Why Calories Count: Do Some Diets Work Better Than Others?

Although some diets may be easier for you to stick to or be more satiating, the bottom line is that you need to eat less to reduce body weight.

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One problem in studying the effects of dietary composition is that it is not possible to vary the proportion of one component without changing the others. At the extremes of weight-loss diets, the Atkins and South Beach diets are low carbohydrate but high fat, while the Ornish diet is low fat, high carbohydrate [1]. To compare the effects of such diets outside metabolic wards, researchers must deal with study subjects whose dietary and other behaviors are not easily controlled.

Investigators do everything they can to encourage compliance with study protocols. But they confront a major challenge: Telling free-living people what you want them to do does not necessarily mean that they will follow your instructions or tell you the truth about what they are eating. And you have no easy way of getting around this problem. Because dietary intake methods all depend on accurately disclosing what subjects consume -- something impossible for most people to do -- the lack of an easy way to measure true calorie consumption in weight control studies must be considered "the fundamental flaw of obesity research [2]."

But that's not the only problem. When conducting clinical trials that compare one diet to another, researchers also face challenges in enrolling enough study subjects to satisfy statistical requirements, getting study subjects to stick to the prescribed diets, and retaining participants in the study throughout its length. Furthermore, clinical trials of diet and weight loss are expensive to conduct, and few are able to last long enough to observe whether initial weight losses were regained. These considerations make it especially difficult for investigators to evaluate the results of dietary studies objectively and for others to interpret the significance of the findings. Keep these caveats in mind as we take a look at some of the studies attempting to find out whether varying the proportions of protein, fat, and carbohydrate makes any difference to weight loss in real life.


Atwater Values indicate that fat has more than twice the energy value of either protein or carbohydrate. It makes sense to think that cutting down on fat would help with weight maintenance or loss. In the United States the various editions of the Dietary Guidelines have long promoted lower-fat diets: "Avoid too much fat" (1980, 1985), "Choose a diet low in fat" (1990, 1995), "Keep total fat intake between 20 to 35 percent of calories" (2005), and "Reduce intake of solid fats" (2010). The more recent editions have focused on limiting saturated fat and cholesterol intake rather than total fat per se in recognition of the potential role of these components in heart disease risk. But the newer guidelines also recognize that from the standpoint of body weight, calories from fat are no different from calories from any other source.

This is a shift from the earlier recommendations that reshaped the marketplace. In the early 1990s, advice to reduce fat intake was all that food companies needed to hear to start making low-fat versions of many common foods -- low-fat cheese, mayonnaise, and peanut butter, for example -- along with oxymoronic products such as fat-free half-and-half and fat-free (but equally caloric) cookies. Such products are not necessarily healthier than the products they replace, and rarely taste as good.

But the relationship of dietary fat to obesity is still of much interest. For one thing, it takes hardly any energy to store excess fatty acids as body fat, whereas it takes a bit more energy to make fatty acids from excess dietary carbohydrate. For another, proponents of low-fat diets cite experimental observations demonstrating a connection between fat intake and overweight:

  • Laboratory animals fed high-fat diets generally become obese.
  • Populations consuming low-fat diets maintain lower body weights.
  • Some clinical studies show that reducing dietary fat can result in modest weight loss [3].

Some experts, however, view such evidence as not at all specific to fat, as it could just as easily relate to high-calorie diets from any source. Low-fat diets are necessarily high in carbohydrate -- the calories have to come from something. The range of protein in diets is typically 10 percent (low) to 20 percent (high) of calories; it can't be more, because foods are low in protein -- we don't need much. The real issue in real diets is carbohydrate v. fat. Few studies of such difference control for calories. Overall, studies of dietary patterns typically find no association between either the amount or the type of fat in the diet and subsequent weight gain over periods of several years [4].

Presented by

Marion Nestle is a professor in the Department of Nutrition, Food Studies, and Public Health at New York University. She is the author of Food Politics, Safe Food, What to Eat, and Pet Food Politics. More

Nestle also holds appointments as Professor of Sociology at NYU and Visiting Professor of Nutritional Sciences at Cornell. She is the author of three prize-winning books: Food Politics: How the Food Industry Influences Nutrition and Health (revised edition, 2007), Safe Food: The Politics of Food Safety (2003), and What to Eat (2006). Her most recent book is Feed Your Pet Right: The Authoritative Guide to Feeding Your Dog and Cat. She writes the Food Matters column for The San Francisco Chronicle and blogs almost daily at Food Politics.

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