Nutritionists call diet shakes "meal replacements" because one shake is supposed to be the equivalent of one meal. Meal replacements also come in the form of nutritional bars and pre-packaged entrees. But do they really help you lose weight?
Steven Heymsfield, MD, of Columbia University, conducted a study which seems to prove that they do. Heymsfield and his colleagues looked at the findings of six studies of different types of meal replacements. They discovered that overall weight loss for the 249 individuals on meal replacement diets was greater than the weight loss experienced by the 238 individuals who followed low-calorie diets.
A study of United States Army volunteers showed that soldiers following a meal replacement program experienced greater weight loss over a six month period when the use of the meal replacements was combined with education-based weight management. Only 59% of the volunteers in the study, however, continued with the diet for the entire period.
Another study of 90 obese men and women also found that meal replacements resulted in more weight loss than those on a food-based diet. The participants were randomized to either a meal replacement program consisting of 3-5 meal replacements plus one meal daily or to a 1,000 kcal/daily food diet. After 16 weeks, the study showed that the group of individuals on meal replacements experienced a much greater rate of weight loss (93%) than those who were on the food-based diet (55%). Again, many people participating in the study dropped the diet. Only the people who continued were counted.
Weight loss seems as if it should be very easy. In order to lose a pound a week, simply eat 500 fewer calories every day. But in the real world, busy schedules and a wide array of food choices makes losing weight very hard.
Meal replacements work on the premise that many of us don't know how many calories we actually consume in a day. While packaged foods usually list calorie content, most of our meals do not. In many cases, we can eat a 700-800 calorie meal without realizing it. If we eat three 750-calorie meals in a day, we've consumed 2,250 calories. An average woman only needs 1,800-2,200 calories per day, and an average man only needs about 2,000-2,500 calories per day. Add in snacks, sweets, alcohol, and soda, and most of us consume many more calories than we actually need.
If you replace one or two meals per day with something with a known amount of calories, you will likely reduce the number of calories you consume. Instead of eating a 750-calorie meal, you'll drink a 250-calorie shake - reducing your calorie intake by 500. Do that every day for a week, and you should be able to lose one pound.
David Allison, PhD, obesity researcher at the University of Alabama at Birmingham, has studied meal replacements as a means of losing weight.
"I think they are a reasonable approach and can play a valuable role in weight loss," he said.
Allison evaluated 100 people who were randomly chosen to use either a soy-based meal replacement or a low-calorie diet for three months. He learned that those who were on meal replacements lost more weight and lost more inches around their waists than those who were following a low-calorie diet.
Another study, conducted by Dana Rothacker, PhD, assessed the long-term effectiveness of diet shakes on women who used them for one year. After three months, women who drank diet shakes had lost about the same amount of weight as women who followed low-calorie diets. But after a year, the women on the meal replacement plans were more likely to maintain their weight loss, while those who were on the low-calorie diets had regained much of their weight.
No Magic Bullet
Meal replacements aren't magic. People who stop using meal replacements regain their weight when they return to a higher-calorie diet. And some critics say meal replacements don't teach people how to make healthy food choices.
"People [on meal replacements] haven't learned how to deal with real food," Allison said. When they stop using the meal replacements, they often return to an unhealthy diet. In order to maintain a normal weight, one must either learn lifelong healthy eating habits or stay on the meal replacement plan indefinitely - and not many people want to do that.
The United States Food and Drug Administration (FDA) doesn't regulate meal replacements since they are only dietary supplements, so advertisements for them may make claims which aren't supported by scientific research. There are also no standards for the ingredients of meal replacements. While some diet shakes may be nutritionally sound and even include vitamins and minerals, others may contain very few healthy nutrients and are no more healthy than replacing your meal with a can of soda.
Allison recommends that anyone considering using meal replacements get nutritional advice from a health care provider.
Reliance on a manufactured product may deprive you of the variety that is normal in day-to-day meal planning. Meal replacement products could also provide you with a high intake of foods that you might otherwise rarely eat. For example, scientists are still studying whether heavy consumption of soy may influence the development of some cancers, so you may want to beware of soy-based meal replacements.
Diet shakes or meal replacements will help jumpstart weight loss in many people. Reaching a short-term weight-loss goal can be very satisfying and can provide the encouragement necessary to make permanent changes in the way you eat. Meal replacements can promote weight loss especially if they are used along with the goal of learning lifelong healthy eating choices.
If your goal is long-term weight loss, then you need to find a diet that will help you simply eat less food to shed pounds and then easily maintain your new body weight. Usually, this is a diet plan that allows you to eat your favorite meals and naturally fit to your lifestyle.
American Dietetic Association
National Institutes of Health
Canadian Council on Food and Nutrition
Canadian Society of Exercise Physiology