Do calories from a chocolate bar, for example, have the same effect on your waistline as the same number of calories from an orange? Putting it another way—and getting to a oft-invoked question in the debate—will you be more successful losing weight with calories from a low-fat diet than with the same number of calories from a low-carbohydrate diet? Or might the reverse be true? (As protein typically occurs in low amounts in foods—10 to 15 percent in the average diet—a low-fat diet is necessarily a high-carb diet, and vice versa.)

Heart of the debate

No one disagrees about the basic definition of a calorie. From the Latin calor (heat), a calorie is a specific amount of heat energy. In the labeling of food products in the United States, a food calorie actually refers to a kilocalorie, or 1,000 calories. Technically speaking, one kilocalorie—thus one food calorie in that chocolate or orange—is the amount of energy needed to raise 1 kilogram of water from 15°C to 16°C. (From here on, when we use the term calorie we mean food calorie.)

The debate boils down to whether all sources of calories are the same for maintaining body weight. That is, is a calorie a calorie regardless of—or dependent on—the amount of protein, fat, and carbohydrate in a person's diet? The answer is not clear, in part, because we are unable to sense or measure calories easily. When measured in a calorimeter, a tabletop device for calculating the heat of combustion of a substance, a food calorie is roughly equivalent to the value of a food as an energy source. But the human body is not as thorough as a calorimeter—it doesn't digest all food components—and therefore we lose some calories in our waste. Experts must correct calorimeter measurements for such losses, but accurate corrections require difficult, tedious studies.

Atwater values

The first to accurately measure the number of usable calories in foods was the USDA scientist Wilbur Atwater and his contemporaries over a century ago. Their measurements formed the basis of today's Atwater values: four calories per gram for proteins and carbohydrates, and nine for fats. These values, with some modifications, are the basis of calorie levels in USDA food tables, food-packaging labels, and restaurant menus.

For most foods, estimates based on Atwater values are close enough. But certain foods, again, retain some of their calories as they pass through our gut. For example, the fat in almonds and certain other nuts is incompletely digested. In a 2012 study, USDA scientist Janet Novotny and her colleagues found that the measured energy content of a 28-gram serving of almonds was actually 32 percent less than the Atwater values estimate.

Not in the mix

Nutritionists have long tried to determine the proportion of total food energy we waste versus what we use for basic physiological functions, physical activity, fat storage, and body heat. Their measurements have shown that heat losses from metabolizing protein are greater than losses from metabolizing fat or carbohydrate. That is, we burn more calories while metabolizing protein than while metabolizing the other two so-called macronutrients. This observation suggests that our bodies might use calories from diets higher in protein less efficiently, resulting in less fat storage or greater weight loss than from a similar number of calories consumed in diets high in fats or carbs.

To lose weight, eat less; it works every time.

In 1964, investigators at the Institute for Metabolic Research in Oakland, California tested these ideas in a study involving five obese patients sequestered in a hospital metabolic ward. They gave each patient a liquid diet containing a precise number of calories calculated to induce weight loss. Every few weeks they changed the diets, varying the amounts of the three macronutrients. Patients initially ate a diet with 34 percent of calories from protein, 52 percent from fat, and 14 percent from carbs. Those numbers then changed to 27, 13, and 60 percent, respectively, and finally to 14, 83, and 3 percent, respectively.

The investigators reported that all patients in the study lost weight at a constant rate regardless of the macronutrient proportions. "It is therefore obvious," they wrote in the journal Metabolism, "that the significant factor responsible for weight loss is reduction of calories, irrespective of the composition of the diet."

A burning issue

But what about people living freely, that is, not participating in tightly controlled studies? Later studies attempting to compare diets of varying composition in free-living volunteers have sometimes found low-carb (i.e., high-fat) diets to be most effective in inducing weight loss. But researchers in these studies were not able to measure calorie intake accurately enough to exclude simple calorie reduction as the cause.

Taking a different approach, a 2012 study reported in the Journal of the American Medical Association concluded that the body may indeed use calories from low-carb diets less efficiently than those from low-fat diets, with greater weight loss as a result.

The investigators had a group of obese volunteers lose 10 to 15 percent of their weight by reducing their calorie intake to 60 percent of estimated needs with a carefully controlled diet for 12 weeks. They then fed the volunteers enough calories to maintain their newly reduced body weights through one of three pre-prepared diets: low-fat, low-glycemic index (that is, low in readily absorbed sugars and simple starches), or very-low-carbohydrate. During the following four-week test period, investigators measured, among other factors, total energy expenditure—or calories burned both during activity and during resting metabolism.

The researchers reported that volunteers on the very-low-carb (very-high-fat) diets burned up more energy—100 to 300 calories per day—than those on the low-fat (high-carb) diet. The investigators did not determine the source of this energy loss, but their results implied that very-low-carb diets resulted in more calories wasted in metabolism. (Although subjects consuming the very-low-carb diet expended the most energy, other metabolic measurements taken during this study suggested that low-glycemic index diets are healthiest for maintaining long-term weight loss.)

"The results of our study," the scientists noted in JAMA, "challenge the notion that a calorie is a calorie from a metabolic perspective."

Real-world scenario

We are not so sure. For one thing, some experts not involved with the study suspect the greater weight loss might have been through loss of water from fat-free mass, rather than actual fat loss. Also, the study involved highly obese volunteers eating measured, pre-prepared meals during and after a weight loss that still left them obese. The relevance of this study for maintaining weight loss in the real world remains to be determined. Most studies that find significant differences in weight loss in people eating different proportions of protein, fat, or carbs rarely last more than six months. Longer-term studies of a year or more seldom show clear advantages of low-carb diets.

Until research convinces us otherwise, we believe a calorie is a calorie.

Maintaining long-term weight loss is a challenge for many dieters. The human body seems programmed with numerous hormonal and neural mechanisms to maintain energy stores, perhaps as an evolutionary trait to protect against famine. It seems unlikely that even large variations in the protein, fat, or carb content of a diet will prove to be the simple key to success in either weight maintenance or weight loss. The idea, also proposed, that low-carb/high-fat diets are more satiating remains to be confirmed. As a result, most scientific reviews conclude that a diet of any composition will lead to weight loss if it reduces calories sufficiently.

Amount counts

From our reading of the research, we conclude that, while the precise nature of the relationship between diet composition and weight maintenance needs more research, the number of calories consumed relative to those expended matters more to weight loss than where the calories come from. To lose weight, eat less; it works every time. At the same time, we can think of many good reasons to cut down on the sugars and easily absorbed carbs of soda, potato chips, and other junk food, and to eat a greater proportion of whole foods such as fruits, vegetables, and whole grains.

If anything, the biggest challenge to weight loss is to find ways to cope with biological imperatives to eat more, not less, in the context of an "eat more" food environment that offers large numbers of calories, everywhere, at all times of day, at relatively low cost. It is this food-friendly environment that has brought us to the sobering point where one-third of Americans today are classified as obese.

One point is clear: Larger portions have more calories. The easiest way to prevent weight gain is to eat less by choosing smaller portions, fewer snacks, and healthier meals in general. It also helps to be physically active and to monitor weight status with regular weighing. Until research convinces us otherwise, we believe a calorie is a calorie.

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