Nutrition Bites - Freeze-dried Blueberries and Insulin Sensitivity

Diabetes, specifically type II diabetes, has been on the rise, and researchers have focused their efforts on finding ways to control and prevent this disease. Diet and lifestyle can have a profound effect on a person’s risk of developing type II diabetes, thus it is crucial to determine how to modify diet for disease prevention.

Blueberries have been a popular topic in the field of nutrition due to the antioxidant properties found in their bioactive phenolic compounds, such as anthocyanins. Previous studies using mice models found supplementation with whole blueberries increased glucose uptake in vitro, and reduced inflammatory genes after ingestion. Since most people don’t consume enough fruits and vegetables in their diet, supplementing with the bioactive compounds found in blueberries may be a convenient and feasible dietary intervention to increase insulin sensitivity.

April J. Stull et al. conducted a double-blinded, randomized, and placebo-controlled clinical study to evaluate the effects of dietary supplementation of freeze-dried whole blueberry powder on insulin action (1). A total of 32 men and women (≥ 20 y old), who were classified as obese (BMI between 32 and 45 kg/m2), and insulin resistant participated in the study. Participants were excluded if they were diabetic, taking medications known to affect glucose metabolism, had untreated thyroid or chronic liver, renal, or cardiovascular disease, had a history of drugs and/or alcohol abuse, or psychiatric disease prohibiting adherence to study protocol, had a history of allergic reactions to blueberries, were consuming berries, grapes, and wine > 3 times/wk, or had fluctuations in body weight more than 5 percent in the preceding 2 months.

After participants were characterized as being insulin resistant, they were randomized into two groups, and instructed to consume 1 smoothie at breakfast meals and another smoothie at dinner meals (at least 6 hours apart) for a total of 6 weeks. The smoothie either contained 45 g of blueberry powder (equivalent to the amount of bioactives in about 2 cups of fresh whole blueberries), or an identical smoothie without the addition of blueberry bioactives. Participants were instructed to maintain their current body weight and physical activity level, and worked with a nutritionist to eliminate about 500 kcal/d from their daily intake to compensate for the calories contained in the smoothie.

At the end of the 6 weeks, 67 percent of the participants randomized to the blueberry bioactive smoothie group had at least a 10 percent or greater favorable change in insulin sensitivity compared to only 41 percent of the placebo participants. The percent change in insulin sensitivity was greater in the blueberry bioactive group (22.2 + 5.8 percent) than in the placebo group (4.9 + 4.5 percent). These findings suggest that dietary supplementation with whole blueberries can positively impact whole-body insulin action. Therefore, consumption of blueberry-supplemented smoothies is an attractive and convenient dietary intervention to increase insulin sensitivity for individuals who are at high risk for type II diabetes.

Stull AJ, Cash KC, Johnson WD, et al. Bioactives in Blueberries Improve Insulin Sensitivity in Obese, Insulin-Resistant Men and Women. J Nutr; 2010; 140(10): 1764- 1768.

Deborah Fetter, Department of Nutrition, University of California, Davis.