Vitamin D: an essential component for skeletal health

Physiological Basis for Using Vitamin D to Improve Health

Vitamin D is essential for life-its sufficiency improves metabolism, hormonal release, immune functions, and maintaining health. Vitamin D deficiency increases the vulnerability and severity of type 2 diabetes, metabolic syndrome, cancer, obesity, and infections. The active enzyme that generates vitamin D [calcitriol: 1,25(OH)₂D], CYP27B1 (1α-hydoxylase), and its receptors (VDRs) are distributed ubiquitously in cells. Once calcitriol binds with VDRs, the complexes are translocated to the nucleus and interact with responsive elements, up- or down-regulating the expression of over 1200 genes and modulating metabolic and physiological functions. Administration of vitamin D₃ or correct metabolites at proper doses and frequency for longer periods would achieve the intended benefits. While various tissues have different thresholds for 25(OH)D concentrations, levels above 50 ng/mL are necessary to mitigate conditions such as infections/sepsis, cancer, and reduce premature deaths. Cholecalciferol (D₃) (not its metabolites) should be used to correct vitamin D deficiency and raise serum 25(OH)D to the target concentration. In contrast, calcifediol [25(OH)D] raises serum 25(OH)D concentrations rapidly and is the agent of choice in emergencies such as infections, for those who are in ICUs, and for insufficient hepatic 25-hydroxylase (CYP2R1) activity. In contrast, calcitriol is necessary to maintain serum-ionized calcium concentration in persons with advanced renal failure and hypoparathyroidism. Calcitriol is, however, ineffective in most other conditions, including infections, and as vitamin D replacement therapy. Considering the high costs and higher incidence of adverse effects due to narrow therapeutic margins (ED50), 1α-vitamin D analogs, such as 1α-(OH)D and 1,25(OH)₂D, should not be used for other conditions. Calcifediol analogs cost 20 times more than D₃-thus, they are not indicated as a routine vitamin D supplement for hypovitaminosis D, osteoporosis, or renal failure. Healthcare workers should resist accepting inappropriate promotions, such as calcifediol for chronic renal failure and calcitriol for osteoporosis or infections-there is no physiological rationale for doing so. Maintaining the population's vitamin D sufficiency (above 40 ng/mL) with vitamin D₃ supplements and/or daily sun exposure is the most cost-effective way to reduce chronic diseases and sepsis, overcome viral epidemics and pandemics, and reduce healthcare costs. Furthermore, vitamin D sufficiency improves overall health (hence reducing absenteeism), reduces the severity of chronic diseases such as metabolic and cardiovascular diseases and cancer, decreases all-cause mortality, and minimizes infection-related complications such as sepsis and COVID-19-related hospitalizations and deaths. Properly using vitamin D is the most cost-effective way to reduce chronic illnesses and healthcare costs: thus, it should be a part of routine clinical care.

The Complex ABCG5/ABCG8 Regulates Vitamin D Absorption Rate and Contributes to its Efflux from the Intestine

SCOPE

Most people are vitamin D insufficient around the world. Vitamin D intestinal absorption should thus be optimized. The role of the ATP-binging cassette G5/G8 (ABCG5/G8) heterodimer in vitamin D intestinal efflux is investigated.

METHODS AND RESULTS

Both cholecalciferol and 25-hydroxycholecalciferol apical effluxes are increased by ABCG5/G8 overexpression in human Griptite cells. Mice deficient in ABCG5/G8 at the intestinal level (I-Abcg5/g8^-/- mice) display an accumulation of cholecalciferol in plasma in females and in liver in males compared to control animals. I-Abcg5/g8^{- / -} mice display a delay in cholecalciferol postprandial response after gavage compared with controls. 25-Hydroxycholecalciferol transfer from plasma to lumen is observed in vivo in intestine-perfused mice, and the lack of intestinal ABCG5/G8 complex induces a decrease in this efflux, while vitamin D bile excretion remains unchanged.

CONCLUSION

Overall, it is showed for the first time that the ABCG5/G8 heterodimer regulates the kinetics of absorption of dietary vitamin D by contributing to its efflux back to the lumen, and that it also participates in vitamin D transintestinal efflux.

