Vitamin D has a beneficial effect on bone health throughout life, through several key functions:
- It assists in calcium absorption from food in the intestine [1]Heaney, R.P., et al., Calcium absorption varies within the reference range for serum 25-hydroxyvitamin D. J Am Coll Nutr, 2003. 22(2): p. 142-6.
- It has a downward regulatory effect on parathyroid hormone level [2]Steingrimsdottir, L., et al., Relationship between serum parathyroid hormone levels, vitamin D sufficiency, and calcium intake. JAMA, 2005. 294(18): p. 2336-41.
resulting in reduced bone loss [3]Bischoff-Ferrari, H.A., et al., Fall prevention with supplemental and active forms of vitamin D: a meta-analysis of randomised controlled trials. BMJ, 2009. 339: p. b3692.
[4]Ceglia, L., et al., Multi-step immunofluorescent analysis of vitamin D receptor loci and myosin heavy chain isoforms in human skeletal muscle. J Mol Histol, 2010. 41(2-3): p. 137-42.
- It ensures correct renewal and mineralization of bone [5]Priemel, M., et al., Bone mineralization defects and vitamin D deficiency: histomorphometric analysis of iliac crest bone biopsies and circulating 25-hydroxyvitamin D in 675 patients. J Bone Miner Res, 2010. 25(2): p. 305-12.
- It has a direct stimulatory effect on muscle tissue and thereby may reduce the risk of falling [3]Bischoff-Ferrari, H.A., et al., Fall prevention with supplemental and active forms of vitamin D: a meta-analysis of randomised controlled trials. BMJ, 2009. 339: p. b3692.
- It increases bone mineral density [6]Bischoff-Ferrari, H.A., et al., Dietary calcium and serum 25-hydroxyvitamin D status in relation to BMD among U.S. adults. J Bone Miner Res, 2009. 24(5): p. 935-42.
The primary source of vitamin D comes from sun exposure: vitamin D is made in the skin when it is exposed to ultraviolet B rays. The type 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).
Very few foods are naturally rich in vitamin D. Food sources include oily fish such as salmon, sardines and mackerel, eggs, liver, and in some countries fortified foods such as margarine, dairy foods and cereals. See list of vitamin D-rich foods.
In children and adults, exposure of the hands, face and arms to the sun for 10 to 15 minutes per day is usually sufficient for most individuals. However, how much vitamin D is produced from sunlight depends on the time of day, geographic latitude, the colour of the skin, the age and use of sunscreen, among other variables. Notably, in the elderly the skin produces 4-times less vitamin D when exposed to the sun, as compared to younger people [7]Holick, M.F., Sunlight, UV-radiation, vitamin D and skin cancer: how much sunlight do we need? Adv Exp Med Biol, 2008. 624: p. 1-15.
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As the half-life of vitamin D is three to six weeks, there is a seasonal peak of vitamin D status in northern latitudes in September, with the lowest point in early spring. In other words, getting sufficient vitamin D during the summer may not secure vitamin D status in the winter months and early spring.
There is concern that the increasingly indoor lifestyles of people of all ages have resulted in low levels of vitamin D in populations around the world [8]Mithal, A., et al., Global vitamin D status and determinants of hypovitaminosis D. Osteoporos Int, 2009. 20(11): p. 1807-20.
, which may be detrimental to bone health. For further information see vitamin D insufficiency below, and the IOF Map of Vitamin D Status.
Vitamin D recommendations from various medical organizations which are focused on the care of patients with or at increased risk of osteoporosis generally recommend higher intakes or higher 25 (OH)D levels than the National Academy of Medicine (NAM) recommended daily allowances for seniors.
|
Age group in years |
Public Intake Recommendations for Vitamin D (National Academy of Medicine - NAM) |
Public Intake Recommendations for Vitamin D (IOF) |
|---|---|---|
|
0-1 |
* |
Not assessed |
|
1-59 |
600 IU/day |
Not assessed |
|
60-70 |
600 IU/day |
800 - 1000 IU/day |
|
71+ |
800 IU/day |
800 - 1000 IU/day |
|
Target 25(OH)D level |
50 nmol/l for bone health at all ages |
50 nmol/l for bone health at all ages |
|
* adequate intake is 400 IU/day |
||
IOF recommends that older adults aged 60 years and over take a supplement at a dose of 800 to 1000 IU/day, as this is associated with greater muscle strength and improved bone health [9]Dawson-Hughes, B., et al., IOF position statement: vitamin D recommendations for older adults. Osteoporos Int, 2010. 21(7): p. 1151-4.
