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Why “calories in, calories out” is an oversimplification

“Treat obesity as physiology, not physics”, urges Gary Taube in his Nature column, and he’s right. In this short article, I show you why the source of calories matters.

I’ve been taught, as a medical student, that the body treats all calories alike. The source of calories makes no difference. What makes an impact is the balance of “calories in” and “calories out”. You’ve probably heard it too. This is overly simplistic. It’s not entirely wrong, but it lacks important nuance. The current dogma reduces nutrition to mathematics – plus and minus, whereas it should understood in terms of metabolism. Proteins, carbs and fats are not just sources of energy, a fact the presiding paradigm fails to integrate.

Calorie sources – proteins, carbs, fats – have unique interactions with our biology. Proteins triggers one set of processes, while carbs trigger another set, and fats other still. They’re not the same. Even within these categories there are considerable metabolic differences. Yes, they can all be turned to ATP – Adenosine Triphosphate – the energy currency of our cells, but so what? Both Gandhi and Hitler turned to compost eventually, but you wouldn’t dare say that their impact on our earth was the same. Similarly, different calorie sources trigger different hormones in the body before being turned into calories.  Why this matters will be evident in a moment.

Our bodies are continously balancing anabolic (growth) and catabolic (break down) processes. If anabolism takes over, you gain weight. Otherwise if catabolism is more prominent, you’ll lose weight. Weight lost or gained could be muscle mass, bone mass, fat mass, connective tissue, water etc. The composition depends on several factors. Some hormones are catabolic – like cortisol & adrenaline – and promote breakdown of tissue to provide energy. Other hormones are anabolic – like testosterone & insulin – and stimulate tissue synthesis. This tissue can be fat, muscle, or anything else depending on the hormone at work. The reason I’m telling you this is, as I wrote earlier, different foods bring about different hormones that do different things. “A calorie is a calorie” does not account for this.

If in excess, all calories will stimulate anabolism. Some calorie sources, however, go the extra mile to make you fat. There is evidence, which I’ll share with you promptly, that you can gain weight to a certain extent without increasing caloric intake. Before this, though, some clarification is necessary. You cannot grow without the calories to account for it. This is physically impossible, and so it is not what I’m implying. I’m not arguing that carbohydrates are so bent on making you obese that they are breaking the laws of physics. What I am arguing, however, is that hormones such as insulin can influence your body to use a larger portion of the calories you’re already consuming for anabolic purposes.

If your body is using, say, 60% of the calories you eat for anabolic processes – to maintain or gain weight – then I argue that insulin can make it use 70% or more of its calories for anabolism. Effectively causing you to gain weight without increasing caloric intake. The next paragraph shows how this is possible by reviewing a study done on the anabolic hormone testosterone. Then I’ll argue that what is true for testosterone, in this case, also applies to insulin and therefore all foods that stimulate insulin production.

If you want to read the study before reading on, here it is: The Effects of Supraphysiologic Doses of Testosterone on Muscle Size and Strength in Normal Men.

This 10 week long, randomized and double-blinded study was done on 40 men. All 40 men ate the same standardized diet, at 36 kcal/kg body weight. One half got testosterone injections once a week, while the other half was given placebo. Both halfs were further divided into one exercising and one non-exercising group. So in total this study had 4 groups. To keep the calories-to-body weight ratio constant at 36kcal/kg, their diets were adjusted every 2 weeks. If they had gained weight, their calories were increased. Researchers regularly checked that participants followed their diets. You may not be surprised by the results, but they do allow us to argue that there is certainly more to weight gain than a simple surplus of calories. The testosterone & exercise group gained 6kg / 13,2lb, followed by the testosterone & non-exercise group with their 3,6 kg / 7,7 lb. Neither of the placebo groups had statistically significant weight gain.

You might be inclined to think that the weight gain can be explained entirely by caloric intake, since it was adjusted every 2 weeks on the basis of changes in weight. Pause, however, to consider the order of events. First, the men gained weight, then, their calories were adjusted. Therefore, increased caloric intake did not cause weight gain, it merely facilitated it. The number of calories do not explain why growth began in the first place. We must look at the nature of calories. This study shows that at a given caloric intake, your body may maintain its weight, or it may decide to use those calories differently and grow because it was influenced by anabolic hormones. It’s interesting to note that the group that likely expended the most energy also gained the most weight. Exercise, like food, can influence the release of certain anabolic hormones.

Insulin is an anabolic hormone. In fact, some bodybuilders are using it for its anabolic effects. Certain foods release more insulin than others. It follows, then, that the foods that increase insulin will steer the body into anabolism, more so than other foods. This is my entire argument. This is why obesity is physiology, not physics. Why it is metabolism, not mathematics. This is why “what” you eat matters as much as “how much” you eat. There are other reasons as to why calorie composition makes a difference. Not all sources of calorie are burned with same efficieny, for example. In addition to anabolic stimuli, certain foods might alter brain chemistry and influence eating behaviour, see for example “Fructose may spur overeating“. I also recommend you read “A calorie is a calorie” violates the second law of thermodynamics.

I’d like to hear your thoughts, dear reader. Leave a comment below, or share this article if you found it interesting.

A big thanks to Brad Pilon for helpful criticism and comments. See here.


Nadeem J. Qureshi