This review was conducted to clarify both the complex interrelationship between adiposity and vitamin D and the clinical implications of vitamin D status on metabolic abnormalities associated with ...obesity. Obesity increases the risk of vitamin D deficiency, a finding consistently reported across all ages and in different population groups. According to genetic studies, this is driven by the effect of higher adiposity, which causes a reduction in circulating concentrations of 25-hydroxyvitamin D 25(OH)D, used as an indicator of vitamin D status. Conversely, higher concentrations of 25(OH)D do not appear to affect the risk of obesity. Evidence from clinical trials using vitamin D supplementation to achieve weight reduction is limited. Some trials, however, have suggested that concomitant supplementation with vitamin D and calcium potentially reduces central fat deposits, especially in individuals with low dietary calcium intakes. Adiposity has important implications for the efficacy of vitamin D supplementation, and increases in 25(OH)D concentrations are generally lower in obese than in normal-weight individuals. Active hormonal vitamin D has many mechanistic effects, both physiologically and biochemically, that could counteract the harmful effects of obesity on metabolism and reduce the risks of metabolic abnormalities and tissue damage consequent to adiposity. Whether improvements in the overall obesogenic metabolic profile can be achieved by vitamin D supplementation, however, is still unknown.
Our knowledge of vitamin D has come a long way since the 100 years it took for doctors to accept, between 1860 and 1890, that both sunlight and cod liver oil (a well-known folk remedy) cured and ...prevented rickets. Vitamins D2/D3 were discovered exactly a hundred years ago, and over the last 50 years vitamin D has been found to have many effects on virtually all human tissues and not just on bone health, while mechanisms affecting the actions of vitamin D at the cellular level are increasingly understood, but deficiency persists globally. Observational studies in humans have shown that better provision of vitamin D is strongly associated, dose-wise, with reductions in current and future health risks in line with the known actions of vitamin D. Randomised controlled trials, commonly accepted as providing a ‘gold standard’ for assessing the efficacy of new forms of treatment, have frequently failed to provide supportive evidence for the expected health benefits of supplementation. Such RCTs, however, have used designs evolved for testing drugs while vitamin D is a nutrient; the appreciation of this difference is critical to identifying health benefits from existing RCT data and for improving future RCT design. This report aims, therefore, to provide a brief overview of the evidence for a range of non-bony health benefits of vitamin D repletion; to discuss specific aspects of vitamin D biology that can confound RCT design and how to allow for them.
Although geographic ecological studies and observational studies find that ultraviolet B exposure and 25-hydroxyvitamin D 25(OH)D concentrations are inversely correlated with 15-20 types of cancer, ...few randomized controlled trials (RCTs) of vitamin D support those findings. The poor design of some RCTs may account for that lack of support. Most vitamin D RCTs to date have considered the vitamin D dose, rather than initial, final, or changes in, serum 25(OH)D concentrations. Here a model is developed for use in designing and analyzing vitamin D RCTs with application to cancer incidence. The input variables of the model are vitamin D dose, baseline and achieved 25(OH)D concentrations, known rates of cancer for the population, and numbers of participants for the treatment and placebo arms is estimated-vitamin D dosage and numbers of participants are varied to achieve desired hazard ratio significance, using information from two vitamin D RCTs on cancer incidence conducted in Nebraska with good agreement between the model estimates and reported hazard ratios. Further improvements to the conduct of vitamin D RCTs would be to start the trial with a moderate bolus dose to achieve the desired 25(OH)D concentrations, and bloodspot 25(OH)D assay use in summer and winter annually to monitor seasonal and long-term changes in 25(OH)D concentration and compliance, and to allow dosage adjustment for achievement of desired vitamin D status.
