Vitamin D for Health: A Global Perspective

Vitamin D (D represents D₂, D₃, or both) is a secosterol produced endogenously in the skin from sun exposure or obtained from foods that naturally contain vitamin D, including cod liver oil and fatty fish (eg, salmon, mackerel, and tuna); UV-irradiated mushrooms; foods fortified with vitamin D; and supplements.^, 

During exposure to sunlight, 7-dehydrocholesterol (7-DHC) in the skin is converted to previtamin D₃. The 7-DHC is present in all the layers of human skin.^{,  ,}  Approximately 65% of 7-DHC is found in the epidermis, and greater than 95% of the previtamin D₃ that is produced is in the viable epidermis and, therefore, cannot be removed from the skin when it is washed. Once previtamin D₃ is synthesized in the skin, it can undergo either a photoconversion to lumisterol, tachysterol, and 7-DHC or a heat-induced membrane-enhanced isomerization to vitamin D₃ (Figure 1).^,  The cutaneous production of previtamin D₃ is regulated. Solar photoproducts (tachysterol and lumisterol) inactive on calcium metabolism are produced at times of prolonged exposure to solar UV-B radiation, thus preventing sun-induced vitamin D intoxication.^,  Vitamin D₃ is also sensitive to solar irradiation and is, thereby, inactivated to suprasterol 1 and 2 and to 5,6-trans-vitamin D₃. Cutaneous vitamin D₃ production is influenced by skin pigmentation, sunscreen use, time of day, season, latitude, altitude, and air pollution.^{,  ,  ,}  An increase in the zenith angle of the sun during winter and early morning and late afternoon results in a longer path for the solar UV-B photons to travel through the ozone layer, which efficiently absorbs them. This is the explanation for why above and below approximately 33° latitude little if any vitamin D₃ is made in the skin during winter.^,  This is also the explanation for why—whether being at the equator and in the far northern and southern regions of the world in summer, where the sun shines almost 24 hours a day—vitamin D₃ synthesis occurs only between approximately 10 am and 3 pm.^,  Similarly, in urban areas, such as Los Angeles, California, and Mexico City, Mexico, where nitrogen dioxide and ozone levels are high, few vitamin D₃–producing UV-B photons reach the people living in these cities.^,  Similarly, because glass absorbs all UV-B radiation, no vitamin D₃ is produced in the skin when the skin is exposed to sunlight that passes through glass.

Once formed, vitamin D₃ is ejected out of the keratinocyte plasma membrane and is drawn into the dermal capillary bed by the vitamin D binding protein (DBP).^,  Vitamin D that is ingested is incorporated into chylomicrons, which are released into the lymphatic system, and enters the venous blood,^,  where it binds to DBP and lipoproteins transported to the liver.^{,  ,}  Vitamin D₂ and vitamin D₃ are 25-hydroxylated by the liver vitamin D-25-hydroxylase (CYP2R1) to produce the major circulating vitamin D metabolite, 25(OH)D, which is used to determine a patient's vitamin D status.^{,  ,}  This metabolite undergoes further hydroxylation by the 25(OH)D-1α-hydroxylase (CYP27B1) in the kidneys to form the secosteroid hormone 1α,25-dihydroxyvitamin D [1,25(OH)₂D] (Figure1).^{,  ,}  The 25(OH)D bound to DBP is filtered in the kidneys and is reabsorbed in the proximal renal tubules by megalin cubilin receptors.^,  The renal 1α-hydroxylation is closely regulated, being enhanced by parathyroid hormone (PTH), hypocalcemia, and hypophosphatemia and inhibited by hyperphosphatemia, fibroblast growth factor-23, and 1,25(OH)₂D itself.^{,  ,} 

