The Atkins’diet has four phases. During the induction phase, you eat no more than 20 grams of carbohydrates per day, mostly from salad greens and other acceptable vegetables. This means no bread, pasta, rice, fruit, milk, yogurt, cereal, juice, cookies, sweets, or potatoes, among other foods. However, you can eat as much as you like from foods that are pure protein and fat, such as meats, eggs, oils, and butter. The induction phase lasts at least two weeks, and usually results in rapid weight loss.

During the next three phases (ongoing weight loss, pre-maintenance, and lifetime maintenance), you will slowly reintroduce carbs until your weight loss slows and, when you reach your goal, stops. Then, you will monitor your weight, and if you gain, you take away carbs again until you lose the weight again. This means that your calorie intake (and your weight) is moderated by adjusting your carbohydrate intake. Protein and fat are not restricted. You are also advised to take a multivitamin and drink eight 8-ounce (236 milliliter) glasses of water each day.

In 2003, a study in the New England Journal of Medicine reported that people following the Atkins’ diet lost more weight than a control group after three and six months, but that both groups had lost approximately the same amount of weight after one year. Since then, studies have continued to show that compared to other diets, a low-carbohydrate diet leads to greater weight loss at six months, but similar amounts of weight loss at one year. A 2009 study looked at 13 different studies and concluded that after one year on a low-carbohydrate diet, dieters tend to lose as much weight—if not more—as dieters on a traditional high-carbohydrate diet.

On the PointsPlus program, you are allotted a daily PointsPlus target, which is based on a number of factors including your age and body weight. Every food has a PointsPlus value, and you can eat whatever foods you want, as long as you stay within your daily range. However, Weight Watchers encourages eating what it calls Power Foods—foods that are particularly nutritious and satisfying, such as lean protein, whole grains, and low-fat or fat-free dairy products. PointsPlus values are based on the amount of fat, carbohydrates, protein, and fiber in a food. The way your body processes these major nutrients are also taken into consideration. Fruits and most vegetables have zero PointsPlus values to encourage consumption of these nutrient-dense and satisfying foods.

The PointsPlus program also emphasizes exercise. When you exercise, you earn activity PointsPlus values, which may be swapped for food.

Weight Watchers recommends an average loss of 0.5-2 pounds (0.2-0.9 kilograms) a week for safe and sustained weight loss.

A 2003 study in the Journal of the American Medical Association found that people following the Weight Watchers program lost significantly more than those on a self-help, weight-loss program. At the end of one year, the people on the Weight Watchers program lost an average of 9.5 pounds (4.3 kilograms), while the people in the self-help program lost an average of 2.9 pounds (1.3 kilograms). After two years, the Weight Watchers group lost an average of 6.4 pounds (2.9 kilograms) and the self-help group lost an average of 0.4 pounds (0.2 kilograms). Again, losing weight, followed by some weight gain, is typical. However, a net weight loss after two years—no matter how much—is desirable.

Overall, the Weight Watchers PointsPlus program is a healthy one. In addition to cutting the amount of calories you consume and encouraging you to exercise, the diet encourages taking a daily multivitamin, drinking lots of water, and eating plenty of fruits, vegetables, and low-fat dairy products. It also takes effort to log all the food you eat and exercise that you do. Fortunately, computer- and smart phone-based programs make it easier to calculate the PointsPlus value of foods and log activities. While the traditional version of the Weight Watchers program requires attendance at a weekly meeting, these days you can also choose an online-based program.

The South Beach Diet has three phases. During the two weeks of Phase 1, breads, rice, potatoes, pasta, baked goods, and even fruit are restricted. You can eat enough meat, chicken, turkey, fish, vegetables, eggs, cheese, nuts, and salads with olive oil dressing to make you feel full. The goal of phase one is to eliminate cravings and kick-start weight loss.

Phase 2, which you will stay on until you reach your weight-loss goal, is less restrictive. You can add carbs back into your diet, but in moderation. The focus is on whole-grain carbs, but you can now splurge on refined carbs every once in a while.

After you have reached your weight-loss goal, you will enter into Phase 3, “the stage that lasts the rest of your life.” In this phase, you are encouraged to focus on monounsaturated and polyunsaturated fats instead of saturated and trans fats, and whole-grain carbs instead of refined ones.

Although exercise is not a main focus of this diet, Dr. Agatston recommends you take a brisk, 20-minute walk each day. Dr. Agatston claims that you will lose 8-13 pounds (3.6-5.9 kilograms) in Phase 1, and 1-2 pounds per week in Phase 2.

The South Beach Diet relies on Dr. Agatston’s own research. A randomized, clinical trial published in a 2004 issue of the Archives of Internal Medicine compared the Modified Low Carbohydrate diet (South Beach Diet) with the National Cholesterol Education Program diet. Sixty participants were randomized and the study lasted for 12 weeks. Participants in the Modified Low Carbohydrate diet had significantly greater weight loss over 12 weeks. However, since the study was small and, more importantly, since Dr. Agatston himself conducted the study, more research is needed to determine whether the South Beach Diet is effective.

A 2010 study published in the New England Journal of Medicine compared the effectiveness of different types of diets and found that a diet similar in make-up to the South Beach Diet—it had a modest increase in protein content and a modest reduction in glycemic index—led to an improved diet adherence and weight-loss maintenance compared to other diets.

While the strengths of this diet focuses on healthful fats and whole grains, Phase 1 is extremely restrictive and may cause many enthusiastic dieters to give up. Also, the promise that you will lose 8-13 pounds (3.6-5.9 kilograms) during the first two weeks is questionable. Even if it were true, most health professionals recommend no more than a couple of pounds a week for safe, long-term weight loss.