Blueberries Decrease Insulin Resistance

Reference

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

Design

Double-blinded, randomized, and placebo-controlled clinical study.

Participants

32 obese, middle aged, nondiabetic, and insulin-resistant subjects of both genders.

Scoring

An 18-unit relative Mediterranean diet score was utilized that rated 9 key components of the Mediterranean diet and allowed estimation of a score for relative Mediterranean diet adherence.

Study Medication and Dosage

Participants were randomized to consume either a smoothie containing 22.5 g blueberry bioactives (n=15) or a smoothie of equal nutritional value without added blueberry bioactives (n=17) twice daily for 6 weeks. The blueberry bioactives used were “made from a 50/50 mixture of 2 varieties of highbush blueberries, Tifblue (Vaccinium ashei) and Rubel (Vaccinium corymbosum). The whole blueberries were freeze-dried, [and] milled” into a powder. The daily dose was equal to 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, following standard protocols at baseline, the middle, and end of the study. Serum inflammatory biomarkers and lipids were measured before each clamp procedure. Serum inflammatory markers included high sensitivity C-reactive protein (hsCRP), tumornecrosis factor-a (TNFa), and monocyte chemoattractant protein1 (MCP-1).

Key Findings

Daily doses of freeze-dried blueberry powder enhanced insulin sensitivity by the end of the study without 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).

Practice Implications

Blueberries are a new and attractive option to add to our current assortment of things that improve insulin sensitivity. The best-proven and safest ways to increase insulin sensitivity are still exercise and weight loss. Weight reduction reduces insulin resistance in both children and adults, especially in combination with exercise. ^{1, 2, 3}

Adding large amounts of cereal fiber to the diet also increases insulin sensitivity. In one experiment, slightly more than 1 ounce a day of oat bran produced significant changes in insulin sensitivity after just 3 days.⁴ High-fiber rye breads have a similar beneficial action and have also been shown to lower cholesterol. ^{5, 6} Any weight gain during these experiments cancels out the benefit.⁶

Low vitamin D levels adversely affect insulin sensitivity.⁸ A study published in April 2010 calls into question whether vitamin D will be useful for the general population; low vitamin D levels were only associated with insulin sensitivity in African American women, not Caucasian.⁹ Obviously there is no reason not to supplement all patients with vitamin D, but it may prove to affect insulin sensitivity in a portion of patients.

Obviously there is no reason not to supplement all patients with vitamin D, but it may prove to affect insulin sensitivity in a portion of patients.

The same might be true of chromium. For years we have given patients with blood sugar problems supplemental chromium. A double-blind, placebo-controlled trial published in July 2009 by Yale University researchers calls this practice into question. After 6 months of supplementation at either 500 or 1,000 mcg/day, insulin sensitivity was no different than in those who had taken placebo. The authors concluded, “Chromium supplementation does not appear to ameliorate insulin resistance or impaired glucose metabolism in patients at risk for type 2 diabetes, and thus is unlikely to attenuate diabetes risk.”¹⁰ However, a paper published 2 months earlier from Louisiana State University reported that some people do respond to chromium. In this study clinical improvement was “more likely in insulin-resistant subjects who have more elevated fasting glucose and A(1c) levels.”¹¹ The participants in the Yale study were only at high risk for 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, let us assume that fresh or frozen blueberries will be as effective at improving insulin sensitivity as the powders utilized in the current study. Given that blueberries are thought to provide benefit against a wide range if health disorders, it will be reasonable to suggest daily consumption of blueberries to a large number of patients, especially those with reduced insulin sensitivity.¹²