The 1,25(OH)₂D performs many of its biologic functions by regulating gene transcription through a nuclear high-affinity vitamin D receptor (VDR).^,  This active metabolite of vitamin D binds to the nuclear VDR, which binds retinoic acid X receptor to form a heterodimeric complex that binds to specific nucleotide sequences in the DNA known as vitamin D response elements. Once bound, a variety of transcription factors attach to this complex, resulting in either up-regulation or down-regulation of the gene's activity.^{,  ,}  There are estimated to be 200 to 2000 genes that have vitamin D response elements or that are influenced indirectly, possibly by epigenetics, to control a multitude of genes across the genome.^,  A recent microarray study on the influence of vitamin D status and vitamin D₃ supplementation on genome-wide expression in white blood cells before and after vitamin D₃ supplementation found that an improved serum 25(OH)D concentration was associated with at least a 1.5-fold alteration in the expression of 291 genes. This study suggested that any improvement in vitamin D status will significantly affect the expression of genes that have a variety of biologic functions of more than 80 pathways linked to cancer, autoimmune disorders, and cardiovascular disease, which have been associated with vitamin D deficiency.

One of the major physiologic functions of vitamin D is to maintain serum calcium and phosphorus levels in a healthy physiologic range to maintain a variety of metabolic functions, transcription regulation, and bone metabolism (Figure 1).^,  The 1,25(OH)₂D interacts with its VDR in the small intestine to increase the efficiency of intestinal calcium absorption from approximately 10% to 15% up to 30% to 40% and intestinal phosphorus absorption from approximately 60% to 80%. It also interacts with VDR in osteoblasts to stimulate a receptor activator of nuclear factor κB ligand, which, in turn, interacts with receptor activator of nuclear factor κB on immature preosteoclasts, stimulating them to become mature bone-resorbing osteoclasts (Figure 1).^,  The mature osteoclast removes calcium and phosphorus from the bone to maintain blood calcium and phosphorus levels. In the kidneys, 1,25(OH)₂D stimulates calcium reabsorption from the glomerular filtrate.^, 

The VDR is present in most tissues and cells in the body.^{,  ,  ,  ,  ,  ,  ,  ,  ,}  Many of these organs and cells, including the brain, vascular smooth muscle, prostate, breast, and macrophages, not only have a VDR but also have the capacity to produce 1,25(OH)₂D.^{,  ,  ,  ,  ,  ,  ,  ,  ,}  This production probably depends on the availability of circulating 25(OH)D, indicating the biological importance of sufficient blood levels of this vitamin D metabolite.^{,  ,} 

The estimated 2000 genes that are directly or indirectly regulated by 1,25(OH)₂D^{,  ,  ,  ,  ,  ,  ,  ,  ,  ,}  have a wide range of proven biological actions, including inhibiting cellular proliferation and inducing terminal differentiation, inhibiting angiogenesis, stimulating insulin production, inducing apoptosis, inhibiting renin production, and stimulating macrophage cathelicidin production.^{,  ,  ,  ,  ,}  In addition, 1,25(OH)₂D stimulates its own destruction in the kidneys and in cells that have a VDR and responds to 1,25(OH)₂D by enhancing expression of the 25(OH)D–24-hydroxylase (CYP24A1) to metabolize 25(OH)D and 1,25(OH)₂D into water-soluble inactive forms that are excreted in the bile (Figure 1).^{,  ,  ,} 

Vitamin D metabolism is enhanced during pregnancy and lactation. The placenta is formed at 4 weeks of gestation.^,  From this time to term, 25(OH)D is transferred across the placenta, and the fetal cord blood concentration of 25(OH)D is correlated with the mother's concentration. However, the active metabolite 1,25(OH)₂D does not readily cross the placenta.^,  The fetal kidneys and the placenta provide the fetal circulation with 1,25(OH)₂D by expressing CYP27B1 (Figure 2).

