Is a Calorie Just A Calorie?

Eat your vegetables
We are what we eat. Hiroshi Watanabe/Getty Images

There is a lot of confusion today, both in the public media and in the medical literature, about whether “a calorie is just a calorie.” Some experts insist that, yes, a calorie is a calorie, and your weight is determined simply by how many calories you eat vs. how many you burn. Others insist, equally vociferously, that calories are not all equal, and that what we eat is just as important (if not more important) than how much we eat.

These two types of experts are talking past each other. They are both making legitimate points, and in their own ways both are correct. Where the experts go wrong is in the shorthand they use - in trying to simplify their messages, they have come up with messages that appear to be in conflict.

Let's try to clear up the unnecessary confusion.

Yes, A Calorie Is A Calorie

Traditional physiology, and traditional dietary science, tell us that if we take in more calories than we burn we will gain weight, and vice versa. This teaching is based on a simple, irreducible fact: a calorie of fat yields the same amount of energy as a calorie of carbohydrate, and the same as a calorie of protein. You take in the energy you take in, you burn the energy you burn, and the difference determines whether you will store excess energy or use stored energy, and thus whether you are adding to or taking from your fat stores.

This is simple physics.

So, to maintain our weight we just need to make sure that, in an average day, we are balancing our intake and outflow of calories. If we eat more calories than we burn we will gain weight. If we burn more calories than we eat, we will lose weight. Calories in, minus calories out, equals calories stored (or lost).

Case closed. So all you need is your Fitbit or Apple Watch to estimate how many calories you’re burning and how many you’re consuming, plus the willpower to keep the needle in the neutral (or negative) position, and you’re all set.

But Different Kinds of Calories Are Different

The above irreducible truth notwithstanding, the kinds of calories you eat will indeed have different effects on your weight.

This is because of human physiology. We do not simply absorb the carbs, fat and protein we eat, then burn what we need and store the rest. Our bodies handle different kinds of foods quite differently, and our physiology changes and adjusts to different kinds of foods. These differences have a significant effect on how much we end up weighing.

Here are just a few examples:

  • When you eat high-fiber foods, you’re only absorbing 75 - 80% of the calories you take in. The rest are excreted in the stool.
  • It takes more calories to burn protein than carbs or fat; so fewer of the calories from protein are available for energy or storage. This is called the thermal effect of food metabolism.
  • Foods with a high glycemic index (i.e., many kinds of carbs) cause rapid spikes and drops in insulin levels. Insulin preferentially converts glucose to fat, so the insulin spike assures that more energy is stored. And the subsequent rapid drop-off in insulin levels is a powerful stimulant of hunger. This is why we can become ravenously hungry a few hours after a high-carb meal.
  • Different foods have greatly different effects on satiety - a sense of fullness. When you eat eggs, meat, beans and fruit you feel much fuller much sooner than if you are eating candy or potato chips, and you take in fewer calories.
  • Eating protein tends to kill the appetite. One reason low-carb diets work is that many people on this diet simply don’t eat as many calories in a day.
  • Even different sugars have different effects. While glucose and fructose are both sugars, a high-fructose diet tends to stimulate the appetite more than a high-glucose diet (apparently by stimulating the hunger hormone ghrelin).

Clearly, when it comes to gaining or losing weight, different kinds of calories have different effects on our physiology. So not all calories are alike.

A Reconciliation

How can it be true that all calories are equal, but at the same time, the kind of calories we eat can make such a difference in whether we gain or lose weight?

It is a matter of semantics, a matter of the language being used by our two varieties of dietary experts. They are creating confusion with their oversimplified slogans.

Yes, the number of calories we absorb from our guts, minus the number of calories we burn, determines whether we are burning or storing fat. And in this simple equation it doesn’t matter whether those calories are carbs, protein or fat.

However, the kind of foods we eat has a far more complex effect on this simple equation than merely the calorie count contained in that food. The kind of food we eat - the type of calories we’re consuming - has a major impact both on the total number of calories we take into our bodies (by altering food absorption, and stimulating or suppressing our appetites), and the number of calories we use (by altering our metabolisms).

So, while the determining fact is indeed “calories in minus calories out,” the type of food we eat affects this simple equation (and our weight) in ways that go far beyond merely the calorie count itself.

Sources:

Jensen MD, Ryan DH, Apovian CM, et al. 2013 AHA/ACC/TOS guideline for the management of overweight and obesity in adults: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines and The Obesity Society. Circulation 2014; 129:S102.

Ebbeling CB1, Swain JF, Feldman HA, et al. Effects of dietary composition on energy expenditure during weight-loss maintenance. JAMA. 2012 Jun 27;307(24):2627-34. doi: 10.1001/jama.2012.6607.

Mozaffarian D1, Hao T, Rimm EB, et al. Changes in diet and lifestyle and long-term weight gain in women and men. N Engl J Med. 2011 Jun 23;364(25):2392-404. doi: 10.1056/NEJMoa1014296.

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