Like carotenoids, flavonoids are a class of antioxidants found in fruits and vegetables and in some plant-based beverages including tea and red wine. To study the possible benefit of dietary flavonoids for preventing Type 2 diabetes in women, dietary flavonoid intake, as well as insulin resistance and inflammation, were measured in 38,018 healthy U.S. women over an average of nine years. The results showed no relationship between intake of flavonoids and risk of developing Type 2 diabetes. However, there was a modest benefit for consumption of apples and tea.
The relationship between vitamin C and E intake and diabetic retinopathy (eye disease) was assessed in two U.S. studies of populations of 998 and 1,353 adults with Type 2 diabetes. There were no significant associations for dietary intake or blood level of the vitamins with the occurrence of retinopathy. However, one of the studies showed that those taking supplements of the vitamins for more than three years had a reduced risk of retinopathy.
In summary, population studies have shown mixed results as to possible benefits of antioxidants to people with diabetes. Some show a benefit, others show no relationship, and none show harm.
Clinical trials. Population studies are limited in that they show only associations between dietary factors and medical conditions—they do not show cause and effect. For instance, high carotenoid levels in the blood are markers for high fruit and vegetable intake: Therefore, the beneficial effects on glucose metabolism associated with carotenoids in the Australian study noted above could actually be due to other substances in the fruits and vegetables rather than the carotenoids. To prove conclusively that a dietary factor has an impact on a medical condition requires further clinical trials with better-controlled conditions. Ideally, studies to determine the potential benefit of antioxidant nutrients on disease prevention should be large and be carried out over many years. They should also be placebo-controlled (control subjects receive an inert pill which is indistinguishable from the antioxidant treatment pill) and double-blind (neither the subjects nor the investigators know who is getting the antioxidant or the placebo pills until the results are analyzed).
A number of smaller studies of people with diabetes have shown positive results with antioxidant treatments, while others have shown no benefit. Two Italian studies found that treatment of people with Type 2 diabetes with vitamin E decreased HbA1c, a measure of long-term blood glucose control, and improved markers of heart function. However, Swedish investigators found that two years of antioxidant treatment in children with Type 1 diabetes (beginning at diagnosis) had no effect on blood glucose levels, HbA1c, and insulin doses as compared to similar children receiving placebo treatment. And in a small study in Scotland, people with Type 2 diabetes who were given a low-flavonoid base diet enriched with large amounts of onions and tea (which are high-flavonoid foods) were found to have lower levels of oxidative damage to the DNA of white blood cells compared to the same subjects on the diet without the onions and tea.
In smokers and people with cardiovascular disease and/or hypertension (high blood pressure), vitamin C is known to improve the ability of blood vessels to relax and allow increased blood flow. This may be important to people with diabetes since they have a high rate of complications involving the blood vessels. In fact, studies have shown that intravenous infusion of vitamin C improves vascular relaxation in people with both Type 1 and Type 2 diabetes. However, oral consumption of large amounts of vitamin C by people with Type 2 diabetes in separate English and Swedish studies had no such effect. And treatment of 49 people with diabetes in the Boston area with combined vitamins C and E supplements for six months showed improvement in vascular function for people with Type 1 diabetes but not for those with Type 2.