A significant number of Australians are deficient in vitamin D. Indeed, it is a fallacy that Australians receive adequate vitamin D from casual exposure to sunlight. Here, we outline the causes and outcomes of vitamin D deficiency, the people who are at risk, and recommendations for management of deficiency.
The term “vitamin D” encompasses two molecules:
Cholecalciferol (vitamin D3), formed in the skin through the action of ultraviolet (UV) light on 7-dehydrocholesterol to produce cholecalciferol.
Ergocalciferol (vitamin D2), produced by UV irradiation of the plant steroid ergosterol. Ergosterol is the major form of supplemental vitamin D currently available in Australia.
Vitamin D3 and D2, made in the skin or ingested, are transported to the liver and metabolised to 25-hydroxyvitamin D (25-OHD), the major circulating form. Further hydroxylation occurs in the kidney to form the highly biologically active 1,25-dihydroxyvitamin D (1,25-(OH)2D). This compound promotes:
Exposure to sunlight: For people living in Australia and New Zealand, the main source of vitamin D is through exposure to sunlight. It has been shown that whole body exposure to 10–15 minutes of midday sun in summer (about 1 minimal erythemal dose [MED], or the amount of sun exposure which just produces a faint redness of skin) is comparable to taking 15 000 IU (375 μg) of vitamin D (cholecalciferol) orally.2 On this basis, exposure of hands, face and arms (around 15% of body surface) to around 1/3 MED should produce around 1000 IU of vitamin D (cholecalciferol). The amount of sun exposure to produce 1/3 MED varies with latitude, season, time of day and skin type (Box 1).
Box 1
Recommended sun exposure times (minutes) which result in 1/3 MED for people with moderately fair skin* at different times of day
Less vitamin D is synthesised in winter, in those who have dark skin or are older, and in those who dress with little potential for skin exposure to sunlight for cultural reasons or for sun protection. Therefore, serum vitamin D levels are lower in winter than in summer.5 Short exposures may be more efficient at producing vitamin D. Once previtamin D3 (a chemical precursor of vitamin D3) and vitamin D3 (cholecalciferol) have been formed, continued exposure to UV results in their degradation to relatively inert over-irradiation products.2
It is therefore prudent to expose hands, face and arms to 1/3 MED of sunlight most days. Box 1 shows approximate exposure times for various regions, months and skin types. But there is a caveat: deliberate exposure to sunlight between 10:00 and 14:00 (or 11:00 and 15:00 daylight saving time) in the summer months is not advised. If adequate sunlight exposure to generate sufficient endogenous cholecalciferol is not possible, then a vitamin D supplement of at least 400 IU (10 μg) per day is recommended.
Diet: Vitamin D3 is found in small quantities in a few foods, such as fatty fish (North Sea salmon, herring and mackerel). Liver, eggs and fortified foods, such as margarine and some low-fat milks, also contain very small amounts of vitamin D.
For most Australians, adequate vitamin D is unlikely to be achieved through diet alone. Intake is estimated as 2–3 μg/day (< 100 IU). Recommended dietary intakes are currently under review in Australia. Daily reference intakes for vitamin D devised by the Food and Nutrition Board of the Institute of Medicine of the United States National Academy of Science6 are shown in Box 2. These are similar to the recommendations of the Food and Agriculture Organization of the United Nations (FAO).7
Box 2
Recommended adequate intake (AI) for vitamin D from the US Food and Nutrition Board6*
Broad-spectrum sunscreens protect the skin against UVA and UVB radiation (wavelengths, 320–400 nm and 290–320 nm, respectively). Radiation in the UVB range is responsible for skin carcinogenesis as well as the conversion of 7-dehydrocholesterol into previtamin D3.2 Glass blocks UVB rays required for previtamin D3 synthesis. Concerns were raised that sunscreen use may cause vitamin D deficiency, especially among older people.2 Yet sunscreens are essential to prevent skin damage with longer periods of sun exposure. For incidental sun exposures (< MED), it may be reasonable to omit the sunscreen for the time required.
