Many physicians and those charged with regulating the nation’s health have been obsessed with salt since Lot’s wife was transformed into a pillar of the stuff. On March 7, 2018 the JAMA published online a well performed study of sodium intake, the investigators used 24 hour urinary sodium excretion as a marker of intake, which documents what everybody already knew, Americans ingest a lot of sodium every day –Estimated 24-Hour Urinary Sodium and Potassium Excretion in US Adults. The study is not behind a paywall and you can read it in its entirety if you so wish. “Estimated mean sodium intake was 3608 mg per day. The findings provide a benchmark for future studies.” Men ingested more sodium than women, which is what you’d have predicted before the study was done.
There is an accompanying editorial, Measurements of 24-Hour Urinary Sodium and Potassium Excretion – Importance and Implications, which focuses on both sodium and potassium intake; it too is not behind paywall and can be read at leisure. Let’s just concentrate on sodium.
The study is pretty straightforward and documents what even Lot knew – Americans consume more sodium than is said to be advisable. The editorial reminds us that 2300 mg/day is held by the Institute of Medicine (they’ve changed their name, but I’m sticking with the original one) to be the upper limit of intake consistent with long term good health. The two editorialists remind us that “Excessive sodium intake is etiologically related to hypertension and cardiovascular diseases, and these conditions contribute significantly to morbidity, mortality, and health care costs.” This statement is true but require more analysis than it ever gets.
Let me give you a digressive analogy and then I’ll get back to sodium. Suppose you had two population half of which had the fairest skin possible while the other half had the darkest. Now you divide them into two halves each with the same number of light and dark skinned people. One half gets a lot of exposure to sunlight while the other half entirely avoids such exposure. After a number of years you observe that the incidence of skin cancer is statistically greater in the group exposed to sunlight. You don’t dig deeper into the two groups, but instead issue a recommendation that everybody avoid exposure to sunlight. Then some smart aleck comes along and examines the data and points out that the increased incidence of skin cancer was almost entirely observed in the fair skinned population. He then says that it would make greater sense to advise the light skinned people to avoid sunlight more than the dark skinned population. The message is that a change that affects only one half of a group may be statistically significant for the whole group even if half the group is unaffected by that maneuver..
Back to salt (salt is the main source of dietary sodium). The reason high salt diets are bad for your health is that they increase blood pressure. The increase in cardiovascular disease associated with high salt diets is entirely due to hypertension. But the general population is divided into two groups. Those who are salt sensitive and those who are not. The former group’s blood pressure will vary directly with salt intake. The later’s blood pressure will not be influenced by dietary salt intake. How do you tell one from another? Take their blood pressure. As long as BP is normal there is no need to restrict salt. If it goes up salt intake should be limited.
The salt intake recommended by the IM is almost impossible to follow by anybody who likes food. People use salt because it makes food taste better. Its use should be restricted only for a very good reason. What I’m saying is that physicians who issue these ukases should use good judgement and should individualize their advice rather than tilting at every salt shaker in the country. That physicians should tailor their care to fit each patient is drummed into the head of every medical student from day one. Why some of the leaders of the profession have forgotten this admonition deserves study by the IM. Some people need salt restriction other don’t; it’s as simple as that.