Saturday, March 21, 2009

Diet: GI Diet

The Glycemic Index Diet (Low Glycemic Diet)

By Kathleen M. Zelman, MPH, RD, LD
WebMD Expert Review

What It Is

The "glycemic index diet," "GI diet," and "low glycemic diet" are more than diet books. They refer to a system of ranking carbohydrate foods according to how much a certain amount of each food raises a person's blood sugar levels.

Originally developed as a tool to help diabetics manage blood sugar control, the glycemic index has found its way into the mainstream weight loss market. The glycemic index is the basis for many popular diet plans, such as SouthBeach, The Zone, Sugar Busters, Glucose Revolution, and Ending the Food Fight.

Specifically, the glycemic index (GI) measures how much a 50-gram portion of carbohydrate raises a person's blood-sugar levels compared with a control (that is, white bread or pure glucose). Virtually all carbs are digested into glucose and cause a temporary rise in blood glucose levels, called the glycemic response. This response is affected by many factors, including the quantity of food; the amount and type of carbohydrate; the cooking method; degree of processing, and more. Each food is assigned an index number from 1-100, with 100 as the reference score for pure glucose. Typically, foods are rated high (greater than 70), low (less than 55) or moderate (56-69).

The glycemic index diet's popularity has been fueled by claims that low-GI foods can help control appetite and weight and may be useful for people with diabetes or pre-diabetes. The idea is that low-GI foods are absorbed more slowly, allowing dieters to feel full longer and making them less likely to overeat.

Dieters are encouraged to choose carbohydrate foods with a low glycemic index, which tend to be (but are not necessarily) healthier, nutrient-rich, less refined, and higher in fiber -- like whole fruits, vegetables, and beans.

In contrast, higher GI foods "trigger a rise in blood sugar, followed by a cascade of hormonal changes, which tend to make you hungry again sooner because they are metabolized quicker than low-GI foods," explains David Ludwig, MD, PhD, author of Ending the Food Fight.

The blood sugar spikes of high-GI foods are especially problematic for people with diabetes because their bodies have trouble regulating blood sugar. The theory that helped launch all the glycemic index diets is this: If it works to help control blood sugar in diabetic people, then it should work for weight control.

Because the GI diet is an approach to weight loss rather than a specific plan, guidance on fats, protein, alcohol, weight maintenance and exercise vary depending on the plan. Ludwig and most GI diet plans also advocate getting regular exercise and eating moderate amounts of lean protein and healthy fats -- similar to the recommendations of the U.S. government's 2005 Dietary Guidelines.

What You Can Eat

Quality, not quantity, of carbs is the mantra of a glycemic index diet. The idea is to feel fuller by enjoying plenty of low-GI "smart" carbs -- whole grains, whole fruits, vegetables and legumes -- along with lean protein and healthy fats. You'll avoid the high GI foods, which tend to be made with white flour and heavily processed.

But confusion can arise when you check the GI score for a food like carrots, which can range from 16 to 92, or for sugar or candy, which can get a lower score than a potato!

The truth is that a low GI score is no guarantee of healthy fare. Low-GI foods include candy bars (a Snickers scores 55) and potato chips (54). Common sense should warn dieters that these treats are not healthy components of a weight-loss diet. Likewise, there are nutritious, high-GI foods, like corn, baked potatoes and fruit juices, that can certainly be part of a healthy weight loss plan.

Ludwig's plan in particular calls for no calorie counting, just a basic understanding of the principles of the glycemic index or glycemic load (a similar calculation in which the glycemic index is multiplied by three times the amount of carbohydrate in a serving).

"There is a psychological benefit when a dieter is not restricted and allowed to eat to satisfy their appetite with healthy foods," says Ludwig.

How It Works

Glycemic index diets have become a popular weight loss tool based in part on the theory that high-GI foods raise blood sugar levels, cause the body to secrete excess insulin, and lead to the storage of fat. Nevertheless, a huge debate exists in the nutrition community about the value of the glycemic index for weight loss.

One of the reasons the glycemic index is controversial is because of the variability of the GI scores, which can be altered by many factors, ranging from ripeness to cooking method.

"The riper a banana, the higher the score," explains Rachel Johnson, PhD, MPH, RD, professor and dean at the University of Vermont. "Al dente pasta is higher than more cooked pasta. Add fat to foods and you can lower the GI, or if a product is made with fructose instead of sucrose or table sugar, it is absorbed more slowly and therefore gets a lower GI score."

The GI response to a given food also varies widely from person to person. It can even vary within the same person from day to day, according to research reported in the June 2007 issue of Diabetes Care.

"We tested the response when an individual ate the same food on three different occasions, and even within the same person, the range in glucose and insulin was huge," says Tufts University's Alice Lichtenstein, DSc, one of the authors of the study.

