Vitamin D and Health: Current Perspectives

Abstract

Throughout the chapter, we will discuss the chemical characteristics of vitamin D, its sources, its metabolism in the liver and kidneys. We will also discuss its endocrine and genomic mechanisms of action. We will describe the roles of vitamin D on the maintenance of calcium phosphate homeostasis, its role on the intestinal absorption of calcium as well as its action at the renal and bone levels and on the feedback exercised at the level of the parathyroid glands. A chapter will be devoted to the reference values of serum 25(OH)D as well as the thresholds used in children and adults. We will describe the techniques for the determination of 25(OH) D. We will describe the intervention studies showing positive effects of vitamin D. Finally, we will cite the epidemiological data on the status of vitamin D with the various determinants linked to hypovitaminosis D. We will end with recommendations in terms of supplementation.

1. Introduction

Vitamin D is a fat-soluble vitamin, considered a pro-hormone (steroid) rather than a vitamin of the secosteroid group due to its structure and functions. It exists in two forms, vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol). Vitamin D or calciferol has two origins: the first exogenous through food (ergocalciferol D2 of plant origin and cholecalciferol D3 of animal origin) and the second endogenous through skin synthesis from a precursor. Vitamin D3 (cholecalciferol) is synthesized by the epidermis from a precursor, 7-dehydrocholesterol or pro-vitamin D 3. Under the influence of ultraviolet rays brought by the sun, which are the major source of vitamin D, ultratviolet rays induce photolysis of 7-dehydrocholesterol into pre-vitamin D3 in the cells of the deep layers of the epidermis. This is then isomerized into vitamin D3 (or cholecalciferol) [1].

In a situation of strong sunlight, the excess pre-vitamin D3 formed is transformed into inactive metabolites.

Vitamin D3 is found in rare food sources of animal origin (marine fatty fish, liver, butter, egg yolk and certain meats (see Table 1) [2].

Foods	Content in μ g/100 g
Cod liver oil	200
Fresh or smoked salmon	15 à 20
Herring	17
Sardine, rainbow trout	10
Tuna	5
Egg yolk	2 μ g/egg
Liver	1.5
Fortified milk	0.75–1
Margarines	5
Butter	0.15 μ g/15 g

Table 1.

Food sources richest in vitamin D.

Vitamin D2 (ergocalciferol) is the vitamin of plants, it is absent in our diet, with the exception of certain mushrooms. It can be used in therapy, is also produced by the pharmaceutical industry Sterogyl).

The term vitamin is inappropriate for vitamin D which should rather be considered as a prohormone.

There are three main sources: exposure to ultraviolet rays (natural or artificial), food and proprietary medicinal products. Ultraviolet B radiation from the sun (whose wavelength is between 290 and 315 nm) is the major source of vitamin D. In a situation of intense exposure, the excess pre-vitamin D3 formed is transformed into an inactive compound (no vitamin D poisoning following excessive and prolonged exposure to the sun) [3].

2. Vitamin D metabolism

Vitamin D is absorbed slowly (about 3 days) throughout the small intestine, associated with bile salts and free fatty acids. Part of the synthesized or ingested vitamin D will be stored in adipose tissue and part released into the bloodstream.

Vitamin D (D2 or D3) is transported in the blood by a carrier protein, vitamin D binding protein (VBP), to the liver to undergo primary processing there.

Vitamin D Binding Protein, the main transport protein for vitamin D in plasma, is able to regulate the release of calcidiol in target tissues and increase its half-life.

In the liver, vitamin D is hydroxylated on carbon 25 by the enzyme 25-hydroxylase to form 25OHD (25-hydroxy-cholecalciferol or calcidiol).

The exact nature of this 25-hydroxylase is not entirely clear, but cytochrome P450 CYP2R1 seems to be the best candidate.

This hydroxylation is not regulated, the greater the amount of vitamin D synthesized or ingested, the greater the amount of 25OH D formed. This allows its use in routine practice to assess a subject’s vitamin D reserves.

Vitamin D is fat soluble and some is stored in fatty tissue. Thus, storage will be greater in obese subjects than in lean subjects, and hepatic production of 25OHD will be reduced, 25OH D circulates in the blood, carried by Vitamin D Binding Protein, with a half-life of around three to 4 weeks [4].

The 25OHD-Vitamin D Binding Protein complex is filtered by the renal glomerulus and undergoes endocytosis in the brush border of proximal tubule cells. Once the DBP-25OH D complex has entered the cell, DBP is degraded and 25OHD is taken up by intracellular binding proteins, the best known of which are IDBP1 and IDBP3.

It is probably the latter that transports 25OH D to the mitochondria where it is hydroxylated on carbon 1 to form 1,25OH2D or calcitriol.

