Relationship Between Hypovitaminosis C, Obesity, and Features of Metabolic Syndrome—A Narrative Review

This narrative review aimed to review plausible mechanisms for the role of vitamin C (ascorbic acid [AA]) in the maintenance of healthy weight and energy metabolism; examine the evidence for inadequate vitamin C (plasma AA <50 µmol/L), hypovitaminosis C (≤23 µmol/L), and vitamin C deficiency (≤11.4 µmol/L) in the disrupted homeostasis of obesity and metabolic syndrome; and ascertain whether vitamin C supplementation or dietary intervention could potentially treat obesity and the associated features of metabolic syndrome. Vitamin C hypovitaminosis and deficiency are prevalent in developed countries, despite the widespread availability of vitamin C-containing fruit and vegetables and vitamin supplements. Western diets are characterized by highly processed, macronutrient-rich foods, which are deficient in dietary fiber and micronutrients. This contributes to postprandial oxidative stress and gut dysbiosis, leading to profound effects on insulin sensitivity, hyperglycemia, levels of endotoxemia, fatty acid oxidation, adipocyte hypertrophy, and regulation of metabolism and energy balance. The existing in vitro and in vivo preclinical data demonstrate the effectiveness of vitamin C as both a prophylactic and a therapeutic intervention for obesity and metabolic syndrome. The outcomes in human intervention studies are more modest, with improvements in insulin sensitivity, lipid profile, metabolic inflammation, weight, hypertension, gut permeability, and hepatic steatosis. Some clinical studies are limited by the lack of baseline plasma AA concentrations, or the inability to optimize plasma AA in participants with hypovitaminosis C or metabolic syndrome. The addition of vitamin C to physical activity and dietary interventions may improve the efficacy of treatments for obesity and metabolic dysfunction. However, more data are required to understand the synergism between vitamin C supplementation, medical nutrition therapy, adequate exercise, and pharmacological intervention in weight control and metabolic syndrome management.