Continuous glucose monitoring to assess the ecologic validity of dietary glycemic index and glycemic load 1 – 3

Background: The circumstances under which the glycemic index (GI) and glycemic load (GL) are derived do not reflect real-world eating behavior. Thus, the ecologic validity of these constructs is incompletely known. Objective: This study examined the relation of dietary intake to glycemic response when foods are consumed under free-living conditions. Design: Participants were 26 overweight or obese adults with type 2 diabetes who participated in a randomized trial of lifestyle modification. The current study includes baseline data, before initiation of the intervention. Participants wore a continuous glucose monitor and simultaneously kept a food diary for 3 d. The dietary variables included GI, GL, and intakes of energy, fat, protein, carbohydrate, sugars, and fiber. The glycemic response variables included AUC, mean and SD of continuous glucose monitoring (CGM) values, percentage of CGM values in euglycemic and hyperglycemic ranges, and mean amplitude of glycemic excursions. Relations between daily dietary intake and glycemic outcomes were examined. Results: Data were available from 41 d of monitoring. Partial correlations, controlled for energy intake, indicated that GI or GL was significantly associated with each glycemic response outcome. In multivariate analyses, dietary GI accounted for 10% to 18% of the variance in each glycemic variable, independent of energy and carbohydrate intakes (P , 0.01). Conclusions: The data support the ecologic validity of the GI and GL constructs in free-living obese adults with type 2 diabetes. GI was the strongest and most consistent independent predictor of glycemic stability and variability. Am J Clin Nutr 2011;94:1519–24. INTRODUCTION Dietary GI and GL are related constructs that refer to the rate at which available carbohydrate raises blood glucose during the postprandial period. GI is a food-classification system based on the rise in blood glucose after consumption of a test food, relative to a standard (eg, glucose or white bread) containing the same amount of carbohydrate (1). GL is calculated as the multiplicative product of GI and the amount of carbohydrate and thus is influenced by carbohydrate source and quantity (2). The postprandial AUC for glucose increases in a dose-response fashion with increasing carbohydrate portions of a food and varies by GI when carbohydrate load is held constant (3, 4). The clinical utility of GI and GL has been the subject of considerable debate. Some researchers question evidence regarding the relevance of dietary GI and GL to health and raise several concerns about the methods used to determine GI (5, 6). Of particular relevance to the current study, the experimental constraints under which GI values are derived may not reflect real-life conditions. The test setting requires the subject to consume a specified portion (ie, containing 50 g available carbohydrate) of a single item, to be in a fasting state, and to consume the test food within a specified period of time. In addition, the optimal measure for assessing clinical significance (eg, incremental AUC above fasting, glycemic excursions) remains a topic of debate (5, 6). In the context of that debate, the current study was undertaken to examine the relations of dietary GI and GL (and other dietary variables) to glycemic response (as assessed with a CGM) when foods are consumed in self-selected amounts, at self-selected times, and in self-selected combinations by free-living overweight and obese individuals with type 2 diabetes. SUBJECTS AND METHODS