Vitamin D and obesity: current perspectives and future directions

In recent years, new functional roles of vitamin D beyond its traditional role in calcium homoeostasis and bone metabolism have emerged linking the fat-soluble vitamin to various non-communicable diseases. Vitamin D deficiency (25-hydroxyvitamin D (25(OH)D) < 25-30 nmol/l) and sub-optimal status (25(OH)D < 50-100 nmol/l) are increasingly associated with unfavourable metabolic phenotypes, including insulin resistance, type 2 diabetes and CVD; conditions also commonly linked with overweight and obesity. Early studies reported poor vitamin D status in the morbidly obese. More recently, it has been observed that a graded relationship between vitamin D status and BMI, or specifically adiposity, exists in the general population. A number of hypotheses have been proposed to explain the potential mechanisms whereby alterations in the vitamin D endocrine system occur in the obese state. Plausible explanations include sequestration in adipose tissue, volumetric dilution or negative feedback mechanisms from increased circulating 1,25-dihydroxyvitamin D3. Others hypothesise that heavier individuals may partake in less outdoor activity, may also cover-up and wear more clothing than leaner individuals, thus decreasing sun exposure and limiting endogenous production of cholecalciferol in the skin. Moreover, in some but not all studies, BMI and adiposity have been negatively associated with the change in vitamin D status following vitamin D supplementation. It therefore remains unclear if body size and/or adiposity should be taken into account when determining the dietary requirements for vitamin D. This review will evaluate the current evidence linking vitamin D status and supplementation to overweight and obesity, and discuss the implications for setting dietary requirements.

Vitamin D and colorectal cancer: molecular, epidemiological and clinical evidence

In many cells throughout the body, vitamin D is converted into its active form calcitriol and binds to the vitamin D receptor (VDR), which functions as a transcription factor to regulate various biological processes including cellular differentiation and immune response. Vitamin D-metabolising enzymes (including CYP24A1 and CYP27B1) and VDR play major roles in exerting and regulating the effects of vitamin D. Preclinical and epidemiological studies have provided evidence for anti-cancer effects of vitamin D (particularly against colorectal cancer), although clinical trials have yet to prove its benefit. In addition, molecular pathological epidemiology research can provide insights into the interaction of vitamin D with tumour molecular and immunity status. Other future research directions include genome-wide research on VDR transcriptional targets, gene-environment interaction analyses and clinical trials on vitamin D efficacy in colorectal cancer patients. In this study, we review the literature on vitamin D and colorectal cancer from both mechanistic and population studies and discuss the links and controversies within and between the two parts of evidence.

Vitamin D modulates adipose tissue biology: possible consequences for obesity?

Cross-sectional studies depict an inverse relationship between vitamin D (VD) status reflected by plasma 25-hydroxy-vitamin D and obesity. Furthermore, recent studies in vitro and in animal models tend to demonstrate an impact of VD and VD receptor on adipose tissue and adipocyte biology, pointing to at least a part-causal role of VD insufficiency in obesity and associated physiopathological disorders such as adipose tissue inflammation and subsequent insulin resistance. However, clinical and genetic studies are far less convincing, with highly contrasted results ruling out solid conclusions for the moment. Nevertheless, prospective studies provide interesting data supporting the hypothesis of a preventive role of VD in onset of obesity. The aim of this review is to summarise the available data on relationships between VD, adipose tissue/adipocyte physiology, and obesity in order to reveal the next key points that need to be addressed before we can gain deeper insight into the controversial VD-obesity relationship.

Dietary and pharmacological compounds altering intestinal calcium absorption in humans and animals

The intestine is the only gate for the entry of Ca to the body in humans and mammals. The entrance of Ca occurs via paracellular and intracellular pathways. All steps of the latter pathway are regulated by calcitriol and by other hormones. Dietary and pharmacological compounds also modulate the intestinal Ca absorption process. Among them, dietary Ca and P are known to alter the lipid and protein composition of the brush-border and basolateral membranes and, consequently, Ca transport. Ca intakes are below the requirements recommended by health professionals in most countries, triggering important health problems. Chronic low Ca intake has been related to illness conditions such as osteoporosis, hypertension, renal lithiasis and incidences of human cancer. Carbohydrates, mainly lactose, and prebiotics have been described as positive modulators of intestinal Ca absorption. Apparently, high meat proteins increase intestinal Ca absorption while the effect of dietary lipids remains unclear. Pharmacological compounds such as menadione, dl-butionine-S,R-sulfoximine and ursodeoxycholic acid also modify intestinal Ca absorption as a consequence of altering the redox state of the epithelial cells. The paracellular pathway of intestinal Ca absorption is poorly known and is under present study in some laboratories. Another field that needs to be explored more intensively is the influence of the gene × diet interaction on intestinal Ca absorption. Health professionals should be aware of this knowledge in order to develop nutritional or medical strategies to stimulate the efficiency of intestinal Ca absorption and to prevent diseases.

