Between the end of March and the beginning of October, most people can produce all of the vitamin D they require through a combination of being exposed to the sun and having a balanced diet.1 But during the autumn and winter months, it is necessary for people to get all of their vitamin D from their diet, because the sun is not strong enough to stimulate production of it in the skin.2
It is, however, very difficult for people to get enough vitamin D from food alone, with only oily fish such as salmon, sardines, herring and mackerel offering any kind of significant source (red meat, eggs and some fortified foods also contain small amounts of vitamin D).1 The relative lack of sunlight compared to the summer months, caused mainly by reduced daylight hours, combined with the difficulty of deriving enough vitamin D from food, mean that seasonal vitamin D deficiency is very prevalent. Indeed, in winter, somewhere in the region of 30 to 40% of the UK population across all age ranges is classed as being vitamin D deficient.3
For this reason, the most recent advice from Public Health England is that all people should consider taking a daily vitamin D supplement during the autumn and winter.1
Sunlight and seasonal vitamin D deficiency
The main source of vitamin D is dermal synthesis3, through which the skin produces vitamin D in response to being exposed to natural sunlight. For dermal synthesis to work properly, light of the so-called critical wavelength has to reach the surface of the skin, but in the UK, this can be unreliable. Light of the correct wavelength cannot pass through glass windows, and, crucially, it is absorbed completely by the atmosphere during the winter months.3 This means that dermal synthesis of vitamin D in winter is not possible for people in countries such as the UK which sit at higher latitudes on the Earth’s surface, because the sun is at too low a position in the sky for the rays to break through.4 Even during the summer months, geographical position can affect a person’s ability to make vitamin D4: there is more critical wavelength sunlight available to drive dermal synthesis in southern parts of the UK than there is in the north, for example.
Seasonal affective disorder (SAD)
Not getting enough vitamin D in winter might not only be damaging to physical health.5 Some studies have suggested that low vitamin D levels might have a link to mental health and seasonal affective disorder, or SAD. One study from the University of Georgia, the University of Pittsburgh and the Queensland University of Technology reported that vitamin D could have a regulatory role in the development of SAD5, as opposed to just being one of many factors. The team behind the research say that, based on their investigations, low vitamin D in winter was likely to contribute to SAD, adding that “there are several reasons for this, including that vitamin D levels fluctuate in the body seasonally, in direct relation to seasonally available sunlight.” Vitamin D is also involved in the synthesis of serotonin and dopamine within the brain, both chemicals linked to depression, according to the researchers.
Getting enough vitamin D in winter
As people won’t be able to produce all the vitamin D they need during the autumn and winter months - and because it still plays a vital role in maintaining health - it is important that the lack of vitamin D produced by dermal synthesis is added to in other ways. The best way of doing this and the main one recommended by the likes of the NHS and the Scientific Advisory Committee on Nutrition (SACN)6 is to take a daily dose of a vitamin D supplement, which will help to prevent deficiency and keep bones, teeth and muscles healthy. For most people, the recommended intake is 10 micrograms1, although a GP may prescribe more in some circumstances. SACN found that musculoskeletal health is put at risk when the blood’s level of vitamin D drops below 25nmol/L6. It concluded that the best way of avoiding vitamin D levels falling below this benchmark was to take a daily supplement. The only other way for the body to obtain further vitamin D is through the diet, although as most foods contain low levels of the vitamin, this can be difficult. Oily fish such as mackerel, herring and salmon contain vitamin D, as do fortified foods which have it added, such as margarine and some breakfast cereals and brands of yoghurt.