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Liu S, et al. Long-term beta-carotene supplementation and risk of type 2 diabetes mellitus: A randomized controlled trial. JAMA 1999;282:1073-1075.
Recent data suggest a protective role of carotenoids in the development of type 2 diabetes mellitus (DM), possibly via an antioxidant effect, but no randomized trial has directly assessed the efficacy of beta-carotene to prevent type 2 DM.
We enrolled a total of 22,071 healthy U.S. male physicians aged 40-84 years in a randomized, double-blind, placebo-controlled trial from 1982 to 1995. Subjects received beta-carotene (50 mg on alternate days) or placebo. More than 99% of the participants had complete follow-up of median duration 12 years.
A total of 10,756 subjects were assigned to beta-carotene and 10,712 were assigned to placebo. Incidence of type 2 DM did not differ between groups; 396 men in the beta-carotene group and 402 in the placebo group developed type 2 DM (relative risk 0.98; 95% confidence interval, 0.85-1.12). The lack of association between beta-carotene supplementation and incidence of type 2 DM persisted despite multivariate adjustment. There was no evidence of benefit when the period of risk was subdivided into years of follow-up or increasing duration of treatment.
In this trial of apparently healthy men, supplementation with beta-carotene for an average of 12 years had no effect on the risk of subsequent type 2 DM.
Most of the data we have about carotenoids (a category which includes the carotenes, lycopene, cryptoxanthin, and lutein) come from extrapolating backward from brightly colored fruits and vegetables. Findings from the Third National Health and Nutrition Examination Survey (Ford ES, et al. Diabetes mellitus and serum carotenoids. Am J Epidemiol 1999;149:168-176), for example, showed that among 1,665 subjects, serum beta-carotene level and degree of glucose intolerance were directly and linearly correlated.
The investigators matched their groups clinically: Even their body mass indices were identical at baseline (24.9 Kg/m2). Reported adherence to the beta-carotene regimen was 85% after five years and 78% after 12 years.
In this re-analysis of the Physicians’ Health Study, the equivalent of two carrots a day (according to the authors’ other work, higher than the average American consumption) of beta-carotene did not prevent diabetes. But these subjects did not receive carrots—they received supplements. And beta-carotene supplements have been associated with an increased incidence of death from lung cancer and stroke, or no change at all.
Weaknesses of this study include the fact that the participants self-reported their diagnoses, and medical records were not checked. (Diagnosis denial is an effective coping mechanism among many patients until reality intervenes.)
Intensified, concentrated components of food, such as beta-carotene, probably do have a medicinal use. Beta-carotene for diabetes prevention is not one of them. Carrots, mangoes, and habañeros are likely to be better daily medicine.