Vitamin D is a fat soluble vitamin which can be derived from food sources and sunlight. While it is most widely recognised for its role in skeletal health, epidemiological, laboratory and clinical data have emerged in recent years implicating low vitamin D status in other serious disorders such as auto-immunity, metabolic disease, type 1 and type 2 diabetes, cardiovascular disease and its antecedents, cancer and neurocognitive impairment. 

Recent intake and status studies have suggested that sub-optimal vitamin D status is common in Ireland and other northerly countries, raising concerns that this deficit is a contributor to Ireland’s heavy chronic disease burden. If this is the case, it is important that this issue be addressed by the development and implementation of effective public health policy and clinical guidelines in this area. 

Sources of vitamin D

Physiologically, vitamin D exists in several different forms, the most potent of which is vitamin D3. It occurs naturally in a small number of foods such as eggs, oily fish and liver; and is also found in fortified foods like breakfast cereals and milks. 

The other primary source of vitamin D is sunlight, specifically UVB irradiation at a wavelength of 290-315nm.¹ Although it’s intuitive to assume that oral intake is the main source of vitamin D, this is not really the case. While it is true that many people are now heavily reliant on the vitamin D that they consume in food and supplements, this is actually a relatively recent departure from our evolutionary past. 

Sunlight and vitamin D

Historically, most people at northerly latitudes received the majority of their vitamin D from sunlight exposure during the summer months, accumulated this vitamin D in their liver and fat tissue and then deployed this ‘biological store’ of the vitamin to meet their physiological requirements over the darker winter months. In this scenario, dietary intake really only provided small top-up doses of vitamin D to keep levels from dropping into functionally deficient ranges before they could be replenished the following spring. 

To put the relative contributions of diet and sunlight into context, average oral intakes of vitamin D in Ireland are currently estimated at 4.2µg/d, with 72% of men and 78% of women aged 18 to 64 years reporting average daily intakes less than 5µg/d.² In contrast to this low provision from diet, exposure of the skin to sunlight in summertime has been estimated to generate 70-125µg/d of vitamin D endogenously.³

Unfortunately, recent years have witnessed a sharp decline in summer sunlight exposure and an increased use of high factor sun protection due to legitimate concerns about the increased risk of skin cancer with extended UV irradiation. This has created a situation where our main source of vitamin D has been critically diminished, precipitating a much greater reliance on oral vitamin D intake than at any time in our ancestral past. And because dietary intakes, even among people consuming fortified foods tend to be very low,⁴ the prospect of widespread vitamin D deficiency has risen substantially in recent decades.

Deficiency 

The biological thresholds which define vitamin D deficiency, insufficiency and adequacy remain contentious. While some agencies have suggested that serum 25(OH)D levels of ≥50nmol/l define adequacy,⁵ diversity of opinion persists in this area.⁶ For example, others have contended that the  range 50-80nmol/l should be termed ‘insufficiency’, and that true adequacy is only achieved at serum levels which exceed 80nmol/l or beyond.⁷ It is noteworthy in this context, that humans are the only primate whose serum 25(OH)D typically falls below 100nmol/l, with low-land gorillas, chimpanzees and other non-human primates typically having serum 25(OH)D levels of 110-130nmol/l.⁸

While parathyroid hormone (PTH) release (and hence osteoclastic bone demineralisation) is essentially minimised at serum 25(OH)D levels of 80nmol/l,⁹ it has been suggested that levels above this 80nmol/l threshold are required to provide the extra vitamin D required for optimal control of cell division, cell differentiation and other important metabolic and biochemical pathways pertinent to various chronic diseases.^10,11

Intakes

It has been recognised for some time that vitamin D intakes in Ireland are low.¹² In 2001, analysis of the NSIFCS dataset again highlighted low levels of intake across the Irish adult population.¹³ Subsequent analysis of data from the National Adult Nutrition Survey⁴ revealed that while intakes had risen in the intervening years, consumption of vitamin D remained low in all population groups. 

