# A Calorie is Just a Calorie, Except When It Isn’t

Thermodynamics is a funny subject. The first time you go through it, you don’t understand it at all. The second time you go through it, you think you understand it, except for one or two small points. The third time you go through it, you know you don’t understand it, but by that time you are so used to it, it doesn’t bother you anymore.
— Arnold Sommerfeld

Thermodynamics and “All Else Equal”

The first law of thermodynamics is a version of the law of conservation of energy which tells us, more or less, that energy can change form but cannot be created or destroyed. This unassailable bedrock principle of physics has been abused by nutritionists for decades. “Calories in, calories out!” they say. “A calorie is a calorie!” Want to lose body mass? Simply decrease energy (calorie) intake or increase output. Want to gain mass? Do the opposite.

One cannot argue with physics.* The laws of thermodynamics hold as much for the human body as for any other isolated system. It is true that if all other factors are held constant, a change in caloric intake will result in a corresponding change in body mass.

The however, if weight management and body composition were as simple as reciting tautologies from physics 101, we’d all look amazing naked and the obesity crisis would be a thing of the past. Unfortunately, it’s not that simple.

All Else Not Equal: Calories In vs Calories Out

The wrench in the system is a simple phrase from the above: if all other factors are held constant. In the human body, no factors are ever held constant. The systems of the body are well calibrated by millennia of evolution to adapt to changes in order to maintain equilibrium in the internal environment. This phenomenon is called homeostasis. It means if you make a change to a system (say, by reducing caloric intake), the body is going to do stuff differently to adapt to that change.

For example, say you increase your calorie output by increasing your activity level. All else equal, you should lose weight. But all else isn’t equal. Your body responds to the caloric deficit by being hungrier. You eat more. Homeostasis achieved. The reverse occurs on typical calorie-restriction diets. Your activity level and resting metabolic rate (calories burned at rest) fall to compensate.

All Else Not Equal: Hormones and Food Reward

Beyond homeostatic processes, there are other factors that make “all else equal” useless when the thermodynamic system in question is the human body. When it comes to consumption, while a calorie is just a calorie when it comes to thermodynamics, different calorie sources can actually impact consumption.

Its true that 200 calories of chicken and broccoli and 200 calories of ice cream and potato chips provide exactly the same amount of energy. However, the ice cream and chips are much more likely to make you eat more food. There are several hypotheses about why this is the case.

One idea is called the food reward hypothesis, and it goes something like this: certain types of food provide more motivational value i.e. reinforce behavior. If you’re at all familiar with behavioral psychology, this concept is not new to you. If you do something (eat a tasty potato chip) that provides you with some reward (hedonic pleasure, dopamine response, whatever — it doesn’t really matter what the reward is), it reinforces the behavior. You are likely to do the same thing again. You have learned through the reward you got from the behavior. Some types of foods — those high in fat, sugar, and salt — provide much more reward value and thus much more reinforcement to eat more of them. The same mechanisms drive addiction to other stimuli (drugs, alcohol, gambling, etc), why wouldn’t they work for food as well?

Another common hypothesis to explain why eating some foods tends to make you eat more food is the insulin hypothesis. It’s built on the idea that some foods (carbohydrates, especially simple ones like sugar, typically) cause certain hormonal reactions in the body which result in more consumption. The rough sketch of the insulin hypothesis is this: You eat simple carbohydrates, which are quickly digested into glucose (a simple sugar the body uses for energy), which floods your bloodstream. Your body notes the increase in blood glucose and releases insulin, a storage hormone which tells your cells to store the glucose away for later. The spike in insulin quickly results in blood glucose dropping to below its initial level. The body says “Oh, no! We’re out of glucose!” Bang, you’re hungry again.

I tend to favor the food reward hypothesis (after years of being a die-hard insulin hypothesis man), but the truth may be some combination of the two and likely involves other complexities not-yet-discovered.

OMG That’s All Too Complicated Plz Send Help (tl;dr)

Yes, but in the end it really boils down to a few things:

- The body is complicated, and simplistic explanations/solutions are likely to fail
— Calorie control technically can work, but holding “all else equal” is very hard
— The quality of calories consumed impacts quantity of calories consumed and thus body mass/composition

Focus on calorie quality. The rest will probably work itself out.