INTRODUCTION

Vitamin D, the so-called sunshine vitamin, is in fact not really a vitamin but a hormone which the body can make using sunlight. Historically [1], vitamin D deficiency was associated with the childhood disease of rickets characterized by severe growth retardation and the bending or bowing of the legs. Rickets was presumably a product of the Industrial Revolution with a high level of urbanization and child labor resulting in minimal exposure to the sun. Severe vitamin D deficiency also caused some young women to have a deformed pelvis with the resultant difficulty in birthing, which incidentally gave rise to the practice of Cesarean sections. The suggestion that rickets was due to a lack of sunlight was advanced in 1822, but it was not until 1919 that a cure attributed to exposure to radiation from a mercury arc lamp gave strong support to this hypothesis. By the 1930s and 1940s the fortification of food with synthetically made vitamin D was popular. This was long before the photochemistry of the cutaneous (in the skin) production of vitamin D and the biochemistry and action of its metabolites were understood. With the almost complete disappearance of rickets, there was little interest in the possibility of residual or sub-clinical deficiency. Only recently has a serum marker for the vitamin D status been validated, and there has been renewed interest in the possibility of vitamin D deficiency and its implications which is quite recent and is in part due to the modern understanding of the multiplicity of biochemical actions of vitamin D metabolites. Today, research on the role of vitamin D metabolites in health and illness has gone well beyond their role in calcium homeostasis and bone health. They are implicated in cancer prevention, hypertension, rheumatoid arthritis, multiple sclerosis, and early-onset diabetes (type 1).

It is the nature of the human species that most of the vitamin D required is generated by the action of the sun. Natural food sources are very limited and provide only small amounts unless large quantities of oily fish are eaten. Humans are thought to have evolved in equatorial Africa and to have migrated from this area only about 80,000 years ago [2]. The dark skin of our ancestors is thought to have been a protective feature, reducing the destruction of folate by UV light, protecting the sweat glands from damage and increasing reflectivity of solar energy [3,4]. Even though dark skin reduces the efficiency of vitamin D production, our hunter-gatherer ancestors were exposed to very high levels of UV throughout most days. Migrations eventually took groups into the northern latitudes of Europe and Asia where the dark skin was a disadvantage because of reduced vitamin D production due to low winter levels of UV radiation. The lighter skin color associated with those living in northern latitudes that subsequently evolved [3,4] provided better utilization of solar UV and presumably allowed greater buildup of vitamin D stores during the summer months. The effect of severe vitamin D deficiency on both children and females of child-bearing age provides a plausible mechanism for selection and adaptation.

Compelling evidence-based arguments can now be made that many individuals have only marginal serum levels of the critical metabolite of this vitamin, and that deficiency is present in a significant fraction of the world's population [1,5,6]. This deficiency results from inadequate sun exposure and from the failure to eat large enough quantities of dark, oily fish and, where available, fortified foods. This review will examine a number of questions concerning vitamin D, its role in health, the dangers of deficiency, the need for supplements, and the currently expanding appreciation of its importance in biochemical processes other than those related to calcium homeostasis.

SOURCES AND METABOLISM OF VITAMIN D [1,5]

Humans acquire vitamin D from the action of sunlight and from food. The skin contains a cholesterol derivative, 7-dehydrocholesterol (provitamin D), which ultraviolet light (UVB, 290-315 nm) converts to vitamin D which is then either stored in body fat or converted in the liver to 25-hydroxyvitamin D, which we will denote as 25(OH)D. Vitamin D from dietary sources is also converted in the liver to 25(OH)D. Circulating 25(OH)D is converted, mostly in the kidney, to another derivative, 1,25(OH)2D, also called calcitriol, or vitamin D hormone, which regulates serum calcium and phosphorus levels by controlling the intestinal efficiency of absorption. Many tissues and cells in the body have receptors for vitamin D hormone, and it has been recognized for at least two decades that this hormone is a potent inhibitor of cellular proliferation and an inducer of cell maturation. This may have very important implications in connection with the incidence and progression of cancer. Vitamin D hormone receptors are known to exist, for example, in breast, prostate and colon tissue.