An important issue when assessing the adequacy of vitamin D intake is selecting an appropriate threshold to define the desired level of intake. Again, this is complicated by the derivation of vitamin D from sunlight, where the physiological requirement from dietary intake is essentially dependent on the adequacy or deficiency of endogenous cutaneous synthesis. This uncertainty is reflected in the recommended dietary allowance for vitamin D in Ireland, which stands at 0-10µg/d for those aged 18 to 64 years,and 10µg/d for those aged 65 years and over.¹⁴

While the Food Safety Authority of Ireland has since published guidance that those aged five to 50 years should take a 5µg/d vitamin D supplement, and that those aged over 50 years should take a 10µg/d supplement,¹⁵ this provision has received little attention in terms of public health promotion and has not been universally adopted or actively endorsed in clinical practice.

Vitamin D toxicity

Part of the conservatism which surrounds the recommendation of vitamin D supplementation arises from the documented cases of vitamin D toxicity described in the literature.¹⁶ It is important to note however, that these cases have almost exclusively arisen from very high oral intakes, usually precipitated by industrial accidents where the vitamin was added to foods at levels at least an order of magnitude higher than that intended.^17,18

The reality is that vitamin D toxicity is very difficult to induce by high dietary intakes, or indeed by supplementation with the preparations currently available on the Irish market. In fact, while toxicity has been observed at serum levels as low as 355nmol/l,¹⁹ such intoxication more typically occurs only at levels exceeding 600nmol/l.²⁰ This is much higher than the serum levels achieved through the use of even ‘high-dose’ supplements currently available in this country. 

Further to this issue of toxicity, numerous studies have now yielded data demonstrating the safety of vitamin D supplementation at daily doses at or exceeding 100µg/d.^{11, 16, 21,22} The safety of supplementation is further supported by the fact that blood levels of vitamin D typically rise in a proportionate 1:1 ratio with increases in oral consumption, with each 1µg/d increment in intake generally yielding a 1nmol/l increase in serum 25(OH)D levels.⁷

This suggests that toxicity will not arise from supplementation at 5-50µg/d; a point supported by the fact that vitamin D intoxication from sunlight exposure (which produces significantly more vitamin D than these supplemental doses), has not been described in the literature.²³

Vitamin D status in Ireland

Analyses of Irish data several years ago indicated that vitamin D insufficiency was widespread in the Irish adult population, especially amongst older adults and women.^24,25 Subsequent analysis of data from the National Adult Nutrition Survey has shown that low serum vitamin D levels are endemic in the Irish population, with 40% of adults falling below 50nmol/l, and 76% recording 25(OH)D levels less than 75nmol/l.²⁶ Modelling studies in this area have estimated that the oral dose of vitamin D required to maintain year-round serum 25(OH)D concentrations of > 37.5, > 50, and > 80nmol/l in 97.5% of the Irish adult population are 19.9, 28.0, and 41.1µg/d, respectively.²⁵

While early studies clearly implicated vitamin D deficiency in the pathogenesis of osteoporosis,²⁸ more recent work in this area has highlighted the possible role of low vitamin D status in multiple serious disorders²⁹ including auto-immune disease,³⁰ diabetes,³¹ cardiovascular disease,³² cancer³³ and neuro-cognitive disorders.³⁴ More importantly, recent studies have consistently reported an association between low vitamin D status and all-cause mortality.^35,36

Ireland continues to have higher rates of osteoporosis, diabetes, cardiovascular disease, cancer and other chronic degenerative disorders than most other countries. These are multi-factorial conditions with complex aetiologies but the epidemiological, clinical and laboratory data implicating low vitamin D status in these disorders, as well as our high prevalence of sub-optimal vitamin D status suggest that addressing this issue in the clinical and public health context could yield a significant health dividend for the Irish population. 

While the issue of food fortification remains under review,³⁷ there is much that can be achieved now by the strengthening, promotion and implementation of the vitamin D supplementation guidelines which already exist in this country. 

References on request