Nutrition and its Biochemistry forms the foundation of exercise physiology as it relates to energy metabolism. You can eat junk and get by for some time but you will never reach your athletic potential doing so. Unfortunately, and many people don't take it seriously, heart disease and chronic conditions like type 2 diabetes are greatly influenced by the foods you eat - especially over the long term. Exercise is not enough to circumvent the negative effects from eating a poor diet on a regular basis. Vitamins will not make up for a bad diet overall, especially one with a great amount of overly processed food stuffs high in calories but usually lacking in the micronutrients and other substances (fiber, etc..) needed for optimal health.
There are very fit cyclists at the age of 20 that eat very poorly, perform well on the bike who show coronary artery damage and partial blockage, and arterial and venous damage from plaque or venous semi-lunar valve damage as seen with doppler type scans or other tools by their mid 20's while still performing well on the bike. This damage does not manifest itself clinically for several decades but the damage is still visible now and continuing poor eating habits only hastens the process. Heart disease and other diseases highly influenced by dietary habits don't just happen overnight but the damage is measurable now from pretty early on in young adulthood with poor habits.
Genetics plays a role of course, but the damage is still being done albeit at a very slow but consistent rate with poor diet, poor sleep habits, etc...
Food is medicine. Some of the medicine is admittedly more tasty than others