Fifty-Plus Fitness Library

Library : Year 2 | Session 5: Wellness Through Understanding--Overweight, Diabetes, Hypertension

Glycemic Load, Diet and Health

from Harvard Women's Health Watch
June 2001

Until recently, there was little question that the best approach to healthy eating was to follow official guidelines - such as the USDA Food Guide Pyramid - which emphasize carbohydrates as a way to discourage fat consumption. But as we wrote a few months ago (Harvard Women's Health Watch, December 2000, pp.1-2), this view of carbohydrates and fats is too simplistic. It might even contribute to the very conditions that we hope to avoid by eating a good diet in the first place, such as obesity, diabetes, and cardiovascular disease.

For one, not all fats are bad. In fact, certain fats are heart-healthy. In addition, not all carbohydrates act the same. Some are quickly broken down in the intestine, causing the blood sugar level to rise rapidly. Such carbohydrates have a high glycemic index (GI).

Because rapidly rising blood sugar levels have various adverse effects, we advised eating plenty of fruits and non-starchy vegetables and few high-GI carbohydrates, such as refined grains and starches. We also endorsed a food pyramid where fruits and nonstarchy vegetables, not refined grains, occupy the bottom tier. The purpose of this advice is to reduce overall glycemic load (GL). GL is a relatively new way to assess the impact of carbohydrate consumption that takes GI into account but gives a fuller picture than does GI alone. Several HWHW readers wanted to know more about it.

GL REFLECTS ACTUAL CARBOHYDRATE BURDEN

A GI value tells you only how rapidly a particular carbohydrate turns into sugar. It doesn't tell you how much of that carbohydrate is in a serving of a particular food. You would need to know both things to understand a food's effect on blood sugar. That is where GL comes in. The carbohydrate in a carrot, for example, has a high GI. But there isn't a lot of it, so a carrot's glycemic load is relatively low. Calculating GL allows researchers to better relate carbohydrate intake to its health effects.

Researchers at Harvard, including Harvard Women's Health Watch advisory board member JoAnn E. Manson, M.D., have closely examined the relationships among carbohydrates and heart disease and diabetes in women. The Nurses' Health Study, for example, found that women with the highest dietary GL have double the risk for heart disease when compared to those with the lowest GL.

WHY IT'S NOT AS SIMPLE AS GI The glycemic index originated as a research tool. It assigns a numerical value to a food indicating how much and how rapidly 50 grams of its carbohydrate content will raise blood-sugar levels, compared to 50 grams of a reference food (glucose or white bread). The reference food is given an arbitrary value of 100, and the GI value of a particular food is expressed as a percentage of that value. Many things contribute to the GI of a given food, including its fat and fiber content and how much it's been processed.

But carbohydrates differ in quantity, as well as in GI ranking, from one kind of food to another. The shortcoming of GI values is evident when you compare foods of different carbohydrate densities. For example, the GI of a baked potato is121% (assuming white bread is the standard reference food). This has earned the potato, which is largely carbohydrate, a place on the "avoid" list in publications and on Web sites promoting the GI approach to food choices.

The GI of carrots, as noted earlier, is also high: 131%. But this unfavorable GI rating is based on the blood-sugar effect of eating 50 grams of carbohydrate from carrots - the amount contained in a pound and a half of them - which few people would consume in one sitting. A serving of carrots, therefore, just doesn't have much carbohydrate, so its impact on blood sugar is much less than that of a serving of potato.

Avoiding carrots because of their GI ranking would be a big mistake, particularly given all the vitamins and minerals they contain and the low GL of each serving (see "How to Calculate GL," below). The GI of potatoes, on the other hand, is not a misleading measure because potatoes are carbohydrate-dense. Their GL is also fairly high.

HOW TO CALCULATE GL GL is the amount of carbohydrate in a serving of food multiplied by that food's GI. Thus, a ½ cup serving of carrots(which has 8 grams of carbohydrate) has a glycemic load of about 10 (8 x 131%,or 1.31 = 10.48).

As reported recently in the American Journal of Clinical Nutrition (March2001), Nurses' Health Study researchers - aided by blood samples and a food-frequency questionnaire - used GL measures to assess the impact of carbohydrate consumption on 280 postmenopausal women. They found that high-GL diets (and, by extension, high GI foods and greater total carbohydrate intake), correlated with lower HDL concentrations and higher triglyceride levels, a marker for heart disease. The strongest association was in overweight women, i.e., those whose body mass index (BMI) was over 25. Increased risk started, on average, at a daily GL of 161.

Calculating overall dietary GL is difficult outside a research setting. But knowing a food's GL can help you make comparisons that can improve the quality of your carbohydrate choices. In general, it's a good idea to replace processed and refined-grain carbohydrates, such as those found in many snacks and desserts, with fruits and non-starchy vegetables. These foods, as well as whole grains and beans, are rich in nutrients and contain fiber, which slows digestion and moderates blood sugar levels. Also, try substituting, for example, whole grain bread for white bread; wild rice for white rice; and beans or lentils for potatoes.

Dietary GL may not become an everyday calculation, but we wouldn't be surprised to see labels that exclaim "Low Glycemic Load!" - like "Low Cholesterol!" - beckoning from grocery shelves in the near future.

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