In a similar way in which the injury and illness section is written, this is not the advice of a professional in the field, just some observations made by a competitive athlete. The few things we do suggest here are generally just common sense.
As you will often hear on television the best approach to eating is a balanced healthy diet. You should try to eat at least five items of fruit and vegetables a day and also avoid really fatty foods where possible. We all need a certain amount of fat to survive - just aim to go for the healthier option where possible.
If you are very conscious of what you are eating then aim to eat between 25% and 30% of you calories in the form of fat. Basically, from the athletes point of view if you are being sensible you should burn off the odd cake or biscuit you eat without any trouble.
Clearly, an athlete is going to use a lot of energy and lose a lot of fluid through sweat. It is therefore important, particularly on hot days to drink enough fluid. Some isotonic drinks may have benefit in terms of boosting energy, but many athletes prefer just to take on water. Nutritionalists suggest that non-active people should drink about 3-4 pints of water a day, though few do. Your urine gives an indication of your levels of hydration - if it is a dark yellow you should be drinking more.
You will often hear athletes saying that they are taking this vitamin or that supplement. Whilst, these things may well be benefical, they can also sometimes be unnecessary and as they can be expensive do little more than burn a whole in your pocket.
Having said that there is an argument (particularly if you are eating on a limited budget, say as a student) of ensuring that you get a good level of vitamins with a multivitamin tablet once a day - as a kind of vitamin insurance. These can be relatively cheap (from only 2-3 pence a day) particularly if you buy own brand labels.
This is just a back up to a good diet and should not replace it. Think of it as a safety net as you would household, car or travel insurance.