Vitamin D Deficiency and Impact on Bone Health
Vitamin D has a crucial role in maintaining normal bone and muscle health, though a significant proportion of the population are deficient. Indeed, in Ireland, 13.1% of the population older than 50 are vitamin D deficient (<30 nmol/l) based on findings from The Irish Longitudinal Study of Ageing (TILDA). In the winter, deficiency affected nearly one in four (24%) with the greatest prevalence (37%) in those aged over 80. Furthermore, about 15% of children and younger Irish adults have been found to be deficient.
Vitamin D is not a nutrient in the traditional sense, but a pro-hormone that is largely derived from cutaneous synthesis after UVB exposure. However, little or no vitamin D synthesis occurs in Ireland between the months of November and March giving rise to the so-called “vitamin D winter”. Dietary sources are limited and contributing only a small proportion to overall levels, but include fortified foods (milks, breakfast cereals), oily fish and eggs. For this reason, low vitamin D status is particularly common in those with limited sun exposure. A significant prevalence of vitamin D deficiency is observed in individuals with darker skin, who require prolonged sun exposure for adequate synthesis due to the presence of melanin. Notably, a study conducted in Ireland found that 67% of Southeast Asians were deficient in vitamin D.
Vitamin D is required for the adequate absorption of calcium and phosphate from the gut. In deficiency, as little as 10% of calcium is absorbed rising to 40% in those who are vitamin D replete. This can result in secondary hyperparathyroidism (SHPT), which is associated with bone loss at cortical sites such as the hip, distal radius and humerus. SHPT also blunts the bone density increases in patients with osteoporosis who are on bisphosphonate or denosumab therapy.
A 25-hydroxyvitamin D level of less than 30 nmol/l is generally considered to represent deficiency, can result in SHPT and when severe causes rickets in children and osteomalacia in adults. There is a risk of vitamin D inadequacy when levels are between 30 and 50 nmol/l, which may be deleterious to bone health, though other factors like calcium and phosphate intake interact with vitamin D and play a role. Indeed, higher calcium intake can partially compensate for lower vitamin D status in reducing SHPT. A recent large study of older Irish adults (n=4139) identified that one third with vitamin D deficiency had SHPT, as did nearly 15% with 25(OH)D levels between 30 and 50 nmol/l. This SHPT was associated with lower bone density at the hip consistent with other studies. Based on the same study, it was estimated that 8.5% of Irish adults (aged ≥50 years) may have SHPT in the Winter. In frailer and older adults, this prevalence is likely to be significantly higher. In one study (n=165), nearly one third (30.1%) of Irish patients admitted to hospital with a hip fracture had high parathyroid (PTH) levels due to low vitamin D status.
For patients with low bone density or a diagnosis of osteoporosis, serum 25(OH)D level should be at least ≥50 nmol/l. In about 5-10% with SHPT and 25(OH)D levels between 50 and 75 nmol/l aiming for a level ≥75 nmol/l is also advised. Indeed, a target level of ≥75 nmol/l is recommended for older adults by some bodies including the American Geriatric Society, particularly for those who are frail and at risk of falls and fractures. Maintaining a 25(OH)D level ≥50 nmol/l is also essential for patients with osteoporosis who are on potent anti-resorptive treatments (e.g. denosumab or zoledronic acid) in order to prevent hypocalcaemia.
In most cases, oral supplementation with 800 to 1,000 IU of cholecalciferol (vitamin D3) daily is sufficient to maintain levels ≥50 nmol/l. However, higher doses may be required if there is poor gut absorption, obesity or liver disease. A variety of vitamin D3 tablets are licensed to treat or prevent deficiency and include daily (800 IU or 1,000 IU), once-weekly (7,000 IU) or once-monthly (25,000 IU). Therapy with 50,000 IU of vitamin D3 once weekly for about 6-8 weeks can be used for more rapid correction of deficiency. More recently, there is the option of supplementing with calcifediol (25 hydroxyvitamin D) which has better gut absorption and results in a faster rise in serum 25(OH)D. This is available as a once monthly (255 mcg) tablet and should be especially considered in those with malabsorption syndromes or significant liver disease.
While the above recommendation applies to adults with low bone density, widespread supplementation of the population is not supported by vitamin D “mega-trials”. However, these included few patients with vitamin D deficiency in whom they were underpowered to examine for differences in fracture rates. However, given the high prevalence of deficiency in the Irish population, a targeted approach has been adopted by the Food Safety Authority of Ireland (FSAI). For fair skinned individuals (aged 12 to 65), daily supplemental vitamin D of 600 IU is recommended during the winter months (end of October to March). For older adults (≥65 years) and individuals with darker skin, supplementation (600 IU daily) is recommended all year round.
It is also important to maintain adequate calcium intake for optimal bone health. In those with low bone density, a calcium intake of up to 1000 mg daily is generally advised (include dietary and supplements). European guidelines also recommend 950 mg daily for adults (≥25 years). Most dietary calcium is derived from dairy products (a portion typically contains 200-250 mg), with three portions recommended per day. However, dairy intake in Irish adults is low at just under two portions daily. Furthermore, the vast majority (97%) of older Irish adults (aged ≥60) in a large study (n=4444) were found to consume less than three dairy portions daily. For those aged ≥65 who consume less than one dairy portion per day, a 500 mg calcium supplement daily is recommended by the FSAI.
Combined vitamin D and calcium supplements may have a modest affect in reducing the incidence of hip or any fractures. However, meta-analyses that examined for fracture outcomes also included patients who had adequate calcium intake and vitamin D status. In particular, the benefit of combined supplements appears greatest in those with poorest vitamin D status and lowest dietary calcium intake. In one large study (n=3270) of ambulatory older adults in nursing homes, supplementing with vitamin D and calcium reversed SHPT and fractures rates at the hip by 43% and at non-vertebral sites by 32%. However, not all patients need supplemental calcium, and an individualised approach should be taken. For those with inadequate dietary calcium, supplements can be used to achieve target intake of about 1000 mg daily.
In conclusion, there is a high prevalence of vitamin D deficiency in Ireland which contributes to secondary hyperparathyroidism, bone loss, and fracture risk, especially among older adults and those with limited sun exposure. Achieving a serum 25(OH)D level of at least 50 nmol/L (and ensuring adequate dietary calcium) can mitigate these risks. Addressing vitamin D deficiency is a significant priority at both the public health and individual health levels, requiring targeted interventions to improve awareness and supplementation.
Written by Dr Kevin McCarroll (Consultant Physician & Geriatrician, Bone Health Unit, St James’s Hospital, Dublin and Dr Donal Fitzpatrick (Consultant Physician & Geriatrician, Mater Misericordiae University Hospital, Dublin)
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