Good Calories, Bad Calories

general had gone far enough in pushing a national low-fat diet plan. Both the Washington Post and the New York Times quoted Jacobson scolding the authors of Diet and Health for lacking “the courage” to tell Americans straight out that a healthy lifestyle required much “greater reductions” in total fat, saturated fat, and cholesterol. In the Post article, Arno Motulsky, chairman of the NAS committee that compiled the report, acknowledged that one intention of Diet and Health was to convince Americans further of the existence of a scientific consensus on the benefits of reducing fat in the diet. “Many people may be confused by the vast amount of advice about what to eat,” he said. “Some may have delayed making changes in their diets until they are more convinced that scientists have reached consensus. We hope our report will help these individuals move from inaction and complacency to action.” The public face of the controversy had now shifted entirely. It was no longer about the validity of the underlying science, which was no less ambiguous than ever, but about whether Americans should be eating low-fat diets or very low-fat diets.

One striking fact about this evolution is that the low-fat diets now being recommended for the entire nation had only been tested twice, as I’ve said, once in Hungary and once in Britain, and in only a few hundred middle-aged men who had already suffered heart attacks. The results of those trials had been contradictory. The diets tested since then had been exclusively cholesterol-lowering diets that replaced saturated fats with unsaturated fats.

The rationale for lowering the total fat content of the diet to 30 percent was the tangential expectation that such a diet would help us control our weight. In 1984, the year of the NIH Consensus Conference, Robert Levy and Nancy Ernst of the NHLBI had described the state of the science this way: “There has been some indication that a low-fat diet decreases blood cholesterol levels,” they wrote. “There is no conclusive proof that this lowering is independent of other concomitant changes in the diet (for example, increased dietary fiber or complex carbohydrate…or decreased cholesterol or saturated fatty acid level)…. It may be said with certainty, however, that because 1 g fat provides about 9 calories—compared to about 4 calories for 1 g of protein or carbohydrate—fat is a major source of calories in the American diet. Attempts to lose weight or maintain weight must obviously focus on the content of fat in the diet.” Though this was an untested conjecture (however obvious it might seem), the official healthy diet of the nation was now a low-fat diet. A new generation of diet doctors, the most influential of whom was Dean Ornish, were even prescribing 10-percent-fat diets, if not lower.

Another striking aspect of the low-fat diet recommendations is how little any individual might benefit from lowering his cholesterol.^*19 Keys and others had argued that heart disease had to be prevented because its first symptom was often a fatal heart attack. But in twenty-four years of observation, the Framingham Heart Study had detected no relationship between cholesterol and sudden cardiac death. The likelihood of suffering a fatal first heart attack was no less for those with a cholesterol level of 180 mg/dl than for those with 250. “The lack of association between serum cholesterol level and the incidence of sudden death suggests that factors other than the atherosclerotic process may be of major importance in this manifestation of coronary artery disease,” explained Thomas Dawber.

There is also little to gain from lowering cholesterol even in less catastrophic manifestations of the disease. This was made clear in 1986, when Stamler published a reanalysis of his MRFIT data in JAMA. As Stamler reported it, the MRFIT investigators had continued to track the health of the 362,000 middle-aged men who had originally been screened as potential candidates for MRFIT, including death certificates. Stamler reported that the cholesterol/heart-disease association applied at any level of cholesterol, and so anyone would benefit from lowering cholesterol.

Using the MRFIT data, however, it is possible to see how large or small that benefit might be (see chart, below). For every one thousand middle-aged men who had high cholesterol—between, say, 240 and 250 mg/dl— eight could expect to die of heart disease over any six-year period. For every thousand men with cholesterol between 210 and 220, roughly six could expect to die of heart disease. These numbers suggest that reducing cholesterol from, say, 250 to 220 would reduce the risk of dying from a heart attack in any six-year period from .8 percent (eight in a thousand) to .6 percent (six in a thousand). If we were to stick rigorously to a cholesterol- lowering diet for thirty years—say, from age forty to seventy, at which point high cholesterol is no longer associated with an increased risk of heart disease—we would reduce our risk of dying of a heart attack by 1 percent.

The data from the MRFIT trial showing the relationship between heart-disease mortality and cholesterol levels in the blood.

The data from the MRFIT trial showing the relationship between total mortality—i.e., death by all causes—and cholesterol levels in the blood.

Whether we would actually live longer by lowering our cholesterol is, of course, a different question. People die from myriad causes. Though Stamler neglected to include total mortality data in his JAMA article, a second group of MRFIT investigators did include it in an article published in The Lancet just a month earlier.

Their data revealed that for every thousand men with cholesterol around 240 to 250 mg/dl, twenty to twenty- three would likely die of any cause within six years. For those whose cholesterol was approximately 220, between nineteen and twenty-one were likely to die. In other words, for every thousand middle-aged men who successfully lower their cholesterol by diet from, say, 250 to 220, at most four (although perhaps none) can expect to avoid death during any six-year period. Nineteen or twenty of these men can expect to die whether they diet or not. For the remaining 98 percent, they will live regardless of their choice. Moreover, lowering cholesterol further would not help. The death rate for men whose cholesterol is below 200 appears little different from that of men whose cholesterol falls between 200 and 250. Only for those men whose cholesterol is above 250 mg/dl does it appear that lowering cholesterol might improve the chances of living longer.

There is another way to interpret this statistical association between cholesterol, heart disease, and death. The association, as documented by Framingham, MRFIT, and other studies, only says that, the higher our cholesterol, the greater our risk of heart disease. It does not tell us whether the benefit from lowering cholesterol is shared by the entire population or only by a small percentage. The latter is the implicit assumption of the above analysis. But what if the benefit of lowering cholesterol is indeed shared democratically among all who do it? Perhaps we may all live longer by lowering our cholesterol. But how much longer?

Between 1987 and 1994, independent research groups from Harvard Medical School, the University of California, San Francisco, and McGill University in Montreal addressed the question of how much longer we might expect to live if no more than 30 percent of our calories came from fat, and no more than 10 percent from saturated fat, as recommended by the various government agencies. All three assumed that cholesterol levels would drop accordingly, and that this low-fat diet would have no adverse effects, which was still speculation rather than fact.

The Harvard study, led by William Taylor, concluded that men with a high risk of heart disease—such as smokers with high blood pressure—might gain one extra year of life by shunning saturated fat. Healthy nonsmokers, however, might expect to gain only three days to three months. “Although there are undoubtedly persons who would choose to participate in a lifelong regimen of dietary change to achieve results of this magnitude, we suspect that some might not,” the Harvard investigators noted.