Optimal vitamin D levels in Crohn's disease: a review

Vitamin D deficiency is common among patients with Crohn's disease. Serum 25-hydroxyvitamin D (25(OH)D) is the best measure of an individual's vitamin D status and current cut-off ranges for sufficiency are debatable. Several factors contribute to vitamin D deficiency in Crohn's disease. These include inadequate exposure to sunlight, inadequate dietary intake, impaired conversion of vitamin D to its active metabolite, increased catabolism, increased excretion and genetic variants in vitamin D hydroxylation and transport. The effects of low 25(OH)D on outcomes other than bone health are understudied in Crohn's disease. The aim of the present review is to discuss the potential roles of vitamin D and the possible levels required to achieve them. Emerging evidence suggests that vitamin D may have roles in innate and adaptive immunity, in the immune-pathogenesis of Crohn's disease, prevention of Crohn's disease-related hospitalisations and surgery, in reducing disease severity and in colon cancer prevention. The present literature appears to suggest that 25(OH)D concentrations of ≥75 nmol/l may be required for non-skeletal effects; however, further research on optimal levels is required.

Is there a role for vitamin D in supporting cognitive function as we age?

Globally, an estimated 46 million people are currently living with dementia and this figure is projected to increase 3-fold by 2050, highlighting this major public health concern and its substantial associated healthcare costs. With pharmacological treatment yet to reach fruition, the emphasis on evidence-based preventative lifestyle strategies is becoming increasingly important and several modifiable lifestyle factors have been identified that may preserve cognitive health. These include good cardiovascular health, physical activity, low alcohol intake, smoking and a healthy diet, with growing interest in vitamin D. The aim of the present paper is to review the evidence supporting the potential roles of vitamin D in ageing and cognitive health in community-dwelling older adults. Furthermore, to describe the utility and challenges of cognitive assessments and outcomes when investigating vitamin D in this context. Evidence indicates that serum 25-hydroxyvitamin D (25(OH)D) may impact brain health. There is a biological plausibility from animal models that vitamin D may influence neurodegenerative disorders, through several mechanisms. Epidemiological evidence supports associations between low serum 25(OH)D concentrations and poorer cognitive performance in community-dwelling older populations, although an optimal 25(OH)D level for cognitive health could not be determined. The effect of raising 25(OH)D concentrations on cognitive function remains unclear, as there is a paucity of interventional evidence. At a minimum, it seems prudent to aim to prevent vitamin D deficiency in older adults, with other known common protective lifestyle factors, as a viable component of brain health strategies.

Vitamin D receptor Cdx-2-dependent response of central obesity to vitamin D intake in the subjects with type 2 diabetes: a randomised clinical trial

This study aimed to investigate the effects of daily intake of vitamin D-fortified yogurt drink (doogh) on central obesity indicators in subjects with type 2 diabetes (T2D) and the possible modulation of this effect by vitamin D receptor (VDR) Cdx-2 genotypes. A total of sixty T2D subjects were randomly allocated to two groups to receive either plain doogh (PD; n 29, containing 170 mg Ca and no vitamin D/250 ml) or vitamin D3-fortified doogh (FD; n 31, containing 170 mg Ca and 12·5 μg/250 ml) twice a day for 12 weeks. 25-hydroxyvitamin D (25(OH)D), glycaemic as well as adiposity indicators were evaluated before and after the intervention. VDR-Cdx-2 genotypes in extended number of T2D subjects in the FD group (n 60) were determined as AA, GA and GG. After 12 weeks, in FD compared with PD, serum 25(OH)D increased (+35·4 v. -4·8 nmol/l; P<0·001) and mean changes of waist circumference (WC; -1·3 v. +1·6 cm; P=0·02), body fat mass (FM; -1·9 v. +0·60 %; P=0·008), truncal fat (TF; -1·1 v. 0·13 %; P=0·003) and visceral adipose tissue (-0·80 v. +0·37 AU; P<0·001) decreased significantly. Circulating 25(OH)D was raised only in the AA group (34·8 nmo/l in AA group v. -6·4 nmol/l in AG and -1·6 nmol/l in GG groups; P<0·001), which was accompanied by a significant decrease in changes of WC (P=0·004), FM% (P=0·01) and TF% (P<0·001) in the AA genotype. Daily intake of vitamin D-FD for 12 weeks improved the central obesity indices in T2D subjects, and the improvement was more pronounced in the carriers of the AA genotype of VDR-Cdx-2.

