Vitamin D and Metabolic Health: What Recent Evidence Shows

Vitamin D status is linked to bone health, immune function, and a range of metabolic processes. In northern latitudes, low sun exposure during winter months and lifestyle factors contribute to widespread insufficiency. This review summarizes recent findings on vitamin D, with emphasis on metabolic outcomes relevant to adults.

Physiology and measurement

Vitamin D is obtained from cutaneous synthesis and dietary intake. It is hydroxylated in the liver to 25‑hydroxyvitamin D, the circulating marker used to assess status, and then converted in the kidneys to the active form, 1,25‑dihydroxyvitamin D. The 25‑hydroxyvitamin D concentration reflects stores over several weeks and guides supplementation.

Prevalence of insufficiency

Insufficiency remains common in Canada and other northern regions, particularly in winter, among people with darker skin, limited outdoor activity, or conditions affecting fat absorption. Obesity is associated with lower circulating 25‑hydroxyvitamin D concentrations, possibly due to volumetric dilution and sequestration in adipose tissue.

Bone health and beyond

The role of vitamin D in preventing osteomalacia and supporting bone mineralization is well established. Interest has grown in cardiometabolic outcomes, including insulin sensitivity, blood pressure, and lipid metabolism. Observational studies frequently link low vitamin D status with higher risk of type 2 diabetes, metabolic syndrome, and cardiovascular disease. However, association does not prove causation. Randomized trials provide a more reliable guide to supplementation benefits.

What trials suggest

Large randomized trials and meta‑analyses show that routine vitamin D supplementation in generally healthy adults does not produce large changes in weight or body fat. Some studies report modest improvements in insulin sensitivity or inflammatory markers among individuals with documented deficiency or specific risk profiles. In people with diabetes, supplementation can have small effects on glycemic markers, but results are heterogeneous. The overall picture suggests that correcting deficiency is reasonable for general health and may provide metabolic benefits in selected groups, while not serving as a primary weight‑loss strategy.

Dosing considerations

Common recommendations for adults fall in the 600–800 IU per day range for maintenance, with higher doses used short‑term to correct deficiency. Many guidelines consider up to 2000 IU per day safe for most adults, though dosing should consider baseline levels, body weight, sun exposure, and dietary intake. Serum 25‑hydroxyvitamin D can be rechecked after three months of supplementation to confirm adequacy. Toxicity is rare at typical doses but can occur at very high intakes over time, leading to hypercalcemia and its complications.

Populations at risk

Groups at higher risk include older adults, individuals with malabsorption syndromes, people with darker skin living at northern latitudes, those with chronic kidney or liver disease, and individuals who rarely go outdoors. In these populations, a proactive approach to testing and supplementation is reasonable. For pregnant individuals and adults over 75, some expert groups support empiric supplementation even without testing.

Practical points

Food sources of vitamin D are limited, which makes supplementation a practical strategy in many cases. Fortified dairy alternatives, fatty fish, and egg yolks can contribute but often are insufficient on their own in winter months. Sensible sun exposure contributes to vitamin D status but must be balanced with skin cancer prevention. Weight loss in individuals with obesity can increase circulating 25‑hydroxyvitamin D concentrations, which is one of several health benefits of sustained fat loss.

Summary

Vitamin D plays a clear role in skeletal health and may influence metabolic outcomes, particularly in those who are deficient. Supplementation is safe at commonly recommended doses and can be tailored to an individual’s baseline level and risk profile. It is not a stand‑alone therapy for weight loss, but maintaining adequate vitamin D is a reasonable part of an overall health plan.

References

Demay MB et al. Vitamin D for the prevention of disease: Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2024. PMID: 38828931.

Office of Dietary Supplements, NIH. Vitamin D Health Professional Fact Sheet. Updated 2025.

Lopez P et al. Resistance training and body fat percentage: meta‑analysis. Sports Med. 2022;52: (PMCID: PMC9285060).

Ribeiro‑Souza AC et al. Vitamin D and glycemic control: systematic review and meta‑analysis (recent).

Hanley DA et al. Canadian review on vitamin D dosing and safety (historical context).