A calorie is a calorie, so the sound bite goes. Calories in/calories out proponents make the claim that weight gain is simply a matter of storing more energy than you burn.
Well. In a way, that's true. But the statement is overly simplistic, and as it turns out, irrelevant.
The formula that calories in/calories out proposes appears, at face value, to look something like this:
Change in Calories (in a body) = Calories eaten - Calories burnt
So, if a person, say, who burns 3000 calories a day eats the equivalent of 2500 calories a day will end up in a deficit of -500 calories (which should make that person lose weight).
But as a matter of fact, the story is more complicated. Here are two things to consider.
1- The human body does not burns all the calories it takes in. Have a look at this article here about a study that showed that we do not use all the calories from higher fat and higher fiber foods. Almonds, the article describes, have 20% of their calories showing up in poop. That's a lot of unused calories, giving high fat and high fibre foods a metabolic advantage. Let's revise the formula:
Change in Calories = Calories eaten - Calories burnt - (x% of high fat and high fibre calories)
2 - Let's look at the idea of calories burnt. We burn calories when we exercise, and we also burn calories when we are at rest. The human body has what is known as a basal metabolic rate, which is a specific amount of calories it would burn when completely at rest.
The Calories in/Calories out formula then can be expanded to look like this:
Change in Calories = Calories eaten - Calories burnt during activity - Calories burnt at rest (BMR) - (x% of total high fat and high fibre calories)
Looking good. But does it say it all?
Here's a link to an article about another study that showed that people on low carb diets burned more calories than people on low fat diets. This is just amazing. Low carb diets INCREASE the basal metabolic rate. By 350 calories a day! That's around half a meal's worth of calories a day, burnt, simply by eating more fat and protein instead of carbohydrates.
So, put simply: a calorie provided by carbohydrates would result in a lower BMR than a calorie provided by fat or protein.
So let's look at our formula below, one more time, with the above statement in mind:
Change in Calories = Calories eaten - Calories burnt during activity - Calories burnt at rest (BMR) - (x% of total high fat and high fibre calories) - 350 Calories (if on a low carb diet)
Let's take 2 people with identical body composition and activity levels and give them a controlled diet. Let's assume the BMR for both people is 1500 calories a day and they are totally at rest
Person 1: Eats 2000 calories a day from almonds (high fat)
Person 2: Eats 2000 calories a day from sugar/starch (high carb)
Applying the formulas:
Person 1: Change in Calories = 2000 (calories from food) - 0 (activity calories) - 1500 (calories burnt at rest) - 400 calories (20% of almond calories excreted) - 350 (calories burnt/BMR increase due to high fat diet)
Person 1 calorie deficit: 250 calories (lost)
Person 2: Change in Calories = 2000 (calories from food) - 0 (activity calories) - 1500 (calories burnt at rest)
Person 2: calorie surplus = 500 calories (stored)
Person 1 loses weight. Person 2 puts on weight.
So there you have it. A calorie is a calorie, but the source of the calorie is what can make a food fattening or not. A low carb/high fat diet gives you a metabolic advantage over the traditionally touted low fat diet. You can eat the same amount of calories, but loose more weight.
There are more pieces to the puzzle, of course. We haven't even discussed the role of insulin in fat regulation here, but the point I'm trying to make is that there is a real danger when we go around equating all the sources of our energy. When it comes to the the human body, a calorie from carbohydrates is not the same as one from protein or fat. Not all food is the same when considered from the hormonal and metabolic perspective, even if their calories appear to be identical on paper.