That's a very difficult question. Exercising vigorously on a poor diet can be very bad for you. You're taking your body to it's limits on poor fuel, causing a lot of stress. Your body would also not have the adequate nutrition to rebuild once it has been torn down after a workout. On the flip-side, eating a healthy diet without exercise is not too beneficial itself. Not only is it more difficult to keep a strict diet when you're not using your body to see the results/consequences of what you eat, but being sedentary is extremely troubling for the body as well. I do think though, that more damage would be done to the body by exercising vigorously on a very poor diet then being sedentary with a good diet. I enjoy exercising more than I do eating healthy, but If I HAD to give up one I suppose it would be exercise. Even saying that makes me feel sick though. ***Edit*** I was thinking about this question last night as I was drifting to sleep. I changed my mind. I'd rather have a bad diet and still have exercise. I would just feel absolutely useless if I couldn't work my body regularly. The thought alone makes me feel weak and gross.