ABCB1 (P-glycoprotein) regulates vitamin D absorption and contributes to its transintestinal efflux

Efficient intestinal absorption of dietary vitamin D is required in most people to ensure an adequate status. Thus, we investigated the involvement of ATP binding cassette subfamily B member 1 (ABCB1) in vitamin D intestinal efflux. Both cholecalciferol (D₃) and 25-hydroxycholecalciferol [25(OH)D₃] apical effluxes were decreased by chemical inhibition of ABCB1 in Caco-2 cells and increased by ABCB1 overexpression in Griptites or Madin-Darby canine kidney type II cells. Mice deficient for the 2 murine ABCB1s encoded by Abcb1a and Abcb1b genes ( Abcb1^-/-) displayed an accumulation of 25(OH)D₃ in plasma, intestine, brain, liver, and kidneys, together with an increased D₃ postprandial response after gavage compared with controls. 25(OH)D₃ efflux through Abcb1^-/- intestinal explants was markedly decreased compared with controls. This reduction of 25(OH)D₃ transfer from plasma to lumen was further confirmed in vivo in intestine-perfused mice. Docking experiments established that both D₃ and 25(OH)D₃ could bind with high affinity to Caenorhabditis elegans P-glycoprotein, used as an ABCB1 model. Finally, in a group of 39 healthy male adults, a single-nucleotide polymorphism (SNP) in ABCB1 (rs17064) was significantly associated with the fasting plasma 25(OH)D₃ concentration. Thus, we showed here for the first time that ABCB1 is involved in neo-absorbed vitamin D efflux by the enterocytes and that it also contributes to vitamin D transintestinal excretion and likely impacts vitamin D status.-Margier, M., Collet, X., le May, C., Desmarchelier, C., André, F., Lebrun, C., Defoort, C., Bluteau, A., Borel, P., Lespine, A., Reboul, E. ABCB1 (P-glycoprotein) regulates vitamin D absorption and contributes to its transintestinal efflux.

Calcium and vitamin D in human health: Hype or real?

The incidence and prevalence of vitamin D deficiency are increasing worldwide. It is estimated that over 50% of the world's population have low vitamin D (i.e., hypovitaminosis D; serum levels below 30 ng/mL). In vitamin D inadequacy, human physiological systems work inefficiently. In humans, 80% of the vitamin D is synthesized in the presence of ultraviolet rays from sunlight; for the rest, we rely on diet and nowadays, supplements. The latter becomes important when one is exposed to less than optimal amounts of sunlight, inability to generate vitamin D in the skin efficiently, and/or having conditions that lead to decreased intestinal absorption or increased catabolism of vitamin D. The normal serum 25-hydroxyvitamin D [25(OH)D] level is around 30 ng/mL (75 nmol/L) and the optimal range is between 30 and 60 ng/mL (75-150 nmol/L). In 2011, the Institute of Medicine (IOM) suggested that 600IU of vitamin D is adequate for people below age 71 who are not exposed to sunshine. Although this might be relevant to the ambulatory healthy white Caucasians to achieve serum 25(OH)D level of 20 ng/mL, but it is insufficient for other ethnic groups. Moreover, the IOM recommendations are not suitable for those who live outside North America. Vitamin D requirements are higher during adolescence, pregnancy and lactation, and in many other disease conditions. Most clinicians consider 30 ng/mL as the minimum serum level of 25(OH)D necessary to maintain good health. In the absence of sunlight exposure and with daily oral intake of 600IU vitamin D, very few people would reach serum 25(OH)D level above 30 ng/mL. While an additional daily intake of 1000IU of vitamin D are required for people with lighter-skin color, those with darker complexion and the elderly, require a minimum of 2000IU/day to maintain serum 25(OH)D levels over 30 ng/mL; 5000 IU/day supplement is considered as the safe daily upper limit of supplementation. Vulnerable groups such as the disabled and/or house-bound, obese, with gastrointestinal abnormalities and/or malabsorption syndromes, institutionalized people (e.g., nursing homes, prisons, etc.), and pregnant and lactating women need approximately 4000IU per day for optimal physiological activity. Vitamin D is essential for gastrointestinal calcium absorption, mineralization of osteoid tissue and maintenance of serum ionized calcium level. It is also important for other physiological functions, such as muscle strength, neuromuscular coordination, hormone release, subduing autoimmunity, and curtailing the development of certain cancers.

