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When a Calorie Isn't a Calorie Isn't a Calorie

If your New Year’s resolutions include losing weight, then you must read this.

Conventional wisdom has it that weight loss and weight maintenance can be captured by a simple equation: Calories consumed vs. calories burned. If you take in more calories than you burn through exercise, you gain weight. If you take in fewer, then you lose weight. Simple, right?

Simple, yes, but right—not so much. A recent study published in the Journal of the American Medical Association shows just how wrong this idea is. Why? Because this simple model of weight loss fails to take into account the way different types of macronutrients impact the body.

The goal of the study was to answer a simple question: Why do only 1 out of 6 people who lose weight maintain their weight loss for at least one year? The standard explanation is that people simply return to poor eating habits, which causes them to regain all the weight lost. An alternative explanation is that weight loss triggers biological adaptations that promote regaining weight that was lost while dieting, such as a slowed metabolism and increase in hunger.

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The study was a small one conducted by physicians at Brigham and Women’s Hospital and Children’s Hospital in Boston, MA. It involved 21 obese and overweight men and women between the ages of 18 and 40 years. Three diets were compared:

• A low-fat diet that emphasized whole grains, vegetables, and fruits

• A low-glycemic index diet that replaced some grains and starchy vegetables with healthy fats and non-starchy vegetables

• A low-carbohydrate diet that modeled the Atkins diet.

The number of calories consumed was held constant across the groups. The participants were also given multivitamins and mineral supplements to ensure proper intake of micronutrients.

The study began with a run-in phase where all participants consumed a diet that consisted of 45% carbohydrate, 30% fat, and 25% protein. They stayed on this diet until they achieved 10-15% weight loss. Then they were randomly assigned to the other three test diets (low fat, low carbohydrate, or low glycemic index), and stayed on each one for four weeks.

The researchers looked at a number of physiological measures, including resting energy expenditure, total energy expenditure, leptin, insulin production and sensitivity, cholesterol, thyroid function, CRP, cortisol, and others. Resting energy expenditure is a measure of how much energy a person burns just doing nothing. Total energy expenditure measures the number of calories burned in one day taking into account level of physical activity. Leptin is a hormone that suppresses appetite. Insulin is a hormone that shuttles glucose into cells where they it be burned. A cell that is insulin resistant (or insulin insensitive) does not allow insulin to be absorbed, and hence glucose builds up in the blood stream. CRP is a measure of inflammation, and cortisol is a stress hormone.

The results were quite striking. Keep in mind that the participants were eating the same number of calories on each of these diets. So the differences cannot be due to increased food intake.

The low-fat diet yielded increased insulin and leptin resistance, as well as reduced energy expenditure (lowered metabolism). Lowered energy expenditure was not due to reduced physical activity as this did not change during the course of the study. Instead, the physiological changes that occurred promoted weight gain. It was as though the dieter’s bodies felt they needed to restock their fat reserves after losing weight this way.

The low-carbohydrate diet had beneficial impact on energy expenditure and several metabolic functions (most notably insulin sensitivity), but increased cortisol excretion and CRP. These metabolic profiles indicate a high level of metabolic stress on the body. This was the case even though the dieters reported no significant differences in hunger across the three diets.

The low-glycemic diet showed all of the metabolic benefits of the low-carbohydrate diet—but without the harmful effects of physiological stress and chronic inflammation.

Based on these results, the researchers drew the following two conclusions. First, “The results of our study challenge the notion that a calorie is a calorie from a metabolic perspective.” (p. 2631) Second, “These findings suggest that a strategy to reduce glycemic load rather than dietary fat may be advantageous for weight-loss maintenance and cardiovascular disease prevention.“ (p. 2633)

The best example of a low-glycemic load diet is a Mediterranean diet.

Enjoy and Happy New Year!

Copyright Dr. Denise Cummins December 30, 2013

Dr. Cummins is a research psychologist, a Fellow of the Association for Psychological Science, and the author of Good Thinking: Seven Powerful Ideas That Influence the Way We Think.

More information about me can be found on my homepage.

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Denise Dellarosa Cummins, Ph.D., is the author Good Thinking, The Historical Foundations of Cognitive Science, and Evolution of Mind.


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