Key points

Introduction

There have been almost 400 scientific articles published on the relationship of vitamin D to allergic inflammation. Simultaneously, there has been growing clinical recognition that vitamin D deficiency occurs frequently in many patient populations. Therefore, the key questions clinical allergists must now ask are how do knowledge of vitamin D actions and the frequency of vitamin D deficiency influence clinical practice; should allergy patients be tested for vitamin D status; and, if deficient, should they receive vitamin D supplementation?

Introduction

There have been almost 400 scientific articles published on the relationship of vitamin D to allergic inflammation. Simultaneously, there has been growing clinical recognition that vitamin D deficiency occurs frequently in many patient populations. Therefore, the key questions clinical allergists must now ask are how do knowledge of vitamin D actions and the frequency of vitamin D deficiency influence clinical practice; should allergy patients be tested for vitamin D status; and, if deficient, should they receive vitamin D supplementation?

Calcitriol

The biologically most active form of vitamin D is calcitriol, 1,25-dihydroxy cholecalciferol (vitD3). A bioequivalent molecule, 1,25-dihydroxy ergocalciferol (vitamin D2), is formed from plant sterols. VitD3 is a steroid hormone acting via the nuclear vitamin D receptor (VDR) to activate or repress the expression of hundreds of genes that function in every tissue. For example, calbindin, important in embryonic lung differentiation, is vitamin-D dependent. Approximately 3% of all human genes are regulated by vitamin D, either solely or as a coregulator, often with vitamin A. Secosteroids in food, found primarily in fish oil, artificially fortified foods, and, to a lesser extent, in plants, are first 25-hydroxylated by the liver to produce calcidiol, and then acted on by the kidney, antigen processing cells, lung epithelial cells, and other cell types, including almost all immune system cells. This second hydroxylation, catalyzed by 1-α-hydroxylase, is the rate-limiting step in vitD3 synthesis, and is tightly controlled by a complex web of feedback loops.

VitD3 can also be synthesized in skin from cholesterol, via photolysis by ultraviolet light. Long ago, sunlight was known to be important for health because of the obvious variation in rickets incidence and in infectious diseases with increased sunlight. Importantly, the indoor existence of many people today easily can lead to vitamin D deficiency. Institutionalized or hospital inpatients, poorly nourished individuals, and people living closer to the poles are at risk for vitamin D deficiency, as are persons of low socioeconomic status, elders, dark-skinned people, patients with cystic fibrosis or other chronic pulmonary diseases (including asthma), and tobacco users. Cannell and colleagues, and Pearce and Cheetham, believe that most of the population in temperate zone countries now has vitamin D deficiency. Deficiency also commonly occurs in the Middle East and the tropics. Despite these opinions, there are still so few long-term, randomized, vitamin D studies, that the Institute of Medicine (IOM) stated that current evidence indicates only about 2.5% of the North American population is actually vitamin D deficient. The IOM report has been criticized as being overly conservative. A comprehensive review of vitamin D studies has been done, as well as a review of the impact of nutrition on asthma development.

Illnesses associated with calcitriol deficiency

The rare visible consequences of vitD3 deficiency are osteomalacia with fractures and pediatric rickets. Other potential consequences are less visible, including autoimmune illnesses, cancers, diabetes, metabolic syndrome, hypertension, infection susceptibility, adverse pregnancy outcomes, neurocognitive dysfunction, and a 26% higher all-causes mortality, all of which are associated with low vitamin D levels. For example, a large data-mining study of unselected middle-aged people found that calcidiol levels less than 52.5 nmol/L were associated with significant increases in both cardiovascular and all-causes mortality. The Third National Health and Nutrition Examination Survey (NHANES III) analyzed more than 14,000 adults and found a strong association between higher calcidiol levels and better pulmonary function. Finally, one possible cause of the historic rise in asthma cases is vitamin D deficiency. Low serum vitamin D levels are associated with poor asthma control, and low levels have been found to be common even in the Mediterranean region. In the United States, a meta-analysis found 61% of young asthmatics have low serum calcidiol levels, whereas 86% of young urban black asthmatics are calcitriol deficient, compared with 19% of nonasthmatic controls. A final factor may be the recent rise in obesity because body fat content is inversely correlated with serum calcidiol level.

