Good Calories, Bad Calories

Let’s recall that hypertension is a disease of civilization, an observation that dates back to the late 1920s. Just as physicians in Europe and the United States took to measuring blood pressure in their patients with the availability of an instrument that could do so easily and reliably (the sphygmomanometer), missionary and colonial physicians throughout the world took to measuring blood pressure in native populations. Within a decade, noted the British physician Cyril Donnison in 1938 in Civilization and Disease, hypertension was already among the best-documented examples of a disease that seemed specific to Western societies and the more affluent social classes elsewhere. The average blood pressure in isolated populations eating traditional diets was inevitably low, but not dissimilar to the average blood pressure of Europeans and Americans who had not yet reached middle age. Hypertension was never seen in these populations, and blood pressure, if anything, dropped lower with age, which is the opposite of what happens in developed nations. In 1929, Donnison reported that he had measured the blood pressure in a thousand Kenyan nomads and found it similar to that of Europeans for those men under forty, but not so after that: “It tends to come down in the African,” Donnison wrote, “whereas in the white races it continues its tendency to rise until the eighth decade.” The Kenyan nomads in their sixties had an average systolic blood pressure forty points lower than that of European men of the same age. Over the next forty years, these observations would be confirmed in isolated populations throughout the world.

With exposure to Western lifestyles and diets, however, blood pressure among these native populations began to rise with age, as it does in Europe and America, and the average blood pressure and the incidence of hypertension increased as well. In Kenya and Uganda, British physicians considered hypertension to be nonexistent among their African patients in the late 1930s. By the 1950s, more than 10 percent of native Africans checking into hospitals for any reason were diagnosed with clinical hypertension. That number had risen to over 30 percent by the mid-1960s. By the 1970s, hypertension was considered as frequent in the native African populations as it was in Europe or America. In some urban populations, hypertension rates as high as 60 percent were reported.

Until the salt hypothesis began receiving serious attention in the 1960s, the investigators paid little attention to nutritional explanations for the rise in blood pressure that accompanied Western diets and lifestyles. Instead, they debated whether it was the stress and tension of what they considered civilized life that led blood pressure to rise, as Donnison believed. Once the salt hypothesis raised the possibility that diet was responsible, investigators began to perceive the presence or absence of hypertension in isolated populations purely as a test of the salt hypothesis. Since hypertension only appeared in these populations when they gained access to Western diets, which frequently included salt-rich processed foods, the investigators saw their studies as confirming the salt hypothesis. By the 1990s, the absence of hypertension in isolated populations eating their traditional diets was still the most compelling evidence in support of the hypothesis.

Of course, the same societies that ate little or no salt ate little or no sugar and white flour, so the evidence supported both hypotheses, although the investigators were interested in only one. The notion that the refined-carbohydrate hypothesis could explain many of the other chronic changes in health among these populations was rarely discussed. In two cases—Gerald Shaper’s studies of nomadic tribes in Kenya and Uganda, and Ian Prior’s studies of South Pacific Islanders—the investigators first implicated refined carbohydrates as a possible cause of the emergence of hypertension in their populations, because sugar and flour constituted the conspicuous additions to the diet with Western influence. Then they embraced salt as the culprit, after they became aware that investigators in the U.S. believed salt to be the problem. In the early 1970s, when the Harvard hypertension specialist Lot Page and his colleagues set out to study “the antecedents of cardiovascular disease” in the Solomon Islands, they, too, considered their research to be solely a test of the salt hypothesis, so salt was the only aspect of the Solomon Islanders’ diet that they assessed. In what came to be considered a seminal study in the field, they concluded naturally enough that suspicion of the cause of high blood pressure among the islanders “falls most heavily on salt intake.”