This hydroxylation is catalyzed by another cytochrome P450 enzyme, 1 alpha-hydroxylase (CYP27B1), present on the inner membrane of mitochondria.

A second hydroxylation then takes place in the kidneys under the action of the enzyme 1-alpha-hydroxylase, to give the active form of vitamin D: 1,25(OH)2D (1,25 dihydroxy-cholecalciferol or calcitriol).

This hydroxylation is tightly regulated and stimulated by parathyroid hormone (PTH), by hypophosphatemia or low dietary intakes of calcium or phosphate, and by Insulin Growth Factor (IGF). It is inhibited by FGF-23 and hyperphosphatemia and by 1,25(OH) 2D itself [2].

1,25 (OH) 2D is the active metabolite of vitamin D, its half-life in serum is four hours.

1,25(OH)2D production sites have been identified in the brain, keratinocytes, placenta, prostate, macrophages and osteoblasts that express CYP27B1.

This extra-renal production locally ensures a sufficient supply of 1.25(OH) 2D to target cells [5].

The formation of 1,25(OH) 2D is strictly regulated unlike 25(OH) D, its concentration is 500 to 1000 times lower and is relatively stable.

3. Mechanisms of action of vitamin D

The 1.25(OH)2D can exert:

• “Endocrine” actions: the 1,25(OH) 2D produced by the kidney is transported systemically to its target tissues.

• “Autocrine” actions: many tissues express 1-alpha-hydroxylase as well as Vitamin D receptor (VDR) and 24-hydroxylase. 25(OH)D enters these tissues and is hydroxylated there to 1,25(OH)2D where it acts locally at the cellular level, the excess being metabolized into an inactive compound under the action of 24-hydroxylase.

Whatever the mode of action, the mechanism is then the same.

3.1 Endocrine actions of vitamin D

In the target cell, 1,25(OH) 2D binds to the cytosolic receptor, the Vitamin D Receptor, present in many tissues. This VDR-1,25(OH) 2D complex itself binds to the retinoic acid receptor (RXR) at the cell nucleus. Finally, the RXR-VDR-1,25(OH)2D complex binds to DNA at specific vitamin D response sites [2].

The best known endocrine effect concerns calcium-phosphate and bone metabolism, which will be detailed later.

3.2 Intracrine genomic actions of vitamin D

The second mode of action highlights an intracrine production of 1,25(OH) 2D, which does not participate in phosphor-calcic metabolism but which does have metabolic or even clinical effects. This mode of action would concern many tissues, and 1,25(OH) 2D would control the expression of several genes (300 to 1000), which regulate the proliferation of healthy and cancerous cells, namely their differentiation, their apoptosis and their angiogenesis, also the regulation of genes involved in immunomodulation.

4. Roles of 1,25(OH) 2D

4.1 Classical effects of vitamin D

Maintenance of calcium phosphate homeostasis: Bone contains 99% calcium, circulating calcium constitutes a small part of the body’s calcium. For bone mineralization to be optimal, the calcium-phosphate product must remain stable. Phosphataemia is regulated, on the one hand by the kidney and on the other hand by Fibroblast Growth factor.

FGF-23 which decreases the proximal tubular reabsorption of phosphates and inhibits the synthesis of 1,25(OH) 2D.

4.2 At the intestinal level

The intestinal absorption of calcium takes place mainly at the level of the duodenum and the proximal portion of the jejunum. In the intestinal cell, 1,25(OH) 2D induces the synthesis of a calcium channel, via the transient receptor potential TRPV6 protein, at the level of the luminal brush border and of a calcium transporter protein inside the intestinal cell enterocyte. This active process is preponderant when the calcium or phosphorus intakes are low or in physiological (growth, pregnancy) or pathological (granulomatosis, hyperparathyroidism) conditions where the plasma concentration of 1,25(OH) 2D is high, thus making it possible to increase the fraction of calcium and phosphorus absorbed in relation to the amount ingested. This will ensure an optimal mineral climate for the bone and its mineralization [6].

4.3 At the renal level

The main effect of 1,25(OH) 2D at the level of the proximal convoluted tubule is the negative feedback on its own synthesis with inhibition of 1-alpha-hydroxylase activity and stimulation of CYP24A1 which accelerates its catabolism.

1,25(OH) 2D increases the reabsorption of calcium at the level of the cells of the distal tubule. Its stimulating effect on the tubular reabsorption of phosphates is secondary to the inhibition of PTH secretion [7].

4.4 At the bone level

Vitamin D controls the transcription, differentiation and mineralization of osteoblasts. 1,25(OH)2D also controls the expression of certain proteins such as collagen and osteocalcin which possess VDREs.