Measuring vitamin D

Serum vitD3 is fully bound by vitamin D binding protein (VDBP), which is present in great excess and has immunomodulatory properties independent of vitD3. VDBP is secreted into the airway where it assists in macrophage activation, chemotaxis, and oxidative killing. Serum VDBP levels inversely correlate with lung function of patients with chronic obstructive pulmonary disease (COPD). VDBP levels are elevated in the airways of children with severe, poorly treated, unresponsive asthma; however, it is not known if this is cause or effect.

Vitamin D status can be accurately determined by measuring calcidiol, 25-hydroxy cholecalciferol, in blood. There are two common measurement units for calcidiol, with a conversion factor of 1 ng/mL = 2.5 nmol/L. There is very active debate about the optimum serum calcidiol range, with the current consensus being 50 to 75 nmol/L, or greater than 50 nmol/L. However, based on observational studies, Grant has proposed optimum levels of 75 to100 nmol/L. According to the IOM, adequate calcidiol serum levels can normally be sustained by vitamin D supplements of 400 IU per day in infants, 600 IU per day for ages 1 to 70, and 800 IU per day over age 70. However, in pregnancy, 4000 IU per day is required, and some people may require supplements greater than 5000 IU per day. See later discussion on supplementation.

Vitamin D affects immune function

The first evidence for immune effects of vitD3 was the 1983 discovery that human peripheral mononuclear cells and activated T-cells had VDRs, and that vitD3 deficiency was a cause of delayed hypersensitivity anergy. The following year, vitD3 was shown to be a stimulatory hormone for monocyte activation. By 2004, vitD3 had been found to be an important selective immune system regulator, with complex effects that depend on the specific function being evaluated and on other environmental cofactors, especially calcium. Both T-helper type (Th)1 and Th2 CD4+ cells increase their expression of VDR on activation. In addition, these cells respond to vitD3 by reducing proliferation and altering production of Th1 and Th2 cytokines. The primary effects of vitD3 are to inhibit specific immunity; increase production of innate antimicrobial peptides, cathelicidin, and defensin β2; and promote the development and function of NK T-cells. VitD3 is also essential for production of prohibitin, which regulates antioxidant defenses and apoptosis, and for stimulation of antiinflammatory mast cells. Most importantly, vitD3 inhibits the initiation of most Th1-mediated and Th2-mediated inflammatory diseases. A complicating finding is that there is evidence for different effects of vitD3 at different doses. Kuo and colleagues studied both human peripheral monocytes and a monocyte-derived cell line in vitro. They found that vitD3 suppressed Th1 cytokines at low doses and stimulated Th2 cytokines at high doses.

VitD3 had long been known to affect macrophage, dendritic, and Th1 cells. However, it was not until 2004 that vitD3 effects on Th2 cells were found. Changes in Th2 cell migration and tissue homing led to reduced Th2 function at inflammation sites. In a mouse VDR genetic knockout model (KO), compared with wild type (WT) mice, it was impossible to induce allergic asthma, implying a critical role for vitamin D in regulation of allergic inflammation. These KO mice natively show a Th1 bias, which is normalized by vitD3 supplements, and, experimental asthma can be induced in vitD3-treated KO mice just as successfully as in WT mice. Dietary vitD3 deficiency shifted the WT T-helper cell bias toward Th1, just as in the KO mice. These findings could indicate that vitD3 supplements would worsen allergic asthma. However, vitD3 supplementation of WT mice had no effect on either asthma induction or asthma severity. On further analysis, the vitD3 receptor KO mice could develop peripheral Th2 inflammation, but, probably due to deceased cell trafficking, they did not develop lung inflammation.