There are two forms of vitamin D, D2 and D3. Vitamin D3 is also called cholecalciferol, whereas vitamin D2 is called calciferol or ergocalciferol. The same conversion is used for both to convert from grams to International Units (IU), i.e. 100 IU = 2.5 micrograms (µg). However, these two forms are thought to have different biological activity, with D3 having between 1.7 and 2 times the conversion efficiency to 25(OH)D for approximately equivalent amounts [7]. However, this area remains uncertain and it is common practice not to differentiate between the two forms. Vitamin D2 is not a natural component of human biochemistry but can be manufactured, for example, by UV irradiation of a lipid extracted from yeast. Thus its existence in fortified food and therapeutic prescriptions is mainly for the sake of synthetic convenience. Supplements may contain either form, and sometimes this is not clear from the label. Typical supplement users probably consume 200-800 IU/d.

Fish are the primary natural food source of dietary vitamin D (the D3 form), with 100 grams of herring or salmon providing 1000 IU or 640 IU respectively. A teaspoon of cod liver oil provides about 400 IU, an egg only about 100 IU [5]. For those who consume only limited amounts of these foods, fortified foods and sunlight are the only sources. If one avoids fortified dairy or cereal products, and in addition minimizes exposure to the sun, deficiency becomes a real possibility. Babies who are nourished exclusively by nursing must get their vitamin D from the mother's milk or from sun. Breast milk is a very poor source of vitamin D and if sun exposure is limited, serious deficiencies can develop. A rebound is in fact being seen in the incidence of rickets [8], even in the US. In addition, the fear of skin cancer has promoted the extensive use of sunscreens which essentially eliminates any solar vitamin D generation. A sunscreen SPF of 8 reduces vitamin D3 production by about 98% [1]! To put these numbers in perspective, consider that an adult with white skin wearing a bathing suit generates about 10,000 IU of vitamin D3 in 15-30 minutes when exposed to the summer sun [9]. This is 25-50 times what is in the typical multivitamin. Lengthy sun exposure does not produce toxic levels because vitamin D is also photolabile and as it builds up it is converted (also by UVB) to compounds that do not lead to bioactive metabolites.

It may surprise some readers to learn that in the northern latitudes (>35°-40°N) the amount of UVB in sunlight is low to negligible in the winter months, except at higher altitudes, and contrary to popular belief, sunbathing in the winter in Boston or Edmonton does not generate significant Vitamin D [10]. The same is true in latitudes below about 35°S. Even the sunny French Rivera and Spain have low levels of UVB in the winter. The latitude effect is caused by increased light scattering and ozone absorption due to the tilt of the earth's axis. Thus there is a large and expected seasonal variation of vitamin D status in many populated regions. A number of correlations of latitude with disease incidence have been reported which my be due to vitamin D deficiency [1]

ESTABLISHING DEFICIENCY AS WELL AS HEALTHY LEVELS OF VITAMIN D

To establish daily requirements and the prevalence of deficiency, it is desirable to have a marker, ideally a blood marker. The concentration of vitamin D3 in the blood turns out to be uninformative. The consensus today is that the serum concentration of the metabolite 25(OH)D is the most informative measure of the vitamin status and should be used to define deficiency, sufficiency and perhaps toxicity [11]. Most labs offer this test. Given this consensus on a marker, the challenge is to establish a level below which deficiency exists and a level for optimum health, and to relate these levels to vitamin D intake, both orally and from sun exposure. A number of different approaches have been used.

