Oxidative stress may be implicated in the aetiology of diabetes (Ho & Bray 1999). Carotenoids and vitamins C and E are important components of the human defence system against oxidative stress (Stahl & Sies 1997). Studies in humans on the effects of vitamin E on glucose or insulin metabolism are controversial (e.g. Paolisso et al. 1993; Facchini et al. 2000; Sanchez-Lugo et al. 1997; Ylönen et al. 2003b). Two prospective cohorts have provided evidence for a protective effect of vitamin E in the development of type 2 diabetes in nonsupplement users (Salonen et al. 1995; Mayer-Davis et al. 2002). Fruit and vegetable intake have been inversely related to the development of type 2
diabetes in three cohort studies (Colditz et al. 1992; Feskens et al. 1995; Ford et al. 2001), providing indirect evidence on the putative protective effect of carotenoids. A long-term randomised trial did not show any effect of beta-carotene supplementation on the incidence of type 2 diabetes (Liu et al. 1999). In a long-term cohort serum beta-carotene was not related to the incidence of type 2 diabetes (Reunanen et al. 1998).
Cross-sectional findings on the associations between dietary and serum carotenoids and measures of glucose and insulin metabolism are inconsistent (e.g. Ford et al. 1999; Facchini et al. 2000; Ylanen et al. 2003b). High dietary intake of vitamin C was related to a lower incidence of type 2 diabetes in a long-term cohort study (Feskens et al. 1995). Some minerals such as potassium, magnesium, chromium and zinc may affect glucose and insulin metabolism, but their putative effects on the development of glucose intolerance and type 2 diabetes remain to be elucidated (Costacou & Mayer-Davis 2003).
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