•Vitamin D metabolites circulate bound to DBP and, to a lesser extent, albumin.•25OHD binds to DBP with high affinity and is thus strongly influenced by DBP.•For some cells unbound or ‘free’ 25OHD ...may be the most bioavailable form of 25OHD.•DBP may play a pivotal role in the intracrine synthesis of 1,25(OH)2D by immune cells.•Free or bioavailable 25OHD is influenced by DBP concentration and binding affinity.
The last five years have witnessed a remarkable renaissance in vitamin D research and a complete re-evaluation of its benefits to human health. Two key factors have catalyzed these changes. First, it now seems likely that localized, tissue-specific, conversion of 25-hydroxyvitamin D (25OHD) to 1,25-dihydroxyvitamin D (1,25(OH)2D) drives many of the newly recognized effects of vitamin D on human health. The second key factor concerns the ongoing discussion as to what constitutes adequate or optimal serum vitamin D (25OHD) status, with the possibility that vitamin D-deficiency is common to communities across the globe. These two concepts appear to be directly linked when low serum concentrations of 25OHD compromise intracrine generation of 1,25(OH)2D within target tissues. But, is this an over-simplification? Pro-hormone 25OHD is a lipophilic molecule that is transported in the circulation bound primarily to vitamin D binding protein (DBP). While the association between 25OHD and DBP is pivotal for renal handling of 25OHD and endocrine synthesis of 1,25(OH)2D, what is the role of DBP for extra-renal synthesis of 1,25(OH)2D? We hypothesize that binding to DBP impairs delivery of 25OHD to the vitamin D-activating enzyme 1α-hydroxylase in some target cells. Specifically, it is unbound, ‘free’ 25OHD that drives many of the non-classical actions of vitamin D. Levels of ‘free’ 25OHD are dependent on the concentration of DBP and alternative serum binding proteins such as albumin, but will also be influenced by variations in DBP binding affinity for specific vitamin D metabolites. The aim of this review will be to discuss the merits of ‘free 25OHD’ as an alternative marker of vitamin D status, particularly in the context of non-classical responses to vitamin D.
This article is part of a Special Issue entitled ‘16th Vitamin D Workshop’.
The vitamin D receptor is highly expressed in the gastrointestinal tract where it transacts gene expression. With current limited understanding of the interactions between the gut microbiome and ...vitamin D, we conduct a cross-sectional analysis of 567 older men quantifying serum vitamin D metabolites using LC-MSMS and defining stool sub-Operational Taxonomic Units from16S ribosomal RNA gene sequencing data. Faith's Phylogenetic Diversity and non-redundant covariate analyses reveal that the serum 1,25(OH)
D level explains 5% of variance in α-diversity. In β-diversity analyses using unweighted UniFrac, 1,25(OH)
D is the strongest factor assessed, explaining 2% of variance. Random forest analyses identify 12 taxa, 11 in the phylum Firmicutes, eight of which are positively associated with either 1,25(OH)
D and/or the hormone-to-prohormone 1,25(OH)
D/25(OH)D "activation ratio." Men with higher levels of 1,25(OH)
D and higher activation ratios, but not 25(OH)D itself, are more likely to possess butyrate producing bacteria that are associated with better gut microbial health.
To evaluate whether objectively measured sleep characteristics are associated with mortality risk independent of inflammatory burden and comorbidity.
The Osteoporotic Fractures in Men Sleep Study ...(conducted in 2003-2005) included community-dwelling older men (n = 2531; average standard deviation {SD} age = 76.3 (5.5) years). Sleep measures from in-home polysomnography and wrist actigraphy and assessments of serum inflammatory markers levels (C-reactive protein, interleukin-6, tumor necrosis factor α, tumor necrosis factor α soluble receptor II, and interferon-γ) were obtained. Vital status was ascertained over an average (SD) follow-up of 7.4 (1.9 SD) years.
Three of the seven main sleep measures examined were independently associated with greater inflammatory burden. Mortality risk associated with prolonged (≥10% total sleep time) blood oxygen desaturation and short (<5 hours) sleep duration was attenuated to nonsignificance after adjusting for inflammatory burden or medical burden/lifestyle factors. Severe blood oxygen desaturation (adjusted hazard ratio aHR = 1.57, 95% confidence interval CI = 1.11-2.22), sleep fragmentation (aHR = 1.32, 95% CI = 1.12-1.57), and a lower percentage of sleep in rapid eye movement (aHR per SD = 0.90, 95% CI = 0.93-0.97) were independently associated with mortality.
Short sleep duration and prolonged blood oxygen desaturation were independently associated with inflammatory burden, which attenuated associations between these sleep characteristics and mortality. Medical and life-style factors also substantially attenuated most sleep-mortality associations, suggesting complex relations between sleep, inflammation, and disease. Sleep fragmentation, severe blood oxygen desaturation, and the percentage of sleep time in rapid eye movement were independently related to mortality risk. Future studies with repeated measures of mediators/confounds will be necessary to achieve a mechanistic understanding of sleep-related mortality risk.
Adequate sleep timed appropriately during the circadian night is important for numerous biological processes and systems. New evidence suggests that both sleep timing and duration may be important ...for optimal bone health as well. This review examines the diurnal variation of bone turnover markers (BTMs) and the importance of circadian clock genes in regulating bone mass. In addition, this review explores the evidence for a link between shift work (and its associated disturbances in sleep duration/quality and circadian alignment) and alterations in bone metabolism and bone health. Finally, we review how commonly used medications and over-the-counter substances (e.g. caffeine, melatonin) complicate the relationship between sleep and circadian disorders and bone health.
Direct assessment of skeletal muscle mass in older adults is clinically challenging. Relationships between lean mass and late-life outcomes have been inconsistent. The D3-creatine dilution method ...provides a direct assessment of muscle mass.
