Understanding Glycemic Index:
Not Everyone Needs the Same Information
The glycemic index (GI) measures how fast a food is likely to raise your blood sugar. Carbohydrates that break down quickly during digestion, releasing glucose rapidly into the bloodstream, have a high GI; carbohydrates that break down more slowly, releasing glucose more gradually into the bloodstream, have a low GI.
The common wisdom is that for most people, foods with a low GI have significant health benefits. The belief is that these foods require a lower insulin demand and can contribute to long-term blood glucose regulation. From here, all sorts of claims about satiety, nutrient absorption, fat storage, and personal energy level get made. But what is the bottom line? How important is GI in determining which foods make up a healthy diet?
The answer seems to be, for people with hypoglycemia, blood sugar spikes play a role of increased importance in regulating—and compromising—overall health. For non-diabetic people with normal blood sugar levels, including many fad dieters, worrying excessively about GI will not really improve health or lower weight much.
Low GI for Overall Health?
There is evidence that reducing the proportion of energy derived from carbohydrate or reducing the glycemic index (GI) of the carbohydrate improves the rate of fat loss and cardiovascular risk factors. The proposed mechanisms include higher satiety, higher metabolic rate, reduced postprandial (post-meal) blood glucose and/or blood insulin and higher fat oxidation. The International Journal of Obesity reports, "Exchanging high for low GI foods, without changing the macronutrient ratio, may be optimal because it is simple, cost-effective and often associated with higher intake of whole grains and greater food volume, and factors that maximize the chance of sustained weight loss."
Yet there is the age-old problem of any food measurement tool that lists carrots as less healthy than cheese pizza should probably be discarded. One of the limitations of the GI scale is that a GI value tells you only how rapidly a particular carbohydrate turns into glucose. It doesn't tell you how much of that carbohydrate is in a serving of a particular food (glycemic load, or GL).
The glycemic index, then, should only be applied to foods with a reasonable carbohydrate content, as the test relies on subjects consuming enough of the test food to yield about 50 g of available carbohydrate. Many fruits and vegetables contain very little carbohydrate per serving, and the average person is not likely to eat 50 g of carbohydrate from these foods. This glycemic load problem applies to carrots.
Weight Loss Studies
How do GI and GL measure up with regard to weight gain and waste circumference? In a population-based prospective cohort study looking at five European countries, a total of 89,432 participants, aged 20 to 78 years were followed for 2 to 12.5 years. All subjects were free of self-reported cancer, cardiovascular diseases, and diabetes at baseline.
GI and GL were calculated, adjustment was made for demographic and lifestyle factors, follow-up duration, and other dietary factors. Associations of GI and GL with subsequent changes of weight and waist circumference broke down as follows: with every 10-unit higher in GI, weight increased by 34 g per year. Waist circumference increased by 0.19 cm per year. With every 50-unit higher in GL, weight increased by 10 g per year and waist circumference increased by 0.06 cm per year.
These findings do not support an effect of GI or GL on weight change. The positively significant association between GI, not GL, and subsequent gain in waist circumference may imply a beneficial role of lower GI diets in the prevention of abdominal obesity, but, the researchers conclude, "[F]urther studies are needed to confirm this finding given the small effect observed in this study."
In a 12-week crossover study published two years ago, no differences were reported in the body weights of 19 obese women divided into low and high GI diets. There were also no differences in appetite and food intake after representative test meals. The study "provides no evidence to support an effect of a reduced GI diet on satiety, energy intake or body weight" in obese women. "Claims that the GI of the diet per se may have specific effects on body weight may therefore be misleading."
Later that year, a follow-up study found that the high GI meal resulted in greater satiety overall, suppressing hunger, desire to eat, and prospective consumption compared with the low GI meal. These researchers concluded: "A LGI diet may not be suitable for optimal satiety and appetite control in overweight women."
What About Diabetes?
One study of 63 pregnant women looked specifically at gestational diabetes. Women who have high blood glucose that is discovered for the first time during pregnancy are said to have gestational diabetes mellitus (GDM). Women with GDM are not able to make or use all the insulin their bodies need during pregnancy. Treating GDM is important for the health of pregnant women and their developing fetuses. Can a low-glycemic index diet reduce the need for insulin in gestational diabetes mellitus?
Women who followed the low-GI diet were much less likely to need insulin to control their GDM. Of those using the low-GI diet, 29% needed to start insulin. Of those using the other diet, 59% reached a point where they needed insulin, but nearly half of these women were able to avoid starting insulin by switching to a low-GI diet. Women in both groups were equally likely to have safe, successful pregnancies. And so following a low-GI diet during pregnancy can help women with GDM control their blood glucose without using insulin.
Low-GI diets have been helpful for people with type 2 diabetes. As in the gestational diabetes study, this regulation of blood sugar makes perfect sense and so can make the GI of certain foods valuable information to have. Some examples of low-GI foods are 100% stone-ground whole wheat bread, rolled or steel-cut oatmeal, pasta, sweet potatoes, corn, lima beans, peas, and most fruits and non-starchy vegetables. Some examples of high-GI foods are white bread, corn and bran flakes, white rice, potatoes, pretzels, popcorn, crackers, melons, and pineapple.
Conclusions
The clinical utility of a low glycemic index diet for appetite and food intake control is controversial. Complicating the issue are psychological and behavioral influences related to eating. Many of the low-GI foods listed above are high in fiber and vitamins and minerals, and so worth eating in abundance regardless of their GI. There is no blanket rule suitable for everyone with regard to the importance of GI, but rather different informational needs due to different nutritional considerations. "Faster" carbs (higher numbers) are great for raising low blood sugars and for covering brief periods of intense exercise. "Slower" carbs (lower numbers) are helpful for preventing overnight drops in the blood sugar and for long periods of exercise. Diabetics and intense exercisers certainly can benefit from such info, but fad dieters may be wasting their time.
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