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The FTC says it is difficult to lose more than a pound a week without cutting back on calories or increasing exercise. Likewise, the WIN says that losing a half pound to two pounds a week and keeping the weight off is best accomplished by eating healthy foods in moderate proportions and exercising most days of the week. Talk with your health care provider about weight loss and ask for information on how to choose a safe weight-loss program.

No product can bring about permanent weight loss, the FTC says. Most diets end in failure because people are unwilling to change their lifestyle for the long term, seeking instead a quick fix. Healthy (and feasible) weight loss is no more than a pound a week. Any more than that without a doctor's supervision may not be safe. No product that is worn (rings, earrings, shoe inserts) or rubbed on the skin can cause permanent weight loss, the FTC says.

In the Atkins diet, you eat a low-carbohydrate, high-protein, high-fat diet. People like this diet because they get to eat "forbidden" high-fat foods, such as bacon cheeseburgers, steak, cheese omelets, and butter. But by limiting fruits and vegetables, you may not get the vitamins, minerals, and other essential nutrients you need. Talk with your healthcare provider before starting any diet.

These diets overwork your kidneys and can cause dehydration, headaches, and bad breath. They also can make you feel nauseous, tired, weak, and dizzy. On these diets, you lose water because your kidneys try to get rid of the excess waste products of protein and fat that your body makes. A buildup of these waste products in your blood can cause your body to produce high levels of uric acid. Uric acid is a risk factor for gout, which a painful swelling of the joints, and kidney stones. This condition can be risky for pregnant women and people with diabetes.

Low-fat or nonfat foods, particularly snack foods, may still have plenty of calories, the NIDDK says. These foods often have extra sugar or other ingredients added to make them taste better. Sugar, extra flour, and thickeners all have calories, which can add up if the foods are eaten in excess. The number of calories in a food depends on the amount of carbohydrate, protein, and fat it contains. Carbohydrate and protein have about 4 calories per gram, and fat has more than twice that amount, at 9 calories per gram. Good alternatives for snacking are fruits and vegetables, most of which are naturally low in fat and calories.