Older people who are institutionalised or housebound are at a particularly high risk of vitamin D deficiency. For example, up to 80% of women and 70% of men living in hostels or nursing homes in Victoria, New South Wales and Western Australia were frankly deficient in vitamin D, and 97% had a 25-OHD level below the median value of the healthy reference range.8,9 There also appears to be a significant prevalence of mild vitamin D deficiency in younger adults, particularly during winter.5
Groups for whom low vitamin D levels have been documented include:
Circulating 25-OHD and 1,25-(OH)2D levels decrease with age. This may be a result of age-related factors, such as reduced capacity to produce vitamin D, diminished sunlight exposure,2 reduced intake, decline in renal function, disorders associated with abnormal gut function, or reduced synthesis or enhanced degradation of 25-OHD (Box 3).
Box 3
Causes of vitamin D deficiency
Mild vitamin D deficiency: Defined as serum 25-OHD levels in the range 25–50 nmol/L, mild deficiency leads to increased parathyroid hormone secretion and high bone turnover.
Moderate vitamin D deficiency: Defined as serum 25-OHD levels of 12.5–25 nmol/L,18 moderate deficiency has been associated with reduced bone density,19 high bone turnover20 and increased risk of hip fracture in older people.13,21
Severe vitamin D deficiency: Serum 25-OHD levels of < 12.5 nmol/L, resulting in osteomalacia, are rare in Australia and New Zealand. Patients with severe deficiency may present with bone and muscle pains, weakness and pseudofractures. Bone histomorphometry shows classic findings of thickened unmineralised seams, increased osteoclast surfaces, smearing of tetracycline labels and prolonged mineralisation lag times. Cortical thinning occurs because of secondary hyperparathyroidism.
Vitamin D deficiency is an independent predictor of falls in older women in residential care in Australia.8 It is also linked with falls and fragility fractures in both women and older men.22,23 In Australia, a randomised clinical trial of vitamin D supplementation in non-institutionalised people is required to determine whether falls and fracture can be prevented in this group.
Some calcium supplements and multivitamin preparations contain vitamin D, but the levels of 32–200 IU per tablet are too low to treat vitamin D deficiency.
Halibut or cod liver oil capsules contain 400 IU of cholecalciferol (vitamin D3) per capsule. However, although these are cheap (0.08 cents each), they also contain vitamin A (4000 IU), which may not be beneficial (epidemiological studies suggest that vitamin A excess may be associated with an increased fracture risk24).
The single pure vitamin D preparation in Australia is Ostelin 1000 (Boots), which contains ergocalciferol (vitamin D2) (1000 IU or 25 μg) and costs about 24 cents per capsule.
A larger dose (50 000 IU) of cholecalciferol is available in New Zealand.
Some commercial 25-OHD assays do not measure 25-OHD2 (the vitamin D2 form) as well as they measure 25-OHD3 (the vitamin D3 form) and may not be optimal for monitoring replacement therapy.
Vitamin D is stored in fat and muscle and slowly released, particularly during winter.25 In vitamin D-deficient patients, it is necessary to replete the vitamin D stores. While the daily requirement for vitamin D is 400–600 IU, a larger dose is needed to treat patients with deficiency. As vitamin D is distributed in the body fat compartment, which is larger than the plasma and extracellular fluid compartment, large doses are needed before changes in serum 25-OHD are seen. This may explain the lag time seen with vitamin D supplements before normalisation of serum 25-OHD levels. Vitamin D status should be assessed 3–4 months after commencing treatment. Recommendations for management of vitamin D deficiency states are summarised in Box 4.
Box 4
Recommendations for management of vitamin D deficiency states
High dosages of Ostelin (3000–5000 IU daily for 6–12 weeks) can be used to replete body stores. Indeed, oral doses of 10 000 IU per day have been given without adverse effects for at least 90 days in postmenopausal women.26 Oral dosing can be effective in individuals with malabsorption, unless it is very severe, but higher doses are required.