The maternal (decidual) and fetal placental (trophoblastic) components of the placenta have CYP27B1 activity; cultured human syncytiotrophoblasts and decidual cells synthesize 1,25(OH)₂D₃. The spatiotemporal organization of placental CYP27B1 and the VDR across gestation has also been characterized, confirming that the enzyme and receptor are localized to the maternal and fetal parts of the placenta. Serum levels of DBP increase 46% to 103% during pregnancy, suggesting that DBP may play a role in directing vitamin D metabolism and function during pregnancy.^{,  ,}  The DBP has a much higher binding affinity for 25(OH)D than for 1,25(OH)₂D, and in kidney epithelial cells, DBP plays a pivotal role in conserving 25(OH)D by facilitating the recovery of 25(OH)D from the glomerular filtrate.^, 

Transplacental transfer of calcium to the fetus is also facilitated by expression of all the key mediators of vitamin D metabolism in the placenta. Hormones involved in fetal growth and that influence CYP27B1 activity include insulin-like growth factor 1 and human placental lactogen, PTH-related protein (PTHrP), estradiol, and prolactin.^{,  ,  ,}  The PTHrP acts as a calciotropic hormone during fetal life and in lactation.^{,  ,}  The exact role of circulating PTHrP in pregnancy is unknown, but its rise may stimulate renal CYP27B1 and contribute to the increase in 1,25(OH)₂D concentration and, indirectly, the suppression of PTH levels.^{,  ,}  The PTHrP arises from several sources, including the breast, myometrium, decidua, amnion, and fetal parathyroids. Several roles of PTHrP are postulated from animal studies, including fetal chondrocyte maturation, fetal calcium transfer, and stimulation of CYP27B1 activity.^{,  ,}  Furthermore, the carboxy terminal of PTHrP (osteostatin) may suppress osteoclastic activity and may have a possible bone protection role in the mother during pregnancy.^{,  ,  ,} 

Calcitonin, an important component of calcium homeostasis during pregnancy,^,  is known to promote transcription of CYP27B1 and may, therefore, be a key determinant of placental vitamin D metabolism. Thus, PTHrP and calcitonin, as well as other factors, cause 1,25(OH)₂D levels to increase, being 2-fold higher in serum of women in the third trimester of pregnancy than in nonpregnant or postpartum women.^,  Normally, 1,25(OH)₂D regulates its own metabolism via a feedback loop such that elevated concentrations induce the expression of CYP24A1, with concomitant down-regulation of CYP27B1.^{,  ,}  This process results in a reduction of 25(OH)D and 1,25(OH)₂D levels.^{,  ,}  However, during pregnancy, this process becomes uncoupled, resulting in elevated maternal concentrations of circulating 1,25(OH)₂D.^,  The placental methylation of the CYP24A1 promoter reduces the capacity for CYP24A1 induction and down-regulates basal promoter activity and abolishes vitamin D–mediated feedback activation. This epigenetic decoupling of vitamin D feedback catabolism also plays an important role in enhancing 1,25(OH)₂D bioavailability at the fetomaternal interface.

Epidemiologic evidence has suggested a link between fetal life events and susceptibility to disease in adult life.^{,  ,}  This paradigm, referred to as fetal programming or developmental origins of health and disease, may have a profound effect on public health strategies for the prevention of major illnesses.^,  The role of vitamin D in implantation tolerance and placental development has been studied. The 1,25(OH)₂D₃ regulates key target genes associated with implantation, such as Homeobox A10 (HOXA10), whereas the potent immunosuppressive effects of 1,25(OH)₂D₃ suggest a role in placental development. Increasing expression of CYP27B1 and VDR in first-trimester human trophoblasts and deciduas may be related to the immunosuppressive effects of 1,25(OH)₂D₃ and may help improve implantation tolerance. Placental development plays a critical role in pregnancy health, and its link to maternal vitamin D deficiency may explain related adverse outcomes.^,  In neonatal rats exposed prenatally to low maternal serum 25(OH)D levels, there was a general slowing of cardiac development, with significantly lower heart weights, decreased citrate synthase and 3-hydroxyacyl CoA dehydrogenase activity, and a 15% lower myofibrillar protein content. A 2-month-old human infant with dilated cardiomyopathy and severe vitamin D deficiency had dramatic improvement of her ejection fraction (17%-66%) after vitamin D supplementation. In addition, maternal vitamin D deficiency in rats stimulated nephrogenesis in offspring, with a 20% increase in nephron number but a decrease in renal corpuscle size observed between replete and deficient rats, despite there being no difference in body weight or kidney weight and volume.^,  These findings support the role of vitamin D influencing fetal programming and placental development.

