Each study covers a different aspect of diet. Together, the studies show that diabetes risk may rise if you drink too many sodas and sweetened fruit drinks, fall if you eat more fruits and vegetables, and may not be affected by how much fat you eat.
"Until we have more information, we have to assume that calories trump everything else, and that our No. 1 goal for the reduction of new cases of type 2 diabetes should be to reduce the intake of high-energy, low-benefit foods," especially in young people at high risk of diabetes, write Mark Feinglos, MD, CM, and Susan Totten, RD, from Duke University Medical Center.
Here's a quick look at each of the studies, published with Feinglos and Totten's commentary in the Archives of Internal Medicine.
Increased Risk With Sugary Drinks?
Sugary sodas and fruit drinks may be linked to increased risk of type 2 diabetes in African- American women, the first study shows.
The study included nearly 44,000 African-American women who were followed from 1995 to 2005. They completed dietary surveys when the study began and again in 2001.
None of the women had diabetes at the study's start; a decade later, the group had reported 2,713 new cases of type 2 diabetes.
Women who drank at least two regular soft drinks per day were 24% more likely to be diagnosed with type 2 diabetes than women who drank less than one soft drink per month. Weight gain appeared to account for some of the increased risk in soda drinkers.
Women who drank at least two sweetened fruit drinks per day were 31% more likely to be diagnosed with type 2 diabetes than women who drank less than one sweetened fruit drink per day, the study also states.
The researchers, who included Julie Palmer, ScD, of Boston University, note that sweetened fruit drinks "are often marketed as a healthier alternative to soft drinks" but may have at least as many calories as a regular soda.
Diet sodas, orange juice, and grapefruit juice weren't linked to increased risk of diabetes. It's possible that the natural sugars contained in orange and grapefruit juice may have different metabolic effects than the high-fructose corn syrup that is added to regular sodas and most sweetened beverages.
Beverage Industry Reaction
WebMD contacted the American Beverage Association, the trade group representing companies that make and distribute nonalcoholic beverages in the U.S, for its response to the study.
"We agree that type 2 diabetes is an important public health problem, particularly among African- American women, but it is important to recognize that beverage consumption is not an identified risk factor for the disease," says Maureen Storey, PhD, the American Beverage Association's senior vice president for science policy.
Storey points out that the study recommends that women trying to lose weight may find it easier to do so if they switch from regular sodas to diet sodas. She also notes that the study's link between fruit-drink consumption and type 2 diabetes "is very weak or nonexistent. Therefore, avoiding these drinks may have no effect on diabetes risk."
Lastly, Storey says it's not clear whether the researchers controlled for total energy intake -- the total number of calories the women consumed from all sources. An imbalance between energy intake (calories consumed) and energy output (calories burned) can lead to weight gain over time, "and that, aside from family history, is the most important factor in development of type 2 diabetes," says Storey.
Fruits and Vegetables May Cut Diabetes Risk
Eating more fruits and vegetables may cut diabetes risk, according to another study in the journal.
The study included nearly 22,000 adults in Norfolk, England. When the study started, they got a checkup, provided blood samples, and completed a diet and lifestyle survey.
Over the next 12 years, 735 of the participants developed diabetes.
After controlling for multiple other lifestyle factors, including vitamin supplement intake, the diagnosis of diabetes was 62% less likely in people with the highest blood levels of vitamin C and 22% less likely in those with the highest intake of fruits and vegetables. Participants with the highest vitamin C levels ate five to six servings of fruits and vegetables per day.
"Because fruit and vegetables are the main sources of vitamin C, the findings suggest that eating even a small quantity of fruit and vegetables may be beneficial and that the protection against diabetes increases progressively with the quantity of fruits and vegetables consumed," write the researchers, who included Anne-Helen Harding, PhD, of Addenbrooke's Hospital in Cambridge, England.
Low-Fat Diet: No Impact?
The third study set out to see whether a low-fat diet would lower diabetes risk in healthy postmenopausal women. But that turned out to be a tall order.
The study included nearly 46,000 postmenopausal U.S. women. The researchers, who included Lesley Tinker, PhD, of Seattle's Fred Hutchinson Cancer Research Center, split the women into two groups.
One group was assigned to cut their dietary fat to 20% of their daily calories, down from about 38% at the study's start. Women in that group also got intensive nutritional and behavioral counseling and regular group meetings to help them meet the low-fat goal.
For comparison, women in the other group weren't told to cut back on their dietary fat. They got a pamphlet with the federal government's dietary guidelines, but no counseling or group meetings.
None of the women in either group was asked to lose weight or get more exercise.
The study lasted for about eight years, and during that time, women in both groups were equally likely to report being diagnosed with diabetes. So Tinker's team concludes that cutting back on fat without exercising or losing weight may not be enough to curb diabetes risk.
But there is a catch: The women in the low-fat group cut back on fat, but not as much as they were supposed to.
Diet surveys showed that a year after the study began, the women in the low-fat group got about 24% of their daily calories from fat (mostly from saturated fat), and that this percentage edged up to almost 29% (still mostly saturated fat) by the study's sixth year -- well over the 20% goal. Trends in body weight were similar, with the low-fat women initially showing an average 5.3-pound weight loss, but regaining most of that weight by the end of the study.