Junkfood Science: GI — more ‘bad carb’ myths

October 27, 2007

GI — more ‘bad carb’ myths

One of the more popularized beliefs is that you can give yourself type 2 diabetes by eating sugars or ‘bad carbs’ because they cause blood sugars and insulin levels to surge. No matter how many times researchers have shown this not to be the case, myths surrounding dietary sugars and carbohydrates, especially those that come in the color white, continue, with each generation 'refining' their explanations.

A study just published in the American Journal of Clinical Nutrition adds to the body of evidence on whether foods that cause blood sugars and insulin secretion to spike can increase risks for developing diabetes.

To attempt to accurately predict foods’ blood sugar effects, the researchers used glycemic index and glycemic load. The glycemic index, first suggested as a way to manage blood sugars among diabetic patients in 1981, is a measure of foods’ immediate effects on blood sugars. The thinking is that foods that break down quickly (thought to be refined sugars and simple carbs) will have a high GI, while those that digest more slowly (thought to be complex carbohydrates, high fiber and high-fat foods) will have a lower GI. The index was created by measuring the change in blood sugar levels after a person eats a set amount of a single carbohydrate food (usually 50 grams) compared to a control food with the same amount of digestible carbohydrate, such as white bread or glucose. Glycemic load considers the GI and the proportion of carbohydrate content of foods eaten.

The study subjects were 7,321 white London civil service workers, average age of 50, first recruited to the Whitehall II study in 1985-8. After clinical examinations, they were followed at 2.5 years intervals and detailed dietary information was obtained from food frequency questionnaires. The researchers calculated the calorie-adjusted average GI and GL, using the 2002 international table of GI food values. These subject had no history or diagnosis of diabetes by doctors and were followed for 13 years, well into the years at highest risk for developing diabetes.

Incidences of diabetes were determined by actual blood tests and glucose tolerance tests. Blood was collected to determine fasting blood sugars and the test subjects were then given 75 grams of anhydrous glucose to drink and their blood retested 2 hours later to measure glucose and insulin levels.

The researchers reported their findings:

After 65,774 person-years of follow-up, 329 incident cases of diabetes were identified ... Dietary GI was not associated with the risk of incident diabetes. Further adjustment for employment grade; physical activity; smoking status; intake of alcohol, fiber, and carbohydrates, WHR and BMI did not alter the findings. Hazard ratios across tertiles of GL showed an inverse association with diabetes risk in the base model (P for trend 0.011). The weak protective effect of high GL remained after adjustment for employment grade, physical activity, smoking status, and alcohol intake, but it was not significant after further adjustment for carbohydrate and fiber intakes and in a model additionally adjusted for BMI and WHR.

In the actual data, fasting blood glucoses and insulin levels were slightly lower among those in the highest tertiles of GI and GL, and incidences of diabetes were slightly lower, too. Before and after adjusting for all of the possible confounding factors (sex, age, employment grade, physical activity, smoking, alcohol, fiber, BMI, etc.), the risk for diabetes was 6% lower among the highest GI tertile and 30% to 20% lower in the highest GL tertile, respectively. All too small to be significant, but clearly showed that low GI and GL levels did not reduce risks for developing diabetes.

The researchers discussed the conflicting evidence from both observational and clinical trial studies in the published literature and an important point deserves our attention: the need to differentiate the management of the symptom of high blood sugars in people with diabetes versus prevention. Also, all of the controlled trials on humans to date suggesting that low GI diets may help insulin sensitivity have been done on those who already have impaired glucose tolerance, not the prevention of diabetes in the general population.

The misnamed U.S. Diabetes Prevention Program trial, for example, screened 133,683 people and included only the 2.2% found to have impaired glucose tolerance.

There is also a widespread misconception that simple carbs are bad, and that complex carbs are good because they don’t spike blood sugars or insulin levels as much. Not only is this of little concern to nondiabetics, in reality, many complex carbohydrates have higher GI levels than pure sugar. Baked potatoes and corn flakes have higher glycemic indices than jellybeans and soft drinks. “Although hundreds of scientific articles have studied [GI] and many popular books have espoused it, the practical significance is still debatable,” said Dr. David Klurfeld, professor and chairman of the Department of Nutrition and Food Science at Wayne State University and editor-in-chief of the Journal of the American College of Nutrition, in a series of articles on GI in Nutrition News Focus. “The glycemic index can be influenced by the amount of fiber and carbohydrate but also by fat....The type of starch, particle size, maturation of a fruit or vegetable, cooking time, and many other factors affect the glycemic index.” It is also based on single foods and people eat foods together. “No expert panel has endorsed using the GI to choose foods, and the American Diabetes Association believes it is of little use [even] to diabetics in making food choices,” he wrote.

Similarly, many believe that only carbohydrates in the diet stimulate insulin production and that high-carb diets are responsible for obesity and illness, but this is a “a very undeserved reputation based on false and twisted truths,” explains Kathy Goodwin, R.D. “The truth is that all ingested foods stimulate insulin production.” And even population studies completely contradict such fears, she said. In Japan, for instance, high carb foods like white rice [with a GI higher than pure sucrose] is a daily staple, yet Japan “has one of the lowest rates of obesity, heart disease, cancer and diabetes in the world.” Again and again, the science supports there being nothing magical in the foods we eat or that there is one perfect diet.

As Dr. Elliot J. Rayfield, M.D., professor of medicine at Mount Sinai School of Medicine, explains:

Because diabetes is often referred to as a sugar disease, many people mistakenly think that it is caused by eating too much refined sugar. While it is true that sugar and other simple carbohydrates can produce a rapid rise in blood glucose in the absence of adequate insulin, they do not cause diabetes.

© 2007 Sandy Szwarc. All rights reserved

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