Individual variety in nutrition and response to food and exercise is really interesting, and it seems to occupy a weird gray area between science and craft. I'd like to learn more about this.

One example I've seen (more from the fitness/nutrition community than the scientific community) is that people respond differently to carbohydrates -- I think it's variation in insulin response. Try eating a big meal of pancakes and syrup. Some people will feel sluggish soon after; some people will be energized. Some people are always mildly unwell on a high-carb diet: tired, hungry, gastric distress. Some people do fine. (I'm in the mildly unwell camp, only I didn't know it until I experienced what it was like not to be mildly unwell.)