Why are low-salt diets harmful?

Sodium — the main extracellular cation, which is responsible for osmotic pressure, blood volume, blood pressure, and water-salt balance. The body loses sodium daily through:

• urine (the main route — about 90-95% of all sodium excretion);
• sweat (especially in heat or during physical activity);
• feces;
• skin and mucous membranes.

With normal kidney function, a healthy person excretes about 3-3.5 g of sodium per day. The kidneys constantly adapt sodium excretion depending on its intake, aldosterone levels, sympathetic nervous system activity, and other factors.

Since sodium is not stored in large reserves in the body, its balance is a constant influx and excretion. If intake does not cover losses:

• the volume of extracellular fluid decreases;
• blood pressure drops;
• the renin-angiotensin-aldosterone system (RAAS) is activated to enhance sodium reabsorption in the kidneys;
• thirst increases.

Table salt (NaCl) contains:

• 39.3% sodium (often rounded to ~38-39%);
• 60.7% chlorine.

If 3–3.5 g of sodium is excreted per day, then the recommendation of 9 g of salt per day is quite physiologically justified to compensate for basic losses.

However, in a healthy physically active person, especially in a hot climate, with high physical loads or on a low-carbohydrate diet (which itself increases sodium excretion through the kidneys), the need for salt can easily reach 9–12 g per day.