Reading time: 5 – 8 minutes
In the United States, 23.6 million children and adults — 7.8% of the population — have diabetes and one-third of U.S. adults are obese [1-2]. Obesity is associated with an increased risk of developing type 2 diabetes as well as insulin resistance, a condition that precedes the development of type 2 diabetes. New research published in the Journal of Nutrition now suggests that blueberries can help improve insulin sensitivity in obese, non-diabetic and insulin-resistant people .
People with type 2 diabetes have high blood sugar because their bodies have lost their innate ability to regulate blood sugar levels. This regulation depends on the hormone insulin, which is produced in the pancreas and controls blood sugar. Normally, insulin patrols the blood, monitoring glucose levels. When these levels get too high, insulin signals cells in the liver, muscle and fat tissue to take up glucose from the blood and store it for later use. It is stored in the form of glycogen in the liver and muscles. Obese individuals often develop insulin resistance. In this scenario, the muscle, fat and liver cells react to insulin much like the townspeople replied to the boy who cried wolf; they ignore it. Because blood sugar levels are constantly too high, insulin becomes ineffective. This eventually results in type 2 diabetes.
Type 1 diabetes is a completely different disease on the cellular level, it just has the same outcome. Type 1 diabetes is an inherited autoimmune disease in which the immune system destroys the cells that make insulin, so the body has none.
Blueberries are high in antioxidants and dietary supplementation with whole blueberries has been shown to reduce blood glucose levels in mice . Few studies, however, have been done in humans and most people don’t eat enough fruits and vegetables to reap the nutritional benefits. Thus, scientists from Pennington Biomedical Research Center at Louisiana State University tested how freeze-dried blueberry powder added to a smoothie affected a number of biomarkers in obese individuals .
Why smoothies? Researchers have to “blind” treatment during a clinical trial. If participants knew they were getting blueberries, they may behave differently — eating healthier or exercising. Thus, freeze-dried blueberry powder was added to a smoothie in order to mask the treatment.
For their randomized, double-blinded, and placebo controlled study, researchers recruited 32 obese (body mass index (BMI) between 32 and 45 kg/m) insulin resistant but non-diabetic adults (over 20 years of age) from the Greater Baton Rouge area. Fifteen of them got the smoothie with 45g of blueberry powder, the equivalent of 2 cups of fresh blueberries; the remaining 17 got a placebo smoothie that looked and tasted the same, and had the same nutritional value and number of calories. Participants drank their smoothies twice a day for six weeks, over the course of which they were weighed, serum inflammatory biomarkers were measured and whole body insulin sensitivity was assayed with hyperinsulinemic-euglycemic clamps, the accepted standard for this type of measurement. They were asked not to alter their exercise routines or diets in any other way, and were asked to refrain from eating any other berries or grapes or products derived from them.
The researchers found that in people drinking the blueberry smoothies, whole body insulin sensitivity increased significantly without a change in body weight. Sixty-seven percent (10 out of 15) of those in the blueberry group had at least a 10% increase in insulin sensitivity, while only 41% (7 out of 17) of those in the placebo group reached this level. Moreover, the percentage change in the blueberry group was higher than in the placebo group. Nothing else that was measured was changed; energy and nutrient consumption, biomarkers of inflammation, lipid profile, blood pressure and cholesterol levels, as well as body weight, stayed constant over the six weeks of the trial in both groups. This is important since any change in the fat in fatty tissue can affect whole body insulin sensitivity; by keeping body weight constant, the researchers ensured that any effect they saw was due to the blueberries.
Blueberries, like other berries, are high in polyphenols. These compounds have antioxidant properties and are thought to be cardioprotective. In another recent study, forty eight obese individuals with metabolic syndrome — which means they have additional risk factors for developing heart disease and diabetes, like high blood pressure and low HDL levels, drank a freeze-dried blueberry drink similar to the one described here . The authors of that work reported that blueberries induced significant decreases in systolic and diastolic blood pressure and plasma oxidized LDL levels.
Dr. William Cefalu, senior author of the study and chief of the Joint Program on Diabetes, Endocrinology and Metabolism at Pennington Biomedical Research Center emphasized :
People need to understand that we replaced other calories in their diet with the smoothie containing the blueberries. If a patient starts adding other foods and calories to the diet they are already eating, this behavior will ultimately lead to weight gain. What we did was within the participants’ own diets, and we replaced 500 calories with 500 calories containing the components of blueberries. So, this design suggested that there indeed was something in the blueberries that had a favorable effect.
The cellular mechanisms by which blueberry extracts achieve these effects were not investigated but the authors conclude by noting that blueberries should continue to be evaluated for enhancing insulin sensitivity in an already insulin resistant population. The Pennington Biomedical Research Center is urging for additional research to determine whether the same effects would be found in people with type 2 diabetes.
- Diabetes Statistics. American Diabetes Association. Accessed 2010 Nov 17.
- Obesity and Overweight. Centers for Disease Control and Prevention. Accessed 2010 Nov 17.
- Stull et al. Bioactives in blueberries improve insulin sensitivity in obese, insulin-resistant men and women. J Nutr. 2010 Oct;140(10):1764-8. Epub 2010 Aug 19.
- DeFuria et al. Dietary blueberry attenuates whole-body insulin resistance in high fat-fed mice by reducing adipocyte death and its inflammatory sequelae. J Nutr. 2009 Aug;139(8):1510-6. Epub 2009 Jun 10.
- Basu et al. Blueberries decrease cardiovascular risk factors in obese men and women with metabolic syndrome. J Nutr. 2010 Sep;140(9):1582-7. Epub 2010 Jul 21.
- No More Singing the Blues: Blueberries Improve Pre-Diabetes. Louisiana Medical News. Accessed 2010 Nov 17.