Macronutrient Intake: Are All Calories Created Equal?

“A calorie is a calorie”, an adage purported for decades in the fitness/health subculture.

I think it’s fair to assume the war of “clean” and “dirty” foods is actually rooted in the postulation that all calories are indeed not created equally. Physicists and scientists alike will often shun such an inane theory given their rational approach to thermodynamics.

On the other hand however, certain dieticians and “health advisors” will argue that certain calories are more prone to cause weight gain and/or elicit different metabolic responses.  So which side of this debate is right?

Well frankly/theoretically speaking, a calorie is just that--a calorie.

What exactly is a calorie you ask? Scientifically speaking, a calorie (specifically a dietary/food calorie) is the amount of energy required to raise the temperature of one kilogram of water by one degree Celsius. Energy, as most people know, remains constant in systems but may come in various forms including: chemical, heat, light, nuclear, electrical, kinetic, etc.

Thus, when we are talking about calories, we are specifically concerned with the heat energy of a particular food. As you are likely familiar, certain macronutrients contain a respective amount of calories per gram, as is readily available on food labels. Just for reference, here is the approximate amount of energy (i.e. calories) in the three primary macronutrients (on a per gram basis):

• Carbohydrates - 4 kcal
• Proteins - 4 kcal
• Fats - 9 kcal

But it can’t be this simple, can it? I mean come on; nutrition is a field with decades of research focused on deciphering how the body responds to various caloric intakes, so there has to be some complexity to the issue at hand. What I would argue is that all calories are indeed the same (i.e. “created equal”), but not all macronutrients are. See the difference here? If not, I will elaborate.

Macronutrient variability

When we look at the intricacies of the human body and the biochemical reactions that occur, it seems like an oversimplification to say that the body treats all calories the same. Again, the phrasing of this postulation needs some refining. The body does see a calorie as just a calorie, but what differs in nutrition is how the body reacts metabolically to different macronutrients.

Essentially, what this means is that while a gram of protein and a gram of carbohydrate may both contain ~4 calories, the body will elicit different responses to the ingestion of each despite the identical energy content. A simple example of this is the heightened insulin response seen with carbohydrate ingestion.

Different macronutrients, different functions

So as was alluded to above, the body exhibits specific reactions to each of the macronutrients we ingest. This is why people seem to twixt the concept of macronutrients and calories, since a diet comprised of purely carbohydrates but equivocal amount of total calories as a diet composed of, say, purely protein would have differing effects physiologically.

If you were to subsist solely on sugar, you would never really provide opportunity to activate muscle-building cell signals like the mammalian target of rapamycin (mTOR) pathway. Rather, you would be chronically stimulating secretion of insulin from the pancreas and probably experiencing some wild blood glucose swings.

Now on the contrary, someone subsisting purely on protein would indeed be stimulating the mTOR pathway more efficiently than the carbohydrate-only dieter, but the issue is further convoluted by the fact that muscle protein synthesis (and many other biochemical pathways) has a peak rate/limit.  That’s a whole other topic in and of itself however and is beyond the scope of this article.

What macronutrients do share

It’s probably rather intuitive to most people, but the commonality among all macronutrients is that they serve the main purpose of providing the cell with energy. Yes, the hormonal and metabolic pathways that are involved in the ingestion of fats, carbs, and proteins may differ, but at the end of the day they ultimately serve as a fuel source for the body.

In fact, this is one of the reasons that “hard-gainers” are often pushed to eat calorically-dense foods, which are usually rather high in fats, because such individuals just need the extra energy to stimulate growth. Of course the inverse of this is often suggested to individuals who are trying to reduce energy (read: calorie) intake (i.e. eat foods high in volume but low in calories).

Take-home message

Many people are highly biased when it comes to the debate of calories being equal, so it’s only natural that I will likely receive some flak for this article. Again, I look at this from a rational approach and just find it odd to say a calorie is not the same as any other calorie; it’s analogous to me saying that ATP is not ATP, which just makes no sense.  However, it is safe to say that the metabolic response to proteins is not identical to carbohydrates or fats.