Genetic control of vitamin D levels and the relation to asthma

Human genome-wide linkage evaluation has shown strong genetic regulation of serum calcidiol levels, but not calcitriol levels. Single nucleotide polymorphisms (SNPs) that reduce calcidiol levels usually affect activity of one of the two hydroxylases or the serum VDBP. The second hydroxylation to active calcitriol is the rate-limiting step in synthesis. Production of the 1-hydroxylase is under extensive feedback control by cytokines and kinases that, by increasing vitD3 levels, help terminate ongoing immune inflammation. SNPs in genes for the 25-hydroxylase can directly affect presence of asthma, and SNPs in the VDR gene affect asthma morbidity and lung function, as well as number of positive allergen tests and IgE elevation.

Concomitant effects of vitamin D and calcium

VitD3 and calcium work together for normal homeostasis, including immune functioning. Because vitD3 is necessary for intestinal absorption of both calcium and phosphate, insufficient calcidiol has a multiplier effect. VDR KO mice develop rickets, osteomalacia, and secondary hyperparathyroidism that are improved by calcium and phosphate supplementation. In humans, calcium with vitD3 is effective in reducing hip fractures, unlike single supplements. Dietary experiments with IL-10 KO mice that develop irritable bowel disease (IBD) show that combined calcium and vitD3 can completely prevent IBD, whereas supplements of only vitD3 or calcium are less effective. Similar effects of calcium and vitD3 supplements were found in a mouse multiple sclerosis model. The IOM recommends daily calcium doses from 200 to 1300 mg, up to maximum daily doses of 1000 to 3000 mg, depending on age, sex, and pregnancy status. The US Preventative Services Task Force recently reviewed the combined use of calcium and vitamin D supplements specifically for fracture prevention, and concluded that, although there is good evidence for an effect to prevent falls in all people over age 65, the evidence is inadequate to recommend supplement use for fracture prevention.

In the past year, the cardiovascular safety of calcium supplements, but not calcium-containing foods, was questioned. A subsequent detailed reanalysis was done of the two epidemiologic studies and the meta-analysis that had raised concerns. The review board found that there were sufficient problems with the methodology of those three studies and that many other good studies did not show elevated risks from calcium supplements. The board concluded that current calcium supplement guidelines should not be changed.

Vitamin D effects on allergy and asthma

In vitro studies prove that vitD3 supplementation suppresses dendritic cell maturation and subsequent Th1 cell development by blocking IL-12 signaling. Wjst then proposed that infant vitD3 supplements, which had eliminated rickets, might actually be a reason for the rapid increase in allergy prevalence in developed countries. Wjst cited several nutritional supplement studies as supporting data for a possible link between vitD3 supplementation and increased risk of food allergy sensitization, but acknowledged that there is still insufficient information to conclude how early vitD3 exposure affects the developing immune system. On the other hand, in adult mice with experimental asthma, prenatal vitD3 deficiency causes an increase in their allergic sensitized lymphocytes.

One effect of vitD3 that could be relevant for asthma is the induction of innate natural killer (NK) T-cells and anti-infectious peptides. In mice, inadequate vitD3 during embryogenesis causes a reduction in NK cells that cannot be remedied after birth. Production of intracellular protective cathelicidins and defensin β2 is also vitD3 dependent. In humans, naturally varying seasonal vitD3 levels have been found to inversely correlate with incidence of viral respiratory infections, with calcidiol levels greater than 95 nmol/L being maximally protective. In children younger than age 5 in Hawaii, viral bronchiolitis, respiratory syncytial virus, and pneumonia vary with both season and skin pigmentation; whereas, in England, monthly incidence of astrovirus and norovirus infections inversely correlates with calcidiol levels. Vitamin D treatment trials improved tuberculosis patients with VDR mutations and substantially reduced winter viral infections in postmenopausal black women.

