Vitamin D helps regulate calcium and phosphate needed for bones, teeth and muscles. NHS guidance lists oily fish, egg yolks and some fortified foods as sources, alongside production in sun-exposed skin. Supplement needs depend on age, health and sun exposure. Seasonal UK advice should not automatically be transferred to every climate. Ask about your circumstances instead of choosing a high dose because deficiency is widely discussed. Long-term excess supplementation can raise calcium and damage kidneys and the heart. Supplements do not replace sun protection, and taking one does not establish a diagnosed deficiency.