Vitamin D stimulates osteoclasts, thereby releasing the minerals contained in the bone matrix. For this, it is first recognized by osteoclasts, which then overexpress the RANK ligand (Receptor Activator of Nuclear Factor-kBLigand). This ligand then binds to its RANK receptor located on the pre-osteoclasts and the union of the receptor to its ligand causes the transformation of the pre-osteoclast into a mature osteoclast. The osteoclasts then move calcium and phosphorus from the bone into the bloodstream and thus increase the calcium phosphate product [8].

4.5 At the level of the parathyroid glands

There is an expression of calcium-sensitive receptors capable of detecting variations in serum calcium, a drop in the latter will lead to an increase in the synthesis and secretion of PTH. 1,25(OH) 2D exerts a negative feedback on the parathyroid glands by inhibiting the synthesis and secretion of PTH. It also exerts a feedback control on the growth of parathyroid cells [9].

4.6 Regulatory mechanisms

The renal production of 1.25 (OH) 2D is finely regulated and stimulated by the parathormone, but also the states of hypocalcemia and hypophosphatemia. To these two factors is added IGF 1.

Renal production of 1,25(OH) 2D is inhibited by Fibroblast Growth Factor 23 (FGF23), hyperphosphatemia and 1,25(OH) 2D itself.

FGF 23 is secreted by bone. It thus performs a negative retrocontrol by reducing the synthesis of 1.25 (OH) 2D, and by directly reducing intestinal phosphocalcic absorption and renal reabsorption.

1,25(OH) 2D decreases its own production in a negative feedback phenomenon.

Finally, there is a vitamin D activation pathway via an enzyme, 24 hydroxylase (CYP-24) which induces the production of inactive compounds (24.25 (OH) 2 vitamin D, 1, 24.25 (OH) 3 vitamin D) then transformed into inactive calcitroic acid [10].

5. Vitamin D status

5.1 Reference values of serum 25(OH)D

The measurement of 1,25(OH) 2D is not appropriate for assessing vitamin D status. It is 25(OH) D which represents the body’s store of vitamin D, which must be measured to know whether or not an individual has a vitamin D deficiency. This observation is now consensual [2].

25(OH)D values are expressed in nmol/L (equal to ng/ml × 2.5).

The 25(OH)D assay method is radioimmunological, it uses antibodies specific for 25(OH)D.

Currently, there is no standard definition of vitamin D status.

Many scientists and experts discuss the fact that the reference values of 25(OH) D are inadequate and low and that the various recommended supplements are insufficient, which has led to a new reflection for the establishment of these reference values [7].

To establish reference values for a biological variable, it must be measured in a large number of healthy people, assumed to be representative of the reference population, then a reference interval is calculated corresponding in general to 95% of the population, and from this interval the lower and upper limits of the biological variable are established.

This cannot be done for vitamin D because, in healthy subjects, the value of 25(OH) D is variable and will depend on the population studied, the sampling season, the latitude and the temperature. Altitude of the place of residence of the subjects, their age and their phototype.

Given this established fact, it is therefore not easy to establish a reference population representative of the general population [10].

The true definition of deficiency, which is currently looming, is the concentration of 25(OH)D below which, in a healthy population, PTH increases statistically.

Lips [11] proposed to define vitamin D insufficiency by concentrations of 25(OH) D for which there may be deleterious effects for health and in particular for bone (secondary hyperparathyroidism and or increased bone remodeling).

JC Souberbielle et al. in 2008 found several approaches used to define 25(OH)D concentrations associated with optimal vitamin D status and therefore to define vitamin D insufficiency [11].

They propose to separate them into several categories.

5.1.1 Define vitamin D insufficiency by the threshold below which there is secondary hyperparathyroidism

Secondary hyperparathyroidism is an increase in PTH secretion to compensate for a downward trend in ionized calcium. One can in fact observe serum PTH within normal limits and have secondary hyperparathyroidism, especially since, for a given PTH assay technique, the reference values can be significantly different depending on the reference population recruited [12].

5.1.2 Vitamin D and the frequency of certain diseases

Many observational studies have been able to show a positive relationship between the low concentration of 25(OH) D and the frequency of certain diseases (diabetes, rheumatoid arthritis, cancers, multiple sclerosis, tuberculosis, cardiovascular events).

Generally, subjects in the upper “quantile” of 25(OH)D (corresponding to a concentration of 25(OH)D > 30 ng/ml) have a lower relative risk than those in the lower “quantile” [13].

These observational studies obviously have a lower place “less evidence-based medicine” than interventional studies. They do not make it possible to establish a causal link between vitamin D deficiency and the various diseases mentioned [14].

5.1.3 Intervention studies showing positive effects of vitamin D

Interventional studies have shown a reduction in the relative risk of developing certain pathologies after vitamin D supplementation.

