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The sources and functions of vitamin D

Alison Wall
BSc
Health Visitor and Public Health Nurse
Child Protection Lead
Watford and Three Rivers PCT

Vitamin D is an organic steroid known for its beneficial effect on bone health. There are two sources of vitamin D - sunlight, or ultraviolet B rays (UVB), and certain foods, such as:

  • Margarine.
  • Oily fish, such as salmon and mackerel.
  • Eggs.
  • Cheese.
  • Liver and meat.
  • Some fortified cereals.

In the UK, we obtain about a quarter of our required intake of vitamin D through the diet. Oily fish is the best dietary source - 90g of salmon provides 425IU (international units) of vitamin D, while the same weight of herring provides 765IU, and 90g of canned sardines contributes 255IU to the diet. We obtain most vitamin D from margarine, which is fortified by law. Certain cereals are also fortified, although some companies have recently discontinued supplementation to comply with other EU countries. Only groups who consume at least three portions of marine fish a week, such as the Inuit, consume an adequate amount of vitamin D.

The role of sunlight
Most of our vitamin D comes from sunlight. However, there is a problem, as we do not receive enough sunlight in the winter months to manufacture vitamin D, and increasing levels of air pollution block its transmission. It is only between April and October that the sun's rays are near enough to have the desired effect, as we live so far from the equator. The sun's rays do not penetrate during winter in northern Europe.(1)
Oliver Gillie has written extensively on the dilemma of avoiding sunburn on the one hand without depriving ourselves of the opportunity to manufacture the vitamin on the other.(1)

Functions of vitamin D
Vitamin D, also known as calciferol, is a fat-soluble vitamin that can be found in two forms, known as D2 (ergocalciferol) and D3 (cholecalciferol). Armas et al undertook a prospective cohort trial using both to measure their potency over a 28-day period.(2) They found that D2 potency is approximately one-third that of D3, and that D2 has a much shorter duration of action, breaking down more quickly than D3. Health workers need to be aware of this when making prescribing decisions.
It has been generally known and accepted that vitamin D promotes the absorption of calcium and phosphate from the diet, which is essential for the formation of bones and teeth. However, it seems to have a complex range of functions. The main cause of rickets is the failure to convert calcidiol to calcitriol when deficiencies of vitamin D exist.
Wharton and Bishop recommend supplementation in the second and third trimesters of pregnancy as a preventive measure.(3) They also note that breastmilk in northern Europe is generally deficient in vitamin D.
More recent research has uncovered the vital role that vitamin D plays in apoptosis (programmed cell death).  Vitamin D lowers insulin resistance, which is one of the key features linked to heart disease, and influences cell growth, repair and destruction of cells that are worn out. It has been linked to protection against a range of cancers, including breast, colon, ovary and prostate in particular. An enzyme in breast tissue appears to convert vitamin D into the cancer-fighting chemical calcitriol.
Vitamin D is also used by the parathyroid gland to regulate calcium levels in the blood. Calcium is linked to blood pressure stabilisation, although the mechanism is not completely understood.
Mental illness may also be influenced by vitamin D status in pregnancy. Studies have looked at the possibility of vitamin D deficiency compromising brain growth in utero. The way that vitamin D influences cell growth and the immune response may be linked in some way with multiple sclerosis.(1,4-6)

Dangers of overdose
It is very difficult to overdose on vitamin D. However, if large doses are continually taken then there is a risk of bone resorption, hypercalcaemia and hypercalciuria. The first symptoms are anorexia, nausea and vomiting. Symptoms such as polydipsia (excessive thirst leading to excessive fluid intake), weakness and pruritus follow later. Treatment of overdose involves discontinuing the vitamin, eating a low-calcium diet, and using steroids. There is one reported case from the USA where a two-year-old child had an unintentional overdose from an imported supplement.(7)
The US and the UK advise different daily requirements. The US Food Standard Agency states that taking a supplement of up to 1,000IU is not likely to cause harm, whereas the UK guidelines quote half this amount (see Table 1).

