Everyone agrees to the importance of a healthy diet, but there seems to be a lot of controversy over what exactly a healthy diet is (for example the Atkins thread on this board). By "diet", I mean a consistent, life-long, and sustainable eating plan, and not a crash diet to lose a few pounds.
Here's my take:
1) Balance. Carbohydrates 60% (by calories, not grams), protein 20%, fat 20%. This is not an exact ratio. Carbs may decline to 40-45% in more inactive periods.
2) Food choice carbohydrates. The best carbs are those that are unprocessed: fuits, raw vegetables. The next best are grain based: pasta, cereals, rice, whole wheat breads. The worst are pre-packaged snack foods (chips, cookies, eic.) with high-glycemic carbs and high levels of saturated and trans fats.
3) Food choice protein. Best choices are skinless chicken breast (baked or broiled), 10% fat ground beef, turkey, etc. Textured vegetable protein can be added to ground beef to reduce the saturated fat. Soy or whey protein isolate can added to the diet if protein supplementation is desired. Most pork cuts have the majority of their calories as saturated fat (bad). Avoid packaged lunch meats.
4) Food choice fats. There are good fats and you need them! Unsaturated fats from olive oil, raw nuts, avocados should be a part of your diet. Read labels! Avoid all foods with hydrogenated or partially-hydrogenated oils--they're killers! The trans-fatty acids in these foods are the most potent heart disease agents in the Western diet. Eating one bag of chips is like smoking one cigarette: the first one doesn't kill you but 10, 000th will.
5) Do not go on "diets". If you follow a healthy eating plan, don't smoke, exercise regularly, and drink plenty of water, then you will gradually achieve the optimum weight for your unique metabolism.