We will mention only some of them, the others being developed in a later chapter.

• Reduction of the risk of fractures for a 25(OH)D threshold of 75 to 100 nmol/l or 30 to 40 ng/ml [15].

• Reduction of the risk of falls for a 25(OH)D threshold greater than 60 nmol/l [16].

• Reduced risk of cancer [17].

• Reduced risk of falling teeth [18].

• Reduction of bacilliferous character in patients with pulmonary tuberculosis [19].

• Reduction in blood pressure after 3 months of UVB exposure during which 25(OH)D increased from 58 to 151 nmol/L (32.2 to 60.4 ng/ml), or after 1200 mg of calcium and 800 IU/D of vitamin D3 for 2 months during which 25(OH) D increased from 25.7 to 64.8 nmol/l (10.3 to 25.9 ng/ml) [20].

Significant improvement in Langerhans beta cell function and lower elevation of HBA1c in patients at high risk of type 2 diabetes after 16 weeks of supplementation with cholecalciferol (2000 IU/d) (threshold of 25(OH) D passing 24 to 31 ng/ml) [21].

Rather than the term normal or reference values, we will now more often use the notion of “desirable” values, the threshold value of 25OHD defining vitamin D insufficiency (i.e. the minimum concentration that must have) being:

• For Lips in its 2001 review [11]: 50 nmol/L (20 ng/mL)

• For Holick in his 2002 review [2]: 50 nmol/L (20 ng/mL) to avoid secondary hyperparathyroidism and 75 nmol/L (30 ng/mL) to take advantage of other vitamin effects D (anti-cancer for example – see following chapters).

• For Zittermann in his 2003 review [22]: 100 nmoL / L (40 ng/mL)

5.2 The thresholds used: children and adolescents

“Normal” serum 25(OH)D values were defined by the absence of signs of deficiency, rickets in children and osteomalacia in adults. Subsequently, normality was defined by the mean ± 2 standard deviations of the 25(OH) D values recorded in a population of healthy subjects, i.e. from 15 to 55 ng/ml for the European and North American populations [7].

Several vitamin D thresholds have been adopted to define insufficiency and deficiency. There is currently no consensus defining the threshold for vitamin D insufficiency. Some authors recommend the threshold of 30 ng/ml, while others prefer the threshold of 20 ng/ml.

The different definitions currently used are:

• Insufficiency: concentration of 25OH D ≤ 20 or 30 ng/ml (50 or 75 nmol/l)

• Deficiency: a concentration of 25(OH) D ≤ 10 (25 nmol/l) or 12 ng/ml

Thresholds have recently been defined, so-called reference values, varying from one individual to another and from one laboratory to another. Most laboratories set the lower limit at 15 or 20 ng/ml.

According to Zittermann. A [22], the available data could suggest that the sufficiency threshold is 40 ng/ml. On the other hand, Hill et al. [23] determine a threshold of 24 ng/ml in adolescents and cannot set one in adolescents, while Esterlé et al. find a lower threshold at 16 ng/ml [24].

All in all, it is currently not possible to retain a precise point of inflection in children for 25(OH)D values corresponding to maximum suppression of PTH [25].

The significance of PTH variations, during periods of rapid bone growth such as adolescence, may be different from that retained in adults.

Tylavsky et al. [26], in a follow-up of adolescents at the start of puberty (Tanner stage 2) and supplemented with calcium, show that, if PTH is indeed negatively correlated with 25(OH) D, it is also, but in an apparently paradoxical way, positively with Bone Mineral Density (BMD) and bone mineral content (BMC) .

Thus, adolescents with a serum concentration of 25(OH) D at the lower limit of normal (18 ng/ml) and an increased PTH, have a better gain in bone surface area (+8%) and BMC (+11%) than those with a concentration of 25(OH)D at 34 ng/ml.

Absorptiometry studies also make it possible to find the “deficit” threshold at a higher level than the “deficiency” threshold used for rickets. For cortical bone, Toola et al. [27], and Cashmann et al. [28], observed a decrease in BMD respectively for 25(OH)D values <16–18 ng/ml and 20–24 ng/ml. For trabecular bone, on the other hand, some studies do not show any mineralization defect [29], except for very low 25(OH)D concentrations, below 8 ng/ml [30].

Finally, cases of deficiency rickets, documented radiologically, have been described in infants of African origin with serum 25(OH)D concentrations as high as 16–18 ng/ml [31].

In 2012, the Nutrition Committee of the French Society of Pediatrics, during a review, and in the light of knowledge recently acquired in children, retained the threshold of 20 ng/ml as the one defining vitamin deficiency D [32].

