Vitamin D is an essential vitamin that occurs in very few foods, but the human body produces it when exposed to the ultraviolet B (UVB) rays that we get from sunshine.
People can get enough vitamin D from spending time outside, from eating certain foods, especially dairy products, fortified foods, and oily fish, and by taking supplements. Milk usually has Vitamin D added to it.
People who cover up their bodies to avoid sun exposure and those who spend a lot of time indoors or who live in a place where the days are short in winter are more at risk of a deficiency. The amount of Vitamin D you get from the sun depends on where you live in relation to the equator.
Why do we need vitamin D?
Vitamin D is a fat-soluble vitamin that is necessary for a range of functions. It helps the intestine to absorb calcium, which is necessary for healthy bones and teeth. It also contributes to cell growth and boosts the health of muscles, nerves, and the immune system.
The United States (U.S.) Food and Nutrition Board (FNB) recommend that an adult aged between 19 and 50 years should have around 600 International Units (IU) a day of vitamin D.
A deficiency can result from:
- inadequate exposure to the right kind of sunlight
- inadequate intake from foods and supplements
- having darker skin
- having had gastric bypass surgery
- disorders in the gastrointestinal tract that limit absorption
- liver or kidney disorders
- some hereditary disorders
Smog, long winter days and too much time spent indoors can lead to a deficiency.
A deficiency can lead to:
Rickets: a disease in children and infants that interferes with normal bone formation. Bones become distorted, and they bend wrongly because they are soft.
Osteomalacia: softening of the bone caused by demineralization, or loss of mineral, and mainly a loss of calcium from the bone. It affects adults and it can cause severe musculoskeletal pain.
Osteoporosis: reduced bone mineral density and increased bone fragility.
Vitamin D deficiency has been linked to a number of other diseases:
- hypertension, or high blood pressure
- heart failure
- ischemic heart disease
- tuberculosis (TB)
- periodontal disease
- multiple sclerosis (MS)
- chronic pain
- seasonal affective disorder (SAD)
- peripheral artery disease
- cognitive impairment
A lack of vitamin D has also been linked to the following:
However, not all of these findings have been confirmed by research.
There have been many warnings about excessive sun exposure and the increased risk of skin cancer, but some sun exposure is necessary, to boost the synthesis of vitamin D.
The amount of sun exposure varies. People who live in Florida will not need to be in the sun as long as a person in Boston to get the same benefit. The sun is also more intense at higher altitudes, for example, on a mountain.
Experts suggest that for people with a Fitzpatrick skin type III, which is a medium-fair skin, spending 6 minutes outside in the summer and 15 minutes in the winter is enough to produce around 1,000 IU of vitamin D.
A person in Boston with a similar skin type could get the same amount of vitamin D if they spend 6 minutes in the sun at noon in the summer. In winter, however, a person with the same skin types in Boston would need about 1 hour to synthesize 1,000 IU of D.
Older people and those with darker skin may need longer to generate the same about of vitamin D. This is because melanin “competes” for UVB with substances in the skin that trigger vitamin D production. People with darker skin will need longer in the sun to synthesize the same amount of vitamin D.
A tanning bed cannot provide the type of exposure needed to create vitamin D. According to the Skin Cancer Foundation, a tanning bed provides UVA but not UVB. Both types of UV light increase the risk of skin cancer, so using a sun bed increases the risk without offering any health benefits.
UV light cannot pass through glass, so driving or sitting in a window will not provide vitamin D. The early morning sun will not produce enough UVB either. Exposure to the sun at lunchtime is best.
Sunscreen blocks most of the UV rays, including the UVB rays needed to produce vitamin D. While sunscreen is important for preventing skin cancer, spending about 10 or 15 minutes a day without it may be a good idea.
Some sunscreens allow enough UVB rays through to enable vitamin D to synthesize. According to Harvard Health, most people do not use enough sunscreen to block all UVB light completely, anyway.
Where a person is in the world makes a difference to the amount of sun exposure they need. The time of day, time of year, distance from the equator and level of pollution will all make a difference.
The Norwegian Institute for Air Research has produced a sun exposure calculator to calculate how much sun a person needs to get 1,000 IU of vitamin D. The user needs to put in the details of where they are, their skin type, and so on, to find out how many minutes they need.
However, it is important to be aware of the dangers of too much sun exposure. If your skin starts to turn pink, it is probably best to move out of the sun.
Cod liver oil can provide 340 percent of the vitamin D needed in a day.
Other sources of vitamin D include food and supplements.
Vitamin D is available through the diet in the following food items and amounts:
- 1 tablespoon of cod liver oil contains 1,360 International Units (IU), or 340 percent of the recommended daily value (DV)
- 3 ounces of cooked swordfish contains 566 IU, or 142 percent, of DV
- 3 ounces of cooked salmon contains 447 IU, or 112 percent, of DV
- 1 cup of orange juice, fortified with vitamin D, contains 137 IUs, or 34 percent, of DV
- 1 large egg contains 41 IU, or 10 percent, of DV
- 1 ounce of Swiss cheese contains 6 UI, or 2 percent, of DV
Breakfast cereals and milk are often fortified with vitamin D, so they can be a good source.
How much is too much?
It is possible to have too much vitamin D from supplements, but not from sunshine. Too much sunshine, however, predisposes people to skin cancer.
Postmenopausal women who took supplements providing 1,000 milligrams a day of calcium and 400 IU a day of vitamin D have been found to have a 17 percent higher chance of kidney stones over 7 years, compared with women who did not take the supplements.
The FNB recommends a maximum daily level of 4,000 IU for adults aged 19 years and older. A doctor can check the vitamin D levels in a person’s blood, and they may prescribe higher doses.
Excessive sun exposure does not lead to vitamin D overdose, because of the way the skin interacts with sun exposure to produce and degrade the vitamin at the same time.