A calorie is NOT a calorie

Science by Dr. Ben Bikman

A calorie is NOT a calorie

We have all heard the expression, “A calorie is a calorie.” And that view is used as a cudgel to defend what I think are wrong ideas about human nutrition, and certainly in the context of metabolic health. Outside the context of metabolic health, but within the context of the universe, that idea is true: a calorie is a calorie. In other words, energy must be accounted for. It is neither created nor destroyed. It’s that law of thermodynamics that is really the basis of that expression, “A calorie is a calorie.” Again, that idea, on its surface, is absolutely correct, but it is absolutely misunderstood in the context of human nutrition and certainly in the context of what we do with energy in the body.

Now, in a laboratory setting, the term “calorie” really is borne from this idea of how much energy does it take to warm up a certain amount of water by one degree Celsius. So, imagine a little flame burning under a test tube that has water in it, and we have a thermometer in that water. Well, once we’ve used enough heat to push the temperature up one degree Celsius in that water, then we’ve reached what one calorie is going to do. Now, this sentiment is applied to the macronutrients that we eat: carbohydrates, protein, and fat. And if we were using any of those macronutrients as essentially fuel on this fire (imagine wood on a campfire), that’s going to be what’s burning. And we know that fat will give more heat than carbohydrate or protein will, so that’s why we give fat a higher caloric value. Now, this idea then is assumed to have perfect fidelity or to transfer perfectly to what actually happens in the body. But I hate to break it to you: we are not a campfire. We are a little more complicated when it comes to understanding thermodynamics and really whether a calorie is actually a calorie.

And so there are a few ideas that I think we have to account for to challenge that dogma. First of all, when we eat anything, our metabolic rate will go up compared to what it would have been if we hadn’t eaten anything. But, then how do we reconcile that fact when we’re trying to balance “calorie is a calorie” idea? Eating anything will stimulate metabolic rate, but some things will stimulate metabolic rate more than other things. For example, dietary protein has a significantly higher effect on stimulating the metabolic rate than either carbohydrate or fat do. In any of these instances, when we’re eating something and the body is working a little harder to digest it, which it always does, that is a concept called the thermic effect of food [1]. And every food has it, but protein does it more than the other two [2].

But how do we account for that? Because traditional thinking would say, “Well, protein has the same number of calories as glucose does. And so if you’re looking at something that’s pure protein or something that’s pure glucose, they have the same caloric effect on the body.” That is absolutely not true. But it must be true if we hold to the idea that a calorie from one food is comparable to a calorie from another. But when it comes to what the body does with it, we are not a little flame burning in a lab. We are a complex organism or a complex machine. And that protein takes a lot more effort for the body to digest. And so the thermic effect of that protein, digesting it and absorbing it, is significantly higher than, say, the glucose. But in a lab, they have the same caloric value. In the body, they don’t because what actually gets into our blood by way of a caloric amount isn’t the same because the body has had to expend more energy in order to get that energy from the protein.

And while I’m talking about protein, it should never be discussed, like I just was, as a fuel, because the body will only use protein as a fuel in either states of incredible protein consumption, more than what most people would ever eat, or alternatively, when we are actually starving. When you are starving, a state when you’ve run out of body fat, that is when the body is breaking down muscle in order to get muscle protein to convert it into glucose [3]. Even in that conversion, we lose a little bit of energy in the process. But nevertheless, looking at protein as an energy source and giving it a caloric value, I think, is just silly. Giving a caloric value to carbohydrates and fats, which are the main fuels for the body, that’s valuable.

I mentioned eating food and how food stimulates the metabolic rate. What too few realize is that a long fast, a fast that actually goes over a day, starts to increase metabolic rate for a period of time. Now, eventually it would come back down. But in the short term, a fast that’s going about 48 hours, you actually see an increase in your metabolic rate. And that will then, of course, affect when you’re spending more energy, then you start to eat energy, you might not have been able to account for the fact that now your body is burning more energy than it was before. And so, looking at food and trying to count those calories coming in and trying to anticipate the calories going out to fit that idea of “A calorie is a calorie,” the idea, it all starts to fall apart.

My biggest criticism with the dogmatic view that “A calorie is a calorie” is because it fails to account for what the calories we eat do to our hormones. And hormones, in turn, have a profound effect on what our body does by way of burning calories or storing calories or even wasting calories, which is a concept we’ll get to in a moment. So, on the one hand, during a short-term (some may consider it long-term) fast of one to two days, metabolic rate actually increases. That is mostly an effect of an increase in a hormone called epinephrine or adrenaline. When this hormone starts to climb, which it does during a fast, it stimulates the metabolic rate in the body [4]. On the other side of this, or acting in concert with that increase in epinephrine, what happens over a fast is the hormone insulin starts to come down.