In total, a serum concentration of 25(OH) D of 50 nmol/l (20 ng/ml) could, as in adults, be retained as a threshold for vitamin “deficiency” in children and adolescents in below which a mineralization defect may appear. This threshold should be retained because of the possible, undemonstrated, increase in the risk of fractures that it could entail and the obtaining of a too low “peak of bone mass” at the end of growth due to a insufficient bone calcium accretion. This threshold of 50 nmol/l (20 ng/ml) is now used in North America to determine the recommended intakes (RDA) of vitamin D, i.e. those that ensure good bone health at 95.5% of the child and adolescent population, subject to otherwise adequate nutrient intakes, particularly for calcium [33].

Since then, many expert committees have met, benefiting from the expertise of French, English, Swiss and Australian international representatives. These experts mainly advocate a threshold value of 25(OH) D of less than 30 ng/ml. This is obviously not an absolute consensus because some authors stick to the value of 20 ng/ml, others propose a threshold of 40 ng/ml based on sufficient levels of evidence [34].

The latest ESPGHAN report (European society of peadiatrics gastroenterology, hepatology and Nutrtion) of March 2013, recommends the pragmatic threshold of 20 ng/ml defining vitamin D insufficiency in children and adolescents and that of 10 ng/ml to define severe deficiency [35].

6. 1.25(OH)D assay techniques

The 25OH D assay should be considered a routine assay for assessing vitamin D status. This assay is easy to perform but practiced by many laboratories.

The assays available do not all meet the same quality criteria, thus explaining the assay variability found from one laboratory to another.

25OH D assay techniques can be separated into two main families: immunoassays and mass spectrometry-type separation techniques.

Whichever assay method is used, it is important that it recognizes both vitamin D2 (ergocalciferol) and D3 (cholecalciferol) as there are drugs on the market that contain only vitamin D2. The risk is to excessively supplement patients treated with these drugs if the assay kit used only recognizes vitamin D3.

It is necessary to always perform vitamin D assays in the same analysis laboratory because there are several techniques and vitamin D levels can differ significantly from one laboratory to another. Indeed, some measure total vitamin D (D2 and D3) and others still do so only for vitamin D3 [36].

In practice, if the assay is carried out during the day, the sample (decanted) can remain at room temperature. If the assay is performed on another day, it is advisable to store the decanted tube at −20°C.

To date, the results of the DEQAS (International Vitamin D Quality Assessment Scheme) quality control are relatively reassuring, showing that the assay techniques used in most laboratories give results that only differ occasionally and for values between 10 and 40 ng/ml [27].

In a state of vitamin D deficiency, the concentration of the 1,25(OH)2D metabolite will not be low as one might expect, but normal, high or low. 1,25(OH) 2D, even if it is the biologically active form of vitamin D, is therefore not the value to be measured to assess the vitamin status of a subject. It is therefore increasingly recommended to also measure PTH to estimate the reality of a vitamin D deficiency.

Thus, the true definition of hypovitaminosis D corresponds most accurately to the concentration of 25(OH) D below which, in healthy subjects, parathyroid hormone (PTH) increases significantly [4].

7. Epidemiological data on vitamin D status

In adolescents, studies conducted in different populations and at different latitudes have reported a frequent collapse of serum levels of 25-OH vitamin D, especially in winter, as well as a deficiency in food intake, including including in industrialized countries.

7.1 General population

Regardless of the threshold used (20, 30 or 40 ng/ml), vitamin D insufficiency is extremely widespread. The “worldwide” inventory was drawn up by experts from the IOF (International Osteoporosis Foundation) [37].

7.1.1 Europe

In 2008, a prevalence survey of hypovitaminosis D in the Lyon region in winter showed that out of 135 women aged 19 to 49, 94.8% of them had a 25(OH) D level below 30 ng/ml, and 39.3% have a severe deficiency with a level lower than 12 ng/ml [38].

Also in 2008, a similar survey carried out in the Paris region on 50 healthy adults showed that 64% of patients were insufficient at the threshold of 30 ng/ml [39].

In a study carried out in winter in Helsinki (Finland) on 178 adolescent girls aged 14–16 in good health, the prevalence of vitamin D insufficiency was 61.8% for the threshold of 16 ng/ml [27].

7.2 America

Various small population groups have been the subject of work on vitamin D status in Latin America [40].

In the United States, vitamin D status was assessed among hospital staff in Boston in 2002, 32% of staff were found to have a serum 25(OH) D level below 20 ng/ml [41].

A Canadian study carried out in 2007 with a sample of more than 5000 people aged 6 to 79, a vitamin D deficiency (less than 11 ng/ml or 27.5 nmol.l) was found in 4% of population [42].

