If you're a fitness fanatic, you'll need plenty of energy from carbohydrates, and the building blocks of muscle repair and growth from protein.
Eat small portions every 3-4 hours. Make sure you're getting some protein and some carbs in every meal/snack.
Don't fall for excessively low carb diets - they're one of the most unhealthy fads in a world of crazy and destructive diets. Better still, avoid any short term 'diet' and focus on healthy eating all the time.
Carbs to focus on are complex carbs such as sweet potato, bulgar wheat, brown rice, wholemeal bread (in moderation), porridge oats.
Protein sources to focus on are fish, lean meat, eggs (these last two in moderation!), skimmed milk, quinoa.
And eat tons of veg! As wide a variety as you can. Some raw. Snack on raw carrots, raw celery, raw baby sweetcorn, raw young leaf spinach.
As for fruit, eat 3 portions a day. The golden rule is 'more veg than fruit' as the simple sugars in fruit can cause unstable insulin reactions and eventually convert to fat if you eat in excess.
Last but not least, fellow fitness fanatics, drink plenty of water between meals, small amounts each time, around 1.5 - 2 litres a day, depending on the weather, your body size etc. Water hydrates you (obviously!) and good hydration is vital for optimum physical performance.