Nationally known diabetes expert and author Marion Franz, MS, RD, CDE, says the GI numbers don't mean much because the range is tremendous even within the same foods.

"It is hard to know what the glycemic index of a food really is when something like rice can range from a low of 55 to over 100," she says. Further, "there is no difference in the ranking of white and brown rice, or white and whole wheat bread, and clearly whole grain choices are more healthful."

Indeed, the 2005 Report of the Dietary Guidelines Advisory Committee states, "Current evidence suggests that the glycemic index and/or glycemic load are of little utility for providing dietary guidance for Americans."

Franz also takes issue with the actual glycemic index measurements: "The GI does not measure how rapidly glucose levels increase, and studies that compare high- and low- glycemic diets show that the glucose peaks are about the same time" for each, she says.

She explains that blood glucose peaks about the same time with most foods because we rarely eat foods alone, and the addition of fat and/or protein to meals slows down the response time. A high GI-food combined with a low-GI food produces a moderate response.

Still, other experts note there is evidence that eating meals low on the glycemic index can help reduce hunger. In research published in the May 16, 2007, issue of TheJournal of the American Medical Association, Ludwig and colleagues found that foods with lower GI scores seemed to reduce hunger in obese teenage boys.

"The boys were hungrier after they had eaten a high-GI breakfast, which resulted in eating 600 to 700 more calories at lunchtime than when they ate a low GI breakfast," says Ludwig. And boys on a low-glycemic diet lost more weight than other groups in the study.

In the same study, Ludwig and colleagues also found that low-GI diets seemed to be more effective in people whose bodies secrete more insulin. High insulin secreters tend to be "apple-shaped" people who accumulate fat around their waists, rather than those with more lower-body fat or "pear" shapes. He suggests if you are an "apple" who has not done well on other diets, you may succeed with the GI approach.

Regardless of your shape, Ludwig says, a low glycemic diet can be good for your heart: "In our experience, anyone who followed a low GI diet improved their triglyceride and HDL cholesterol levels, both important cardiovascular risk factors," he says.

What the Experts Say

Ludwig, founder of the Optimal Weight for Life program (OWL) at Children's Hospital in Boston, says many children and families have successfully used his 9-week low glycemic plan for weight loss. And many other dieters have reported success using other low glycemic plans.

But Lichtenstein notes that any appetite-reducing effects from the GI diet may stem from the fact that many foods that score low in the glycemic index are also high in fiber, which is filling.

"Diet books have embraced the concept of the glycemic index because it is simple, but there are so many factors that confound the use of the GI as a weight loss tool that it is premature to assume it is the answer," says Lichtenstein.

Franz believes some diet books have distorted the definition of glycemic index. "Even if a high-GI diet gets a slight increase in the glucose peak, it does not stimulate insulin or appetite," she says. While Franz says the glycemic index is useful for diabetics, she thinks it has no real value as a weight loss tool.

Losing weight for the long term is a more complex proposition than choosing low-GI carbs, says American Dietetic Association spokesperson Suzanne Farrell, MS, RD. It's more about what else you're eating and how physically active you are, she says. She says she does not like the GI diet "because it promotes a good food/bad food message. ... It has not been shown to be an effective weight loss tool."

To some experts, the glycemic index is just another gimmick -- but, they say, if this gimmick helps people eat a healthier diet, it can be successful.

Lichtenstein points out that keeping track of GI scores may help some people control calories -- the essence of any successful weight loss plan.

"As long as the food choices are healthy, and it is a diet plan that is sustainable over time and addresses other lifestyle issues such as physical activity, there is no reason not to use this approach, if it works for you," she says.

Johnson thinks a glycemic index diet has merit "as long as you choose healthy carbs that are high in fiber, whole, natural, and less processed to help with satiety and reducing calories." And, she adds, "make sure you get plenty of lean and low-fat protein to pair with the low glycemic index foods. Choose heart-healthy fats, and control portions, because it is easy to overeat even good foods."

She advises consulting a registered dietitian for an individualized calorie prescription and using the glycemic approach to select the healthiest carbs.

Food for Thought

There is no such thing as a simple answer for weight control, and no one approach works for everyone. Any diet you can stick with for the long term is the right one for you.

If you don't get too hung up on the numbers and use common sense to select healthy, wholesome carbs, lean protein, and healthy fats, a glycemic index diet may be right for you -- especially if you are pre-diabetic, carry extra weight around your middle, and have not succeeded on traditional diets. Keep in mind that you must also control portion sizes and total calories, and you must get regular physical activity.

While there is plenty of research on the glycemic index, the results have been inconsistent. Further research is needed to reach consensus on whether the glycemic index works as a long-term weight loss plan.