Rena Mendelson, a professor of nutrition at Ryerson University School of Nutrition in Toronto, rates four of today's most popular diets: Atkins, Glycemic Index (GI), South Beach and Weight Watchers.

Dr. Atkins' New Diet Revolution
CLAIMS: A low-carb, high-protein diet (all the protein and fat that you want) paves the road to weight loss and good health. One study that followed low-carb and low-fat dieters for one year found that both groups lost about the same amount of weight (5.1 to 8.7 kilograms and 3.1 to 8.4 kilograms, respectively), but the low-carb group had a better lipid (triglyceride and "good" HDL cholesterol) profile.

WHAT'S ON THE MENU: The first phase may be from two weeks to many months and stipulates no carbohydrates (no fruit, bread or pasta) but unlimited proteins and fat (seafood, poultry, meat, eggs, cheese, oils and butter) along with a few leafy green vegetables. Followup phases add some carbohydrates (limited nuts; fruits, such as berries, pears, apples, grapefruit and oranges; wine; beans; and veggies) up to a maximum of 40 to 120 grams of carbohydrates per day.
THE SCIENCE: A high-protein diet helps curb appetite and reduce water retention.
PLUS: Causes quick and significant weight loss, largely due to loss of water (fewer carbs results in fewer spikes in blood-sugar levels and less output of insulin, the hormone that makes the body retain sodium and therefore water).
PLUS: Insufficient carbohydrates for the brain. New guidelines indicate that 100 to 130 grams are required each day for optimal brain and nervous-system function and for energy. Lack of fibre increases risk of constipation.
CAVEAT: Many people cannot tolerate this diet because they have strong cravings for carbohydrates and they feel weak.

CLAIMS: Low-glycemic-index foods keep you feeling satisfied longer and help you burn more body fat and less muscle. The glycemic index (GI) measures how quickly the food you eat is converted into sugar (glucose) for the blood. High-GI foods rapidly convert into glucose, while low-GI foods take longer, which leads to a more sustained level of glucose and energy. The GI value of a food may change depending on how and for how long it's cooked and with which foods it's combined.

WHAT'S ON THE MENU: Low-GI foods, many of which are also high-fibre foods, such as beans, pasta, most fruits and veggies as well as low-fat dairy, poultry, lean meat and seafood. On the no list: high-GI foods (potatoes, white bread, full-fat dairy and watermelon) and fatty meats.
THE SCIENCE: The theory is that these foods will help you sustain a sense of satiety: when you feel full longer, you don't eat as much – and lose weight.
PLUS: Puts emphasis on healthier foods and lower fat intake, which helps protect against heart disease and is useful for people who have diabetes.
PLUS: Diet guidelines may be challenging. Since GI values may change according to cooking method and food combinations, it may be difficult to follow.
CAVEAT: None.

South Beach Diet
CLAIMS: Cutting "bad" carbohydrates lowers cholesterol and insulin levels, which helps protect the heart, reduces food cravings and leads to weight loss (3.6 to 5.9 kilograms after the first two weeks), then desired weight maintenance.

WHAT'S ON THE MENU: "Good" carbohydrates (most vegetables, healthier breads, whole grain foods and most fruits and beans) and good fats (lean meat, healthy oils, low-fat cheese and other dairy products). Nixed: refined grains, sweets, juice and potatoes, as well as fatty meats and full-fat cheeses.
THE SCIENCE: Consuming "good" carbohydrates combined with a higher protein intake provides longer, more-even energy and a sense of being full longer. It is logical that weight loss results because you eat less.
PLUS: Teaches you about healthy food choices. Forces you to spend time choosing recipes that include low-GI foods, and when you do, you value meals more. Emphasis on health-promoting omega-3 fatty acids.
PLUS: For some people, preparing food is too time-consuming (most prepackaged foods have a high GI). Perpetuates "good" and "bad" food labels. Some food staples, such as potatoes, are discouraged.
CAVEAT: None.

Weight Watchers
CLAIMS: Healthy weight-loss program based on a point system in which every food has a value and the dieter has a daily point allowance. Members pay for membership, program materials and weekly meetings.

WHAT'S ON THE MENU: Encourages low-calorie and low-fat foods. No foods are forbidden. Dieters are only restricted in the number of points.
THE SCIENCE: Paying more attention to food and eating choices and portion size leads to weight loss.
PLUS: The program helps you lose weight while teaching you healthy lifestyle habits. Provides for flexibility and a variety of food as well as individual control. Helps people change their eating habits. Weekly meetings provide a huge amount of support. Good focus on activity. Website offers recipes, meal plans and other incentives.
PLUS: For some people, cost ($39.95 sign-up fee, then $21.95 per month, with payment plans available). It won't work for nonjoiners. For some, weight loss, about one kilogram per week on average, is not fast enough (this is actually healthy weight loss).
CAVEAT: Weight Watchers itself screens out people who shouldn't be in the program, such as those at a healthy weight.

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