Vegetarianism and Veganism
When it's done sensibly, the experts say that vegetarianism can be a healthy diet. One of the most obvious problems with cutting out meat is that you may not get enough protein.
"The problem for some people on vegetarian and vegan diets is that they cut out the meat but don't replace it," says Geise. "You can't just sustain yourself on vegetables and grains."
However, if you still eat milk and eggs, you should be OK. Vegans, who don't eat animal products of any kind, are in a trickier position, but they can get protein from foods like beans, legumes, nuts, and seeds, Zied says.
Protein isn't the only issue. "If you're a strict vegan, you are risking some nutritional deficiencies," says Zied. "Zinc, iron, and B-12 are only in foods that come from animals."
The only way to get these vitamins and minerals is through artificial means. You can either buy foods that are fortified -- such as some brands of soy milk or veggie burgers -- or take supplements.
There are simply too many fad diets to go through all of them in any detail -- a walk down the diet aisle at a local bookstore will tell you that. Since new ones are coming out all the time, you need to exercise some common sense when evaluating them.
The experts say any diet that hinges on cutting out or sharply reducing one type of food puts you at risk of nutritional problems. Any of these fad diets may help you lose weight in the short term. Whether you cut out high-fat foods, carbohydrates, or food with a high glycemic index, or if you subsist only on grapefruits and bowls of cabbage soup, you'll probably lose weight. But you'd probably lose weight if you decided to forgo all red and yellow foods, or foods beginning with vowels. That doesn't mean it's a sensible nutritional approach. "You have to eat from a wide variety of foods," Geise tells WebMD.
But a lack of variety is not the only risk: Any diet that keeps you at a very low number of calories can be a problem, even if you're eating healthy food.
Rapid weight loss on any fad diet can also cause problems, such as gallstones.
Aside from the nutritional problems, the experts say that very strict diets have other drawbacks. For one, they're hard to stick to. It's easier to stay on a meal plan that's a little more forgiving.