Unlike the molecular and animal model data, which is complex and sometimes seems conflicting, most clinical observation studies show strong support for the idea that adequate vitamin D is a protective factor against asthma. Luong and Nguyen, and Gilbert and colleagues, have reviewed vitD3 effects on asthma. In most human studies, vitD3 reduces the prevalence of asthma, prevents COPD, and does not aggravate allergic diseases. VitD3 corrects, in vitro, the defective IL-10 secretion by CD4+ Treg cells from steroid-resistant asthma patients and makes these cells normally sensitive to steroids. Calcidiol levels in many, but not all, studies are inversely correlated with asthma and wheezing incidences, in both children and adults, and are directly correlated with FEV1 values in asthma. Some studies also show an inverse correlation between calcidiol levels and IgE concentrations or eosinophil counts. Low calcidiol levels in pregnancy are associated with increased asthma and eczema in those children, and vitD3 supplementation of pregnant women reduces asthma risk in their children by 40%. In another study, calcidiol cord blood levels did not predict the presence of asthma at age 5, even though both high and low calcidiol levels were associated with higher total and specific IgE levels. A reason for the negative results of this one study may be the unknown intake of vitD3 during childhood, and that very few study infants had initially low calcidiol levels. Low calcidiol does predict greater risk of asthma-related hospitalization and increased use of asthma medication. Finally, adequate vitamin D levels seem to protect asthmatic children from loss of bone calcium during treatment with oral corticosteroids. If replicated, this single finding would make vitamin D supplementation essential in asthma care.

Vitamin D effects on immunotherapy and tolerogenesis

VitD3 may be effective as an adjuvant during immunotherapy (IT). In both mouse and human IT studies, vitD3 supplements have a suppressive effect on allergic inflammation, and offset the otherwise negative effects of oral corticosteroids on sublingual IT (SLIT) effectiveness in a mouse model and in humans. VitD3 pretreatment of adult mice, before sensitization, significantly enhances the effectiveness of allergy IT, and decreases both lung allergic inflammatory cells and cytokines. In mice, vitD3 acts by stimulating dendritic cells, promoting development of both IL-10 producing and Foxp3+ Treg cells, thereby increasing tolerance. These effects, unlike those produced by IT, are not antigen specific, so there is a potential benefit from using vitD3 in combination with IT. Corticosteroids were believed to have similar nonspecific effects on Treg development and, when used together, corticosteroids and ViD3 might have enhancing effects on IT. However, there was no human study of the effects of corticosteroids on IT. Majak and colleagues tested these ideas by performing a three-arm, randomized, double-blind study in 48 asthmatic children, treated for a year with dust mite SLIT and prednisone 20 mg daily, plus or minus 1000 IU per week of vitD3 or placebo. Their key finding was that daily corticosteroids substantially decreased SLIT effectiveness and that vitamin D3 supplements prevented the deleterious effect of corticosteroids. This pilot study, along with effects of vitD3 on tolerogenesis (see later discussion), suggests that vitD3 supplements may help allergy IT efficacy.

VitD3 inhibits the polarization of uncommitted T-cells to Th1 or Th2 and, instead, stimulates Treg production, leading to tolerogenesis. Mouse cell cultures show vitD3 does so by directly suppressing transcription of interferon-γ and IL-10, respectively, the primary driving cytokines for Th1 and Th2 differentiation. One of the consequences of this tolerogenesis is that Th2 cells become relatively disinhibited compared with Th1 cells, which shifts the immune response to a Th2 bias. Unlike effects on Th1 and Th17 cells, which are clearly inhibitory, studies of the effects of vitD3 on Th2 function are controversial, and show both inhibition and stimulation, depending on the experimental conditions. This in vitro enhancement of Treg function has now been confirmed to occur in asthmatic human airway lymphocytes, in which both Foxp3(+) and IL-10(+) Treg numbers are correlated with serum calcidiol levels. Similar findings have been seen in human blood lymphocytes and this probably explains cases of asthma that do not respond to steroids. It is likely that vitamin D-related Treg function has strong clinical significance for asthma development and control.