Muscle mass was assessed by D3-creatine (D3Cr) dilution in 1,382 men (mean age, 84.2 years). Participants completed the Short Physical Performance Battery (SPPB); usual walking speed (6 m); and dual x-ray absorptiometry (DXA) lean mass. Men self-reported mobility limitations (difficulty walking 2-3 blocks or climbing 10 steps); recurrent falls (2+); and serious injurious falls in the subsequent year. Across quartiles of D3Cr muscle mass/body mass, multivariate linear models calculated means for SPPB and gait speed; multivariate logistic models calculated odds ratios for incident mobility limitations or falls.
Compared to men in the highest quartile, those in the lowest quartile of D3Cr muscle mass/body mass had slower gait speed (Q1: 1.04 vs Q4: 1.17 m/s); lower SPPB (Q1: 8.4 vs Q4: 10.4 points); greater likelihood of incident serious injurious falls (odds ratio OR Q1 vs Q4: 2.49, 95% confidence interval CI: 1.37, 4.54); prevalent mobility limitation (OR Q1 vs Q4,: 6.1, 95% CI: 3.7, 10.3) and incident mobility limitation (OR Q1 vs Q4: 2.15 95% CI: 1.42, 3.26); p for trend < .001 for all. Results for incident recurrent falls were in the similar direction (p = .156). DXA lean mass had weaker associations with the outcomes.
Unlike DXA lean mass, low D3Cr muscle mass/body mass is strongly related to physical performance, mobility, and incident injurious falls in older men.
Abstract
Background
Whether low muscle mass is a risk factor for disability and mortality is unclear. Associations between approximations of muscle mass (including lean mass from dual-energy x-ray ...absorptiometry DXA), and these outcomes are inconsistent.
Methods
Muscle mass measured by deuterated creatine (D3Cr) dilution and appendicular lean mass (ALM, by DXA) were assessed at the Year 14 Visit (2014–2016) of the prospective Osteoporotic Fractures in Men study (N = 1,425, age 77–101 years). Disability in activities of daily living (ADLs), instrumental ADLs, and mobility tasks was self-reported at the Year 14 visit and 2.2 years later; deaths were centrally adjudicated over 3.3 years. Relative risks and 95% confidence intervals (CI) were estimated per standard deviation decrement with negative binomial, logistic regression, or proportional hazards models.
Results
In age- and clinical center-adjusted models, the relative risks per decrement in D3Cr muscle mass/wgt was 1.9 (95% CI: 1.2, 3.1) for incident self-reported ADL disability; 1.5 (95% CI: 1.3, 1.9) for instrumental ADL disability; and 1.8 (95% CI: 1.5, 2.2) for mobility disability. In age-, clinical center-, and weight-adjusted models, the relative risks per decrement in D3Cr muscle mass was 1.8 (95% CI: 1.5, 2.2) for all-cause mortality. In contrast, lower DXA ALM was not associated with any outcome. Associations of D3Cr muscle mass with these outcomes were slightly attenuated after adjustment for confounding factors and the potentially mediating effects of strength and physical performance.
Conclusions
Low muscle mass as measured by D3Cr dilution is a novel risk factor for clinically meaningful outcomes in older men.
Aging is variably but inevitably accompanied by declines in health; concomitantly, in men, circulating sex-steroid levels fall with age.
1
To what extent these two processes are causally linked and ...whether testosterone therapy can prevent or ameliorate important age-related problems have been major issues in men’s health. In 2003, a committee assembled by the Institute of Medicine (IOM) found a paucity of randomized, placebo-controlled clinical trials involving older men and noted a lack of definite evidence that testosterone therapy conferred benefits.
2
The committee recommended that clinical trials be initiated, first to evaluate the efficacy of testosterone supplementation in older men and . . .
ABSTRACT
Annual costs are enormous for musculoskeletal diseases such as osteoporosis and sarcopenia and for bone and muscle injuries, costing billions annually in health care. Although it is clear ...that muscle and bone development, growth, and function are connected, and that muscle loads bone, little is known regarding cellular and molecular interactions between these two tissues. A conference supported by the National Institutes of Health (NIH) and the American Society for Bone and Mineral Research (ASBMR) was held in July 2012 to address the enormous burden of musculoskeletal disease. National and international experts in either bone or muscle presented their findings and their novel hypotheses regarding muscle‐bone interactions to stimulate the exchange of ideas between these two fields. The immediate goal of the conference was to identify critical research themes that would lead to collaborative research interactions and grant applications focusing on interactions between muscle and bone. The ultimate goal of the meeting was to generate a better understanding of how these two tissues integrate and crosstalk in both health and disease to stimulate new therapeutic strategies to enhance and maintain musculoskeletal health.
ABSTRACT
Concern about the risk of bone loss in astronauts as a result of prolonged exposure to microgravity prompted the National Aeronautics and Space Administration to convene a Bone Summit with a ...panel of experts at the Johnson Space Center to review the medical data and research evidence from astronauts who have had prolonged exposure to spaceflight. Data were reviewed from 35 astronauts who had served on spaceflight missions lasting between 120 and 180 days with attention focused on astronauts who (1) were repeat fliers on long‐duration missions, (2) were users of an advanced resistive exercise device (ARED), (3) were scanned by quantitative computed tomography (QCT) at the hip, (4) had hip bone strength estimated by finite element modeling, or (5) had lost >10% of areal bone mineral density (aBMD) at the hip or lumbar spine as measured by dual‐energy X‐ray absorptiometry (DXA). Because of the limitations of DXA in describing the effects of spaceflight on bone strength, the panel recommended that the U.S. space program use QCT and finite element modeling to further study the unique effects of spaceflight (and recovery) on bone health in order to better inform clinical decisions.
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