Epigenetic modification refers to heritable changes in gene expression that are not mediated by alterations in DNA sequence. This hypothesis, first articulated by Barker et al, postulated that in utero epigenetic fetal programming (as a result of environmental events during pregnancy) induced specific genes and genomic pathways that controlled fetal development and subsequent disease risk. The role of vitamin D in epigenetic modification and fetal programming could potentially explain why vitamin D has been reported to have such wide-ranging health benefits. Recent studies have suggested that epigenetic decoupling of vitamin D feedback catabolism plays an important role in maximizing 1,25(OH)₂D bioavailability at the fetomaternal interface.^,  Modified expression of the genes encoding placental calcium transporters, by epigenetic regulation by 1,25(OH)₂D, might represent the means whereby maternal vitamin D status could influence bone mineral accrual in the neonate.^,  Vitamin D deficiency during pregnancy may, therefore, not only impair maternal skeletal preservation and fetal skeletal formation but also influence fetal “imprinting” that may affect chronic disease susceptibility soon after birth as well as later in life (Figure 3).^,  Transgenerational hormonal imprinting caused by vitamin D treatment of newborn rats has been previously reported. A recent study reported that VDR binds to the ɛ germline gene promoter and exhibits transrepressive activity. Inhibition of IgE production by 1,25(OH)₂D was mediated by its transrepressive activity through the VDR-corepressor complex, affecting chromatin compacting around the Iɛ region. Also, the associations of early-life sun exposure and germline variation in VDR and CYP24A1 with non-Hodgkin lymphoma risk was reported in a clinic-based case-control study.

The blood level of 25(OH)D is the best method to determine vitamin D status. Although 1,25(OH)₂D is the biologically active form, it provides no information about vitamin D status because it is often normal or even elevated in children and adults who are vitamin D deficient.^{,  ,  ,  ,  ,}  Recently, the Institute of Medicine (IOM) and the Endocrine Society released separate guidelines for vitamin D requirements.^,  The recommended dietary allowances (RDAs) of the IOM and the Endocrine Society guidelines for vitamin D intake are summarized in Figure 4.

The revised guidelines by the IOM stress that the daily requirements for vitamin D are generally met by most of the population and are appropriate to reach the “sufficient” level of 20 ng/mL (to convert to nmol/L, multiply by 2.496). The IOM guidelines used a population model to prevent vitamin D deficiency in 97.5% of the general population. Also, note that the IOM report focuses only on bone health (calcium absorption, bone mineral density, and osteomalacia/rickets) and found no evidence that a serum 25(OH)D concentration greater than 20 ng/mL had beneficial effects at a population level. However, considering the available evidence on skeletal and extraskeletal effects of vitamin D, the few negative studies, and the lack of toxicity potential of vitamin D supplementation at recommended doses, the US Endocrine Society, which used a medical model, recommended that serum 25(OH)D levels of 30 ng/mL should be attained to avoid other risks connected with an inadequate vitamin D status.^,  Therefore, the Endocrine Society recommended that vitamin D deficiency be defined as a 25(OH)D level of 20 ng/mL or less, vitamin D insufficiency as 21 to 29 ng/mL, and vitamin D sufficiency as 30 ng/mL or greater for children and adults. It suggested that maintenance of a 25(OH)D level of 40 to 60 ng/mL is ideal (this takes into account assay variability) and that up to 100 ng/mL is safe.

