As long as you feel satisfied, I wouldn’t worry about getting enough fat on fasting days, but I do like to make sure I get a decent amount of protein. I think that might be difficult on potatoes, vegetables, and fruit only. I am mostly vegetarian but frequently eat eggs or fish on fast days to make sure I get enough protein, which the body doesn’t store and which it needs for repair. Other high-protein possibilities are tofu, low-fat cheeses (the calorie count can be high because of fat, but low-fat cottage cheese or ricotta can be good for breakfast) or pulses (but the calorie count can be high there because of starch — one really good recipe for pulses that does work is the curried lentils in the FastDiet cook book). I like to eat low-fat cheese or yogurt and fruit for breakfast and fish (baked in foil with lemon juice and herbs), eggs (in, say, a frittata), tofu (in miso soup), and lots of vegetables for dinner on fast days, and mostly skip starches like potatoes, rice, and pasta. Every once in a while I just don’t feel satisfied without a bit of starch and then I eat about one and a half ounces of the rice or potatoes my son is having. This might not work for everyone.