Carotenoids, vitamin A, and their association with the metabolic syndrome: A systematic review and meta-analysis.

Metabolic syndrome, which multiplies a person’s risk for type 2 diabetes by five, is characterized by abdominal obesity, insulin resistance, elevated blood pressure, and dyslipidemia. The pathogenesis of metabolic syndrome involves oxidative stress and inflammation, both of which are largely influenced by diet and lifestyle. Some studies suggest that carotenoids and other antioxidants might influence the risk of metabolic syndrome, but these studies have never been systematically reviewed. 

Methods

In a 2019 article published in Nutrition Reviews, researchers systematically reviewed 20-years of evidence on the associations between carotenoids, vitamin A, and metabolic syndrome. A total of 33 studies (published between 1997 and 2017) that compared carotenoids or vitamin A to the outcome of metabolic syndrome were included in the review. Of these, 15 evaluated serum levels as an exposure variable and 18 evaluated dietary intake as the exposure variable.

Serum levels of carotenoids appear to be inversely associated with metabolic syndrome.

Results

Meta-analysis was conducted only on cross-sectional studies that evaluated serum levels of carotenoids or vitamin A in comparison with metabolic syndrome. The pooled analysis from these 11 studies and 29,673 participants showed a clear inverse association between total serum carotenoids and metabolic syndrome (OR – 0.66; 95% CI, 0.56-0.78). Subgroup analyses showed that the inverse relationship was significant for individual carotenoids (especially beta-carotene) but not for retinol or retinyl esters.  

A qualitative review summarized the rest of the 22 studies (those that were not cross-sectional or that evaluated dietary intake as the exposure variable). It is noteworthy that the only randomized controlled trial of long-term antioxidant supplementation found that 7.5-years of antioxidant supplementation did not affect the risk of metabolic syndrome. However, that study showed that baseline serum antioxidant concentrations were associated with the risk of metabolic syndrome, which is consistent with the results of the meta-analysis here. 

Conclusions

Overall, the authors of this review and meta-analysis concluded that serum levels of carotenoids are inversely associated with metabolic syndrome, but the role of retinol requires further study. Given that many adults consume suboptimal levels of dietary carotenoids (as little as 1-5 mg of beta-carotene per day in a typical Western diet), more research is needed to determine and establish a recommended daily intake. 

Beydoun, M.A., Chen, X., Jha, K., Beydoun, H.A., Zonderman, A.B., & Canas, J.A. (2019). Carotenoids, vitamin A, and their association with the metabolic syndrome: A systematic review and meta-analysis. Nutrition Reviews, 77,  32-45.