Higher single doses of 50 000–500 000 IU orally or 300 000–600 000 IU vitamin D3 intramuscularly can effectively treat vitamin D deficiency.18 One or two annual intramuscular doses of 300 000 IU of cholecalciferol have been shown to reverse vitamin D deficiency states.18 Intramuscular preparations are preferable for malabsorption. However, these formulations are not currently available in Australia, and there is concern about inducing hypercalcaemia or hypercalciuria if excessive doses are used. Vitamin D3 appears more effective than D2 in raising serum 25-OHD level.27
Calcitriol (1,25-(OH)2D3) is not appropriate for treating patients with deprivational vitamin D deficiency as it has a narrow therapeutic window, may result in hypercalcaemia or hypercalciuria and does not increase serum 25-OHD levels.
Vitamin D intoxication produces hypercalcaemia (with anorexia, nausea, polyuria, constipation, weakness, weight loss, headache, depression, vague aches, stiffness, soft tissue calcification, nephrocalcinosis, hypertension and anaemia). In severe cases, hypercalcaemia may lead to irreversible renal and heart failure28 or coma and death.
Vitamin D toxicity can be caused by excess oral intake through supplementation,29 but not by prolonged exposure of the skin to UV light.
No clinical or biochemical evidence of toxicity has been noted with doses up to 4 000 IU (100 μg) daily.30 Although there is little evidence for toxicity with high-dose injections (300 000 IU) at intervals of not less than 3–6 months,18 the safety of such supraphysiological doses is yet to be established.
The contraindication for administration of vitamin D is hypercalcaemia.
Cod liver oil also contains vitamin A, which can be toxic at high doses.
The greatest benefits of vitamin D supplementation are seen in high-risk vitamin D-deficient individuals with demonstrated decreases in bone density. Increases in bone density from 0–4% in vitamin D-insufficient patients and from 10%–40% in vitamin D-deficient patients with osteomalacia have been reported after optimal treatment with vitamin D.31,32 It is unlikely that vitamin D supplementation has any role in vitamin D-replete individuals. However, the optimal level of serum 25-OHD is unknown; threshold values ranging from 50 nmol/L to as high as 110 nmol/L have been suggested.33,34 Some of the benefit of vitamin D supplementation may relate to improved muscle function and lesser likelihood of falling.
In randomised controlled trials, vitamin D plus calcium reduced the risk of hip and other non-vertebral fractures in older people living in institutions or at home.35,36
A comprehensive meta-analysis suggests that vitamin D reduces the risk of vertebral fractures by 37% with no significant reduction in non-vertebral fractures.37
A recent double-blind randomised placebo-controlled trial of oral cholecalciferol (100 000 IU 4-monthly over 5 years) showed that it reduced the risk of first hip, wrist, forearm or vertebral fracture by 33% in people over the age of 65 years living in the community.38
Adequate dietary calcium is likely to be required together with adequate vitamin D to reduce fracture rates, as most studies have used a combination of vitamin D and calcium supplementation.
Abstract
A significant number of Australians are deficient in vitamin D — it is a fallacy that Australians receive adequate vitamin D from casual exposure to sunlight.
People at high risk of vitamin D deficiency include elderly people (particularly those in residential care), people with skin conditions where avoidance of sunlight is advised, those with dark skin (particularly if veiled), and those with malabsorption.
Exposure of hands, face and arms to one-third of a minimal erythemal dose (MED) of sunlight (the amount that produces a faint redness of skin) most days is recommended for adequate endogenous vitamin D synthesis. However, deliberate sun exposure between 10:00 and 14:00 in summer (11:00–15:00 daylight saving time) is not advised.
If this sun exposure is not possible, then a vitamin D supplement of at least 400 IU (10 μg) per day is recommended.
In vitamin D deficiency, supplementation with 3000–5000 IU ergocalciferol per day (Ostelin [Boots]; 3–5 capsules per day) for 6–12 weeks is recommended.
Larger-dose preparations of ergocalciferol or cholecalciferol are available in New Zealand, Asia and the United States and would be useful in Australia to treat moderate to severe vitamin D deficiency states in the elderly and those with poor absorption; one or two annual intramuscular doses of 300 000 IU of cholecalciferol have been shown to reverse vitamin D deficiency states.