‘Member how I said in a prior post that “The form of the calories don’t matter that much in and of themselves. A thousand calories of protein or fat or carbohydrates is still a thousand calories”? In saying that I was conceding an element of truth to a common dieting axiom: “a calorie is a calorie is a calorie.” This axiom is often used by those who tout calorie restriction as the key to successful fat loss. These folks would say that it doesn’t matter whether the calories you take in are in the form of fat, carbohydrates, or protein. All that counts for losing weight is losing calories.
But while it’s true (by definition) that one calorie represents as much energy as another calorie, that is much more to the story than this. As I went on to point out,
the type of calories does have an effect on the body’s metabolism because the body has to do different things in order to burn different macro-nutrients (i.e., protein, fat, and carbohydrates). If you change the ratio of the macro-nutrients you are eating, your body’s metabolism changes in order to digest and/or store them.
I’d like to document that now by citing a classic study published in 1956 by Alan Kekwick and Gaston Pawan (“Calorie Intake in Relation to Body Weight Changes in the Obese,” Lancet, July 28, 1956, 155-161). These researchers divided their test subjects into three groups, each of which ate a thousand calories a day that were principally composed of one of the three macronutrients. One group got a thousand calories a day that were 90% carbohydrate calories, another got a thousand calories a day that were 90% protein calories, and the third group got a thousand calories a day that were 90% fat calories. If the “a calorie is a calorie” maxim applied to weight loss, these groups should have lost the same amount of weight–or at least approximately the same amount of weight.
They didn’t.
The 90% protein group lost an average of .6 pounds per day of the study. The 90% fat group lost .9 pounds per day. And the 90% carbohydrate group actually gained .24 pounds per day.
What explains this?
The basic explanation is that your metabolism adjusts to the input you give it. If you put in primarily fat, it triggers one set of responses as your body gears up to utilize the fat and manage its energy output. If you put in carbs, it triggers a different set of respones. And if you put in protein, it triggers a third set. These have an impact on how much weight a person will lose. As the 1956 Kekwick study showed (and as subsequent studies have reinforced), if you give your body fat in the absence of carbohydrates then your body will go into fat burning mode. If you give it protein in the absence of carbohydrates then it will do the same, though the rate of fat burning will be less efficient.
On the other hand, if you give it primarily carbohydrates then it will slam on the brakes for fat burning and start hoarding the fat it has, even slowing your metabolism so that it can generate excess calories to try to hoard more nutrients since the sudden absence of fat from your diet has convinced your body that some kind of famine is going on and you need to go into emergency survival mode.
Subsequent studies have confirmed and amplified the Kekwick and Pawan results, and I’ll try to document some of those in future entries, but the basics were right there in the 1956 study.
