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This cross-over pilot study tested blood glucose response to low and high glycemic index meals in 12 obese youth with type 2 diabetes or impaired glucose tolerance. Participants demonstrated significantly lower mean daytime blood glucose and a trend toward lower variability, suggesting a clinically relevant impact of reducing glycemic index.
The incidence of type 2 diabetes (T2D) and impaired glucose tolerance (IGT) in youths is rapidly rising. Data from the 1999–2000 and 2001–2002 NHANES (1) indicates that the prevalence of IGT in the US adolescent population is 2.75 million with an additional 39 thousand diagnosed with T2D (1). Recent research has suggested the potential utility of dietary glycemic index (GI) for reducing postprandial hyperglycemia (2, 3) and subsequent insulin demand (4, 5).
A within-subjects cross-over trial investigated the effects of low versus high GI meals on blood glucose (BG). Obese youths between the ages of 7–16 years, diagnosed with T2D or IGT were recruited through a pediatric endocrinology clinic from June-August of 2006. Eligible youths were identified by their health care provider and referred to the study. Informed parent consent and child assent were obtained. The study protocol was approved by the National Institute of Child Health and Human Development Institutional Review Board.
Subjects were served a full day of high glycemic index (GI) and a full day of low GI mixed meals in a supervised clinic setting, matched for macronutrient content, in counterbalanced order (Table[H1]s I and andII).II). Standard washout dinners (sandwich and chips) and snacks (cereal with milk) were consumed at home the evening prior to both conditions. The two conditions were separated by one day, during which participants consumed their typical breakfast and lunch at home, followed by the same wash-out dinner and evening snack. Subjects’ BG levels were assessed using the Medtronic MiniMed Continuous Glucose Monitoring System. Continuous glucose monitoring (CGM) began in the fasting state and continued throughout the study period.
Daytime (8:00 am to 8:00 pm) and nighttime (8:01 pm to 7:59 am) mean BG were calculated. Because the extent of BG variability may be relevant in addition to overall BG (6), a measure of intra-day glycemic variability, four-hour continuous overlapping net glycemic action (CONGA4) was calculated. CONGA4 is defined as the standard deviation of the differences in BG for every 4-hour pair of values; higher values indicate greater BG variability (7). Paired-sample t-tests were used to test for treatment condition differences in mean BG and BG variability for each time period. Data analyses were conducted using SPSS version 13.0. Previous data on the effect of a low GI diet on BG in adults with T2D (8) indicated that 8 sets of paired data provide 85% power to detect a 15% difference in BG. Because we were unable to recruit 8 subjects for each diagnosis (T2D & IGT), data were combined for primary analyses, with subgroup analyses conducted for descriptive purposes only.
Twelve youths participated in this study; data for 1 subject were not included due to CGM failure. Sample characteristics of the 11 subjects were as follows; all were obese (BMI above the 95th percentile for age and sex), 5 were diagnosed with T2D (2 on metformin and one on insulin plus glucophage) and 6 with IGT (all also diagnosed with acanthosis nigricans or PCOS), 4 were male and 7 female, 10 were African American and 1 Latino, age range was 7–15 years (mean=11.48), and mean HbA1c was 7.25 ± 2.85 (8.72 ± 3.57 for T2D and 5.78 ± 0.45 for IGT).
BG results are presented in Table III. The average daytime mean BG was significantly lower for the low GI condition compared with the high GI condition. Participants demonstrated a trend toward lower variability in BG during daytime hours for the low GI condition. There were no significant differences between conditions in mean BG or variability during the nighttime periods.
Subgroup analyses were conducted for descriptive purposes to determine the degree to which the pattern of findings was similar across diagnoses. Findings indicate a consistent pattern of outcomes across both groups (Table III), though not statistically significant for most outcomes due to the reduced sample size. A greater magnitude of difference in BG between conditions was observed for youths with T2D than for youths with IGT.
The results of this study suggest that consuming a low GI diet may have a favorable impact on the BG among youths with T2D or IGT. The difference observed in BG was clinically relevant and if sustained could translate into meaningful health outcomes. These findings are consistent with a recent meta-analysis of low GI diets among adults with type 1 or 2 diabetes indicating a favorable effect (2). Additionally, findings are consistent with the few existing studies using CGM to assess effects of dietary GI – one conducted in 11 adults with T2D (8) and another in 20 youths with type 1 diabetes (9). Previous research has not addressed the utility of a low GI diet in persons with IGT, though beneficial outcomes on weight and satiety have been observed in obese youths (10, 11).
Limitations of this study include its small sample size and short duration. Research of longer duration with larger samples is clearly needed to determine the sustainability and long-term health effects of a LGI diet in youths with T2D and IGT. Additionally, adherence to the conditions outside of the clinic could not be assured. Strengths of this study include the use of CGM, the inclusion of youths with T2D or IGT, implementation within a controlled clinic setting, and the use of primarily unprocessed foods in the low GI dietary condition to ensure an accurate estimate of GI.
Research in adults has suggested that the incidence of T2D can be reduced through lifestyle interventions, preventing progression to T2D by as much as 58% (12). Findings from this study suggest that reducing dietary GI may be an important component of lifestyle interventions for prevention or management of IGT and T2D. Our findings are of particular importance given that children who develop disorders of glucose metabolism are likely to be a particularly high-risk group. Thus, the ability to demonstrate substantial differences in BG with a dietary change lasting a single day suggests that a low GI diet is a promising approach for achieving improved health outcomes.
The authors would like to thank David Greenberg, Donna Franz, Ellie Centenio, and the staff of Mt. Washington Pediatric Hospital for their assistance in the conduct of this study.
This research was supported by the Intramural Research Program of the National Institutes of Health, Eunice Kennedy Shriver National Institute of Child Health and Human Development.
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