And insulin is the fundamental hormone when it comes to understanding human metabolism. Not only does insulin affect metabolic rate, but it also affects fuel use. When insulin is elevated, its theme is to tell a cell, every cell in the body, to store energy and not to waste anything. Insulin wants to tell the cells to pull in nutrients and make things from it, to build something up, or essentially store that nutrient. And it can do that better by depressing metabolic rate, which it does in the body. When insulin is spiked, metabolic rate slows [5]. In contrast, when insulin is low, say for example, through fasting or through a low-carbohydrate diet, we actually have an increase in metabolic rate, a very real, quantifiable increase in metabolic rate. And a cell, rather than being told to store energy, is now breaking it down to share with the body.

So, again, if someone is eating macronutrients or putting in calories that are allowing insulin to stay low, well, then we are expending more calories. Our actual expenditure is higher than it was before. So it’s not really fair to say “A calorie is a calorie” because it fails to account for what the body is going to do with those calories. And if insulin is down, the body is going to use those calories more readily. And then if insulin is up, the body is going to store them more readily. It’s better at storing.

Now, one last idea with regards to insulin and why I believe it’s of supreme importance to understand energy in the body, is because when insulin is low, metabolic rate is higher. The body is burning fat more as that preferential fuel. And when insulin is low, the cells of the body, especially the liver in this case, have no choice but to continue to burn fat. So, normally, a cell will only burn as much energy, say glucose or fats, as it needs to get work done. The cell is being very miserly. It will say, “I have this much need for energy, so I’m only going to burn that much energy.” But when insulin is low, especially the liver cells, as I mentioned, they can’t help it. They can’t help themselves; they keep burning energy and they keep burning fat. Even when the cell is basically burning all the energy that it needs, it can’t stop. Here’s the energetic demand in the cell. The liver meets it, but it keeps going. And as it keeps going, this threshold, once it’s passed, now the liver cells are burning fat because they can’t stop because insulin is low, and it starts to convert them into ketones. That is why a diet or a fasting regimen that keeps insulin low is called ketogenic: because the liver can’t stop burning fat. Even though it’s burning more than it needs, this excess now is being converted. It’s almost like a pressure release valve where it’s turning the fats into ketones.

And ketones themselves have a caloric value almost comparable to what glucose is, so ketones are an energy source for the body. But the one interesting thing about ketones is that they introduce a new outlet when it comes to energy, and that is by wasting the energy. When someone has higher ketones, they start breathing the ketones out and they start excreting the ketones in their urine. Remember, a ketone is essentially a piece of fat that either would have had to be stored or used for energy. Now we’re just wasting the energy. So, that has to be accounted for in a way, but it never is. When someone is invoking the idea that a calorie is a calorie, they can’t help but come to this idea that they have to count for every calorie coming in and they attempt to account for every calorie going out. But they can never perfectly get their metabolic rate right, which changes based on insulin status, fasting status, and what you’re eating in general. And it fails to account for the loss of ketones which are actual caloric molecules. We didn’t have to burn the ketone. We didn’t have to store the ketone. We just wasted it.

I hope some of these ideas are useful. The takeaways would be: trying to improve metabolic health through a perfect reckoning of calories is doomed to fail. Rather than trying to account for each calorie, I believe it’s more useful to consider what that calorie is doing to hormones, especially insulin, because insulin tells the cells of the body what to do with energy.

References:
1 Calcagno, M., Kahleova, H., Alwarith, J., Burgess, N. N., Flores, R. A., Busta, M. L. and Barnard, N. D. (2019) The Thermic Effect of Food: A Review. J Am Coll Nutr. 38, 547-551
2 Sutton, E. F., Bray, G. A., Burton, J. H., Smith, S. R. and Redman, L. M. (2016) No evidence for metabolic adaptation in thermic effect of food by dietary protein. Obesity (Silver Spring). 24, 1639-1642
3 Cahill, G. F., Jr. (2006) Fuel metabolism in starvation. Annual review of nutrition. 26, 1-22
4 Azevedo, F. R., Ikeoka, D. and Caramelli, B. (2013) Effects of intermittent fasting on metabolism in men. Rev Assoc Med Bras (1992). 59, 167-173
5 Ebbeling, C. B., Feldman, H. A., Klein, G. L., Wong, J. M. W., Bielak, L., Steltz, S. K., Luoto, P. K., Wolfe, R. R., Wong, W. W. and Ludwig, D. S. (2018) Effects of a low carbohydrate diet on energy expenditure during weight loss maintenance: randomized trial. BMJ. 363, k4583