Our bodies do not produce vitamin D, but we need it to regulate the absorption of calcium and phosphorous. Vitamin D helps prevent bone disease like osteoporosis or bone deformities such as rickets. It has also been found to improve several health problems, including fighting off fatigue, depression, and cancer.

We can get the hormone from food sources like fatty fish, cheese, and egg yolks, as well as fortified foods, and, of course, sunlight (exposure to UV light). But how much vitamin D do you need? Individuals who have less than 10 nanograms per milliliter of vitamin D are considered to have a vitamin deficiency and could experience various health problems, from weakness to muscle pain. According to Vox, experts suggest we need our vitamin D blood level to be a minimum of 20 nanograms per milliliter. To achieve this, some people take vitamin D supplements. However, the publication claims that many healthy adults have these levels “without trying.” BBC expands on this point, noting that experts claim that those with healthy levels of vitamin D (which is most of us) do not need to take supplements.

This may be true in the warmer months, but the NHS reports that between October and early March, people in the U.K. do not get enough vitamin D from sunlight. Those who live in the northern hemisphere are especially affected during the winter months with shorter daylight hours and weaker sun. In these months, the sun lacks “sufficient levels of UVB radiation for our bodies to make vitamin D,” Daily Mail reports. This means we need to get more vitamin D through our diets, which is difficult. It is for this reason that the Department of Health recommends “a daily supplement containing 10 micrograms of vitamin D during the autumn and winter,” for adults and children over five.

Individuals can choose not to take supplements between late March or early April, to the end of September, because they would be getting enough vitamin D through sunlight. However, those who do not spend sufficient time outdoors could be at risk of a vitamin deficiency. Those at risk include frail individuals, those in care homes, or those cover their entire bodies with clothes. The NHS also reports that dark-skinned people may not get enough vitamin D from sunlight. These individuals should consider taking the recommended 10 micrograms of vitamin D throughout the year.

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