Answer:
Diet
Explanation:
Health & fitness level has multiple factors. Some internal choice factors, some externally ascribed factors. Eg(s) - Diet, Excercise & Heredity, pollution respectively.
In my opinion, biggest factor affecting health is : Diet. As, it can affects health positively & negatively, by major extent. It can also help overcoming other health detrimental factors - eg lack of excercise, heredity issues, pollution related issues etc.