Dietary fat, weight, and the importance of context

In arguing against the low-fat campaign, Michael Pollan cites extensively a review out of the Harvard School of Public Health. The review focuses on the relationship between dietary fat and coronary heart disease, but it also addresses the supposed connection between low-fat diets and weight loss.

As Pollan tells it,

One other little grenade is dropped in the paper’s conclusion: Although “a major purported benefit of a low-fat diet is wieght loss,” a review of the literature failed to turn up any convincing evidence of this proposition. To the contrary, it found “some evidence” that replacing fats in the diet with carbohydrates (as official dietary advice has urged us to do since the 1970s) will lead to weight gain. (45)

The official dietary advice, instead of promoting weight loss, would lead to weight gain. Damning, isn’t it? But as is too often the case in Pollan’s writing, the actual study tells a more complicated story. Here’s what the review says:

A major purported benefit of a low-fat diet is weight loss. But long-term clinical trials have not provided convincing evidence that reducing dietary fat can lead to substantial weight loss. On the contrary, there is some evidence that a diet containing a high amount of refined carbohydrates may increase hunger and promote overeating, which can lead to weight gain and obesity. It is now generally agreed that total energy intake, whether from fat or carbohydrate, relative to energy expenditure, is a more important determinant of body weight than dietary fat per se.

You’ll notice that the review quite specifically implicates refined carbohydrates like sugars, white flour, and white rice as a cause of weight gain. As it happens, these were not the kind of carbohydrates that the official advice told us to eat. Senator George McGovern’s Dietary Goals for the United States encouraged Americans to decrease consumption of sugars and increase consumption of fruits, vegetables, and grains. The National Academy of Sciences’ Diet, Nutrition, and Cancer did not make an explicit recommendation with respect to refined carbohydrates, but it specifically recommended accompanying the decrease in fat intake with an increase in consumption of fruits, vegetables, and whole grains.

Pollan is correct to say that official dietary advice encouraged Americans to replace fat with carbohydrates. His claim that the review implicates carbohydrate consumption in weight gain is more of a stretch, but perhaps still arguable. However, when he puts these claims together to argue that the review claims some evidence that would link the official dietary advice to weight gain, he misses the mark badly.


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