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Which is more important: a food's glycemic index or its fiber?
I am watching my glucose numbers. Should I be more concerned about a food's glycemic index or the amount of fiber in a food? It seems that a food with a good amount of fiber also has a higher glycemic index.
When the Glycemic Index Doesn't Measure Up
The Glycemic Index is of interest to those seeking to help prevent or treat diabetes because it measures the effect that a specific food has on a person's blood sugar after the person eats it. Unfortunately, the results of studies assessing the link between GI and diabetes risk have been mixed.
Glycemic Index and Weight Loss
Glycemic Index (GI) is a concept that has been around for decades and has moved in and out of favor for use with weight loss and for diabetics. It's a great tool in many ways, but it does add another number to learn and there has long been debate as to whether diets that specifically use foods with a low GI are effective.
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There's been a lot of talk about low-glycemic-index diets being better for helping diabetics control their blood sugars, but the studies that have been done tend to be small and of short duration. Back in 2008 researchers in Canada decided to improve on past studies by designing a larger, more long term study to compare the effects of a low glycemic index diet with a high cereal fiber diet (JAMA 2008; 300(23):2742-2753). Their goal was to see if the Hemoglobin A1C (a measure of diabetic control) improved on either diet, and as a secondary goal, they also looked at whether that diet helped improve the participants' cholesterol scores as well.
199 men and women enrolled in the study. All had type 2 diabetes and were taking oral medications to control their diabetes. Their Hemoglobin A1C values at the start of the study were between 6.5% and 8.0%, and none had significant cardiovascular, kidney or liver issues, nor were they being treated for cancer. Just over 85% of the participants were overweight.
The participants were randomly assigned to one of two dietary interventions:
Low GI Diet: emphasized low GI breads such as pumpernickel, quinoa and flaxseed; large flake oatmeal or bulgur/flax hot cereals; white pastas and rice; and legumes such as beans, peas and lentils. Fruits such as apples, pears, oranges, cherries and berries.
High Cereal Fiber Diet: emphasized brown (whole grain) breads, crackers, breakfast cereals, and rice; potatoes with skin; and legumes. Fruits such as bananas, mangos, grapes, watermelon and canteloupe.
In both diets the participants were asked to avoid pastries such as pancakes, muffins, doughnuts and white rolls or bagels; french fries and chips, and were to avoid those fruits not specifically permitted by their dietary intervention.
Interestingly, the overall goal of these two treatments was to maintain a similar overall amount of fiber in the diet while reducing the glycemic index in the Low GI Diet by 10 to 20%.
After six months the researchers compared the participants' blood pressures, cholesterol scores, and Hemoglobin A1C values to their results at the start of the study. Both groups reduced their Hemoglobin A1C: 0.5% for the Low GI Diet versus 0.18% for the High Cereal Fiber Diet. The Low GI Diet participants also improved their cholesterol scores more than those on the High Cereal Fiber Diet.
That said, although their goal was to maintain a similar overall amount of fiber in both diets, in actuality the Low GI Diet averaged 4.6 grams of fiber per day more than the High Cereal Fiber Diet - meaning that the Low GI Diet was higher in soluble fibers (the types of fiber found in apples, pears and many vegetables) while the High Cereal Fiber was higher in insoluble fibers (those in whole grains).
While this study appears to show that a Low Glycemic Index Diet is slightly better at helping type 2 diabetics control their blood sugars, the truth is that a diet high in both soluble and insoluble fiber is going to be better both for your blood sugars and your cholesterol. Eat more vegetables and fruits and choose whole grains to get both soluble and insoluble fibers in your diet - the best of both worlds.
First posted: January 5, 2011