Study: Blueberries reduce insulin resistance

reference

Stull AJ, Cash KC, Johnson WD, Champagne CM, Cefalu WT. Bioactive substances in blueberries improve insulin sensitivity in overweight, insulin-resistant men and women.J Nutr.2010;140(10):1764-1768.

design

Double-blind, randomized and placebo-controlled clinical trial.

Participant

32 middle-aged obese, non-diabetic and insulin-resistant subjects of both sexes.

Rating

An 18-unit relative Mediterranean diet score was used, assessing 9 key components of the Mediterranean diet and allowing the estimation of a relative adherence score to the Mediterranean diet.

Study medications and dosage

Participants were randomized to consume either a smoothie with 22.5 g of blueberry bioactive substances (n=15) or a smoothie with the same nutritional value without added blueberry bioactive substances (n=17) twice daily for 6 weeks. The blueberry bioactives used were “made from a 50/50 blend of two varieties of highbush blueberries, Tifblue (Vaccinium ashei) and Rubel (Vaccinium corymbosum). The whole blueberries were freeze-dried, [and] “ground” into a powder. The daily dose was approximately 2 cups of fresh, whole blueberries.

Primary outcome measures

The main measure was insulin sensitivity, using hyperinulinemic-euglycemic clamps after a 10-hour fast and following standard protocols at baseline, middle and end of the study. Inflammatory biomarkers and lipids in serum were measured before each stapling procedure. Serum inflammatory markers included high-sensitivity C-reactive protein (hsCRP), tumor necrosis factor-a (TNFa), and monocyte chemoattractant protein1 (MCP-1).

Key findings

Daily doses of freeze-dried blueberry powder improved insulin sensitivity by the end of the study, with no significant changes in adiposity, energy intake, or inflammatory biomarkers. The mean change in insulin sensitivity improved in the blueberry group [1.7 +/- 0.5 mg.kg FFM(-1).min(-1)] compared to the placebo group [0.4 +/- 0.4 mg.kg FFM(-1).min(-1)] (P=0.04).

Effects on practice

Blueberries are a new and attractive addition to our current lineup of things that improve insulin sensitivity. The most proven and safest ways to increase insulin sensitivity remain exercise and weight loss. Weight loss reduces insulin resistance in both children and adults, especially when combined with exercise.^{1, 2, 3}

Adding large amounts of grain fiber to the diet also increases insulin sensitivity. In one experiment, just over 1 ounce of oat bran per day led to significant changes in insulin sensitivity after just three days.⁴Rye bread rich in fiber has a similar positive effect and has also been proven to lower cholesterol levels.^{5, 6}Any weight gain during these experiments negates the benefit.⁶

Low vitamin D levels affect insulin sensitivity.⁸A study published in April 2010 questions whether vitamin D will be useful for the general population; Low vitamin D levels were associated with insulin sensitivity only in African American women, not in Caucasian women.⁹Obviously, there is no reason not to supplement vitamin D with all patients, but it may have a negative impact on insulin sensitivity in some patients.

Obviously, there is no reason not to supplement vitamin D with all patients, but it may have a negative impact on insulin sensitivity in some patients.

The same could also apply to chrome. We have been giving additional chromium to patients with blood sugar problems for years. A double-blind, placebo-controlled study published in July 2009 by researchers at Yale University calls this practice into question. After 6 months of supplementation with either 500 or 1,000 mcg/day, insulin sensitivity was no different than in those who took placebo. The authors concluded: "Chromium supplementation does not appear to improve insulin resistance or impaired glucose metabolism in patients at risk of type 2 diabetes and is therefore unlikely to reduce the risk of diabetes."¹⁰However, an article published two months earlier by Louisiana State University reported that some people actually react to chromium. In this study, clinical improvement “was more likely in insulin-resistant subjects who had more elevated fasting glucose and A(1c) levels.”¹¹Participants in the Yale study only had a high risk of type 2 diabetes. Those in the Louisiana study who responded already had diabetes, and the worse their disease, the better the response.

Until proven otherwise, we believe that fresh or frozen blueberries are as effective at improving insulin sensitivity as the powders used in the current study. Since blueberries are believed to help with a variety of health conditions, it makes sense to recommend daily consumption of blueberries to a large number of patients, especially those with reduced insulin sensitivity.¹²