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Recent findings
Research published this year shows that vitamin D supplementation in pregnancy can affect long-term bone health and result in a reduction of osteoporotic fractures in children.(8)
There is evidence that suggests that taking calcium and vitamin D supplements in the elderly will increase muscle strength and reduce the risk of falls and fractures.(4)

Complex picture
Researchers are certain that there is a causal link between vitamin D and bone strength,(9,10) and specifically recommend that we expose our skin to the sun for 10-15 minutes daily without sunscreen. Black skin is less efficient at manufacturing vitamin D than white skin, up to six times less, which is a real issue for ethnic minorities in the UK. Gillie recommends 20 minutes' exposure, three times per week, for white-skinned people, while darker-skinned people should expose their skin for two hours, three times a week.(1) The picture is complex, as currently we follow the Sunsmart advice issued by cancer research, advising caution in the sun, covering up, use of sunscreen and avoidance of midday sun. Suncream with a UV of factor 8 or above will block the UV rays.
In Australia they promote the "slip, slop, slap" slogan, where you are advised to slip something on to cover the skin, slop on sunscreen in generous and sufficient amount and slap on a hat. The climate in Australia, however, is very different from the UK climate. We know that any sun in the winter months is not strong enough to cause the manufacture of vitamin D, and yet we have no health messages about extra supplementation in the winter.

Taking supplements
Supplementation of foods involves agreements with other EU countries, which can take time and requires consistent political will and conviction.
We are tasked to reduce health inequalities, so should we target resource and attention on ethnic minorities, babies and the elderly who appear to have the highest risks as regards vitamin D deficiency? Supplements are estimated to cost about 4p per day, which is not prohibitive. Multivitamin supplements for children provide an average of 200IU (5μg) per day, and supplements for adults twice that amount.
NICE guidelines outline good practice for the elderly and falls prevention - a routine supplementation with calcium and D3 for all elderly people living in residential and sheltered accommodation, as there is evidence that vitamin D reduces fracture risk.(10) RCP guidelines include calcium and vitamin D3 supplementation for the housebound elderly.(11) Research by MacLaughlin stresses that ageing skin takes longer to produce vitamin D, so longer exposure to UVB is required.(12)
The message has to be balanced against the dangers of sunburn, as the evidence points to a clear increased risk of skin cancer with burning. We need to focus attention on the physical and psychological benefits of the sun - exposure without burning.
Government policy is based on the UK Consensus Statement issued in 1994.(6) A new public health policy is needed, and old messages need to be dropped. It could well be that our good health in the future will depend on a new message about achieving balance, and that we rely on public health to get the message right.

References

  1. Gillie O. Sunlight robbery: health benefits of sunlight are denied by current public health policy in the UK. London: Health Research Forum Occasional Reports; 2004.
  2. Armas LG, Hollis BW, Heaney RP.  Vitamin D2 is much less effective than vitamin D3 in humans. J Clin Endocrinol Metab 2004;89(11):5387-91.
  3. Wharton B, Bishop N. Rickets. Lancet 2003;362:1389-400.
  4. Freedman DM, Dosemeci M, McGlynn K. Sunlight and mortality from breast, ovarian, colon, prostate, and non-melanoma skin cancer:a composite death certificate based case-control study. Occup Environ Med 2002;59:257-62.
  5. Royal College of Physicians. Osteoporosis. Clinical guidelines for prevention and treatment. London: RCP; 1999.
  6. UK Skin Cancer Working Party, British Association of Dermatologists. Consensus statement on sunlight and skin cancer. 1992.
  7. Barretto F, Wang-Flores HH, Howland MA, Hoffman RS, Nelson LS. Acute vitamin D intoxication in a child. Paediatrics 2005;116:453-6.
  8. Javaid MK, Crozier SR, Harvey NC, et al. Maternal vitamin D status during pregnancy and childhood bone mass at age 9 years: a longitudinal study. Lancet 2006;367:36-43.
  9. Garland CF, Garland FC, Shaw EK, et al. Serum 25-hydroxyvitamin D and colon cancer: eight year prospective study. Lancet 1989;ii:1176-8.
  10. Ness AR, Frankel SJ, Gunnell DJ, Smith GD. Are we really dying for a tan? BMJ 1999;319(7202):114-16.
  11. National Institute for Clinical Excellence. Falls: the assessment and prevention of falls in older people. Clinical Guideline 21. London: NICE; 2004.
  12. MacLaughlin J, Holick MF. Aging decreases the capacity of human skin to produce vitamin D3. J Clin Invest 1985;76:1536-8.

Resources
Cancer Research UK Sunsmart
W:www.cancerresearchuk.org/sunsmart
Cancer Research UK News
W:www.cancerresearchuk.org/news