Massive publicity accompanied (even preceded) British Medical Journal publication on April 20 of the fifteenth observational study of health outcomes of lower salt diets. A team of experts led and funded by the National Heart, Lung and Blood Institute, all prominent in their previous advocacy of salt-restricted diets, reported on "Long terms effects of dietary sodium reduction on cardiovascular disease outcomes." The researchers used data from the Trials of Hypertension Prevention whose outcome was blood pressure, not cardiovascular risk, but they compared the groups that were on the low-sodium diets with the control group -- adjusting for "demographic information" and sodium excretion -- and concluded that low-salt diets confer a 25% benefit in reduced heart attacks. This was big news and received massive publicity -- more than the earlier 14 studies of the same question -- do lower-sodium diets reduce the risk of cardiovascular events and mortality? None of the earlier studies had found improved health outcomes in populations similar to Americans. Three found additional risks associated with low-salt diets.
The findings muddy the waters of the underlying question, but, paradoxically clarify the course of action. The record is clear -- observational studies are divided confirming the Salt Institute's long-standing conclusion that we need a controlled trial of the question to resolve whether public policy should advocate salt reduction. Moreover, by choosing the TOHP study, these anti-salt researchers embrace the very study design that the Salt Institute has advocated to the US Department of Health and Human Services. SI has suggested the TOHP model for the controlled trial.
Critics of the new study have observed that the findings depend entirely on five deaths among the 3,000-plus subjects and that the reduction in cardiovascular events failed to reach significance when revascularization (surgical intervention) procedures were excluded, and the change in total mortality failed to reach significance also. These are serious criticisms, but no observational study can hope to resolve these matters. Let's not get bogged down on the methods or conclusions of this single study.
The significance of this study is not its authors' conclusion that lower-salt diets will reduce cardviovascular events, but, rather, that it represents a powerful argument for a controlled trial of the question. We cannot make policy on observational studies nor on the latest single study. While the first 14 "health outcomes" studies offers no support for salt reduction in a US-type population, this study virtually demands that the question be resolved to prevent the public being confused by "dueling studies."
tanley Feldman and Vincent Marks' book Panic Nation explores "the myths we're told about food and health." While the genre is growing in reaction to the current global spate of myth-information, the book's appearance in the UK is timely since the Brits seem furthest off-base regarding the scientific issues of salt and health.
Chapter 9 by Dr. Sandy Macnair deals with salt and has been generating news coverage since publication . While much of Dr. Macnair's review tracks earlier work such as ABC-TV's "20/20" and "The (Political) Science of Salt" in Science magazine , a couple of observations may be new even to close readers on the issue.
We're often told that humans today eat "too much" salt -- more than our cave-dwellilng progenitors. Intakes in acculaturated societies range from about 2,300 mg/day sodium to 4,600 mg/day (6 - 12 grams of salt). While most people's healthy kidneys effectively process salt-laden body fluids, excreting the "excess," some people's kidneys malfunction and problems occur. Explained this way, you get the idea that the kidney is processing roughly ten grams of salt a day. Not so, explains Dr. Macnair. The kidneys filter water from the blood in the amount of 170 liters of water a day and, get this, handle 1,500 grams of salt a day. How our bodies can distinguish one or two milligrams out of this 1,500 grams (that's 3.3 POUNDS of salt a day) is absolutely amazing, but the main point is that we shouldn't think that the kidneys are straining to hand more than a gram or two a day.
Dr. Macnair makes another interesting point in his discussion of human evolution. Fourteen million years ago, he says, our ancestors in the Rift Valley in Africa split into two evolutionary streams, one heading off to the jungles to the west to evolve into great apes while those remaining in the Valley, with ready access to salt and the need for food preservation in the seasonal climate, produced a very different diet incorporating unique essential fatty acids that produced brains three times larger than their departed primate cousins and homo sapiens learned how to salt cure meats to ensure their survival.
You can read more in the South African magazine Food Review (as mentioned in our February blog ).
Perhaps Dr. Macnair's compatriots can reconsider the wisdom in his article.