There is very little science in nutrition science. Unfortunately, this is one area where we know very little but make very big claims. Nearly all rules and prescriptions regarding food are based on culture, custom, religion, hearsay, prejudice, or other unsubstantiated data. Things like the famous food pyramid are driven by commercial interests only. The food pyramid urges you to eat what farmers want to sell. It is as simple as that.
Fortunately, we are largely independent of what we eat. Humans are true omnivores and nearly any diet that is unforced will do. Especially for adults. Adult humans are remarkably insensitive to food types as long as the quality is good. Quality in this case means free of toxins, infectious agents, or contaminants. By and large, food in the Western world is of very high quality and unintentional food poisoning is quite rare.
Unforced is another key term. In this context it means the person can pick and choose from a variety of items. The more freedom people have the better. The more a specific diet is forced on them, the more likely they are to get sick. Nearly all vitamins and essential elements have been discovered by forcing people to eat "scientifically" designed diets.
Contrary to what most parents seem to think, kids naturally know what is good for them. If left to their own devices and kept away from artificially sweetened items, they will eat a healthy diet, even though their day to day intake will be far more erratic than that of an adult. It is true that kids will binge on certain foods, but after a short while they will have enough of it and develop a taste for something they lack.
Humans have one great weakness and that is sugar. Pure sugar is rare in nature and it is difficult to find. But sugar is an essential food item and so people are very attuned to finding it and liking it. Craving it is the better word. Marketers of all stripes discovered this rather quickly and in rich cultures without a solid tradition and footing in cuisine -such as America- sweets quickly took over. Coupled with a sedentary life-style, sugar is wreaking havoc and causing illness everywhere.
The result is an epidemic of obesity. Due to advertising and global trade, other cultures that did have a solid tradition -such as France, the mediterranean, and others- are quickly falling prey to the sugar menace. Armed with a small amount of technology, it is easy and cheap to make highly refined sugar products. Furthermore, these items sell so well that the temptation to sell them is equally hard to resist.
Although many texts recommend a carbohydrate rich diet for athletes, I think it is a mistake. Carbohydrates are important before, during, and after hard workouts, but to switch to a year-round high carb diet is asking for trouble. Many athletes have trouble with their diet and weight as is, and suggesting more sugar is making things worse. It has gone so far that many avoid fats, a key ingredient, and the best substance to induce satiety and remove the constant hunger many carb-rich athletes feel.
Whatever you do, please try to eat a normal diet. It doesn't really matter what diet you choose as long as you pick one that has been tried and tested for a few decades at the minimum. No diet has any advantage over another and all the claims to the contrary, whether made in the popular press or in "scientific papers," are downright wrong.
The worst thing you can do is try an artificial diet, or one concocted by a "scientist" or "nutritionist" a short time ago. These diets are nearly always trouble and should be avoided at all costs.
Remember to always eat normal food, and avoid sugary drinks.