Salt-surprising data.

• In the movie "Sleeper" by Woody Allen, the futuristic characters chuckle and say "Remember when it was thought that cigarettes and fat were bad for you?"   It certainly seems that ideas of health go back and forth like other trends.   For instance, there is a growing body of evidence that a high carbohydrate diet, recommended by everyone from the American Heart Association to the strictest macrobiotic, may not be as healthy as we once thought.   Now, some important studies have been done which emphasize the importance of that much-maligned substance, salt.  

• It is common practice to recommend a low salt diet (or more specifically, a low sodium diet, since we are talking here about sodium chloride, or table salt) in the general population, and particularly in hypertension.   It is known that populations with lower salt intake have lower blood pressures generally.   However, there is no good evidence that lowering a salt intake will lower blood pressure significantly, and certainly little evidence that it will modify the ill effects of high blood pressure, like heart attacks and strokes.   Now comes a study from the medical journal Hypertension which shows that hypertensive people who lowered their salt intake most suffered the most heart attacks, almost four times the amount experienced by the group with the highest salt intake!

• This result did not come totally unsurprising.   It is known that about half of hypertensives and a quarter of the population as a whole are "salt-sensitive", that is, their system overreacts to salt by raising blood pressure.   The rest of us don't seem to react much to salt.   However, in this "sensitive" population, increasing potassium, magnesium and calcium intake seems to control the sensitivity.   So, taking adequate doses of these minerals (about 3500 mg potassium, 800-1000 mg for calcium, 1200-1500 mg calcium for post-menopausal women, and 500-800 mg magnesium) seem like a better idea, along with a moderate level of salt intake.

• The kidney has a delicate mechanism (called the renin-angiotensin system) for maintaining pressure in the blood vessels.   Artificially lowering one element of the system (sodium) causes an elevated renin level, and thus the kidney works harder to retain the salt that it has.   In other words, the hypertension is a sign that a hormonal system is out of balance, and thinking we can starve it into submission may be naive.   Sodium is necessary enough that a major adrenal steroid hormone, aldosterone, is devoted to its regulation and retention.   Many studies show the necessity of dietary salt, from population studies showing a tendancy toward shorter life span in people eating low-salt diets, to animal studies showing growth failure with sodium restriction.   Multiple studies earlier this century showed fatigue and mental dulling to result from salt depletion diets.

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