Diabetes is an increasing epidemic; hyperglycemia results from lack of insulin or inadequate insulin secretion following increases in insulin resistance. Huge costs are placed upon sufferers and ...health providers, aggravated as serious and disabling complications develop. Thus, measures to reduce the diabetic burden are public health concerns. Vitamin D, identified ≈100 years ago, promotes calcium absorption and utilization, preventing and curing rickets & osteomalacia. Calcium is necessary for insulin secretion, suggesting vitamin D may contribute to maintaining insulin secretion. Vitamin D, formed in skin in bright sunshine, is scarce in foodstuffs. Data linking hypovitaminosis D to hyperglycemia, type 2 diabetes (T2DM) and metabolic disorders increasing cardiovascular risk metabolic 'syndrome' has accumulated over ≈40 years. Many mechanisms are known whereby hypovitaminosis D could be causal, e.g. by increasing insulin resistance, reducing insulin secretion and increasing autoimmune or inflammatory damage to pancreatic islets. Major questions still to be answered are whether increasing vitamin D status to the maximum seen in healthy people would reduce the risk of diabetes, the severity of the disease or of its complications, including cardiovascular disease. These questions urgently require answers. If on-going/ planned RCTs confirm causality, maintenance of adequate vitamin D status at the population level by food-fortification or supplementation would be cost-effective measures likely to reduce the burden and costs of diabetes to individuals and health services. Additionally, vitamin D(2/3) supplementation is cheap but whether some non-hypercalcemia-inducing analogue may prove safer has not yet been addressed at the population level.
Although observational studies of health outcomes generally suggest beneficial effects with, or following, higher serum 25-hydroxyvitamin D 25(OH)D concentrations, randomized controlled trials (RCTs) ...have generally not supported those findings. Here we review results from observational studies and RCTs regarding how vitamin D status affects several nonskeletal health outcomes, including Alzheimer’s disease and dementia, autoimmune diseases, cancers, cardiovascular disease, COVID-19, major depressive disorder, type 2 diabetes, arterial hypertension, all-cause mortality, respiratory tract infections, and pregnancy outcomes. We also consider relevant findings from ecological, Mendelian randomization, and mechanistic studies. Although clear discrepancies exist between findings of observational studies and RCTs on vitamin D and human health benefits these findings should be interpreted cautiously. Bias and confounding are seen in observational studies and vitamin D RCTs have several limitations, largely due to being designed like RCTs of therapeutic drugs, thereby neglecting vitamin D’s being a nutrient with a unique metabolism that requires specific consideration in trial design. Thus, RCTs of vitamin D can fail for several reasons: few participants’ having low baseline 25(OH)D concentrations, relatively small vitamin D doses, participants’ having other sources of vitamin D, and results being analyzed without consideration of achieved 25(OH)D concentrations. Vitamin D status and its relevance for health outcomes can usefully be examined using Hill’s criteria for causality in a biological system from results of observational and other types of studies before further RCTs are considered and those findings would be useful in developing medical and public health policy, as they were for nonsmoking policies. A promising approach for future RCT design is adjustable vitamin D supplementation based on interval serum 25(OH)D concentrations to achieve target 25(OH)D levels suggested by findings from observational studies.
Many diseases have large seasonal variations in which winter overall mortality rates are about 25% higher than in summer in mid-latitude countries, with cardiovascular diseases and respiratory ...infections and conditions accounting for most of the variation. Cancers, by contrast, do not usually have pronounced seasonal variations in incidence or mortality rates. This narrative review examines the epidemiological evidence for seasonal variations in blood pressure, cardiovascular disease rates and respiratory viral infections in relation to atmospheric temperature and humidity, and solar UV exposure through vitamin D production and increased blood concentrations of nitric oxide. However, additional mechanisms most likely exist by which solar radiation reduces the risk of seasonally varying diseases. Some studies have been reported with respect to temperature without considering solar UV doses, although studies regarding solar UV doses, such as for respiratory infections, often consider whether temperature can affect the findings. More research is indicated to evaluate the relative effects of temperature and sun exposure on the seasonality of mortality rates for several diseases. Since solar ultraviolet-B (UVB) doses decrease to vanishingly small values at higher latitudes in winter, the use of safe UVB lamps for indoor use in winter may warrant consideration.
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