In a prior post
I wrote 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, but the basics were right there in the 1956 study.
