THE first time I questioned the conventional wisdom on the nature of a healthy diet, I was in my salad days, almost 40 years ago, and the subject was salt.
Researchers were claiming that salt supplementation was unnecessary after strenuous exercise, and this advice was being passed on by health reporters. All I knew was that I had played high school football in suburban Maryland, sweating profusely through double sessions in the swamplike 90-degree days of August. Without salt pills, I couldn’t make it through a two-hour practice; I couldn’t walk across the parking lot afterward without cramping.
While sports nutritionists have since come around to recommend that we should indeed replenish salt when we sweat it out in physical activity, the message that we should avoid salt at all other times remains strong. Salt consumption is said to raise blood pressure, cause hypertension and increase the risk of premature death. This is why the Department of Agriculture’s dietary guidelines still consider salt Public Enemy No. 1, coming before fats, sugars and alcohol. It’s why the director of the Centers for Disease Control and Prevention has suggested that reducing salt consumption is as critical to long-term health as quitting cigarettes.
And yet, this eat-less-salt argument has been surprisingly controversial — and difficult to defend. Not because the food industry opposes it, but because the actual evidence to support it has always been so weak.
When I spent the better part of a year researching the state of the salt science back in 1998 — already a quarter century into the eat-less-salt recommendations — journal editors and public health administrators were still remarkably candid in their assessment of how flimsy the evidence was implicating salt as the cause of hypertension.
“You can say without any shadow of a doubt,” as I was told then by Drummond Rennie, an editor for The Journal of the American Medical Association, that the authorities pushing the eat-less-salt message had “made a commitment to salt education that goes way beyond the scientific facts.”
While, back then, the evidence merely failed to demonstrate that salt was harmful, the evidence from studies published over the past two years actually suggests that restricting how much salt we eat can increase our likelihood of dying prematurely. Put simply, the possibility has been raised that if we were to eat as little salt as the U.S.D.A. and the C.D.C. recommend, we’d be harming rather than helping ourselves.
WHY have we been told that salt is so deadly? Well, the advice has always sounded reasonable. It has what nutritionists like to call “biological plausibility.” Eat more salt and your body retains water to maintain a stable concentration of sodium in your blood. This is why eating salty food tends to make us thirsty: we drink more; we retain water. The result can be a temporary increase in blood pressure, which will persist until our kidneys eliminate both salt and water.
The scientific question is whether this temporary phenomenon translates to chronic problems: if we eat too much salt for years, does it raise our blood pressure, cause hypertension, then strokes, and then kill us prematurely? It makes sense, but it’s only a hypothesis. The reason scientists do experiments is to find out if hypotheses are true.
In 1972, when the National Institutes of Health introduced the National High Blood Pressure Education Program to help prevent hypertension, no meaningful experiments had yet been done. The best evidence on the connection between salt and hypertension came from two pieces of research. One was the observation that populations that ate little salt had virtually no hypertension. But those populations didn’t eat a lot of things — sugar, for instance — and any one of those could have been the causal factor. The second was a strain of “salt-sensitive” rats that reliably developed hypertension on a high-salt diet. The catch was that “high salt” to these rats was 60 times more than what the average American consumes.
Still, the program was founded to help prevent hypertension, and prevention programs require preventive measures to recommend. Eating less salt seemed to be the only available option at the time, short of losing weight. Although researchers quietly acknowledged that the data were “inconclusive and contradictory” or “inconsistent and contradictory” — two quotes from the cardiologist Jeremiah Stamler, a leading proponent of the eat-less-salt campaign, in 1967 and 1981 — publicly, the link between salt and blood pressure was upgraded from hypothesis to fact.
h/t George Ellis