Salt, blood pressure and the advice that applies to some people more than others

Sodium is essential. The body needs perhaps 1.5 grams a day, and most adults in industrialised countries consume between 3 and 5 — the World Health Organization recommends under 2 grams of sodium, which is 5 grams of salt, and almost nobody meets it.

The relationship between that intake and blood pressure is real, well demonstrated in controlled feeding studies, and considerably more variable between individuals than public messaging suggests.

What the trials show

The DASH-Sodium trial fed participants controlled diets at three sodium levels. Reducing sodium lowered blood pressure at every level, with the largest effects in those who were hypertensive and in those on the standard rather than the DASH diet. Meta-analyses of sodium reduction trials find average systolic reductions of roughly 4 to 5 mmHg in hypertensive people and 1 to 2 mmHg in normotensive people.

Those are modest averages. At a population level a 2 mmHg shift in mean blood pressure translates into a substantial number of prevented strokes, which is why public health bodies care. At an individual level, it may mean nothing at all, or it may mean 10 mmHg.

Salt sensitivity, and why age matters

Roughly a quarter to a half of hypertensive people, and a smaller proportion of normotensive people, are salt sensitive — meaning their blood pressure changes appreciably with sodium intake. The rest are relatively insensitive.

Salt sensitivity is more common in older adults, in people of African descent, in those with chronic kidney disease, in those with obesity, and in postmenopausal women. The mechanism relates to the kidney's declining ability to excrete a sodium load, which is precisely the change that occurs with age.

So the advice becomes more relevant, not less, as people get older — which is the opposite of how it is often received.

Where the salt actually is

This surprises people every time. In most Western diets, roughly 70 to 75 per cent of sodium intake comes from processed and prepared foods, not from the salt cellar. Discretionary salt added at the table and in cooking accounts for perhaps 10 to 15 per cent.

Which means putting the cellar away is a small intervention that feels large. The bigger contributors are usually:

• Bread. Rarely tastes salty and, because of the quantity eaten, is often the single largest source in the diet.
• Processed meats — bacon, ham, sausages, deli meat.
• Cheese, particularly hard and blue varieties.
• Ready meals, soups and sauces, including many that taste sweet.
• Breakfast cereals, some of which contain more sodium per serving than crisps.
• Restaurant and takeaway food, essentially all of it.

Reading labels is the effective intervention. Sodium is usually given per 100 g: above 0.6 g sodium (1.5 g salt) per 100 g is high, below 0.1 g sodium (0.3 g salt) is low.

The potassium half, which gets less attention

Blood pressure responds to the sodium-to-potassium ratio, not to sodium alone, and most people are as short of potassium as they are oversupplied with sodium. Increasing potassium intake lowers blood pressure in trials, with effects comparable in size to sodium reduction.

Good sources are unglamorous and cheap: potatoes with skin, beans and lentils, bananas, dried apricots, spinach, tomatoes, yoghurt, fish.

Potassium-based salt substitutes deserve a mention because of one striking trial. The Salt Substitute and Stroke Study, conducted in rural China with over 20,000 participants at high stroke risk, replaced regular salt with a 75 per cent sodium chloride, 25 per cent potassium chloride blend, and found significant reductions in stroke, major cardiovascular events and death.

That is a large result from a simple intervention. It carries an important caution: potassium substitutes are not safe for people with chronic kidney disease, or those taking ACE inhibitors, ARBs, potassium-sparing diuretics or aldosterone antagonists, because of the risk of hyperkalaemia. That excludes a substantial share of the older population, and it is a conversation to have with a clinician rather than a decision to make in a supermarket.

Getting used to less

Salt preference is learned and it is re-learnable. Studies of gradual sodium reduction find that taste preference shifts over roughly eight to twelve weeks, after which previously normal food tastes unpleasantly salty. Doing it slowly matters — an abrupt cut makes food taste of nothing and the change does not survive the month.

The practical substitutes are acid and aroma. Lemon juice, vinegar, herbs, garlic, black pepper, chilli, smoked paprika and toasted spices all supply the interest that salt was providing. Browning food properly — a real sear, a proper roast — generates flavour that reduces how much salt is needed to make something taste of something.

The honest summary

If you have hypertension, are over sixty-five, have kidney disease, or are of African descent, sodium reduction is one of the higher-yield dietary changes available to you. If you are normotensive, young and salt-insensitive, it is a modest public health contribution rather than a personal one — and getting more potassium is probably the better use of your attention either way.