In 2008, a large-scale study was published, based on the American National Health and Nutrition Examination Survey (NHANES) cohort, carried out between 1988 and 1994 and then 2000–2004. This cohort comes from the general population, including children and pregnant women. A total of 20,289 samples were taken between 2000 and 2004, and 18,158 between 1988 and 1994. The prevalence of hypovitaminosis D is 47% at the threshold of 31 ng/ml. After adjusting for the age of the patients, and the method used to measure 25(OH)D, it appeared that the mean concentrations of 25(OH)D had fallen significantly in 29% of the young girls and 13% of the old boys from 12 to 19 years old and this in winter. The authors attribute this drop to the patients’ increased BMI, increased protection from the sun, decreased milk intake and an overall decrease in outdoor activity [43].

7.2.1 Middle east and Africa

Despite ample sunshine, the Middle East (15°–36°N) and Africa (35°S–37°N) have the highest rates of rickets cases worldwide.

The majority of these cases are explained by limited exposure to the sun, socio-cultural practices and prolonged duration of breastfeeding without vitamin D supplementation in the Middle East [44], by skin color and lack of calcium intake in Africa [45].

About 30–50% of adults in the United Arab Emirates, Turkey, and India have serum 25(OH)D levels below 20 ng/ml [46]. The high prevalence of vitamin D insufficiency may be due to decreased skin exposure to ultraviolet (UVB) radiation, dark skin pigmentation and low vitamin D intake [47]. Other factors such as mutations in the vitamin D receptor (VDR) polymorphism, low daily calcium intake, obesity, and low social status have all been associated with low circulating levels of vitamin D [48].

In Tunisia, Meddeb et al. in 2002 evaluated the vitamin D status in subjects aged 20 to 60 years. The prevalence of vitamin D insufficiency is 47.6% at the threshold of 37.5 nmol/l (15 ng/ml). Insufficient dietary intake of vitamin D, wearing the veil and high parity explain this insufficiency [49].

7.3 In children and adolescents

In Europe, situations of deficiency or insufficiency exist among children, adolescents and young adults. In the United Kingdom, around 3% of children aged 4 to 6, 4% of boys and 7% of girls aged 7 to 10, 11% of adolescents aged 11 to 14 and 16% of boys and 10% of girls aged 15 to 18 year olds have serum 25(OH)D levels <10 ng/ml(49).

Similarly, more than a third of adolescent girls with an average age of 12.5 living in northern European countries (Denmark, Finland, Ireland and Poland) have been shown to have winter rates below 10 ng/ml and that in almost all of them (93%) these levels were below 20 ng/ml [50].

In France, 10 to 40% of children and adolescents have a concentration of 25(OH) D < 10 ng/ml [51].

Also in France, studies carried out on apprentice jockeys aged 13 to 17 showed a clear variation in seasonal levels, which fell on average from 58.5 nmol/l at the end of summer to 8.24 ng/ ml at the end of winter [52], and 68–72% of these adolescents had levels below 10 ng/ml at the end of winter in the absence of supplementation [53].

Several cases of symptomatic rickets in adolescents have also been collected in recent years, mainly in the north of France and in the Paris region [54].

For the authors of these articles, the main reasons for this trend are: lack of sun exposure in hot countries, pigmented skin, the wearing of covering clothing, the decrease in outdoor physical activity, especially among young people, in favor of the time spent in front of the computer or the television, the overall decrease in milk consumption.

In a study carried out in Boston, 42% of adolescents aged 11 to 18 had levels below 20 ng/ml [55].

In children before puberty, the data are fewer and more contradictory. In the US state of Georgia, (located at 34°N latitude), the average serum levels of girls aged 4 to 8 years were normal (37.2 ng/ml, levels being higher in white children than in black children) according to the distinction used in the United States), and less than 4% of them had levels below 20 ng/ml [29]. However, in the state of Maine (located at 44°N latitude), among girls aged 9 to 11 years, 48% had levels below 20 ng/ml [56].

In New Zealand (between 35° and 46° South latitude), 31% of children aged 5 to 14 years had levels below 15 ng/ml [57], but in Australia, in the state of Tasmania (located at 42° South latitude), less than 10% of 8-year-old children had levels below 20 ng/ml [58].

Even in sunny countries like Greece, 47% of adolescents aged 15 to 18 and 14% of 13 to 14 year olds have a concentration of 25(OH)D < 10 ng/ml [59].

Gordon et al. showed in a 2004 North American study that 42% of adolescents living in urban areas had a 25(OH)D level less than or equal to 50 nmol/l (20 ng/ml), 24.1% a level less than or equal to 37.5 nmol/l (14 ng/ml) and in 4.6% of them a level < 20 nmol/l (8 ng/ml) [60].

Even in sunny areas like Beirut, levels below 20 ng/ml were found in 65% of children aged 10–16 years in winter [61].

