Moderate vitamin E, C, and β-carotene intake reduces type 2 diabetes risk

In a recent study published in Advances in Nutrition, researchers review the effect of vitamins C and E, as well as β-carotene, on the risk of type 2 diabetes (T2D).

Study: Vitamins C, E, and β-Carotene and Risk of Type 2 Diabetes: A Systematic Review and Meta-Analysis. Image Credit: Suriyawut Suriya / Shutterstock.com

About the study

Previously published meta-analyses have discussed the protective effect of β-carotene and vitamin E, not vitamin C, against T2D incidence. Furthermore, these analyses did not focus on randomized controlled trials (RCTs) or dose-response relationships.

In the current review, all relevant information was obtained from Embase, MEDLINE, and the Cochrane Library databases. To investigate the relationships between dietary intake, circulating levels of vitamin C, vitamin E, and β-carotene, and T2D incidence, case-cohort, cohort, nested case-control, and RCT studies were considered. Any study written in languages other than English was not included in the review.

A total of 6,190 articles were obtained in the initial search, 40 of which fulfilled the eligibility criteria and were considered. Both observational and RCT studies indicated low, moderate, or serious risk of bias. 

Diet and T2D

T2Dis associated with β-cell dysfunction and insulin resistance and is one of the most common and globally prevalent metabolic diseases. Several strategies associated with weight loss, diet, and physical activities have been designed to alleviate the risks of T2D.

Adherence to specific dietary patterns, such as the Mediterranean diet, positively reduces the risk of T2D. The Mediterranean diet emphasizes increased consumption of fruits, vegetables, olive oil, and fish, as well as minimal intake of highly processed food and meat-based products. Fruits and vegetables are high in antioxidants, such as vitamin E, vitamin C, and β-carotene, which has many positive biological effects. 

Vitamin C is a water soluble vitamin found in vegetables and fruits and regenerates vitamin E from its oxidized form. Vitamin E is a fat-soluble vitamin found in seeds, nuts, and vegetable oils, which protects cell membranes from lipid peroxidation. Moreover, β-carotene is a provitamin A carotenoid that is often found in fruits and vegetables. 

Antioxidants protect cells from oxidative stress that arises due to the release of free radicals. Oxidative stress increases insulin resistance by inducing inflammatory processes and suppressing insulin signaling.

Several studies have indicated that a high level of dietary antioxidants could alleviate insulin resistance and T2D. Thus, it is important to understand the mechanisms that contribute to the antioxidant properties of each vitamin. 

How do β-carotene, and vitamins C and E affect T2D risk?

Dietary intake of vitamin C, vitamin E, and β-carotene has been found to reduce the risk of T2D; however, this association is non-linear and plateaus at moderate intakes.

Vitamin C is inversely associated with insulin resistance. To date, few studies have determined the effect of vitamin C on β-cell function.

Among the reviewed RCT studies, there was not sufficient data indicating that supplementation of these vitamins reduces the risk of T2D. In contrast to vitamin C and β-carotene, vitamin E supplementation exhibited insulin resistance. 

Nevertheless, a nonlinear dose-response gradient, which indicated that the lowest risk of diabetes was associated with moderate intake of dietary vitamins, was observed. According to Nordic guidelines, the dietary allowance of vitamin C for men and women is 75 mg/day and 90 mg/day, respectively. The recommendation for vitamin E intake is 8-15 mg/day for both men and women. 

These recommendations can be reached through the consumption of half of a red pepper and half of a cup of almonds for vitamins C and E, respectively. Although no reference value has been established for β-carotene, an intake of 3-6 mg/day could reduce the risk of chronic disease.

Mendelian randomization (MR) analyses did not indicate the causal relationship between genetically predicted circulating vitamin E, vitamin C, or β-carotene and T2D manifestation. This finding was also supported by RCT data revealing that supplementation of the these antioxidants did not confer additional benefits in reducing the risks of T2D in healthy adults. 

It is possible that vitamin C, vitamin E, and β-carotene function synergistically to inhibit oxidative stress and, therefore, do not exhibit beneficial effects when consumed individually. Individuals who adhere to the Mediterranean diet consume an adequate amount of antioxidants, which act synergistically to reduce the risk of T2D. 

Conclusions

Sufficient, rather than high, intake of vitamin C, vitamin E, and β-carotene prevents the risk of T2D in healthy adults. Therefore, these vitamin supplements should not be used as a preventive strategy to reduce T2D in healthy individuals.

However, future studies are needed to identify threshold antioxidant levels for smokers and those with genetic susceptibility to diabetes to obtain optimal benefits.