Vitamin D is an essential nutrient and a deficiency leads to rickets and osteomalacia.
Unlike vitamins that are obtained exogenously from the diet, most vitamin D is produced endogenously from the action of sunlight on the skin. It is estimated that 90 per cent of the required vitamin D comes from exposure to sunlight.
The main functions of vitamin D are considered to be related to maintenance of bone health and calcium homeostasis, but it is emerging that it also has effects on immune function, cellular growth, maturation, differentiation and apoptosis.
These additional functions have led to new research into the role of vitamin D and have brought into debate the need for recommendations. Research from the UK and Canada suggests that vitamin D levels are inadequate or suboptimal across much of the population, with consequences for bone health as well as other diseases.
Currently in the UK there is no recommended intake for vitamin D in the diet for those aged four to 64 years old (except for pregnant and lactating women - see box); it is thought that sunlight exposure between April and September produces enough to last through the winter months.
|Dietary reference values for vitamin D (UK)|
7 months-3 years
Reference nutrient intake (microgram/day)
|** No recommendation as it is assumed enough sunlight exposure occurs in summer months to build adequate stores|
Although advice regarding sun exposure to reduce risk of skin cancer recommends covering up, exposure of face and arms to sunshine for about 20 minutes a day from April to September is thought to be enough.
Dietary intake of vitamin D is low for most people, around 2-4 micrograms a day, and few foods containing the vitamin are consumed regularly in large quantities. Consequently, groups for whom recommendations exist may need to take a supplement.
The Food Standards Agency (www.food.gov.uk) recommends a supplement of 10 micrograms per day for pregnant and breastfeeding women, older people (efficiency of endogenous vitamin D production declines with age and older adults may live increasingly indoors), people of Asian origin, those who always cover their skin (e.g. with a burka) and people who rarely go outdoors.
Others at risk of deficiency who may need to consider a supplement include obese people, those with fat malabsorption conditions (Crohn's disease, coeliac disease, cystic fibrosis) and those taking certain medication (anticonvulsants, colestyramine and colestipol).
For children and pregnant and breastfeeding women, Healthy Start vitamin drops and tablets are recommended to prevent infants and toddlers developing rickets or experiencing fits due to poor vitamin D status (www.healthystart.nhs.uk).
Limited sun exposure
Ethnic groups, especially women, who traditionally dress to limit exposure of the skin are at increased risk of poor vitamin D status. Wearing a burka or hijab may mean the whole body is covered, except for the eyes. High rates of vitamin D deficiency have been observed in the UK among some groups of Asian women.
The problem appears to be particularly significant for women who breastfeed their infants for a long time, and the number of cases of rickets has increased in some areas, including Bradford and Birmingham.
Studies in other countries have found alarming consequences of low vitamin D status in females. Adolescent girls wearing concealing clothing for religious reasons have been found to have a higher incidence of vitamin D deficiency, putting them at greater risk of osteoporosis.1
A study of osteoporotic Lebanese women showed lower levels of vitamin D in muslim women than their Christian counterparts.2
In New Zealand, South Asian women were found to be at high risk of hypovitaminosis D; many cited concerns about skin cancer as reasons for sun avoidance, but an indoor lifestyle also contributed to low vitamin D status.3
Similarly, Arab-American women living in Michigan were found to have low serum vitamin D levels, with the lowest levels among women who wore traditional veils.4
All of these studies point to a role for increased use of vitamin D supplements by some at-risk population groups. Breastfeeding and pregnant women, whose requirements are raised, should be a priority with use of vitamin supplements.
Vitamin D and health
Vitamin D has a vital role in maintaining bone health. A deficiency in vitamin D is known to cause rickets in children and osteomalacia in adults. Even marginal vitamin D deficiency could have devastating effects on bone health, reducing peak bone mass.5
Recently, a number of studies have examined the consequences of inadequate vitamin D status in older adults, especially those who are housebound or institutionalised.
Aspects of cognitive decline have been significantly associated with lower vitamin D status,6 with executive function and attention being affected.
Risk of falls increases with age, with at least one in three people over 65 years experiencing one fall per year needing hospitalisation, and 5-6 per cent suffering a fracture. A recent meta-analysis showed that vitamin D supplements reduced fall risk by around 20-30 per cent.7
Clearly, more studies are needed but it is sensible to ensure vitamin D status is adequate in all older adults, and supplementation may be indicated in many.
Other conditions have been linked to vitamin D status. The potential immunomodulatory role of vitamin D has been demonstrated in animal studies of colitis: vitamin D, combined with steroids, showed a significantly better effect on the intestinal function than steroid treatment alone.
Other epidemiological evidence suggests higher levels of vitamin D are associated with reduced mortality from certain cancers (e.g. colon, breast and prostate), and an inverse relationship has been shown between vitamin D levels and BP.
Low vitamin D status has also been associated with increased risk of rheumatoid arthritis, MS and diabetes.
Finally, a meta-analysis of 18 randomised controlled trials on vitamin D supplementation and mortality showed a small but significant reduction (7 per cent) in all-cause mortality.8
- Dr Phillips is an independent registered dietitian based in Devon
1. Hatun S, Islam O, Cizmecioglu F et al. Subclinical vitamin D deficiency is increased in adolescent girls who wear concealing clothing. J Nutr 2005; 135(2): 218-22.
2. Gannage-Yared MH, Maalouf G, Khalife S et al. Prevalence and predictors of vitamin D inadequacy amongst Lebanese osteoporotic women. Br J Nutr 2009; 101(4): 487-91.
3. von Hurst PR, Stonehouse W, Coad J. Vitamin D status and attitudes towards sun exposure in South Asian women living in Auckland, New Zealand. Public Health Nutr 2009 Online first 4 August.
4. Hobbs RD Habib Z, Alromaihi D et al. Severe vitamin D deficiency in Arab-American women living in Dearborn, Michigan. Endocr Pract 2009; 15(1): 35-40.
5. Cashman KD. Vitamin D in childhood and adolescence. Postgrad Med J 2007: 83: 230-5.
6. Buell JS, Scott TM, Dawson-Hughes B et al. Vitamin D is associated with cognitive function in elders receiving home health services. J Gerontol A Biol Sci Med Sci 2009; 64(8): 888-95.
7. Bischoff-Ferrari HA, Dawson-Hughes B, Staehelin HB et al. Fall prevention with supplemental and active forms of vitamin D: a meta-analysis of randomised controlled trials. BMJ 2009; 339: b3692.
8. Autier P, Gandini S. Vitamin D supplementation and total mortality: a meta-analysis of randomized controlled trials. Arch Intern Med 2007; 167: 1730-7.