High Prevalence of Vitamin D Deficiency in Cambodian Women: A Common Deficiency in a Sunny Country

Recent studies have shown that in spite of being generally close to the equator; vitamin D deficiency is common in South East Asian countries. In order to quantify micronutrient status for women and children in Cambodia; a nationally-representative survey was conducted in 2014 linked to the Cambodian Demographic Health Survey. The countrywide median of 25(OH)D was, respectively, 64.9 and 91.1 nmol/L for mothers and children. Based on The Endocrine Society cutoffs (>50<75 nmol/L = insufficiency; ≤50 nmol/L = deficiency); 64.6% of mothers and 34.8% of their children had plasma vitamin D concentrations indicating insufficiency or deficiency. For deficiency alone, 29% of the mothers were found to be vitamin D deficient, but only 13.4% of children. Children who live in urban areas had a 43% higher rate of vitamin D insufficiency versus those who live in rural areas (OR; 1.434; 95% CI: 1.007; 2.041). However, such differences were not observed in their mothers. The high prevalence of vitamin D deficiency is likely in part due to lifestyle choices, including sun avoidance, increasingly predominant indoor work, and covered transport. These survey findings support the need for a broader national Cambodian study incorporating testing of adult men, adolescents and the elderly, and encompassing other parameters such as skeletal health. However, the data presented in this study already show significant deficiencies which need to be addressed and we discuss the benefit of establishing nationally-mandated food fortification programs to enhance the intake of vitamin D.

Vitamin D levels in white coat and sustained hypertension

OBJECTIVE

The plasma levels of vitamin D in patients with white coat hypertension (WCHT) have not been studied previously. The aim of this study was to evaluate vitamin D levels in WCHT and compare with sustained hypertension (SHT) and with normotension (NT).

PATIENTS AND METHODS

Fifty-three normotensive, 42 WCHT, and 59 SHT patients were recruited in this study. The participants were matched for age, sex, and BMI. The vitamin D levels were determined using the electrochemiluminescence immunoassay method.

RESULTS

Plasma vitamin D levels were significantly lower in SHT than in the WCHT and NT groups (26.4±4.9, 34.3±3.6, and 36±5 ng/ml, respectively), and were similar in the WCHT and NT groups. There was a negative correlation between vitamin D levels and blood pressure parameters such as clinic systolic blood pressure (SBP), clinic diastolic blood pressure (DBP), 24-h SBP, 24-h DBP, daytime SBP, daytime DBP, night-time SBP, and night-time DBP (r=-0.554, -0.419, -0.629, -0.427, -0.559, -0.534, -0.607, -0.462, respectively, and all P<0.001) in the entire study group. Clinic SBP (B±SE=-0.97±0.037, P=0.009) and 24-h SBP (B±SE=-0.138±0.055, P=0.013) were identified as predictors for vitamin D levels in the entire study group.

CONCLUSION

Our data show that sustained hypertensive patients have lower vitamin D levels than white coat hypertensive and normotensive individuals. White coat hypertensive patients without other cardiovascular risk factors have higher vitamin D levels than sustained hypertensive patients, suggesting that they have a lower cardiovascular risk.

Pediatric intestinal failure: Predictors of metabolic bone disease

PURPOSE

The purpose of this study was to identify risk factors for the development of metabolic bone disease (MBD) in pediatric intestinal failure (IF).

METHODS

A retrospective single-center study of 36 pediatric IF patients who were screened for MBD was performed. Bone mineral density (BMD) was measured using dual-energy X-ray absorptiometry (DXA). Simple regression analysis was initially performed to screen predictors, followed by multivariate step-wise linear regression analysis to identify risk factors of MBD.

