Researchers at the Centers for Disease Control and Prevention have discovered that people with impaired glucose tolerance or newly diagnosed diabetes also tend to have lower blood levels of the carotenoid antioxidants (alpha-carotene, beta-carotene, cryptoxanthin, lutein/zeaxanthin, and lycopene). There is a growing consensus that glucose intolerance and diabetes are linked to oxidative stress and increased free radical activity. Earlier research has shown that diabetics have low levels of vitamin C and vitamin E and that vitamin E supplementation can help prevent the development of glucose intolerance (a precursor of diabetes) and diabetes. Their study involved 1665 people aged 40 to 74 years. The participants took an oral glucose tolerance test and gave fasting blood samples for analysis of carotenoid content. One thousand and ten (61 per cent) of the study participants had normal glucose tolerance, 277 (17 per cent) had impaired glucose tolerance, 148 (9 per cent) had newly diagnosed diabetes, and 230 (13 per cent) had previously diagnosed diabetes based on World Health Organization criteria.

Analysis of the blood samples showed that people with impaired glucose tolerance had a 13 per cent lower level of beta-carotene (on average) than people with normal tolerance. People with newly diagnosed diabetes had a 20 per cent lower level of beta-carotene than did people with normal tolerance. Lycopene content was 6 per cent lower in the case of impaired tolerance and 17 per cent in the case of newly diagnosed diabetes. The mean level of cryptoxanthin was 23 per cent lower in those newly diagnosed. After adjusting for potentially confounding variables the researchers also found that both baseline fasting and two-hour glucose concentrations were inversely related to beta-carotene and cryptoxanthin concentrations while lycopene concentration was inversely related to the two-hour glucose concentration only. The researchers conclude that a low blood level of carotenoids is associated with diabetes, but caution that more research is required to determine whether diabetes is caused (at least partially) by low carotenoid levels or whether the low levels occur as a result of impaired glucose tolerance or diabetes.
Ford, Earl S., et al. Diabetes mellitus and serum carotenoids: Findings from the Third National Health and Nutrition Examination Survey. American Journal of Epidemiology, Vol. 149, January 15, 1999, pp. 168-76