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.

Prescription drugs such as Xenical block the absorption of fat, but they work only modestly. They are used under a doctor's care along with diet and exercise. Products that claim to make you lose 2 pounds a week would need to block 1,000 calories a day, the FTC says. Incredibly, some products claim to make you lose 2 pounds a DAY. They would need to block more than 7,000 calories a day, but that is several times the amount most people consume.

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. Over the long-term, this diet may increase your risk for heart disease and cancer, because of the emphasis on saturated fat. No long-term studies have been done to determine whether this diet is safe. Short-term studies have shown that people on this diet lose more in the first six months than with a conventional low-fat diet. At the end of a year, however, the number of pounds lost on the Atkins diet is about the same as with a conventional diet.

Over the long-term, this diet may increase your risk for heart disease and cancer. The South Beach diet rates carbohydrates by how quickly they are digested and how they affect the level of glucose in the blood. This rating system is called the glycemic index. Refined carbohydrates like white bread, white rice, and pasta get a bad rating on the glycemic index scale. Unlike the Atkins diet, which bans most carbohydrates, the South Beach diet allows complex carbohydrates like brown rice and whole grains. Most of the early weight loss on this diet is in the form of water and lean muscle mass, not fat, says the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). People often get bored with high-protein/low-carb diets because they crave the carbohydrates they aren't allowed to eat. Another variation is the Zone diet. On this relatively protein-heavy plan, you consume carbohydrates, protein, and fat in a ratio of 40/30/30 in specific serving sizes called "blocks." Although this diet is more moderate than others, it can be difficult or confusing to keep track of the carbohydrates, protein, and fat you are eating. Nutritionists say this diet can help you lose weight because it restricts calories, but it's complicated.

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.