Do High Glycemic Index Foods Cause Obesity and Diabetes?

A look at the false claims made in Sugar Busters and the Zone Diet books.

By James J. Kenney, Ph.D., R.D., F.A.C.N.

Posted January 1, 1999 - Good through December 2010

Introduction
The Pima Paradox
Glycemic Index vs. Insulin Score Vs Satiety Index
Glycemic Index does not accurately predict Insulin Score
Conclusion
References
Post Test

Introduction

Two of the most popular diet books in recent years (Enter the Zone by Barry Sears and Sugar Busters by H.L. Steward et al) claim that diets too high in carbohydrates (CHOs) lead to high insulin levels which prevent the burning of fat which results in obesity. Foods that produce the highest blood sugar response aka glycemic index (GI), such as carrots, potatoes and parsnips are claimed to be the most dangerous because they are assumed to produce the greatest insulin response. Even some RDs have made similar claims.[1] These authors claim that diets higher in protein and fat and much lower in (CHO) are the key to lowering insulin levels and burning off more body fat. Are they right?

The Pima Paradox

If high CHO diets cause hyperinsulinemia, obesity and type 2 diabetes (NIDDM) how do we explain what is happening to the Pima Indians of Arizona? Pima Indians living in northern Mexico weigh 60-65 pounds less than Pima Indians living Arizona even though the Mexican Pima’s diet consists largely of potatoes and corn tortillas, both high GI foods. The Arizona Pimas eat a Westernized diet with much more protein and fat than their Mexican "cousins." In Arizona, more than half the Pimas develop NIDDM by age 50y whereas in Mexico, NIDDM is rare and afflicts mainly the elderly.[2] If a high-CHO diet containing a lot of high GI foods raises insulin levels and promotes weight gain, why aren’t the high CHO eating Mexican Pimas the ones becoming obese and developing NIDDM? Aside from being inactive, the other factor leading to obesity and NIDDM among AZ Pimas is the relatively high calorie density or energy density (ED) of their diet compared to their Mexican cousins.

Part of the reason for the Pima Paradox, is that the Mexican Pimas are much more active. A single bout of exercise improves insulin resistance for a day or so.[3] Exercise depletes liver and muscle glycogen stores and so these tissues more rapidly take up blood sugar after exercise in order to replenish their glycogen stores. When people are sedentary their muscle glycogen stores are so full they resist the action of insulin because they have no use for blood sugar when their glycogen stores are full and they are not moving. Research also shows the body does not increase caloric intake to account for increased daily activity.[4] This is the main reason more active people are less likely to gain weight and develop insulin resistance and NIDDM.[5]

Genetic differences are not a likely explanation since these two Pima populations arose from the same genetic stock. A study of identical twins showed that there was little difference in ad libitum caloric intake on a high fat or a high CHO diet if the ED of the two diets was the same.[6] The claim that hyperinsulinemia leads to obesity and NIDDM, is also questionable. In fact, it is largely the consumption of excess calories in any form that causes hyperinsulinemia and it is an ED diet coupled with inactivity that leads to excessive caloric intake, obesity and NIDDM. Even though it is claimed that high insulin levels prevent fat burning and promote fat storage, which lead to obesity, it is much more true that obesity leads to insulin resistance and high insulin levels.

Glycemic Index vs. Insulin Score Vs Satiety Index

Glycemic Index does not accurately predict Insulin Score

GI is simply a measure of how high the blood sugar rises in response to a given amount of CHO from various sources compared to a standard food (usually white bread or glucose). For example, carrots and potatoes have about the same high GI as sugar and white bread. However, carrots have only 195 calories per pound and a boiled potato has about 450 calories per pound while bread contains around 1250 calories per pound and sugar contains 1725 calories per pound. So the volume of food must increase dramatically for foods with a lower calorie density to be fed at the same CHO level. Since gastric expansion increases the rate of gastric emptying and CHO can only be absorbed after it leaves the stomach, this methodology creates a bias against lower calorie dense foods. Foods that lead to a higher osmolarity in the stomach can delay gastric emptying and this may also slow their absorption and lower the glycemic response. The reason sports drinks don’t have a high concentration of sugar is that it would delay gastric emptying and slow the absorption of water.

The Insulin Score, (IS) or insulin index, is a measure of insulin output in response to a given caloric amount of various foods. Among foods with little fat or protein, the GI correlates fairly well with the IS. Dietary fat delays gastric emptying so any CHO consumed with a lot of fat usually results in a lower GI but because the fat magnifies the insulin output to a given rise in blood sugar, the IS is generally much higher than would be predicted based on the GI. For example, ice cream with its high fat content and high osmolarity has a fairly low GI, much lower than potatoes, rice or carrots. However, dietary fat greatly magnifies the insulin response to a given amount of CHO so the IS for ice cream is only slightly lower than that of potatoes and actually higher than white rice despite its much lower glycemic index. Dietary protein, like fat, also causes more insulin to be released; foods with no CHO at all cause a fairly substantial insulin response even though they have little effect on blood sugar.[7] It should be clear then that GI does not accurately predict the IS when foods vary considerably in their macronutrient composition and/or ED.

Conclusion

The satiety index (SI) is a relatively new concept that measures how full or satiated people feel after consuming a given calorie load from a variety of foods. It is measured by asking people to rate how satiated they are after a meal and by how much food they will eat after a 2-hour delay after consuming the test food. So a high SI food would leave people more satisfied after eating a set amount of calories and they would also eat less 2 hours later when given something else to eat presumably because they were still less hungry. It seems likely that a diet made up of higher SI foods would likely lead to less hunger and a lower calorie intake. The notion that high GI foods lead to obesity ultimately rests on the assumption that GI equals Satiety Index (SI). It turns out that the highest SI food tested was the potato which is also one of the highest GI foods.[8] Clearly then the presumption that all high GI foods lead to overeating and obesity is not correct. Therefore, the theory that high GI foods invariably lead to excessive insulin output which in turn prevents fat burning and promotes fat storage and obesity is of little scientific merit. In fact, insulin output in response to a meal correlates far better with total calories consumed than it does with the relative GI of the various foods in that meal.

References:

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