• The level at which secondary hyperparathyroidism is evident. When 25(OH)D is low, there is a decrease in vitamin D hormone and thus a decrease in calcium absorption and a lower serum calcium concentration. This causes the parathyroid hormone (PTH) serum concentration to increase, and this in turn increases vitamin D hormone production. This keeps the vitamin D hormone concentration nearly constant at the expense of a higher PTH level. This is called secondary hyperparathyroidism (primary hyperparathyroidism generally involves parathyroid gland tumors). Serum calcium may still be within the reference range. The increased PTH level causes increase bone turnover and bone loss. Thus secondary hyperparathyroidism has been proposed as the principal mechanism connecting vitamin D deficiency with the pathogenesis of decreased bone mineral density and the risk of hip fracture in the elderly, and this disorder can also precipitate or exacerbate osteoporosis [6]. As the level of 25(OH)D continues to drop, PTH levels can double or triple. Numerous studies of the relationship between the levels of PTH and 25(OH)D in the blood reveal that at 25(OH)D levels below 50 to 100 nmol/L (nM), the PTH level begins to increase [12-14]. There is no consensus and those trained in the physical sciences would be alarmed at the scatter in some of the data, but it appears that the majority of researchers favor a range between 75 and 100 nM as the threshold below which secondary hyperparathyroidism begins, and this then establishes a threshold for deficiency. Rickets and osteomalacia are seen at levels below about 20 nM, so there is a big gap between the onset of hyperparathyroidism and the level just high enough to prevent what some call the reference disease.
• Keeping in mind our sun-drenched primitive ancestors in Equatorial Africa, some guidance regarding healthy levels of 25(OH)D can be gleaned from the following data based on studies of people living and working in sun-rich environments [14]. Farmers in Puerto Rico were found on average to have levels of 135 nM, whereas lifeguards in St Louis came in at 163 and lifeguards in Israel at 148 nM. Levels over 200 nM have been found in sun-exposed individuals. Those taking vitamin D supplements were excluded from these studies.
• Studies connecting calcium absorption with the serum levels of 25(OH)D in postmenopausal women suggest a level above about 80 nM is desirable [15].
• In two well accepted studies showing fracture prevention with vitamin D and calcium, mean 25(OH)D levels exceeded 100 nM [14].
• In a clinical report [16] on 15 patients with confirmed osteomalacia, the average 25(OH)D level was 13.5 nM with a range of 5-35 nM. PTH was also 3 to 10 times above the reference range.

Other studies could be quoted [1,5,6] but the point is clear that levels of 25(OH)D in the range of 75-100 nM can be justified as probably desirable for optimum health [6,14,17]. Note that there is some variation between laboratories as well as between various assay methods. The results obtained using two commonly used assay methods have been found to differ, on average, by about 30% [18].

THE PREVALENCE OF VITAMIN D DEFICIENCY[19]

There have been a large number of studies concerning the prevalence of low levels of 25(OH)D, some of which are summarized below to provide an indication of the widespread nature of the problem. Studies generally use vitamin D supplementation of 200 IU/d or more as grounds for exclusion, and frequently set a cut-off for deficiency at between 35 and 50 nM 25(OH)D with severe deficiency below 20 nM. There is no general agreement on nomenclature (deficiency, insufficiency, hypovitaminosis D, etc.) or precise cut-off values, but this does not change the picture that emerges.

• In a study [20] of hypovitaminosis D (low levels of vitamin D) in medical inpatients, 290 consecutive patients hospitalized in a general medical service at Massachusetts General Hospital were selected, 150 in March and 140 in September (when the marker would have been expected to be at its maximum). Using 37.5 nM as a cut- off, 57% were found to be vitamin D deficient and of these 22% had serum levels of 25(OH)D below 20 nM. No significant differences were found in the March and September groups. In a subgroup of 77 patients less than 65 years of age without risk factors for hypovitaminosis D, 43% were vitamin D deficient.
• Nesby-O'Dell et al [21] found that 42% of African American women in the US aged 15 to 49 had 25(OH)D levels below 35.7 nM and were described as having hypovitaminosis D.
• Tangpricha et al [22] reported that 32% of healthy young white men and women in Boston aged 18 to 29 were deficient at the end of the winter of 1999, with levels below 50nM.
• Centenarians living in Parma or Mantove, Italy (latitude 43°N) having no acute diseases were studied [23] and it was found that 99 out of 104 had 25(OH)D levels below the sensitivity of the test used (<5 nM).
• Plotnikoff and Quigley recently reported [24] a study of patients presenting with persistent musculoskeletal pain. Elderly patients refractory to the usual therapy had a high prevalence of vitamin D deficiency (<50nM). It is interesting that 90% of the 150 consecutive patients had been evaluated for their persistent muscu1oskeletal pain one year or more before the study and yet none were tested for vitamin D deficiency!
• Vieth et al [25] describe a study involving 796 young women (18-35 years) over one year in Toronto, Canada (latitude 43°N). During this period, the prevalence of low 25(OH)D of <40 nM was 25.6% for non-white, non-black subjects and 14.8% in white women. Of the 435 women studied during the winter half of the year, the prevalence of low 25(OH)D was independent of vitamin D intake up to 200 IU/d.

