The Diet Studies - Popular Diets: Obesity
Content provided by the Faculty of the Harvard Medical School
The diet studies
What do scientists say about how the different diet plans compare? The results of studies often suggest that it's not the diet plan but whether you stick with it that makes the difference. For example, in a 2005 study published in the Journal of the American Medical Association, researchers randomly assigned a group of 160 overweight and obese adults to one of four popular diet plans: the Atkins diet, the Ornish diet, Weight Watchers, and the Zone diet. After one year, nearly half of the participants had dropped out of the study. But those who completed the study lost similar amounts of weight (about 5 to 7 pounds each, on average). They also lowered their blood levels of cholesterol and several other markers linked to heart disease and diabetes to a similar degree. People assigned to the Atkins and Ornish diets were more likely to drop out of the study, suggesting that many people found these plans too extreme. But for certain people, the structure of a restricted plan may be helpful.
A similar study, published in the British Medical Journal in 2006, compared four programs: the Slim-Fast plan (a meal-replacement approach), Weight Watchers, the Atkins diet, and Rosemary Conley's "Eat Yourself Slim" low-fat diet and fitness plan (which includes a weekly group exercise class). All the diets helped people shed pounds and fat, but many — more than half of the participants — dropped out of the study after eight weeks. About 45% of people in the Rosemary Conley and Atkins groups who completed the six-month study lost 10% of their body weight. For the Weight Watchers and Slim-Fast groups, success rates were slightly lower: only 36% and 21%, respectively, lost 10% of their weight.
Another line of evidence about effective weight-loss strategies comes from a few carefully controlled trials in which volunteers with obesity were blindly assigned to either a standard low-fat diet or a low-carb, high-protein diet. Over all, these trials showed that a low-carb, high-protein diet leads to quicker weight loss than a low-fat diet. In the studies that lasted for a year or longer, though, weight loss was about the same regardless of diet type. These studies focused primarily on weight, and were too short to track other important consequences of diet, such as heart disease, diabetes, bone strength, and cancer.
The overall results mask some startling individual differences. In one trial, on both low-carb and low-fat diets, some people lost weight while others gained. In the low-fat group, the range was from 53 pounds lost to 31 pounds gained. In the low-carb group, it was from 65 pounds lost to 18 gained.
The take-home lesson is that it is okay to experiment on yourself. If you give a diet your best shot and it doesn't work, maybe it wasn't the right one for you, your metabolism, or your situation. Don't get too discouraged or beat yourself up because a diet that "worked for everybody" didn't pay off for you. Try another.
The glycemic index and obesityThe glycemic index is a measure of how quickly the carbohydrate derived from a particular food is absorbed into your bloodstream. Of course, the amount of carbohydrate you eat also affects your blood sugar and insulin levels. Doctors originally developed the index to help people with diabetes avoid foods that increase blood sugar too quickly, but some experts think it can also help people avoid obesity-related health problems. The glycemic index ranks foods on a 100-point scale, with 100 being the fastest rate of absorption. Foods with sugars that are absorbed quickly have high glycemic indexes; those with sugars that are absorbed slowly have low glycemic indexes. When sugar is absorbed quickly, it increases the release of insulin, the hormone that transports sugar to the cells for fuel. Spikes in insulin levels, if they occur regularly, may promote insulin resistance, a condition in which the body doesn't respond normally to the hormone. When insulin doesn't function properly, levels of blood sugar and fats rise, increasing the risk for diabetes, coronary heart disease, and — possibly — stroke, kidney failure, and cancer. In general, high-carbohydrate foods have the highest glycemic indexes; proteins and fats have glycemic indexes that are close to zero. Proponents of low-carbohydrate diets cite the glycemic index in justifying systematic reductions in carbohydrates. However, most scientists disagree. In cutting back on all foods with relatively high glycemic indexes, you would end up avoiding many healthy foods, including carrots (glycemic index of 71), sweet potatoes (glycemic index of 54), and brown rice (glycemic index of 55). Some experts argue that people should avoid certain foods that are high on the glycemic index — or should eat them only in moderation — because they may cause insulin resistance and obesity, although this view remains controversial. The principal suspects are foods heavy in simple sugars, such as candy bars and cookies, and processed carbohydrate foods, such as potato chips, breakfast cereals, and many of the low-fat foods that are on the market. In addition to wreaking havoc with your insulin levels, such foods tend to be high in calories, and excess calories translate to excess pounds. |
Glycemic indexes of popular foods | |
| Food | Glycemic index |
| Yogurt, low-fat with artificial sweetener | 14 |
| Milk, whole | 27 |
| Kidney beans | 27 |
| All-bran cereal | 30 |
| Milk, skim | 32 |
| Yogurt, low-fat with fruit and sugar | 33 |
| Apple | 36 |
| Spaghetti, whole wheat | 37 |
| Chocolate | 49 |
| Rice, white | 50 |
| Banana | 53 |
| Rice, brown | 55 |
| Spaghetti, white | 57 |
| Bread, whole wheat | 58 |
| Cheese pizza | 60 |
| Ice cream | 61 |
| Bread, white | 72 |
| Corn flakes | 77 |
| Last updated: | June 20, 2007 |
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Medical content reviewed by the Faculty of the Harvard Medical School. Harvard Health Publications, Copyright © 2007 by President and Fellows of Harvard College. All rights reserved. Used with permission of StayWell.
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