For the UK: https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/ https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-... > During the autumn

For the UK: https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/ https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-... > During the autumn and winter, you need to get vitamin D from your diet because the sun is not strong enough for the body to make vitamin D. Likely applies to many other countries.