In northern India (27°N), 90.8% of healthy girls between the ages of 6 and 18 have vitamin D deficiency [62].

In a study from northern China (Beijing), 89% of Chinese adolescent girls had serum 25(OH)D < 20 ng/ml [63].

However, hypovitaminosis D is very common in the Middle East and does not spare the pediatric age [64]. In 2004, Moussavi M et al. studied the vitamin D status of 14 and 18 year olds, 46.2% of subjects had a 25(OH) D level < 20 ng/ml (72.1% of girls and 18.3% boys) [65].

In Saudi Arabia [66], 80% of children have mean serum 25(OH)D levels below 20 ng/ml.

Other proportions have been reported, namely that 32% of Lebanese girls and between 9 and 12% of boys and adolescents have serum 25(OH)D levels below 10 ng/ml [67].

Finally, in Saudi Arabia, Sedrani SH et al. evaluated the prevalence of hypovitaminosis D in children aged 6 and 17 years, 16.8% had a rate < 50 nmol/l (20 ng/ml) [68].

8. Risk factors and determinants of hypovitaminosis D

8.1 Geographical location

The amount of UV that reaches the earth’s surface depends on many factors including the angle of UVB rays relative to the ozone layer and the distance it has to travel through the atmosphere. The intensity of the ultraviolet rays varies according to the latitude, it is maximum at the level of the equator and attenuates with the increase in latitude.

8.2 Altitude

Also plays a role in vitamin D concentration: sunshine is stronger in the mountains than in the plains. UVB rays are in fact attenuated when passing through the atmosphere: they will therefore be less attenuated if they have a short distance to travel in the atmosphere. In addition, these UVBs are attenuated by suspended particles in polluted areas: these areas are mainly located around large cities and rather in the plains, which will increase the low intensity of UVB on the ground [68].

8.3 Season

The synthesis of vitamin D varies in the same way according to the seasons. During the winter months, the radiation is less intense and of shorter duration, which explains why the synthesis of vitamin D is more difficult during the winter months in many countries of the northern hemisphere.

In the French study by Guillemant J et al. in Chantilly (49° North latitude) in children aged between 13 and 16 years, the seasonal variation is evident, the level of 25(OH) D is at its lowest level in winter compared to summer [68].

In another study by Docio S et al., seasonal variation is evident, in Cantabria (northern Spain (43° north latitude), conducted in 94 children with an average age of 8 ± 2 years, the rate of 25(OH) D is lower in winter than in summer [69].

In a study in Hungary by Bakos et al., UVB in Budapest, latitude 47° North, was sufficient from March to October for photosynthesis, while from November to February it was very insufficient, even after adjusting for clothing temperature and time spent outdoors [70].

• The time of exposure must also be taken into account in the production of vitamin D. Thus in Boston (42° N), photosynthesis of precholecalciferol is effective from 7:00 a.m. to 5:00 p.m. in the summer period, whereas in the spring and in the fall it is only effective from 10 a.m. to 3 p.m. [19].

8.4 Phototype

Skin pigmentation is a main cause of vitamin D deficiency because melanin absorbs UVB rays. This pigmentation acts as a natural sunscreen and the increase in skin pigmentation can reduce vitamin D synthesis under the effect of UVB as effectively as sunscreen.

It is well established that people with phototype 4 have a lower level of 25(OH) D than people with a clear phototype and this has been demonstrated in the American NHANES III study. The results showed a significant difference between whites, Hispanics and blacks. African-Americans are the most deficient in vitamin D and white men are the least, with however a prevalence of 34.9% [43]. Similarly, in the 2008 study by G Guardia, the proportion of people with vitamin D deficiency was significantly higher in people with black skin color, than in people with light skin color [71].

8.5 Age

Elderly subjects generally have lower 25(OH)D levels than young subjects from the same region. This was demonstrated in the NHANES III study where the average age increased significantly as the quartiles descended from 25(OH) D. The skin of the elderly contains less 7-dehydro-cholesterol, and from it synthesizes less pre-vitamin D3 [68].

8.6 The puberty stage

Puberty is also a period at risk of vitamin D deficiency. Indeed, a study shows that nearly 25% of adolescents aged 10 to 15 years are deficient in 25 OHD (level below 10 ng /ml) in the pre-winter period. The main factor of this hypovitaminosis is the increase in vitamin D requirements in relation to the increased calcium demand of the skeleton [51]. It is important to emphasize that a puberty deficiency will induce a lower peak bone mass than that of non-deficient adolescents, which can have the consequences of early osteoporosis.

8.7 Sex

The studies are contradictory. It would seem that girls have lower rates than boys without this result being consensual [23].

