Vitamin D deficiency impairs immune regulation and systemic metabolic function in humans.

Study: Effect of vitamin D3 supplementation on vascular and metabolic health of vitamin D-deficient overweight and obese children: a randomized clinical trial.

When kids with low vitamin D took higher doses, their blood sugar got better and their blood pressure dropped, which means fixing low vitamin D helps the body work better — supporting the idea that low vitamin D hurts how the immune system and metabolism function.

Study: Alterations in CD4+ T Cell Cytokines Profile in Female Patients with Hashimoto's Thyroiditis Following Vitamin D Supplementation: A Double-blind, Randomized Clinical Trial.

This study found that giving vitamin D to people with an autoimmune thyroid disease helped calm down their overactive immune system by reducing harmful inflammatory signals and boosting helpful regulatory ones. This supports the idea that low vitamin D can mess with immune function.

Study: Correlation of TNF-α and IL-6 expression with vitamin D levels in insulin-resistant type 2 diabetes mellitus patients: exploring the role of vitamin D in inflammation and disease pathogenesis

People with low vitamin D had more inflammation and worse blood sugar control, suggesting vitamin D helps keep the immune system and metabolism working properly.

Study: VITAMIN D DEFICIENCY AS A MODERN MEDICAL AND BIOLOGICAL PROBLEM: ROLE IN THE FUNCTIONING OF THE HUMAN BODY

When your body doesn’t have enough vitamin D, it can’t fight off sickness as well or manage blood sugar and energy properly. This study shows that low vitamin D is linked to weaker immunity and messed-up metabolism.