Diet shakes are called meal replacements by nutritionists and weight-loss experts because, in theory, drinking one shake is intended to replace one whole meal. Other types of meal replacements are nutritional bars and pre-packaged entrees. But do meal replacements work?
According to a scientific study led by Steven Heymsfield, MD from Columbia University, they do. Heymsfield and colleagues pooled data from six separate studies of different types of meal replacements and found that the overall weight loss for the 249 people on meal replacements was greater than for the 238 people in the comparison groups who followed low-calorie diets.
One study with United States Army volunteers found that soldiers started on a meal replacement program, when added to education based weight management, had greater weight loss over a 6 month period. However, only 59% of the volunteers in this study continued with the diet for the study period.
Another study also found that meal replacements resulted in more pounds shed than a food-based diet. The study involved 90 obese men and women who were randomized to either a meal replacement program, which included 3-5 meal replacements plus one meal daily, or a 1,000 kcal/daily food diet. Researchers found that rate of weight loss was greater in the group who were on meal replacements (93%) compared to the group on the food-based diet (55%) after 16 weeks. However, many people in the study dropped the diet and only the people that continued the diet were counted.
Weight loss on paper is very easy. You really only need to eat 500 fewer calories every day to lose a pound a week. The trick is that weight loss in the real world, with busy schedules and abundant food choices, is hard. Meal replacements work on the premise that few of us know how many calories we eat each day. Packaged foods may list calorie content but most meals we eat don’t. Many of our meals may reach 700-800 calories without us realizing it. Three meals a day at 750 calories each equals 2250 calories. An average woman needs only about 1,800- 2,200 calories per day. The average man requires about 2,000-2,500 calories per day. If you add in snacks and the occasional sweets, not to mention alcohol or other calorie beverages like soda, most of us consume far more calories than we need.
Replacing one or two meals per day with a known quantity of calories will necessarily reduce the number of calories you consume. In other words, instead of eating a meal that might have 750 calories, you drink a shake with 250 calories then you will have reduced the usual number of calories you take in by 500. Doing that every day for a week and you will lose one pound as long as nothing else changes.
David Allison, PhD, an obesity researcher at the University of Alabama at Birmingham has studied the use of meal replacements to lose weight.
“I think they are a reasonable approach and can play a valuable role in weight loss,” he says. Allison evaluated the use of a soy-based meal replacement in 100 people who were randomized to use the meal replacement or follow a low-calorie diet for three months. He found that those on meal replacements lost more weight and had a greater reduction in waist circumference than those on a low-calorie diet.
Another study by Dana Rothacker, PhD followed women using diet shakes for weight loss for one year to assess their long-term effectiveness. At three months the women who drank diet shakes lost about the same amount of weight as women on low-calorie diets. But after one year the women who continued to follow the meal replacement plan maintained their weight loss, whereas women continuing on the low-calorie diet had regained much of their weight.
No Magic Bullet
Meal replacements are no magic bullet. As with most diets, the reason people who stop using meal replacements regain their weight is because they return to a higher caloric intake. Plus, critics argue, using meal replacements don’t teach people how to make healthy choices about the rest of the food they eat.
According to Allison, “People [on meal replacements] haven’t learned how to deal with real food.” So when they stop using the meal replacements, they often return to an unhealthy diet. In general, maintaining a normal weight requires learning lifelong healthy eating habits or staying on the meal replacement indefinitely. Few people want to do that.
Because meal replacements are dietary supplements, they are not regulated by the United States Food and Drug Administration (FDA). Therefore, advertisements for meal replacements may make claims about their effectiveness that are not supported by any scientific research. Also, there are no standards for the ingredients of meal replacements. Some diet shakes may be nutritionally sound with necessary vitamins and minerals included. Others may contain few healthful nutrients and are about as healthy as replacing your meal with a can of coke.
Allison recommends that anyone considering meal replacements should talk with a knowledgeable healthcare provider and get some nutritional advice. Then you will be savvy about your choice of meal replacement and what it can do for you. One of the challenges of meal replacement is that your reliance on a manufactured product may deprive you of the variety that we commonly experience in our day-to-day meal planning. Meal replacement may give you a high intake of foods that you might otherwise eat rarely or in moderation. You may, for example, want to be particularly wary about soy-based meal replacements because scientists still don’t know whether and how heavy consumption of soy may influence the development of some cancers. There is data suggesting both increased and decreased risk, so the jury is definitely still “out” on this topic.
For many people, using diet shakes or meal replacements will jumpstart their weight loss. Reaching a short-term weight-loss goal can be very satisfying and may encourage the perseverance needed to make permanent changes in the way you eat. Meal replacements can have a successful role in promoting weight loss especially when incorporated into the goal of learning lifelong healthy eating choices.
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