According to current evidence from biochemical testing, observational studies, and randomized controlled trials (RCTs), serum 25(OH)D levels of at least 20 ng/mL are required for normalization of PTH levels, to minimize the risk of osteomalacia, and for optimal bone and muscle function, with many experts regarding 30 ng/mL as the threshold for optimal bone health.^{,  ,  ,  ,  ,}  The skeletal consequences of 25(OH)D insufficiency include secondary hyperparathyroidism, increased bone turnover and bone loss, and increased risk of low-trauma fractures.^{,  ,  ,} 

The most common etiology of rickets, historically and presently, is vitamin D deficiency. Low maternal 25(OH)D levels were found to correlate with increased fetal distal femoral splaying, determined by ultrasonography measurements of femoral length and metaphyseal width.^,  Children begin to manifest classic clinical signs of rickets between 6 months and 1.5 years that include rachitic rosary, widened epiphyseal plates at the end of long bones, and bowing deformities of the legs. A common early symptom in newborns is excessive sweating due to neuromuscular irritability, and a 25(OH)D level less than 20 ng/mL is common in children presenting with vague limb or back pain.

From a skeletal perspective for adults, evidence from RCTs suggests that vitamin D may be considered a threshold nutrient, with little bone benefit observed at levels of 25(OH)D above which PTH is normalized.^,  A literature review of 70 studies generally found a threshold for a decline in serum PTH levels with increasing serum 25(OH)D levels, but there was no consistency in the threshold level of serum 25(OH)D, which varied from 20 to 50 ng/mL. A study of 4100 older adults (>60 years old) from the Third National Health and Nutrition Examination Survey (NHANES III) found that higher 25(OH)D levels were associated with better lower extremity function. Much of the improvement occurred at 25(OH)D levels ranging from 9 to 16 ng/mL but continued to be seen at levels up to 40 ng/mL. A systematic review revealed that supplemental vitamin D at daily doses of 800 to 1000 IU consistently had beneficial effects on muscle strength and balance. Several RCTs have reported positive effects of vitamin D supplementation on muscle function and fall prevention.^{,  ,}  Adequate calcium intake is imperative to gain optimal benefit from improving the vitamin D status in those with insufficient 25(OH)D levels. In contrast, a study of 173 young Asian Indian females revealed that after supplementation with vitamin D₃ (60,000 IU/wk for 8 weeks followed by 60,000 IU every 2 weeks) and calcium (500 mg twice per day for 6 months), and despite significant improvement in serum 25(OH)D levels, there was no significant change in their skeletal muscle strength. Thus, age, baseline and final 25(OH)D concentrations, and whether and how much calcium supplementation was included in the clinical trial could affect outcome measures related to muscle performance and vitamin D status.

Proximal muscle weakness is a prominent clinical feature of vitamin D deficiency.^,  The relative contributions of vitamin D and calcium for reducing fracture risk remain unclear because improving calcium intake is also associated with suppression of PTH levels independent of vitamin D status.^{,  ,}  A meta-analysis of data from RCTs found a dose-response relationship between a higher vitamin D dose and higher achieved serum 25(OH)D levels, with prevention of falls and fractures. The greatest benefit was observed at 700 to 1000 IU/d or a mean serum 25(OH)D concentration of 30 to 44 ng/mL.^,  Similar results were reported in a more recent meta-analysis of pooled participant-level data from 11 double-blind RCTs of oral vitamin D supplementation, with or without calcium, compared with placebo or calcium alone in persons 65 years or older. Reduction in the risk of fracture occurred only at the highest vitamin D intake level (median, 800 IU/d; range, 792-2000 IU/d), with a 30% reduction in the risk of hip fracture and a 14% reduction in the risk of any nonvertebral fracture. This reduction was independent of the assigned treatment dose of vitamin D, age group, sex, type of dwelling, and study. Several previous meta-analyses have suggested that the dose of vitamin D is irrelevant when vitamin D is combined with calcium.^{,  ,  ,}  In contrast, a pooled subgroup analysis of the 8 double-blind RCTs that used vitamin D combined with calcium indicated that with combined supplementation, the risk of fracture was reduced only at the highest actual intake level of vitamin D. These findings support that a 25(OH)D level of more than 24 ng/mL may be most beneficial for reducing the risk of fractures.

