Making a science of any human attribute is fun because it probably already has a good scientific literature. You can pick anything. For example, out of the sky you could pick "the science of human smiles" and almost certainly find a good literature and write a book about it. Body fat and (separately or together) muscle building weight will have an interesting science. This guy Brad Pilon sounds like an interesting read.

Weight loss science is just another one, but just about losing weight in a 'normal' fashion (no surgery, no disease, etc.). Just one measurement instrument--a scale.

Some of the best literature on weight loss science comes from studies of soldiers where you have an entirely captive audience that you can control their diet, exercise, down to the minute every day.