Evidence check: Vitamin D 25-OH in the context of weight loss

Vitamin D 25-OH is a marker of vitamin D status. Observational studies show that low vitamin D is common in obesity, possibly due to sequestration in adipose tissue or reduced sun exposure. Some research suggests that vitamin D supplementation may modestly aid weight loss when combined with a calorie-restricted diet, but the evidence is inconsistent and effect sizes are small. A 2015 meta-analysis found no significant effect of vitamin D on weight loss. The relationship is likely bidirectional: weight loss can increase vitamin D levels. For individuals with deficiency, supplementation is important for bone health, but expecting significant weight loss from vitamin D alone is not supported by robust evidence. Consumer tests for vitamin D can identify deficiency but should not be marketed as weight loss solutions. Caveat: always consult a physician before supplementing.