With a similar tone and theme, a report from the US Preventive Services Task Force concluded that current evidence is insufficient to assess the balance of benefits and harms of combined vitamin D and calcium supplementation for the primary prevention of fractures in premenopausal women or in men. Furthermore, they concluded that there was insufficient evidence to assess the balance of benefits and harms of daily supplementation with greater than 400 IU of vitamin D₃ and greater than 1000 mg of calcium for primary prevention of fractures in noninstitutionalized postmenopausal women. They recommended against daily supplementation with 400 IU or less of vitamin D₃ and 1000 mg or less of calcium for the primary prevention of fractures in noninstitutionalized postmenopausal women. They also stated that it was unclear whether higher doses of vitamin D and calcium are effective in preventing fractures in postmenopausal women, younger women, or men. The Task Force, however, concluded that vitamin D supplementation is effective in preventing falls in community-dwelling adults 65 years or older, which, in turn, reduces the risk of fracture. This could help explain the observation by the Women's Health Initiative (WHI) that, in the subgroup of long-adherent women who took their calcium and vitamin D, there was a reduced risk of hip but not total fractures. Therefore, what is still unknown is whether adequate intake of calcium, especially from dietary sources, and maintenance of serum 25(OH)D levels of at least 20 ng/mL as recommended by the IOM or at least 30 ng/mL as recommended by the Endocrine Society throughout life will reduce the risk of fracture. Most evidence suggests that adequate calcium and vitamin D intake along with exercise during childhood will maximize bone mineral content that can be sustained in young and middle-aged adults as long as they also have a healthy lifestyle, adequate calcium intake, and a healthy vitamin D status.^{,  ,  ,  ,  ,}  Accruing maximum bone mineral content during childhood, and maintaining peak bone mineral density in young and middle-aged adults, will likely reduce the risk of fracture later in life, when there is a disruption in bone remodeling due to menopause and aging.

Recent recommendations of the European Society for Clinical and Economic Aspects of Osteoporosis and Osteoarthritis (ESCEO) for the optimal management of elderly and postmenopausal women regarding vitamin D supplementation have also indicated that patients with serum 25(OH)D levels less than 20 ng/mL have increased bone turnover, bone loss, and, possibly, mineralization defects compared with patients with serum 25(OH)D levels of 20 ng/mL or greater. Similar relationships have been reported for frailty, nonvertebral and hip fracture, and all-cause mortality, with poorer outcomes at less than 20 ng/mL. Thus, ESCEO recommended that 20 ng/mL be the minimal serum 25(OH)D concentration at the population level and in patients with osteoporosis to ensure optimal bone health. Also, in fragile elderly individuals who are at elevated risk for falls and fractures, ESCEO recommended a minimal serum 25(OH)D level of 30 ng/mL for the greatest effect on fracture. This coincides with the recommendation from the Endocrine Society and with the observation of Priemel et al, who reported that of 675 presumed healthy adults (aged 20-90+ years) who died in an accident, 36% had evidence of osteomalacia. However, Priemel et al observed no osteomalacia in those who had a 25(OH)D level greater than 30 ng/mL.

Observational studies have found a decreased risk of many disorders, including certain types of cancer, mental disorders, infectious disease, cardiovascular disease, type 2 diabetes mellitus, and autoimmune disorders, associated with serum 25(OH)D levels greater than 28 to 32 ng/mL.^{,  ,}  It has, therefore, been argued that 25(OH)D levels should be in the range of 28 to 40 ng/mL to maximize these nonskeletal benefits.^{,  ,  ,  ,  ,}