Sodium is the primary extracellular cation in the human body — it is present at concentrations of approximately 135-145mEq/L in the blood and the extracellular fluid, and it is the primary determinant of the extracellular fluid volume, of the plasma volume, and of the blood pressure. The sodium is actively transported out of the cells by the Na+/K+-ATPase (which pumps the sodium out of the cell in exchange for the potassium into the cell), and this creates the sodium concentration gradient that drives the secondary active transport of the other nutrients (glucose, amino acids, iodide) across the intestinal epithelium and the renal tubular epithelium. The sodium is also the primary determinant of the plasma osmolality — it is the major solute in the extracellular fluid, and it determines the osmotic pressure that drives the movement of the water across the cell membranes and across the capillary walls. Without adequate sodium, the extracellular fluid volume is reduced, the plasma volume is reduced, the blood pressure falls, and the hypotension and the shock develop — the hallmark of the sodium deficiency (hyponatraemia, when the blood sodium is below 135mEq/L, or hyponatraemia when the total body sodium is depleted). The typical dietary sodium intake is 2.5-4.5g daily (from the table salt, the processed foods, and the condiments), and the recommended maximum intake is 2.3g daily for adults — but the sodium deficiency is less common than the sodium excess (which is associated with the hypertension, the cardiovascular disease, and the stroke), and it is most commonly seen in people with the excessive sweating, with the diarrhoea, with the vomiting, with the diuretic therapy, and with the adrenal insufficiency (where the aldosterone deficiency leads to the excessive renal sodium loss).
Sodium and the Blood Pressure
The blood pressure is the product of the cardiac output and of the peripheral vascular resistance — and the sodium is the primary determinant of the blood volume (through its effect on the extracellular fluid volume and the plasma volume), which is the primary determinant of the cardiac output. When the dietary sodium intake is high, the extracellular fluid volume expands, the plasma volume increases, the cardiac output increases, and the blood pressure rises (to maintain the adequate perfusion of the tissues against the increased vascular volume). When the dietary sodium intake is low, the opposite occurs — the extracellular fluid volume contracts, the plasma volume decreases, the cardiac output decreases, and the blood pressure falls. This sodium-blood pressure relationship is one of the most important and most well-established relationships in human physiology, and it explains why the high dietary sodium intake is the primary dietary risk factor for the hypertension and for the cardiovascular disease worldwide.
The clinical importance of the sodium for the blood pressure regulation is underscored by the observation that the reduction of the dietary sodium intake lowers the blood pressure in people with the hypertension. A meta-analysis of 32 RCTs in over 3000 participants found that the reduction of the dietary sodium intake by 1.7g daily (which corresponds to a reduction of 4.4g of salt daily) reduced the systolic blood pressure by 5.8mmHg and the diastolic blood pressure by 2.5mmHg — with the greatest effect seen in the people with the hypertension (who are more sodium-sensitive than the people with the normal blood pressure). This evidence has led to the universal recommendation to reduce the dietary sodium intake as one of the most important lifestyle interventions for the prevention and the treatment of the hypertension.
Practical Application
For general sodium intake, the evidence-based approach is to consume the moderate sodium intake of 2.3-3g daily (which corresponds to 4-6g of salt daily) for the general population, and to limit the sodium intake to 1.5-2g daily (which corresponds to 3-4g of salt daily) for the people with the hypertension or the cardiovascular disease. The sodium is found in virtually all foods, but it is particularly abundant in the processed foods, the fast foods, the canned foods, and the condiments (soy sauce, ketchup, pickles). For comprehensive blood pressure and cardiovascular support, sodium pairs well with the potassium (which antagonises the sodium and which helps to maintain the normal blood pressure — a high potassium intake is protective against the hypertension and the stroke), with the magnesium (which is a vasodilator and which helps to lower the blood pressure through a different mechanism than the sodium-potassium balance), with the calcium (which is involved in the vascular smooth muscle contraction and which may help to lower the blood pressure when deficient), and with the vitamin D (which has cardiovascular protective effects and which is increasingly recognised as an important determinant of the blood pressure and of the cardiovascular risk).
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