by Craig Weatherby
A daily smoothie containing antioxidant-rich blueberry powder increased sensitivity to insulin in people at risk for diabetes.
As a result, the researchers believe that blueberries may reduce the risk of developing the dreaded lifestyle disease, and they called for more research.
The placebo-controlled clinical trial involved people who were obese and had insulin resistance… conditions that increase the risk of developing diabetes.
Participants who consumed a blueberry smoothie daily for six weeks experienced a 22 percent rise in insulin sensitivity compared to only 4.9 percent in the placebo group.
Increased insulin sensitivity is a very good thing that signals an improved ability to metabolize dietary sugars and starches and stabilize blood sugar levels at healthy levels.
This is the first reported clinical trial to test the effect of blueberries' “bio-active” compounds—mostly phenolic antioxidants—on insulin sensitivity in obese, non-diabetic, and insulin-resistant men and women.
Despite its small size (32 volunteers) the trial's results are relatively reliable, thanks to its rigorous (randomized, double blind, placebo-controlled) design.
The beneficial effects of blueberries are generally linked to their polyphenol-type antioxidants… especially anthocyanins and flavanols.
However, it seems increasingly clear that these compounds' apparent benefits result from their effects on key genes.
In addition to cocoa and tea, the best sources of anthocyanins and flavanols are blackberries, Brazilian acai berries, and other dark blue-purple berries. They occur at lower levels in raspberries and strawberries, which provide other beneficial antioxidants.
Study adds to blueberry benefits
Until now, no human study had tested the effects of blueberry polyphenols on insulin sensitivity in people at high risk of developing diabetes.
The study was conducted by researchers from Louisiana State University's Center for the Study of Botanicals and Metabolic Syndrome (Stull AJ et al. 2010).
The Louisiana team recruited 32 obese, non-diabetic, and insulin-resistant men and women whose average age was 51.5 years (Stull AJ et al. 2010).
The participants were randomly assigned to receive a smoothie containing 22.5 grams of blueberry “bio-actives”— polyphenol antioxidants and other components of freeze-dried whole blueberry powder—or a placebo smoothie of equal macro-nutrient (fat, carb, and protein) content.
The blueberry and placebo smoothies were identical in physical appearance and nutrient content, except that blueberry antioxidants were added to the blueberry smoothie.
Each group consumed two smoothies every day for six weeks.
At the end of the study, 67 percent of the people in the blueberry group experienced at least a 10 percent or greater rise in insulin sensitivity, versus only 41 percent of the placebo group.
However, the blueberry group showed no changes in BMI, body fat, or markers of inflammation.
As the researchers wrote, “This study is not conclusive, but it strongly suggests a need to further explore the cellular mechanism for the [beneficial] effect [on insulin sensitivity]. In addition, our study suggests the need for studies of longer duration that will evaluate blueberries and their potential role in improving insulin sensitivity…” (Stull AJ et al. 2010).
The United States Highbush Blueberry Council (USHBC) prepared the freeze-dried whole blueberry powder used in the study.
Funding came from the USHBC, the U.S. NIH, and the National Center for Complementary and Alternative Medicine's Office of Dietary Supplements.
- 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 Aug 19. [Epub ahead of print]