A systematic review of the interrelation between diet- and surgery-induced weight loss and vitamin D status

Obesity is a major global health problem and has been associated with vitamin D deficiency. Intentional weight loss may alter vitamin D status and, conversely, vitamin D supplementation has been hypothesized to aid in weight loss. A systematic literature search in PubMed/Medline identified 3173 articles of which 37 studies (randomized controlled trials (RCT) [n = 17], non-RCTs [n = 20]) are summarized as effect of: (I) diet-induced weight loss on vitamin D status (n = 7), (II) vitamin D supplementation on diet-induced weight loss (n = 11), (III) surgery-induced weight loss on vitamin D status (n = 15), and (IV) vitamin D supplementation after surgery-induced weight loss on vitamin D status (n = 5). While all studies on the effect of diet-induced weight loss on vitamin D status have consistently reported increased vitamin D levels, the targeted percentage of weight loss that is necessary for an increase has varied between 5% and >10%. N = 11 RCTs testing the effect of vitamin D supplementation observe that vitamin D supplementation does not result in increased weight loss, but may affect body fat loss. Vitamin D deficiency and subsequent hyperparathyroidism have been detected in post-surgery patients, and there is evidence that vitamin D supplementation improves these post-surgery complications. We review the current evidence addressing the role of vitamin D status and supplementation in diet- and surgery-induced weight loss. Subsequently, we highlight gaps in current research and suggest directions for future research including differences in vitamin D supplementation dosages, indoor vs. outdoor exercise, and the assessment of vitamin D status in different body pools.