How Much Vitamin D3 Should I Take in a Day?To prevent vitamin D deficiency, NICE, the National Institute for Health and Care Excellence advise that all adults living in the UK, including people at increased risk of vitamin D deficiency, should take a daily supplement containing 400 international units (IU [10 micrograms]) of vitamin D throughout the year. This is particularly important during autumn and winter.Why do I need Vitamin D?Vitamin D plays a number of essential roles in the body including providing support for immune health, muscle health and strong bones and teeth. Vitamin D can be obtained through food sources such as eggs, fortified cereals and oily fish, but is mainly synthesised in the skin through exposure to sunlight and is often referred to as the ‘sunshine vitamin’.Healthy adults should theoretically be able to get enough Vitamin D from being outside in the sunshine, however, Vitamin D deficiency is common throughout the UK. This is as a result of a low level of exposure to sunlight, making Vitamin D synthesis difficult during the winter.