Cranberries (Vaccinium macrocarpon) are a rich source of phytochemicals, including anthocyanins and other flavonoids that may decrease lipid oxidation and protein glycosylation (1). Anthocyanins inhibit aldose reductase in vitro (2). Red wine anthocyanins improved urine and serum glucose levels and free radical control in streptozotocin-injected rats (3). Daily consumption of 200 ml chokeberry (Aronia melanocarpa) juice for 3 months lowered fasting blood glucose, HbA1c, and lipid levels in people who had been diagnosed with type 2 diabetes for 6–17 years (4). Cranberry juice may decrease the side effects of diabetes and increase the quality of life for people with diabetes.
Adults controlling their type 2 diabetes through diet alone were recruited from the Bangor, Maine, community. Fourteen subjects (aged 57.9 ± 10.6 years, 6 women, 8 men, duration of diabetes 6.0 ± 8.5 years) were randomized to the cranberry group; 13 subjects (aged 52.6 ± 13.7 years, 6 women, 7 men, duration of diabetes 4.1 ± 4.9 years) were assigned to the placebo group. Subjects consumed six capsules filled with either cranberry juice concentrate powder or a placebo daily for 12 weeks. Six capsules were equivalent to a 240-ml serving of cranberry juice cocktail. The artificially colored placebo mimicked the cranberry powder in all respects but flavonoid content. Subjects were asked to discontinue use of dietary supplements, but no other diet and lifestyle changes were made during the study.
More than one-half of the subjects had good control of blood glucose levels (<7.0 mmol/l) at the beginning of the study. No differences were found between the treatment groups in fasting serum glucose, HbA1c, fructosamine, triglyceride, or HDL or LDL levels after 6 and 12 weeks. Placebo subjects had higher insulin values throughout the study (160 ± 167 vs. 86 ± 51 pmol/l at week 12, P < 0.05). Different effects might be seen in subjects with poor glucose control, individuals with type 1 diabetes, or people who use medications to control their type 2 diabetes.
Although the National Kidney Foundation recommends drinking one glass of cranberry juice per day to prevent urinary tract infections, people with diabetes may need to consume greater quantities to improve their health. Since most commercial cranberry juice cocktails contain only 27–31% cranberry juice, more concentrated products might improve compliance. The additional heat processing necessary to convert the cranberry juice to a shelf-stable powder may have altered bioactivity. Despite the lack of significant findings in this small study, the role of cranberries and other flavonoid-rich fruits should be investigated further.
This study was supported in part by the Cranberry Institute, the WI Cranberry Board, and the University of Maine Agricultural and Forest Experiment Station.