8.8 Lifestyle

The type of dress and religion is a factor influencing the risk of vitamin D deficiency. Dr. Belaïd’s work has shown the high prevalence of deficiency in patients wearing covering clothing. Indeed, 99% of the 96 patients studied had a vitamin level below 21.2 ng/ml [17].

In Algeria, work done by Lehtihet S, in postmenopausal women wearing covering clothing (veil), showed that 98% had a 25(OH)D level of less than 30 ng/ml [72].

In Turkey (Istanbul 41° N), women aged 14 to 44 were divided according to their clothing style: group I: the usual areas of the skin are exposed to the sun, group II: traditional clothing, hands and face are uncovered, group III: coat completely covering the body (hands and face included). The respective average vitamin D level for each group is 22.4 ± 16.52 ng/ml, 12.76 ± 9.76 and 3.6 ± 2.28 ng/ml). This last comparison suggests that the exposure of the hands and the face makes it possible to synthesize vitamin D but not sufficiently to eliminate a deficiency [73].

8.9 Practice of a sports activity

Moderate physical exercise promotes the accumulation of bone mass, particularly during puberty. The maturity of the skeleton and the peak of bone mass, reached around the age of 20, depend on genetic, hormonal, environmental factors and physical activity. Mechanical forces, by stimulating osteoblastic formation, would promote the local production of growth factors [74].

8.10 Sun protection

This is the subject of numerous prevention campaigns in France and in the rest of the world. This prevention advises using sunscreen, wearing long clothes, a hat and avoiding exposure at the most dangerous times.

Sunscreens work as barriers blocking UV rays. However, many studies have not been able to show a link between the use of sunscreen and hypovitaminosis [3].

8.11 Body mass index

Since vitamin D is fat-soluble, part of it is stored in adipose tissue. Thus the storage will be all the more important as the fat mass is great for a given individual, and the hepatic production of 25(OH) D will be reduced. Obese or overweight subjects have lower serum vitamin D concentrations than lean subjects, this was demonstrated in the American NHANES III study [43].

8.12 Chronic pathologies

The chronic pathologies inducing hypovitaminosis D are the following:

• Chronic renal failure leading to a defect in the transformation of 25(OH) D into 1,25(OH) 2D

• Malabsorption: Crohn’s disease, celiac disease, Whipple’s disease, chronic pancreatitis, cystic fibrosis, biliary obstruction.

• Gastric surgery (By-pass).

• Hepatic insufficiency leading to a lack of hydroxylation of vitamin D in position 25.

• Nephrotic syndrome causing a leak of 25OH D in the urine.

• Sequelae of extensive burns (decreased ability of the skin to synthesize vitamin D3).

• Bone tumors secreting excess FGF 23.

• Primary hyperparathyroidism, which generates an excess of transformation of 25(OH) D into 1,25(OH) 2D, hence a low concentration of 25(OH) D.

• Granulomatoses, sarcoidosis, tuberculosis and certain lymphomas, via macrophages which also excessively transform 25OHD into 1,25(OH) 2D.

• Hyperthyroidism, responsible for an acceleration of 25(OH) D metabolism.

• Genetic diseases: defect in the production of 1,25(OH) 2D (type I pseudo-carential rickets); resistance to 1.25 (OH) 2D (type II pseudo-deficiency rickets due to VDR receptor mutation/deletion); hereditary tubulopathies with urinary loss of phosphates [72].

8.13 Drug treatments

Certain drugs modify the metabolism of vitamin D such as anticonvulsants, glucocorticoids, immunosuppressants in the prevention of transplant rejection and antiretroviral treatments. These drugs increase the transformation of 25(OH) D and 1,25(OH) 2D into inactive compounds at position 24, thus promoting hypovitaminosis D [1].

8.14 Physiological status

In infants, exclusive breastfeeding poses a risk of vitamin D deficiency. It is therefore necessary to supplement them. Since infant milks are supplemented with vitamin D, other infants will be less exposed to deficiencies.

In pregnant women, systematic vitamin D supplementation should be implemented in the 3rd trimester of pregnancy [5].

9. Conclusion

Knowledge about vitamin D has come a long way in recent years. Vitamin D can no longer be considered solely as necessary for the prevention of rickets/osteomalacia. Its role in the prevention of peripheral osteoporotic fractures in the elderly is now well demonstrated (in combination with calcium) and a large number of epidemiological and experimental data support a role in the prevention of numerous conditions (certain cancers and autonomic diseases) – immune, cardiovascular events and hypertension, etc.).

The evaluation of vitamin D status can be easily carried out by the determination of serum 25OHD. However, most recent reviews on the subject suggest that the 25OHD reference values obtained in apparently healthy populations are much too low and that the 25OHD concentration below which there is vitamin D insufficiency is between 50 and 100 nmol/L (20 and 40 ng/ml).