The First Law of Thermodynamics: It is the Foundation for the Two Competing Theories about What Makes Us Fat and How We Can Lose Weight

The first law of thermodynamics, also known as the Law of Energy Conservation, tells us that energy contained in a system must equal the energy coming into it minus the energy going out of it. This is a common sense idea. Any theory that seeks to explain why we get fat and how can we lose weight must ultimately derive from this principle, which is often summarized like this:

Here is a summary of the two different obesity theories that follow logically from the first law of thermodynamics:

Caloric Balance is the theory everyone is taught in school. It assumes that the right side of the equation (the part that deals with Calories In and Calories Out) is "in charge." Raise Calories In (overeat) and/or lower Calories Out (be inactive), and the right side of the equation will get bigger. This forces the left side of the equation to also get bigger to compensate. Hence, it tells us that overeating and inactivity will lead to the storage of excess energy in the body, presumably as fat. Conversely, lower Calories In (eat less) and/or raise Calories Out (exercise more), and the right side of the equation gets smaller. Hence, the left side also must shrink. Thus, eating less and exercising more should lead to weight loss.

This alternative theory was developed in Europe in the 1930s and 1940s. It fell out of fashion after World War II, possibly because it was developed in Germany. It has lain essentially dormant since then, but it was never disproven. Science journalist Gary Taubes recently resurrected Lipophilia in his book, Good Calories Bad Calories. The main idea is that left side of the equation -- not the right side -- is "in charge." In other words, changes in the body's energy stores cause changes in caloric balance. This may sound confusing at first. What it means is that, when some underlying problem messes up our fat tissue and causes it to get bigger, this will then cause us to take in more calories and/or burn fewer calories. When that underlying problem is fixed, the fat tissue will shrink, and this will then cause us to eat fewer calories and/or burn more calories.

Caloric Balance and Lipophilia are both plausible interpretations of the first law of thermodynamics. Both make testable, falsifiable predictions about the real world. And it is pretty likely that only one theory can be correct. So which is it? And how can we tell?

Do excess calories make us fat?	Or do problems in our fat tissue (driven, for instance, by excess insulin secretion) make us fat?

Comparing scientific hypotheses

According to the principles of science, a governing hypothesis must account for all relevant data, not just some of it. This point is crucial. To learn more about how truly rigorous scientific testing should be done, click here. And here is a wonderful essay by Karl Popper on the subject.

Here is our own take on guidelines for comparing these two theories.

Below we compare 11 key predictions made by these two theories derived from the first law of thermodynamics. Click on any link to read more: