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Vitamin D

Physiology of vitamin D

Vitamin D is also essential for the development and maintenance of bone, both for its role in assisting calcium absorption from food in the intestine, and for ensuring the correct renewal and mineralization of bone tissue.  The version of vitamin D made in the skin is referred to as vitamin D3 (cholecalciferol), whereas the dietary form can be vitamin D3 or a closely related molecule of plant origin known as vitamin D2 (ergocalciferol).

Sources of vitamin D

Vitamin D is made in the skin when it is exposed to ultraviolet B rays; in children and adults exposure of the hands, face and arms to the sun for as little as 10 to 15 minutes per day is usually sufficient for most individuals. Vitamin D can also be obtained from food, and dietary supplements. Food sources are rather limited, and include oily fish such as salmon, sardines and mackerel, eggs, liver, and in some countries fortified foods such as margarine, dairy foods and cereals.   Some examples of the approximate calcium levels in foods are shown in Table 3.

Recommended vitamin D intake

Because the sun provides a source of vitamin D in varying amounts for different individuals, dietary recommendations for vitamin D are approximate. Many countries advise a dietary intake of 200 IU/day (5 _g/day) for children and young adults, and 400-600 IU/day (10-15 _g/day) for older persons, to augment that derived via sun exposure.  The FAO/WHO2 dietary intake recommendations are shown below. There is as yet no common definition of ‘optimum’ vitamin D status, although there is emerging evidence and expert opinion that the minimum blood level of 25-hydroxyvitamin D that would be optimal for fracture prevention is 70-80 nmol/l (3). To achieve this, an average older man or woman would need a vitamin D intake of at least 800-1000 IU/day (20-25 _g/day), which is approximately double the intake recommended in most countries.

Age group RNI* (IU/d) RNI (μg/d)

Figures based on Western European, American and Canadian data. Source: FAO/WHO: Human Vitamin and Mineral Requirements, 2002.

0-9 years
 200 5
10-18 years
 200 5
19-50 years
 200 5
51-65 years
 400 10
65+ years
 600 15
Pregnancy 200 5
Lactation 200 5

Factors that interfere with vitamin D synthesis

Dietary or supplemental vitamin D increases in importance during the winter months for populations in northern latitudes, and for elderly people who do not go outdoors much and in whom the capacity for skin synthesis of vitamin D is reduced. Use of sunscreen creams, and a greater degree of skin pigmentation, also reduces the amount of vitamin D that is made in the skin. An increasing body of evidence suggests that on a global level, vitamin D deficiency is widespread, even in very sunny countries such as in the Middle East and parts of Australasia (4).

Vitamin D deficiency

In children, severe vitamin D deficiency results in inadequate mineralization of the bone matrix, leading to growth retardation and bone deformities known as rickets.  In adults, the same condition is known as osteomalacia. In industrialised countries, rickets and osteomalacia are relatively rare conditions. However, milder degrees of vitamin D inadequacy are common, and can predispose to osteoporosis. Maintaining adequate vitamin D status during pregnancy is important, as there is some evidence that mothers deficient in 25-hydroxyvitamin D in pregnancy give birth to children with reduced bone mass, which could in turn be a risk factor for osteoporosis later in life (5).

References

  1. Weaver CM, Proulx WR, Heaney R (1999) Choices for achieving adequate calcium with a vegetarian diet. Am J Clin Nutr 70 (Suppl): 543S-48S.
  2. Food Standards Agency (2002) McCance and Widdowson’s The Composition of Foods, Sixth summary edition. Cambridge: Royal Society of Chemistry.
  3. Dawson-Hughes B, Heaney RP, Holick MF, et al. (2005) Estimates of optimal vitamin D status. Osteoporos Int 16:713-716.
  4. Lim SK, Poor G, Benhamou C-L, et al. (2005) Vitamin D inadequacy is a global problem in osteoporotic women. J Clin Densitom 8 (2):239 (abstract).
  5. Harvey NC, Martin R, Javaid MK, et al. (2006) Maternal 25(OH)-vitamin-D status in late pregnancy and MRNA expression of placental calcium transporter predict intrauterine bone mineral accrual in the offspring. Osteoporos Int 17(Suppl. 2):S9 (OC9).
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