Consuming blueberries or cranberries may improve blood glucose control in people with type 2 diabetes, according to a new analysis published in the journal Nutrition, Metabolism & Cardiovascular Diseases.
Berries have long been considered to have unique nutritional value, for several reasons. Many types of berries are naturally low in sugar compared with other fruits, but are high in polyphenols — a group of highly beneficial nutrients that may reduce the risk for conditions like cancer, heart disease, and type 2 diabetes. Berries are just one of many potential food sources of polyphenols, which are also found in notable amounts in green plants and other fruits and vegetables, certain spices like turmeric, cocoa, chocolate, coffee, tea, and red wine. In addition to potentially helping with blood glucose control and cardiovascular health, diets rich in polyphenols are linked to improved cognitive health and slower progression of chronic kidney disease — both benefits that are particularly relevant to people with diabetes.
Berry consumption linked to improved glucose measures
For the latest analysis, researchers aimed to review what past studies have shown about the effects of blueberry and cranberry consumption on various health outcomes in people with type 2 diabetes, as well as people without diabetes. In their review, they included only studies that were randomized — meaning that they randomly assigned some participants to consume blueberries or cranberries, and compared their outcomes with those of people who weren’t assigned to consume the berries. Out of 2,034 possible studies, the researchers included only 22 in their analysis that they considered to be of high quality.
The researchers found that in people with diabetes, blueberry or cranberry consumption was linked to an average reduction in fasting blood glucose of 17.72 mg/dl. It was also linked to an average reduction in A1C (a measure of long-term blood glucose control) of 0.32%. Berry consumption was not linked to direct measures of insulin resistance — raising the question of how, exactly, consuming these berries had the beneficial effects on glucose levels seen in the studies.
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“The consumption of blueberry and cranberry significantly reduced fasting blood glucose and glycated hemoglobin levels in individuals with diabetes, with high credibility of the evidence,” the researchers concluded.
Cranberries also beneficial for cardiovascular health
Another recent study also showed significant health benefits related to cranberry consumption — this time, in a measure of cardiovascular health in men. Published in the journal Food and Function, the study examined the effects of consuming either 9 grams of freeze-dried cranberry powder — the equivalent of 100 grams (3.5 ounces) of fresh cranberries — or the same amount of a “control” powder that didn’t contain any polyphenols each day in 45 healthy men. Participants were asked to avoid consuming polyphenol-rich foods before the start of the study.
As noted in a Healio article on the study, the 23 men who were assigned to consume cranberries showed a nearly immediate benefit in cardiovascular health on the first day of the study. Two hours after consuming their daily dose of cranberry powder, these participants had an increase in a measure of blood vessel health — called flow-mediated dilation — compared with the 22 participants who consumed the control powder. After a month of their daily powder intake, participants who consumed the cranberry powder showed continued signs of better blood vessel function than the participants who consumed the control powder.
“To our knowledge, this is the first study to investigate improvements in vascular function after daily whole cranberry powder intake in healthy humans,” the researchers wrote. “Moreover, the amounts of cranberry used in this work could realistically be achieved daily, which further underlines the relevance of this study in the context of primary prevention of [cardiovascular disease] in the general population.”
Want to learn more about blood glucose management? See our “Blood Sugar Chart,” then read “Blood Sugar Monitoring: When to Check and Why” and “Strike the Spike II: How to Manage High Blood Glucose After Meals.”