Many more studies could be listed[1,5,6], but the point is clear. Deficiency appears widespread in all age groups, but especially in the elderly. If a cut-off of 75 nM for 25(OH)D, one threshold suggested above, had been used in these and other studies, the prevalence of deficiency would have been much higher. As might be expected, black skinned individuals have the biggest problem followed by Hispanics [19]. In cultures where most of the skin is covered when the individual is outdoors, significant to severe vitamin D deficiency is routinely found. Finally, it is of considerable interest that 200 IU/d in general had an insignificant effect of serum 25(OH)D levels, and yet this dose is the currently recommended adequate intake for young persons.

VITAMIN D INTAKE TO ASSURE THE ABSENCE OF DEFICIENCY

One might think that the current recommendations for vitamin D intake were designed to ensure adequacy. To quote Reinhold Vieth and Donald Fraser of the University of Toronto [9], "In fact, the current recommendations for vitamin D are not designed to ensure anything. They are simply based on the old, default strategy for setting a nutritional guideline, which is to recommend an amount of nutrient similar to what healthy people are eating." The recommended daily allowance for vitamin D does not in fact as yet exist, and instead recommendations are referred to as "adequate intake" (AI). The AI for young adults was chosen to approximate twice the average vitamin intake reported by 52 young women in a study from Omaha, Nebraska in 1997. The use of the term AI is in fact an admission of the weak nature of the evidence used by the Food and Nutrition Board of the US Institute of Medicine. The current AI for young adults is 200 IU, for adults 400 IU and for the elderly, 600 IU/d. These recommendations assume some input from solar generated vitamin D, but as we have seen, this is highly variable.

There have been a number of studies concerning the relationship between vitamin D intake and serum 25(OH)D levels [14]. To keep the levels of this metabolite above 75-100 nM, a total daily intake of about 4000 IU from all sources is required [14]. This translates into adequate sun exposure in the summer months to maintain high summer levels and build up stores of vitamin D, plus supplements. To avoid undesirably low concentrations of serum 25(OH)D, all adults are encouraged by Vieth and Frazer [9] to take 1000 IU of vitamin D3 per day. In fact, at least 25 studies show that 800 IU/d results in an average 25(OH)D level of <80 nM [14], and Vieth et al [17] found that 1000 IU/d resulted in an average 25(OH)D level of about 70 nM. Increasing the dose to 4000 IU is predicted by Vieth et al [17] to yield an average of about 90 nM. For housebound elderly and others with almost no sun exposure at any season, 1000 IU/d would appear to be well below optimum, and 600 IU appears totally inadequate. In connection with the AI of 400 IU/d, Holick [26] found that even a dose of 600 IU/d was insufficient to maintain normal 25(OH)D levels for nuclear submariners submerged for 3 months. The view that the AIs are unrealistically low and that a daily oral intake of about 1000 IU/d is indicated has been put forward by others as well [6,27-30]. Obviously, the biggest problem for the concerned individual is to balance solar generation and supplementation. Fortunately, virtually unlimited solar generation appears safe, aside from skin cancer considerations.

TOXICITY

The maximum suggested dose currently is 2000 IU/d according to guidelines from the 1997 Food and Nutrition Board. Vieth argues in a reply to a letter by Munro [31] that this is unrealistically low. Toxicity has never been observed in cases where the high circulating 25(OH)D is derived from sunlight, and amounts can reach 235 nM, which is vastly more than 2000 IU could generate. If one sunbathes until the skin just shows a slight pink result, the estimated generation of vitamin D is equivalent to an oral intake of between 10,000 and 20,000 IU [14]. Therapeutic oral doses of 50,000 IU, generally D2, are available by prescription and are used to treat severe vitamin D deficiency. In a French study published in 2001 [32], three oral doses of 100,000 IU each of D3 were administered to male adolescents at the end of September, November and January, an intervention which maintained their March 25(OH)D levels at summer values of about 55 nM, as compared to controls that dropped to 20 nM. No side effects were observed. In a recently reported study [33], 2037 men and 649 women received an oral dose of 100,000 IU of D3 every four months for five years to test the hypothesis that there would be a beneficial effect on the incidence of fractures as well as mortality. Both were significantly reduced and no adverse effects were observed. In the study by Heaney et al [34], up to 10,000 IU/d resulted in no adverse effects, including hypercalcemia, and the subjects were carefully monitored because of the high doses used.

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