (7–7.6 g salt/d) in women and 3040–4120 mg/d (9.6–10.3 g salt/d) in men (Land et al. 2014; Nowson et al. 2015).

1.6.4 Dietary sources of sodium in adults

For adults from western countries, approximately 75% of dietary sodium is derived from restaurant or manufactured foods such as breads, processed meat and cereals, ~10% occurs naturally in foods and ~15% is from discretionary use at home (added during cooking or at the table) (James et al. 1987; Mattes and Donnelly 1991; Andersen et al. 2009). In contrast, in developing countries such as China discretionary salt use accounts for the majority of total salt

(approximately 76%) consumed (Anderson et al. 2010).

According to the 2011–13 AHS, key sources of dietary sodium in Australian adults aged 19 years and older were: mixed cereal dishes (e.g. pasta or rice dishes) (14%), breads (13%); processed meats (6%) and cheeses (4%)

14 processed meats (e.g. sausages and frankfurters) were the top three sources of sodium (O'Neil et al. 2012).

1.6.4 Sodium recommendations children

The NH&MRC has also set an AI for children for sodium (as an EAR could not be determined) ranging from 200–400 mg/d (0.5‒1 g/d salt) for 1–3 years old up to 460–920 mg/d (1.15‒2.3 g/d salt) for 14–18 year olds which were derived from adult AIs based on relative energy intakes (National Health and Medical Research Council 2006).

Consistent with adult recommendations and in recognition of the adverse impact on health of consuming excessive amounts of dietary sodium, the WHO advises that the adult sodium levels of 2000 mg/d (salt 5 g/d) should be adjusted

downwards for children based on their energy requirements (World Health Organization 2012a). The NH&MRC recommends an UL for sodium intake in children (Table 1.2) which is in line with these WHO recommendations and (National Health and Medical Research Council 2006) is based on a number of observational studies that have shown that BP tracks with age and is adversely affected by high sodium intake (Bao et al. 1995; Dekkers et al. 2002; Gillman et al. 1993).

15 Table 1.2: National Health and Medical Research Council Nutrient Reference Values Upper Level of Intake (UL) for sodium for Australian children (National Health and Medical Research Council 2006)

Age Upper Level of Intake mg/d (salt

equivalent g/d)

< 12 months UL has not been established and sources of intake should only be through breast milk, formula and food

1‒3 years 1000 (2.5)

4‒8 years 1400 (3.6)

9‒13 years 2000 (5.1)

14‒18 years 2300 (5.9)