Sept. 17, 2010 -- Drinking blueberry smoothies helped obese adults who were pre-diabetic improve insulin sensitivity, researchers report.

Sixty-seven percent of people who drank a blueberry smoothie twice a day for six weeks experienced a 10% or greater improvement in their insulin sensitivity, compared with 41% of people in the placebo smoothie group. The study results are published in the October issue of The Journal of Nutrition.

The findings suggest that compounds found in blueberries, which have also been found to improve heart health, may help people with prediabetes by making the body more responsive to insulin. What the biochemical chain reaction or cellular pathways might be remain unclear. But given the challenges of getting people to eat more fruits and vegetables, researchers suggest a smoothie may be a tasty alternative to help people increase their fruit and vegetable intake and boost their health.
Two Blueberry Smoothies a Day

In the study, researchers led by April Stull, an instructor in diabetes and nutrition from the Pennington Biomedical Research Center at the Louisiana State University System in Baton Rouge, compared 32 obese adults who had high insulin levels but did not have type 2 diabetes. Fifteen participants were randomly assigned to drink a smoothie containing 22.5 grams of blueberry freeze-dried powder twice a day for six weeks, while the remaining participants drank a placebo smoothie that did not contain blueberries.

Participants were asked to fill out food questionnaires and were also asked to avoid eating or drinking other fruits or wines containing berries and grapes throughout the study.

The participants did not change their physical activity levels and the calorie intakes remained the same between the two groups. Diets were adjusted so that drinking the smoothies did not add to the participants’ daily caloric intake, because the researchers did not want anyone to gain weight as a result of drinking the smoothies.

The researchers also measured the participants’ blood pressure, weight, cholesterol, and C-reactive protein levels at the start and end of the study. C-reactive protein is a biomarker that serves as a red flag for inflammation, which could indicate a risk for developing heart disease. Diabetes is also a major risk factor for heart disease.
Blueberries’ Effect on Prediabetes

Blueberry smoothies did not have an effect on the participants’ overall biomarker profile, meaning that blood pressure and cholesterol levels did not change nor differ between the two groups at the end of the study. The participants did not lose or gain weight during the study, either. But the effect of drinking blueberry smoothies on insulin sensitivity was far more pronounced.

Compounds in blueberries, called anthocyanins, have antioxidant properties, which may contribute to health benefits such as improved insulin sensitivity. Researchers say more studies are needed to determine the biological effect of blueberries.