Metabolic syndrome is defined as the presence of at least three metabolic abnormalities including elevated waist circumference, blood pressure, fasting blood glucose, triglycerides, and decreased high-density lipoprotein cholesterol.
Researchers in China who conducted the study say there are many observational studies examining the associations of dietary and circulating vitamin C levels with metabolic syndrome with conflicting results, but no meta-analysis has been done to obtain a conclusion of these studies.
“Affecting 25% of the population in the developed world, metabolic syndrome has been considered an important public health issue in parallel to obesity and diabetes. Although the etiology of metabolic syndrome is not well-understood yet, dietary factors are thought to be involved in metabolic syndrome,” they wrote in Frontiers in Nutrition.
In this meta-analysis, a total of 28 studies from online database were selected, narrowed from the initial 805 articles.
The identified studies were human observational studies and not randomised controlled trials, and they assessed the association between dietary (food) and circulating (serum and plasma) vitamin C level with metabolic syndrome, calculated using relative risk (RR).
These studies comprising about 110,000 subjects and were performed in Asia, Middle East, Europe and West Africa, and South America.
Of the 28 observational studies, 23 studies assessed dietary vitamin C level, and 11 studied circulating vitamin C levels.
Results demonstrated that dietary vitamin C level was inversely associated with metabolic syndrome with an RR of 0.93. An RR greater than 1 typically indicates greater risk of a bad outcome, while a value less than 1 means the risk of a bad outcome is decreased.
Similarly, circulating vitamin C level was inversely associated with metabolic syndrome, with an RR of 0.6.
In animal models, vitamin C has been studied for its association to metabolic syndrome, where it was found to decrease weight gain, blood pressure, glucose, insulin resistance, total cholesterol, triglycerides, and cholesterol.
Vitamin C as an antioxidant is traditionally used to prevent or treat scurvy and known for its immunity benefits.
Researchers say its antioxidant properties help reduce oxidative stress and inflammation which are thought to play a significant role in the pathophysiology of metabolic syndrome.
In human studies, a combination of vitamin C with physical activity, or L-methionine also reported to be beneficial to metabolic syndrome, so they suggest future prospective cohort studies.
“Although vitamin C may be beneficial to the components or complications of metabolic syndrome such as decreased blood pressure, glucose, insulin, insulin resistance, it is not necessarily a reflection of metabolic syndrome prevention,” researchers highlighted.
In this study, the relationship between dietary vitamin C level and metabolic syndrome was only obtained in cross-sectional studies since the number of cohort studies were too small, which may reduce the reliability.
The results may also have been influenced by the seasonal variability of vitamin C levels.
“This is the first meta-analysis of observational studies on the associations of dietary and circulating vitamin C levels with metabolic syndrome. Current evidence suggests that both dietary and circulating vitamin C levels are inversely associated with metabolic syndrome. However, due to the limitation of the available evidence, more well-designed prospective studies are still needed.”
Source: Frontiers in Nutrition
“Vitamin C and Metabolic Syndrome: A Meta-Analysis of Observational Studies”
Authors: Hongbin Guo, et al.