RESULTS

Mean lumbar spine BMD Z-score was -1.16 ± 1.32, and 50.0% of patients had a BMD Z-score less than -1.0. Deficiency of 25-hydroxyvitamin-D (25-OHD <30 ng/ml) was present in the 63.8% of patients, while 25.0% had hyperparathyroidism (intact parathyroid hormone (PTH)>55 pg/ml). Seven patients (19.4%) had bone pain, of which 4 (11.1%) suffered a pathologic fracture. Using multivariate analysis, parenteral nutrition (PN) duration predicted decreased BMD (B=-0.132, p=0.006). Serum 25-OHD nonsignificantly correlated with BMD Z-score (B=0.024, p=0.092). Interestingly, repeat DXA after increasing vitamin D supplementation showed no improvement in BMD Z-score (-1.18 ± 1.49 vs -1.36 ± 1.47, p=0.199).

CONCLUSIONS

Pediatric IF is associated with a significant risk of MBD, which is predicted by the duration of PN-dependence. These findings underscore the importance of BMD monitoring. Better therapies for treating IF-associated MBD are needed.

Vitamin D deficiency and low bone mineral density in pediatric and young adult intestinal failure

OBJECTIVES

The aim of the present study was to determine the prevalence and predisposing factors for vitamin D deficiency and low bone mineral density (BMD) in patients with intestinal failure (IF).

METHODS

A retrospective review of patients with IF managed at the Cincinnati Children's Hospital Medical Center. IF was defined as history of parenteral nutrition (PN) >30 days. Vitamin D deficiency was defined as serum 25-hydroxyvitamin D (25 (OH) D) <20 ng/dL. Reduced BMD was defined using dual x-ray absorptiometry z score ≤-2. A binary logistic regression model was used to test for association of significant risk factors and the outcome variables after univariate analyses.

RESULTS

One hundred and twenty-three patients with median age of 4 years (range 3-22 years) were evaluated. Forty-nine (39.8%) patients had at least a documented serum 25 (OH) D deficiency during the study interval, whereas 10 of 80 patients (12.5%) with dual x-ray absorptiometry scans completed had a low BMD z score. Age at study entry was associated with both 25 (OH) D deficiency (P = 0.01) and low BMD z score (P = 0.03). Exclusive PN at study entry was associated with reduced bone mass (P = 0.03). There was no significant association between vitamin D deficiency and low BMD z score (P = 0.31).

CONCLUSIONS

The risk of 25 (OH) D deficiency and low BMD z score increases with age among patients with IF. Strategies for monitoring and preventing abnormal bone health in older children receiving exclusive PN need to be developed and evaluated.

Vitamin D in the new millennium

The incidence of vitamin D deficiency is rising worldwide, yet in the vast majority of patients, the condition remains undiagnosed and untreated. Current evidence overwhelmingly indicates that supplemental doses greater than 800 IU/day have beneficial effects on the musculoskeletal system, improving skeletal homeostasis, thus leading to fewer falls and fractures. Evidence is also accumulating on the beneficial effects of vitamin D on extraskeletal systems, such as improving immune health, autoimmune disorders, cancer, neuromodulation, diabetes, and metabolic syndrome. The cause-effect relationship of vitamin D deficiency with increasing incidences of nonskeletal disorders is being investigated. Published reports support the definition of sufficiency, serum levels of 25-hydroxyvitamin D [25(OH)D] greater than 30 ng/mL (75 nmol/L). To achieve this, most people need vitamin D supplementation ranging from 600 to 2000 IU/day; consumption up to of 5000 international units (IU) per day of vitamin D is reported as safe. Although light-skinned individuals need 1000 IU/day of vitamin D, elderly and dark-skinned individuals are likely to need approximately 2000 IU/day to maintain serum 25(OH)D levels greater than 30 ng/mL. Other vulnerable patients, such as the obese, those who have undergone bariatric surgery, and those with gastrointestinal malabsorption syndromes, may require higher doses of vitamin D to maintain normal serum levels and be healthy.