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1:7:2003 |
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| A Quick Look at Carbohydrate Quality Because of the harmful effects of carbohydrates of blood sugar control, some popular diets have led people to believe that the best way to eat is to avoid carbohydrates almost entirely. I don't agree with this approach. In fact, based upon results from short-term clinical trials as well as large population-based based studies, diets that have a higher intake of quality carbohydrates are consistently associated with lower risk for diabetes, obesity, cancer and heart disease. The best approach is to focus on quality carbohydrate sources using the glycemic index and the glycemic load index. The Glycemic Index The glycemic index (GI) refers to how quickly blood sugar levels will rise after eating a certain type of food. To determine a food's GI rating, measured portions of the food containing 50 grams of carbohydrate are fed to at least 10 healthy people after an overnight fast. For example, to test boiled spaghetti, the scientists give their subjects 200 grams of spaghetti, which according to standard food composition tables provide 50 grams of available carbohydrate. Finger-prick blood samples are taken at 15-30 minute intervals over the next two hours to construct a blood sugar response curve. The area under the curve (AUC) is calculated and reflects the total rise in blood sugar (glucose) levels after eating the test food. The scientists compare this response with the volunteer's response to a reference food, which may be either glucose or white bread. The GI rating of the test food is calculated by dividing the AUC for the test food by the AUC for the reference food (white bread or glucose) and multiplying by 100. The average of the GI ratings from all ten subjects is published as the GI of that food. Foods with a lower glycemic index will create a slower rise in blood sugar, and foods with a higher glycemic index will create a faster rise in blood sugar. Clinical Research with the Glycemic Index Evidence from clinical studies also shows that replacing high-glycemic-index carbohydrates with a low-glycemic-index carbohydrate sources will improve blood sugar control. Simply replacing products made with white flour and potatoes with whole grain, minimally refined products can have dramatic impact on improving blood sugar levels and is associated with a lower risk for both diabetes and cardiovascular disease. One of the key reasons may be the whole grain foods are rich in magnesium while this vital nutrient has been stripped away in refined flour. In one analysis, the protective effect of whole grain consumption was lost when the relative risk was adjusted for magnesium intake. To provide some general guidelines, here is a chart listing various foods and their glycemic index classification. Table 1 - Classification of Foods by Glycemic Index Scores
The Glycemic Load One of the shortcomings of the glycemic index is that it only tells us about the quality of the carbohydrates, not the quantity. Obviously, quantity matters too, but the measurement of the glycemic index of a food is not related to portion size. That is where the glycemic load (GL) comes into play. The GL is a relatively new way to assess the impact of carbohydrate consumption that takes the glycemic index into account, but provides much more accurate information than the glycemic index alone. A GI value tells you only how rapidly a particular carbohydrate source turns into blood sugar. It doesn't tell you how much of that carbohydrate is in a serving of a particular food. You need to know both things to understand a food's effect on blood sugar. For instance, watermelon has a GI of 72 compared to glucose, but the amount of carbohydrate in a 1/2 cup of is only 6 grams. The GL calculated by multiplying the amount of carbohydrate in a serving of food multiplied by that food's GI (as compared to glucose) as a decimal. Therefore, to calculate the GL for a 1/2 cup serving of watermelon we would multiply 12 times 72 to equal a GL of 4.3. Compare this to 1/2 cup of Grape Nuts(tm) that also has a GI of 71 and but provides 47 grams of carbohydrate yielding a whopping GL of 33 or 1 cup of white rice that also has GI of 72, but provides 36 grams of carbohydrate so its GL is 26. So, while the GI is important it is not as critical as the GL. A GL of 20 or more is regarded as high, a GL of 11 to 19 is medium, and a GL of 10 or less is low. The higher the GL the greater the stress on insulin. Table 2 - Examples of GI, GL, and insulin stress scores of selected foods
Clinical Research with the Glycemic Load Research studies are just starting to utilize the GL as a more
sensitive marker for the role of diet in chronic diseases like diabetes
and heart disease. The preliminary results are showing an even stronger
link in predicting chronic disease than the GI. For example, when
researchers from the Nurses Health Study used GL measures to assess
the impact of carbohydrate consumption on women they found that
high-GL diets (and, by extension, high GI foods and greater total
carbohydrate intake), correlated with even more significantly greater
risk for heart disease than the GI because of lower levels of protective
HDL-cholesterol and higher triglyceride levels. Key References:
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