Poor vitamin D status is a global health problem caused mainly by insufficient exposure to sunlight. It is estimated that 1 billion people have vitamin D deficiency or insufficiency worldwide and it is particularly prevalent among elderly people.
Vitamin D exists in two forms—D2 (ergocalciferol), which is obtained from yeast and plants and D3 (cholecalciferol), obtained from the diet through the vitamin D containing and supplements.
However, the primary source of vitamin D3 (80–90% of the body stores) is via exposure to sunlight which causes the ultraviolet irradiation of the precursor molecule 7-dehydrocholesterol in the skin.
Vitamin D (D2 and D3) are then converted by a number of processes in the liver and the kidney to the biologically active form of vitamin D.
While many people are aware of the importance of vitamin for bone health, what is less known is it’s importance for the immune system. Vitamin D is important for innate immunity, this is the way the body prevents the entry and spread of pathogens (including viruses that can cause disease). Vitamin D stimulates the production of powerful substances in the cells that line the respiratory tract and protect the lungs from infection.

Importance of sunlight
Most people meet at least some of their vitamin D needs through exposure to sunlight when UVB radiation with a wavelength of 290–320 nanometers penetrates uncovered skin
Latitude and season affect both the quantity and quality of solar radiation reaching the earth’s surface, especially in the UVB region of the spectrum
In the UK, which is approximately 52-55 degrees N, exposure to sunlight on a cloudless day produces no vitamin D3 from October through March. Further south for example Ibiza at 38 degrees N and Bangkok at 12 degrees N, sunlight effectively produced previtamin D3 in the middle of winter.
In addition, time of day, length of day, cloud cover, smog, skin melanin content, and sunscreen are among the factors that affect UV radiation exposure and vitamin D synthesis.
Complete cloud cover reduces UVB radiation by 50%; shade and smog reduces it by around 60%. UVB radiation does not penetrate glass, so exposure to sunshine indoors through a window does not produce vitamin D. Sunscreens with a sun protection factor (SPF) of 8 or more also block vitamin D-producing UV rays. While the face may be thought of as an obvious exposure site, a significant number of face creams and make up foundations have a high SPF.
The factors that affect UV radiation exposure and research to date on the amount of sun exposure needed to maintain adequate vitamin D levels make it difficult to provide general guidelines. It has been suggested by some vitamin D researchers, for example, that approximately 5–30 minutes of sun exposure between 10 AM and 3 PM at least twice a week to the face, arms, legs, or back without sunscreen usually lead to sufficient vitamin D synthesis
Despite the importance of the sun for vitamin D synthesis, it is prudent to limit exposure of skin to sunlight as UV radiation is a carcinogen responsible for most of the skin cancers cases that occur annually
People most at risk of vitamin insufficiency include:
those with limited sun exposure: housebound individuals, women who cover up for religious reasons and those with occupations that limit sun exposure
People with dark skins as they have greater amounts of the pigment melanin in the epidermal layer result in darker skin and reduce the skin’s ability to produce vitamin D from sunlight
People who are obese as they may need larger than usual intakes of vitamin D to achieve levels comparable to those of normal weight. Obesity does not affect skin’s capacity to synthesize vitamin D, but greater amounts of subcutaneous fat sequester more of the vitamin and alter its release into the circulation.
Supplements and foods
It is recommended that vulnerable groups, see above, as well as breast fed infants, pregnant women and older people take a daily 10 microgram supplement. Although it could be argued that because very few foods naturally contain vitamin D, everyone in the UK should consider a daily supplement especially during winter. Oily fish (such as salmon, tuna, and mackerel) and fish liver oils are among the best sources. Small amounts of vitamin D are found in beef liver, cheese, and egg yolks. Vitamin D in these foods is primarily in the form of vitamin D3. Some mushrooms provide vitamin D2, levels can be enhanced by leaving them exposed to sunlight.
Boost your intake with these delicious oily fish recipes
Tuna steaks with bean salad (serves 2)

150g green beans
400g cannellini beans, drained and rinsed
Half a red onion, slices
12 cherry tomatoes, halved
2 tsp sunflower oil
2 tuna steaks
Handful each of mint and parsley leaves, torn
Juice and zest of a lemon
1 tbsp balsamic vinegar
1 tbsp olive oil

Put the green beans in a large pan of simmering salted water for 1-2 minutes, then drain and refresh in ice-cold water.

Whisk together the lemon juice and zest, vinegar and olive oil to make a dressing. Place the cannellini beans, onion and cherry tomatoes in a bowl. Cut the green beans in half, add to the bowl, pour over the dressing and toss well.

Heat a griddle or frying pan to hot, rub the oil into the tuna steaks and season well with a pinch of salt and freshly ground black pepper. Once the pan is scorching, cook the tuna steaks for 30 seconds on each side.

Toss the mint and parsley with the salad, serve, and top with the tuna steaks.

Mackerel with broccoli and avocado (serves 2)

4 large handfuls of tenderstem broccoli
4 handfuls of rocket
1 avocado, diced
2 fillets of smoked mackerel, diced
8 cherry tomatoes halved
Juice of ½ a lemon
Handful of fresh dill, torn
4 teaspoons toasted mixed seeds
A dash of virgin olive oil
Freshly ground black pepper

Steam the broccoli for about 2 minutes, until tender but still crunchy. Rinse in cold water and chop. Put the rocket, broccoli, avocado, smoked mackerel and tomatoes into a large serving bowl. Squeeze over the lemon juice, add the dill, seeds, olive oil and black pepper and toss lightly together.

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