Berries, tea, grapes and wine lower diabetes risk

A new study in healthy women suggests that consuming high levels of flavonoids, including compounds found in berries, tea, grapes, and wine, could potentially lower the risk of type 2 diabetes.

The study, published in the February issue of the Journal of Nutrition, indicates that greater intake of these dietary compounds is associated with lower insulin resistance and better blood glucose regulation. The researchers, led by Amy Jennings, PhD, from the department of nutrition, University of East Anglia, Norwich, United Kingdom, say their study is one of the first to examine consumption of different flavonoid subclasses and insulin resistance.

"We found that those who consumed plenty of anthocyanins and flavones had lower insulin resistance. So what we are seeing is that people who eat foods rich in these 2 compounds — such as berries, herbs, red grapes, wine — are less likely to develop the disease," said senior author Aedin Cassidy, PhD, also from the department of nutrition, University of East Anglia, in a statement.

Researchers also found that those who ate the most anthocyanins were least likely to suffer chronic inflammation, which is associated with diabetes, obesity, cardiovascular disease, and cancer. And those who consumed the most flavone compounds had improved levels of adiponectin, which helps regulate a number of metabolic processes, including glucose levels, Dr. Cassidy noted.

Importantly, the difference between the highest and lowest intakes of foods containing these compounds was small, consisting of just one portion of grapes or berries or a couple of oranges, say the authors. Also, the effects on insulin that were associated with high consumption of such foods was equivalent to those observed for other lifestyle factors, such as an hour's walk a day or low-fat diet for a year, they noted.

Nevertheless, Dr. Cassidy said it is not yet know exactly how much of one of these compounds is necessary to potentially reduce the risk of type 2 diabetes. "Dose–response trials are needed to ascertain optimal intakes for the potential reduction of type 2 diabetes risk," she and her colleagues stress.

One of the First Large Human Studies of Flavonoid Subclasses
Researchers note that a previous prospective study, published last year in the American Journal of Clinical Nutrition suggested a 15% reduction type 2 diabetes risk by comparing the highest and lowest quintiles of anthocyanin intake. However, the researchers emphasize that their current study is one of the first large-scale human trials to examine all subclasses of these powerful bioactive compounds to see how they might affect insulin resistance, blood glucose regulation, and inflammation.

The cross-sectional study was conducted in almost 2000 women aged 18 to 76 years from the Twins UK registry. Women who had high glucose levels were excluded. Participants completed a 131-item food-frequency questionnaire, from which flavonoid intakes were estimated using a United States Department of Agriculture database.

The researchers looked at the self-reported intake of 6 subclasses of flavonoids: flavanones, anthocyanins, flavan-3-ols, polymeric flavonoids, flavanols, and flavones.

In multivariable analyses, higher anthocyanin and flavone intakes were associated with significantly lower peripheral insulin resistance (homeostasis model assessment of insulin resistance; quintile 5 [Q5] to Q1 = 20.1, P-trend = .04 for anthocyanins and flavones), as a result of a decrease in insulin concentrations (Q5–Q1 = 20.7 mU/mL, P-trend = .02 anthocyanins; Q5–Q1 = 20.5 mU/mL, P-trend = .02 flavones).

Tea was the main source of overall flavonoid intake, with 4 foods contributing more than 10% of anthocyanin intake (grapes, pears, berries, and wine) and 3 foods making up more than 10% of flavone consumption (oranges, wine, and peppers).

Higher anthocyanin intake was also associated with lower C-reactive protein (hs-CRP) levels (Q5–Q1 = 20.3 mg/L, P-trend = .04), whereas those in the highest quintile of flavone intake had improved adiponectin levels (Q5–Q1 = 0.7 mg/L, P-trend = .01).

Higher intakes of both anthocyanins and flavones were associated with improvements in insulin resistance and hs-CRP, the researchers note.

No significant associations were observed for total or other flavonoid subclasses.

Findings Are Clinically Relevant, Easy to Achieve
Although these findings are from cross-sectional data and require confirmation, they are clinically relevant because of the 0.7-mU/mL difference in insulin observed between the top and bottom quintiles of anthocyanin intake, the researchers note.

The difference in anthocyanin intake between the top and bottom quintiles was 35 mg, which can be readily incorporated into the diet by consuming approximately one portion of grapes (78 g) or berries, such as strawberries (105 g), raspberries (90 g), blueberries (21 g), or blackberries (39 g).

Similarly, the difference in flavones between the top and bottom quintiles was 3.6 mg, equivalent to that found in approximately 2.5 oranges.

These results "are of public-health importance because the intakes associated with these findings are easily achievable through the habitual diet" and make a significant contribution to the knowledge base needed to refine the current fruit and vegetable dietary recommendations, the authors conclude.