Effects of good and bad carbohydrates

Sep 29 2007

The latest common wisdom on carbohydrates claims that eating so-called "bad" carbohydrates will make you fat, but University of Virginia professor Glenn Gaesser says, "that's just nonsense."

Eating sandwiches with white bread, or an occasional doughnut, isn't going to kill you, or necessarily even lead to obesity, he said.

In an article in the October issue of the Journal of the American Dietetic Association, Gaesser analyzes peer-reviewed, scientific research on carbohydrate consumption, glycemic index and body weight and gives the first detailed review of the literature on the correlation between them. His findings run counter to the current consensus on the effects of “good” and “bad” carbohydrates.

Gaesser, author of “It's the Calories, Not the Carbs” and other books, found that diets high in carbohydrates are almost universally associated with slimmer bodies. More importantly, Gaesser found that consuming lots of high-glycemic foods is not associated with higher body weights. In fact, several large studies in the United States revealed that high-glycemic diets were linked to better weight control.

“There is no reason to be eating fewer carbs – they're not the enemy,” says Gaesser, a professor of exercise physiology and director of the kinesiology program in the Curry School of Education.

The description of carbohydrates as “good” or “bad” is based on glycemic index, a measure of the quality of the carbohydrate in terms of how much it raises blood sugar. Foods having a high GI are generally thought to be “bad” because they raise blood sugar more than “good” carbs do. Proponents of the glycemic index claim that this leads to excessive insulin secretion, which can cause weight gain and health problems. Foods such as whole-grain breads are said to offer “good” carbs, because they have a lower GI than white bread, for example. Likewise, a glass of pineapple juice has a high GI compared to apple juice.

Several popular low-carb diets use glycemic index as a key feature for optimum weight control, but it is not a reliable description of carbohydrate quality, Gaesser says. Digestion is a complicated process. It's very difficult to determine the GI of a whole meal, for instance, so it doesn't really make sense to use GI or “glycemic load” — the glycemic index multiplied by the quantity ingested — as a guide to eating.

After looking at hundreds of articles on large-scale studies using surveys or randomized, controlled trials, Gaesser says they show that “people who consume high-carb diets tend to be slimmer, and often healthier, than people who consume low-carb diets.” Even high-glycemic foods have a place in the diet, he said, attributing that to the overall higher quality of a high-carb diet, which includes more fiber-rich and other nutritional foods.

Gaesser also looked for a clear association between carbohydrate consumption and illnesses, such as type 2 diabetes, heart disease and cancer. He found no compelling evidence that avoiding carbohydrates with a high GI helps prevent these diseases and others. People with diabetes, as well as very sedentary women who are obese, may benefit from lowering their consumption of foods with a high GI, Gaesser says.

Reducing any part of the diet — carbs or proteins or fats — will result in modest weight loss in the short term, if calorie consumption is reduced, he points out. But for long-term weight maintenance, a high-carb, low-fat diet is still the best bet, he said.

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