Low Carbohydrate Diets: Take Them With a Grain of Salt, Part 1
James J. Kenney, Ph.D., R.D.
American Council on Science and Health
Date Published: Jan. 29, 2000
Does the relative ability of individual foods and diets to raise blood glucose have a unique or tremendous influence on human health?
According to four of the most popular diet books of recent years -- "The Zone" (1995), whose primary author is Barry Sears, Ph.D.; "Protein Power" (1997), by medical doctors Michael R. Eades and Mary Dan Eades; "Dr. Atkins' New Diet Revolution" (1997), by Robert C. Atkins, M.D.; and "Sugar Busters! Cut Sugar to Trim Fat" (1998), by H. Leighton Steward and associates -- diets high in carbohydrate, or CHO, are responsible for insulin increases that inhibit fat-burning and lead to obesity, type 2 diabetes (noninsulin-dependent diabetes mellitus) and atherosclerosis (specifically, coronary heart disease, or CHD). More recently, actress Suzanne Somers has joined the anti-CHO bandwagon, with "Suzanne Somers' Get Skinny on Fabulous Food" (1999).
A glycemic index, or GI, is the degree to which a food ingested alone increases the concentration of glucose in the blood comparative to a standard, such as white bread. The authors cited above profess that foods with high glycemic indexes -- foods whose ingestion rapidly results in high glucose concentrations - are the most dangerous foods, primarily because of the insulin response their digestion elicits. They further contend that diets relatively high in protein and/or fat are the key to reducing blood insulin, losing weight and improving health.
There is a grain of truth to what these authors say. The trend of scientific evidence from CHO research suggests that, at least for some individuals, both the total quantity and the proportion of the various carbohydrates in a diet may play a complex role in feeding behavior, metabolism and health. But consumers who entertain the assertions of diet-book authors must consider those assertions with more than a grain of salt. For example, the authors named above affirm that beets and carrots are fattening, which is absurd.
The Basis of the Glycemic Index
Glycemic indexes are based on the human physiologic response, in terms of plasma glucose concentrations, to ingestion of a food specimen that contains 50 grams of digestible CHO, comparative to the response to an equivalent intake of either glucose or white bread. The larger the blood-glucose increase, the higher the GI. GIs have been assigned to more than 600 foods, but the typical American supermarket carries over 6,000 foods. So it is likely that many people are regularly consuming many foods that have not been tested regarding GI. Moreover, cooking or otherwise preparing a food can alter its potential blood-glucose effects. As a rule, processing a nonprocessed food amplifies its ability to increase plasma glucose concentrations. In any case, the relationship of high-GI foods and/or a high-CHO diet to the development of obesity, type 2 diabetes and cardiovascular disease has yet to be determined.
The Pima Paradox
Epidemiological studies have not lent much credence to the notion that high-CHO diets in general, or those consisting mostly of high-GI foods, lead to obesity, diabetes and CVD. The contention that the quantity or percentage of CHO in a diet, or the relative ability of individual foods or diets to increase plasma glucose concentrations, is the primary determinant in the development of obesity, insulin resistance and type 2 diabetes is inconsistent with some observations of human populations. For example, although the diet of the Pima Indians living in northern Mexico consists largely of potatoes and corn tortillas -- both high-CHO, high-GI foods -- the Pimas weigh 60 to 65 pounds less than the Pima residents of Arizona, who consume much more protein and fat than do their Mexican cousins.
Foods high in protein and/or fat tend to have GIs below those of high-CHO foods. But by age 50 more than half of the Arizona Pimas become obese and develop type 2 diabetes, whereas among the Mexican Pimas type 2 diabetes is rare and occurs mostly among the elderly. Because the Mexican Pimas and the Arizona Pimas are of the same genetic stock, it is unlikely that genetic factors can account for this disparity. That the Mexican Pimas are much more physically active than their Arizonan counterparts is probably a crucial factor both in the diabetes disparity and in the body-weight disparity.
Scientific findings have positively associated diets high in fat, especially saturated fat, and in refined CHOs, particularly sugars, and the development of insulin resistance. On the other hand, high-complex CHO diets and improvement of sensitivity to insulin appear associated.
Priorities
On Sept. 1, 1998, the American Heart Association released a report stating that diets very low in fat -- diets in which less than 15 percent of calories come from fat -- may carry serious health risks for some people and are not recommended for the public. The authors noted that, according to many research findings, very high CHO diets reduce blood HDL-cholesterol (a compound that tends to retard heart disease) and increase blood triglycerides more than do diets relatively high in unsaturated fat.
But in every study that has shown adverse effects of high-CHO diets on blood lipid concentrations, the high-CHO diet and the diet higher in fat were isocaloric, that is, equally restricted calorically. In similar studies without calorie controls, adverse effects have been minimal -- because for most people, calorie intakes on reduced-fat diets tend to be lower than those on diets with unmodified fat intakes. And in one study in which isocaloric diets were used, following the high-CHO diet, which had a low GI, did not result in a decrease in blood HDL-cholesterol or in an increase in blood triglycerides. Therefore, it is possible that high-CHO diets adversely affect blood lipid concentrations only when they consist largely of processed high-CHO, high-GI foods and one's calorie intake is at least that which one would have on a diet relatively high in fat.
Last year the American Diabetes Association rescinded its advocacy of high-CHO diets for all diabetics and took the stand that the composition of diabetic diets should be individualized. Because body weight, activity level, endogenous insulin output, degree of insulin resistance and risk factors for other diet-related diseases vary widely among diabetics, that their diets should be individualized almost goes without saying. The ADA's rescindment stemmed largely from short-term studies that had associated high-CHO diets (relative to diets higher in unsaturated fat), potentially adverse blood-lipid changes, and little or no improvement in sensitivity to insulin and in blood-glucose concentrations. But it was expected in these studies that the high-CHO diet intake would be calorically equal to the high-fat diet intake. Furthermore, the high-CHO diets in these studies consisted largely of highly processed CHO-rich foods, most of them high-GI foods. It is too early to generalize from the findings of such studies applicably to all type 2 diabetics.
The ADA does not recommend taking GIs into account in diabetic meal planning. It also has maintained that the cumulative GIs of mixed meals cannot be extrapolated reliably. The scientific evidence, however, suggests that such GIs can be dependably extrapolated. In any case, the ADA advises giving top precedence in the planning of diabetic diets to CHO quantity rather than to CHO sources.
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