Ask the expert: Controlling your weight

 The Expert: Dr. Walter Willett

Dr. Walter Willett

Fredrick John Stare Professor of Epidemiology and Nutrition, Departments of Nutrition and Epidemiology, and Chair, Department of Nutrition, Harvard School of Public Health


1. How much control do we have over our body weight? And how much of our weight is controlled by our genes?

Genes do play a role in controlling our weight, but clearly they do not explain the huge increases in overweight and obesity we have seen in the last 30 years. We gain weight when our “calories in” (the food we eat) exceeds our “calories out” (the energy we burn). Given the genetic package we are born with, we can all improve our weight by paying attention to diet and getting regular physical activity, but some people will need to work harder than others to maintain a healthy weight. In other words, if we all eat and work out the same, we will not all look the same. Only a very small percentage of people have such a strong genetic predisposition that they will be obese no matter how hard they try. Even people who are genetically predisposed to obesity can reduce their risk of chronic disease by eating a healthful diet and staying active.

2. What is the best diet for losing weight?

The real issue is not losing weight—people can cut back on calories and lose weight on almost any diet—but keeping weight off over the long run. Thus it is more important to find a way of eating that you can stay with for the rest of your life. For this reason, any eating plan you choose should be satisfying and allow variety, and should also be nutritionally sound.

The most successful dietary weight loss study has been conducted by Dr. Frank Sacks in our department with Kathy McManus, Director of the Dept. of Nutrition at Brigham and Women’s Hospital. This study compared weight loss on a moderate-fat Mediterranean diet to weight loss on a low-fat diet; the Mediterranean diet was high in fruits and vegetables, nuts, and whole grains and it used olive oil as the primary source of fat. The diets emphasized controlling portion sizes to help cut calories. Volunteers assigned to the Mediterranean diet lost an average of 9 pounds in 18 months, whereas volunteers assigned to a low-fat diet gained an average of 6 pounds during that time period. At the study’s end, only 20 percent of volunteers in the low-fat group were still following the diet, compared to 54 percent of those in the Mediterranean group. The Mediterranean group volunteers said that their diet regimen was enjoyable and was more flavorful than a low-fat diet.

If you are interested in following this type of eating plan, I suggest using the Harvard School of Public Health’s Healthy Eating Pyramid as a guide. Like the Mediterranean diet used in the weight loss study, the Healthy Eating Pyramid emphasizes fruits, vegetables, whole grains, nuts and legumes, healthy oils, and daily physical activity. To lose weight, watch your portion sizes and aim for a modest reduction in your daily calorie intake. My new book with Mollie Katzen, Eat, Drink and Weigh Less (Hyperion, 2006), follows the same nutritional principles as the Healthy Eating Pyramid and details many eating strategies for weight control.

3. Can I lose weight through diet alone? Or do I need to exercise, too?Daily physical activity can help you control your weight and can reduce your risk of chronic disease. That’s why it is the foundation of the Harvard School of Public Health’s Healthy Eating Pyramid. Aim for at least 30 minutes a day of physical activity; many people will find that 60 minutes a day is helpful for weight control. For more information on the benefits of physical activity, read the article about staying active.

4. Will going on a low-carbohydrate diet help me lose weight?

Some people who go on a low carbohydrate diet have managed to keep weight off, but the best studies show that on average the weight loss is small after one year. There has been much debate about the impact of low carbohydrate diets on overall health. Our recent 20-year study on 82,080 women found that that a low carbohydrate eating pattern did not increase risk of heart disease; if the protein and fats mainly came from vegetable sources, a low carbohydrate eating pattern actually reduced heart disease. A controlled trial underway at the Harvard School of Public Health may give us a more definitive answer on the possible benefits of low-carbohydrate diets. The study is testing four diets with different amounts of carbohydrate, protein, and fat; it will follow 800 overweight volunteers for two years to see which type of diet is best for weight loss, weight maintenance, and overall good health.

5. Should I cut back on fat to lose weight?

The goal is to cut back on calories, and a common mistake is to focus only on fat. Fat is satisfying, and in the long run low fat diets have generally failed as a way of weight control. For good health, the type of fat is more important than the amount; eat plenty of unsaturated fats, limit saturated fat, and avoid trans fat. For more information on how to choose healthy fats, read the article about Fats & Cholesterol.

6. What are the best fruits and vegetables to eat if I want to lose weight? Which fruits and vegetables should be avoided?

Chose a wide variety of vegetables and fruits every day, but don’t include white potatoes as a vegetable. Potatoes are a starch, and a rapidly-digested one at that; nutritionally, they have more in common with white bread and white rice than with other vegetables, and they should be eaten only occasionally. Go easy on fruits that are higher in carbohydrate, such as oranges, bananas, apricots, cherries, grapes, mangoes, pineapple and pears. Also, avoid fruit juices, since they contain a lot of sugary calories; choose whole fruit instead, since it has more fiber and will make you feel more full.

7. Which type of protein is best for me to eat—vegetable protein or animal protein?

Emphasizing vegetable over animal sources of protein will help reduce risk of heart disease. For example, this means eating nuts, beans, and soy products instead of red meat, which contains a lot of saturated fat and cholesterol. However, a healthy diet can include moderate amounts of poultry, fish, and eggs.

8. Will drinking milk or eating yogurt help me lose weight?

Long term studies do not show a benefit of milk or yogurt for weight loss, which should be no surprise as even low fat versions still contain quite a few calories.

9. Every month, I read about a new type of diet that promises to help me lose weight fast. How can I tell if a diet is safe and effective?

Rigid diets usually fail in the long run. I would be wary of any diet that eliminates entire food groups or drastically reduces one food component (such as diet that is very low in carbohydrates). Diets that recommend skipping meals or replacing meals with special supplements are also suspect.

10. Will supplements such as conjugated linoleic acid, omega 3 fish oil, or vitamin B6 help me lose weight?

None of these supplements have been shown to reduce weight. A multiple vitamin is a good nutritional safety net for most people, and for premenopausal women it is good to use a preparation that includes iron.

11. How can I find out how much I should weigh?

There is no single number that works for everyone. Your body-mass index, or BMI, a ratio of weight to height, is one way to tell if you are at a healthy weight (you can calculate your BMI online). Almost everyone should try to keep their BMI below 25. Also, keep in mind that most of the weight we gain in adulthood is fat. So unless you were clearly underweight at age 18 or 20, it is best not to gain more than 5 or 6 pounds after this age. For someone who is very overweight, it is usually not realistic to go all the way back to your weight at age 18; even a 5 to 10 percent weight loss has important health benefits.

12. How often should I weigh myself?

Once a week is a good interval. It is also a good idea to keep an eye on your waist circumference; even an inch or two increase in waist size is a sign that you may need to cut back or calories, exercise more, or both.