When arguing against a specific scientific claim it is always desirable to be able to say that the claim violates an established law of science. Creationists attempt this with their argument that evolution violates the second law of thermodynamics (it doesn’t). The temptation is that such arguments are short and pithy, they seem conclusive, and they avoid the need to wade through a dense and complex set of scientific evidence and theories.
In the “diet wars” the first law of thermodynamics has been thrown around a lot. Up to now I have been aware of two camps defending their position with thermodynamic arguments. The first (and the one that I find most compelling) is the calorie in vs calorie out camp, that argues that the laws of thermodynamics apply to people too. This means that weight management must be a function of calories in (the total calories consumed by a person) – calories out (the total caloric expenditure, including metabolic processes, waste heat, exercise, and others). Thermodynamics must be obeyed and so if one wishes to lose weight they must burn more calories than they consume.