Although the participant pool was relatively small, the researchers believe their findings are significant enough to suggest that diabetics who regularly enjoy caffeinated beverages and are struggling to maintain their glucose levels should consider reducing or eliminating caffeine in their diets.

The researchers examined how oral caffeine capsules affected carbohydrate metabolism in people with type-2 diabetes. In this population, decreases in insulin sensitivity could result in exaggerated hyperglycemic responses to glucose that would aggravate the glycemic dysregulation that is a hallmark of this disease. Although they found that caffeine did not affect fasting levels of blood glucose or insulin in comparison to placebo, they did find significant effects on both following a meal. The meal, in this case, was the commercial liquid meal supplement known as Boost?®.

"In a healthy person, glucose is metabolized within an hour or so after eating. Diabetics, however, do not metabolize glucose as efficiently," said James D. Lane, Ph.D., associate research professor in the department of psychiatry and behavioral sciences at Duke, and lead author of the study. "It appears that diabetics who consume caffeine are likely having a harder time regulating their insulin and glucose levels than those who don't take caffeine."

The double-blind, placebo-controlled study enrolled 14 habitual coffee drinkers who had at least a six-month history of type-2 diabetes but who did not require insulin therapy as part of their treatment regimen.

Study participants were asked to complete a seven-day diary of caffeinated beverage intake -- including the serving size and time of day for each drink consumed. (Average caffeine consumption from all beverages was about 526 milligrams per day, based on the self-reported diary entries.) Participants were then observed on two different mornings within a two-week period following an overnight fast and abstinence from caffeine.

On the days they were observed, participants took their prescribed diabetes medications according to their usual treatment regimen. After 30 minutes of quiet rest, they provided a blood sample so researchers could record their baseline fasting glucose level. Participants then consumed two 125-milligram capsules of caffeine (or placebo) with water. After a 60-minute interval to allow for caffeine absorption, a second fasting blood sample was taken. Participants were then given an additional 125-milligram capsule (caffeine or placebo) to take with a commercial liquid meal (Boost?®) that contained 75 grams of carbohydrates. The third capsule was provided in order to maintain caffeine levels in the blood. Additional blood samples were taken from participants one and two hours after the meal, while they relaxed.

The researchers determined that caffeine had little effect on glucose and insulin levels during fasting when compared to placebo. However, after consuming the carbohydrates in the liquid meal, those who were given caffeine experienced a 21 percent increase in their glucose level and a 48 percent increase in their insulin level.

"The goal of clinical treatment for diabetes is to keep the person's blood glucose down," Lane said. "It seems that caffeine, by further impairing the metabolism of meals, is something diabetics ought to consider avoiding. Some people already watch their diet and exercise regularly. Avoiding caffeine might be another way to better manage their disease. In fact, it's possible that staying away from caffeine could provide bigger benefits altogether."

Lane and his team hope to begin enrolling participants in larger clinical trials that use brewed coffee -- rather than oral caffeine capsules -- later this year. Other authors of this study include Christina Barkauskas; Richard Surwit, Ph.D.; and Mark Feinglos, M.D., all of Duke University Medical Center.

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White and ISU graduate student Melody Brown undertook studies in the university's Center for Designing Foods to Improve Nutrition. Men and women, between the ages of 19 and 28, participated in the 12-week study, eating salads of spinach, romaine lettuce, cherry tomatoes and carrots, topped with Italian dressings containing 0, 6 or 28 grams of canola oil. Hourly blood samples were collected for 11 hours following each meal, and then sent to food scientists at Ohio State University to be tested with highly sensitive detection equipment.

Essentially no beta carotene absorption was observed when salads with fat-free dressing were eaten. A significantly greater absorption of lycopene, alpha- and beta-carotene was recorded when salads were eaten with full-fat dressings than with reduced-fat dressings.

White's findings are published in the August issue of the American Journal of Clinical Nutrition.

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