[10]Rizzoli, R., et al., Management of osteoporosis of the oldest old. Osteoporos Int, 2014. 25(11): p. 2507-29.
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In people at risk, vitamin D supplements alone may reduce the risk of fracture and of falling provided the daily dose of vitamin D is greater than 700 IU [11]Bischoff-Ferrari, H.A., et al., A pooled analysis of vitamin D dose requirements for fracture prevention. N Engl J Med, 2012. 367(1): p. 40-9.
. In contrast, studies with large annual doses of vitamin D have reported an increased risk of falls and hip fracture [12]Sanders, K.M., et al., Annual high-dose oral vitamin D and falls and fractures in older women: a randomized controlled trial. JAMA, 2010. 303(18): p. 1815-22.
. Thus, a yearly regimen of vitamin D high-dose supplementation should be avoided.
Both vitamin D2 and vitamin D3 are used in supplements and for food fortification. Vitamin D that is taken orally as a supplement is best absorbed if taken with food as it is a fat-soluble vitamin [13]Chen, T.C., et al., Factors that influence the cutaneous synthesis and dietary sources of vitamin D. Arch Biochem Biophys, 2007. 460(2): p. 213-7.
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Clinical trials have shown vitamin D3 to be more efficient that vitamin D2 in reducing falls and fractures [3]Bischoff-Ferrari, H.A., et al., Fall prevention with supplemental and active forms of vitamin D: a meta-analysis of randomised controlled trials. BMJ, 2009. 339: p. b3692.
[14]Bischoff-Ferrari, H.A., et al., Prevention of nonvertebral fractures with oral vitamin D and dose dependency: a meta-analysis of randomized controlled trials. Arch Intern Med, 2009. 169(6): p. 551-61.
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Given increasingly indoor lifestyles, vitamin D insufficiency is seen as a global problem. An IOF Working Group review of global vitamin D status and determinants of insufficiency found that low levels of vitamin D were highly prevalent among adults, with vitamin D deficiency prevalent even in sunny countries in the Middle East and parts of Australasia [8]Mithal, A., et al., Global vitamin D status and determinants of hypovitaminosis D. Osteoporos Int, 2009. 20(11): p. 1807-20.
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Most vulnerable to vitamin D deficiency are:
- Seniors in general and especially those living in nursing homes or institutionalized care
- Individuals living in northern latitudes with minimal sunshine exposure
- Individuals who have a disease that reduces vitamin D uptake from the intestine (i.e. inflammatory bowel disease)
- Individuals who have a darker skin tone
- Individuals who for medical or cultural reasons cannot expose their skin to the sun
Although this issue is rapidly improving, defining universal diagnostic thresholds of vitamin D status is still complicated due to the lack of standardized testing methods and the variability across population groups. However, as a general guidance, vitamin D deficiency can be defined as a 25(OH)D level of less than 50 nmol/l (< 20 ng/ml), where increased bone resorption and increased parathyroid hormone (PTH) levels have been documented. Levels lower than 25 nmol/l (< 10 ng/ml) are considered severe deficiency, and can induce adverse effects such as rickets in infants and osteomalacia in adults. 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 deficiency in pregnancy will impact on the foetus’ bone status, which could in turn be a risk factor for osteoporosis later in life. See maternal nutrition.
|
< 25 nmol/l (<10 ng/ml) |
Severe deficiency |
|
25-49 nmol/l (10-19 ng/ml) |
Deficiency |
|
50-74 nmol/l (20-29 ng/ml) |
Insufficiency |
|
75-110 nmol/l (30-44 ng/ml) |
Adequacy |
Adapted from Three steps to unbreakable bones: vitamin D, calcium and exercise, IOF, 2011
Vitamin D status can be assessed by measuring 25-hydroxyvitamin D in the blood. Vitamin D supplementation can be recommended even without blood test in osteoporotic patients.
International guidelines recommend this measurement should not be used as a screening tool, and should only be targeted to those at risk for severe vitamin D deficiency (see factors listed above) that may need greater doses of vitamin D to correct deficiency.