This conviction that positive caloric balance causes weight gain is founded on the belief that this proposition is an incontrovertible implication of the first law of thermodynamics. “The fact remains that no matter what people eat, it is calories that ultimately count,” as Jane Brody explained in the New York Times. “Eat more calories than your body uses and you will gain weight. Eat fewer calories and you will lose weight. The body, which is after all nothing more than a biochemical machine, knows no other arithmetic.”

For fifty years, clinicians, nutritionists, researchers, and public health officials have used this logic as the starting point for virtually every discussion of obesity. Anyone who challenges this view is seen as willfully disregarding a scientific truth. “Let me state,” said the Columbia University physiologist John Taggart in his introduction to an obesity symposium in the early 1950s, “that we have implicit faith in the validity of the first law of thermodynamics.” “A calorie is a calorie,” and “Calories in equals calories out,” and that’s that.

But it isn’t. This faith in the laws of thermodynamics is founded on two misinterpretations of thermodynamic law, and not in the law itself. When these misconceptions are corrected, they alter our perceptions of weight regulation and the forces at work.

The first misconception is the assumption that an association implies cause and effect. Here the context is the first law of thermodynamics, the law of energy conservation. This law says that energy is neither created nor destroyed, and so the calories we consume will be either stored, expended, or excreted. This in turn implies that any change in body weight must equal the difference between the calories we consume and the calories we expend, and thus the positive or negative energy balance. Known as the energy-balance equation, it looks like this:

Change in energy stores = Energy intake-Energy expenditure

The first law of thermodynamics dictates that weight gain—the increase in energy stored as fat and lean-tissue mass—will be accompanied by or associated with positive energy balance, but it does not say that it is caused by a positive energy balance—by “a plethora of calories,” as Russell Cecil and Robert Loeb’s 1951 Textbook of Medicine put it. There is no arrow of causality in the equation. It is equally possible, without violating this fundamental truth, for a change in energy stores, the left side of the above equation, to be the driving force in cause and effect; some regulatory phenomenon could drive us to gain weight, which would in turn cause a positive energy balance—and thus overeating and/or sedentary behavior. Either way, the calories in will equal the calories out, as they must, but what is cause in one case is effect in the other.