Sodium

Sodium (Na+) is the principal extracellular cation and a critical regulator of fluid balance, nerve conduction, muscle contraction, and active transport across cell membranes. Human physiological sodium requirements are remarkably modest — perhaps 200-500 mg/day — but typical dietary intakes vastly exceed these needs due to processed food composition.

Physiological functions

Sodium's key roles: (1) fluid balance — plasma sodium concentration determines plasma osmolality and, by extension, fluid distribution between intracellular and extracellular compartments and between intravascular and interstitial spaces; (2) electrical excitability — voltage-gated sodium channels initiate action potentials in neurons and muscle; (3) active transport — sodium gradients maintained by Na+/K+-ATPase power secondary active transport of glucose (SGLT1, SGLT2), amino acids, calcium, and phosphate across cell membranes; (4) acid-base balance — sodium handling is coupled to bicarbonate reabsorption and proton excretion in the kidney.

Requirements and current intake

The 2019 NAM DRI set Adequate Intake for sodium at 1500 mg/day for adults 19-50 years (with lower values for younger children and older adults) and established a new concept — the Chronic Disease Risk Reduction (CDRR) intake of 2300 mg/day, representing the level above which reduction would reduce chronic disease risk. The 2020-2025 Dietary Guidelines for Americans recommend less than 2300 mg/day. NHANES data indicate mean US sodium intake of approximately 3400 mg/day for adults.

Blood pressure evidence

Sodium intake and blood pressure are related across a dose-response gradient. Meta-analyses (He et al., Cochrane 2013; Aburto 2013) find approximately 3-5 mmHg systolic and 1-2 mmHg diastolic blood pressure reduction with moderate sodium reduction in normotensive individuals, with larger effects (8-10 mmHg systolic) in hypertensive individuals. The DASH-Sodium trial (Sacks et al., NEJM 2001) demonstrated additive effects of dietary pattern and sodium reduction, with lowest blood pressures at low sodium plus DASH combination.

Cardiovascular outcomes

Sodium reduction's effect on hard cardiovascular endpoints has been more contentious than its blood-pressure effect. The TOHP trials (Cook et al., 2007, 2014 long-term follow-up) showed reduced cardiovascular events with sodium reduction in previously pre-hypertensive adults. The PURE study controversially reported a J-shaped relationship with lowest events at intermediate sodium intake (3-6 g/day), but methodological limitations (spot urine estimation, reverse causation) have led major bodies (AHA, WHO) to maintain recommendations for sodium reduction. The SSaSS trial of salt substitution in China confirmed cardiovascular benefit from reducing sodium via K-enriched salt.

Sources in the US diet

Approximately 70% of US dietary sodium comes from processed foods and restaurant meals rather than salt added at the table. Leading sources per CDC analysis: breads and rolls, pizza, sandwiches, cold cuts and cured meats, soups, burritos and tacos, savory snacks, chicken (processed), cheese, and eggs. This distribution implies that meaningful sodium reduction requires food-system changes (reformulation, labeling, policy) beyond individual behavior change.

Salt sensitivity

Blood pressure response to sodium varies by individual. "Salt-sensitive" individuals — more common among older adults, Black populations, and those with obesity, diabetes, or chronic kidney disease — show larger blood pressure changes with sodium intake. Salt-resistant individuals show smaller effects. Clinical testing of salt sensitivity is impractical; dietary guidelines therefore recommend moderate sodium reduction for all adults.

Lower-sodium diet practicalities

Reducing sodium intake typically requires shifting from processed to whole foods, reading Nutrition Facts labels, choosing low-sodium product variants, and reducing restaurant meal frequency. Potassium-based salt substitutes (e.g., NoSalt, SSS) replace sodium with potassium chloride; these are effective for healthy individuals but contraindicated in CKD and certain medications. The "no salt added" claim on labels indicates less than 5 mg per serving; "low sodium" indicates 140 mg or less; "reduced sodium" indicates 25% less than the reference food.