Author Affiliation: Nutrition Concepts by Franz Inc, Minneapolis, Minnesota.
The importance of the glycemic index (GI) and fiber in diabetes mellitus (DM) nutrition therapy has been controversial. In this issue of the Archives, Jenkins et al1 compared a lower-GI (46%, glucose scale), high soluble fiber (fiber source primarily legumes; 37.6 g of fiber per day) diet with a higher-GI (58%, glucose scale), whole wheat fiber (26.8 g of fiber per day) diet. Both diets decreased hemoglobin A1c, −0.5% and −0.3%, respectively. Lipid values also improved from both diets, and systolic blood pressure decreased by −4.5 mm Hg on the lower-GI, high soluble fiber diet. Absolute coronary risk (10-year percentage) decreased from 10.7% to 9.6% and from 10.4% to 9.9%, respectively. Are the modest benefits from the dietary components, or are they a result of the reduced energy intake of approximately 200 kcal per day? The more important question might be whether people with DM can implement any of these often difficult interventions in the “real world” to the extent it will have an impact on their glycemic control or cardiovascular risk factors.
The role of the GI in DM nutrition therapy has been controversial for several reasons. First of all, the definition is confusing. The GI measures the relative area under the postprandial glucose curve of 50 g of digestible carbohydrates compared with 50 g of a standard food, either glucose or white bread. It does not measure how quickly foods are digested and absorbed into the blood stream as claimed by diet books (and many health professionals). This implies an initial sharp glucose response from high-GI foods and a slower and more gradual glucose response from low-GI foods. Brand-Miller et al,2(p100) in an analysis of the GI, postprandial glucose response, and shape of the curve concluded: “the general glucose curve shape is similar within each food category even though the areas under the curves differ. The notion that low-glycemic index foods produce a sustained rise in blood glucose is not supported.” Although there is a modest difference in the glucose peak from 50 g of high- vs low-GI carbohydrates, the peak occurs at similar times. Furthermore, the meal glucose and insulin responses to a high-GI diet vs a low-GI diet are parallel. There is no quick or sharp glucose or insulin peak response to the high-GI meals.3
Jenkins et al1 reference older, short-term studies for their claim that “low-GI foods have been shown to improve glycemic control in type 2 diabetes mellitus.” However, they neglect to mention that 2 more current trials, each 1 year in duration, reported no significant differences in HbA1c levels from low-GI vs high-GI diets4 or American Diabetes Association (ADA) diets.5 The ADA systematic review of macronutrients in the management of DM also examined the effects of GI on glucose and cardiovascular disease (CVD) risk factors.6 They concluded that, in general, there is little difference in glycemic control and CVD risk factors between low-GI and high-GI diets. A slight improvement in glycemia may result from a lower-GI diet; however, confounding by higher fiber must be accounted for in some of these studies. Furthermore, a standardized definition of a low-GI diet needs to be developed, and low retention rates on low-GI diets must be addressed. As a result, the ADA has deleted a statement implying modest benefit from the GI and/or glycemic load in their nutrition recommendations for DM medical care.
Another major problem with the GI is the variability of responses to carbohydrate foods. For example, Australian potatoes have a high GI, whereas potatoes in the United States and Canada have only a moderate GI. The GI response from white bread varies widely. Although the average GI for white bread was 71, the GI response in trial participants ranged from 44 to 132.7 Furthermore, although often implied, low-GI foods are not necessarily healthy. Foods that are higher in sugars tend to have lower GI than many starches, even starches that are whole grains or unprocessed.2 Fifty grams of sucrose, for example, contain only 24 g of glucose compared with 50 g of glucose in starch, resulting in a lower postprandial glucose curve for sucrose. Several popular candy bars have moderate GI scores, and premium ice cream has a very low GI score.8
The effect of dietary fiber on glycemic outcomes in people with DM is also controversial. Early short-term studies using large amounts of fiber in small number of participants suggested a positive effect of fiber on glycemia. However, results from later studies have shown mixed effects. Diets containing large amounts of fiber (44-50 g per day) are reported to improve glucose outcomes in persons with DM, whereas more reasonable amounts of fiber (≤24 g per day) have not.9 The effects of fiber intakes of 24 to 44 g per day on glycemic outcomes are not well studied. Therefore, it is unknown if persons living in the “real world” can consume the amount of fiber needed to improve glucose outcomes. It is recommended that persons with DM include foods containing 25 to 30 g fiber per day, with special emphasis on soluble fiber sources (7-13 g per day), shown to have beneficial effects on total and low-density lipoprotein cholesterol.8
Legumes, as documented in the study by Jenkins et al,1 are components of a healthy eating pattern for people with DM and the general public. Whether people with DM can eat the amount necessary to improve glycemic control is debatable, and, if legumes do improve glycemia, is it because of their low GI or high soluble fiber content?
Nutrition therapy for DM is effective. However, just as there is no 1 medication or insulin regimen appropriate for all persons with DM, there is no 1 nutrition therapy intervention. A variety of nutrition therapy interventions have been shown to be effective.9 Nutrition education and counseling must be sensitive to the personal needs and cultural preferences of individuals and their ability to make and sustain lifestyle changes.
Correspondence: Ms Franz, Nutrition Concepts by Franz Inc, 6635 Limerick Dr, Minneapolis, MN 55439 (MarionFranz@aol.com).
Published Online: October 22, 2012. doi:10.1001/2013.jamainternmed.871
Conflict of Interest Disclosures: None reported.
Franz MJ. Diabetes Mellitus Nutrition Therapy: Beyond the Glycemic IndexComment on “Effect of Legumes as Part of a Low Glycemic Index Diet on Glycemic Control and Cardiovascular Risk Factors in Type 2 Diabetes Mellitus”. Arch Intern Med. 2012;172(21):1660-1661. doi:10.1001/2013.jamainternmed.871