Maternal dietary vitamin D carry-over alters offspring growth, skeletal mineralisation and tissue mRNA expressions of genes related to vitamin D, calcium and phosphorus homoeostasis in swine

Maternal dietary vitamin D carry-over effects were assessed in young pigs to characterise skeletal abnormalities in a diet-induced model of kyphosis. Bone abnormalities were previously induced and bone mineral density (BMD) reduced in offspring from sows fed diets with inadequate vitamin D3. In a nested design, pigs from sows (n 23) fed diets with 0 (-D), 8·125 (+D) or 43·750 (++D) µg D3/kg from breeding through lactation were weaned and, within litter, fed nursery diets arranged as a 2×2 factorial design with 0 (-D) or 7·0 (+D) µg D3/kg, each with 95 % (95P) or 120 % (120P) of P requirements. Selected pigs were euthanised before colostrum consumption at birth (0 weeks, n 23), weaning (3 weeks, n 22) and after a growth period (8 weeks, n 185) for BMD, bone mechanical tests and tissue mRNA analysis. Pigs produced by +D or ++D sows had increased gain at 3 weeks (P<0·05), and at 8 weeks had increased BMD and improved femur mechanical properties. However, responses to nursery diets depended on maternal diets (P<0·05). Relative mRNA expressions of genes revealed a maternal dietary influence at birth in bone osteocalcin and at weaning in kidney 24-hydroxylase (P<0·05). Nursery treatments affected mRNA expressions at 8 weeks. Detection of a maternal and nursery diet interaction (P<0·05) provided insights into the long-term effects of maternal nutritional inputs. Characterising early stages of bone abnormalities provided inferences for humans and animals about maternal dietary influence on offspring skeletal health.

Modulation of intestinal calcium and phosphate transport in young goats fed a nitrogen- and/or calcium-reduced diet

Feeding ruminants a reduced N diet is a common approach to reduce N output based on rumino-hepatic circulation. However, a reduction in N intake caused massive changes in Ca and inorganic phosphate (Pi) homoeostasis in goats. Although a single dietary Ca reduction stimulated intestinal Ca absorption in a calcitriol-dependent manner, a concomitant reduction of Ca and N supply led to a decrease in calcitriol, and therefore a modulation of intestinal Ca and Pi absorption. The aim of this study was to examine the potential effects of dietary N or Ca reduction separately on intestinal Ca and Pi transport in young goats. Animals were allocated to a control, N-reduced, Ca-reduced or combined N- and Ca-reduced diet for about 6-8 weeks, whereby N content was reduced by 25 % compared with recommendations. In Ussing chamber experiments, intestinal Ca flux rates significantly decreased in goats fed a reduced N diet, whereas Pi flux rates were unaffected. In contrast, a dietary Ca reduction stimulated Ca flux rates and decreased Pi flux rates. The combined dietary N and Ca reduction withdrew the stimulating effect of dietary Ca reduction on Ca flux rates. The expression of Ca-transporting proteins decreased with a reduced N diet too, whereas Pi-transporting proteins were unaffected. In conclusion, a dietary N reduction decreased intestinal Ca transport by diminishing Ca-transporting proteins, which became clear during simultaneous N and Ca reduction. Therefore, N supply in young ruminant nutrition is of special concern for intestinal Ca transport.

Gene expression of matrix metalloproteinase 9 (MMP9), matrix metalloproteinase 13 (MMP13), vascular endothelial growth factor (VEGF) and fibroblast growth factor 23 (FGF23) in femur and vertebra tissues of the hypovitaminosis D kyphotic pig model

The hypovitaminosis D kyphotic pig provides a reliable model to study the initiation of bone lesions caused by maternal vitamin D (D) deficiencies. Matrix metalloproteinases (MMP; specifically, MMP9 and MMP13) and vascular endothelial growth factor (VEGF) are important in endochondral ossification and are potentially regulated by D. Fibroblast growth factor 23 (FGF23) is interrelated with D homoeostasis and bone mineralisation. Relative mRNA expression of MMP9, MMP13, VEGF and FGF23 was measured in pig femur and vertebra. Sows (n 37) were fed diets with 0 (-D), 8·125 (+D) or 43·750 (++D) µg D3/kg throughout gestation and lactation. At weaning (3 weeks), pigs were fed diets with 0 (-D) or 7·0 (+D) µg D3/kg, each with 75 and 95 % (LCaP) or 150 and 120 % (HCaP) of the Ca and P requirements. Pigs at birth (n 27), 3 weeks (n 27) and after the nursery period (7 weeks; n 72) were euthanised for analysis. At 3 weeks, femur MMP9 expression of pigs produced by +D or ++D sows was reduced (P<0·05) to 0·5-fold and VEGF expression to 0·4-fold compared with pigs from -D sows. At 7 weeks, MMP9 expression was reduced (P<0·05) to 0·45-fold in femur and 0·58-fold in vertebra from pigs produced by +D or ++D sows compared with pigs from -D sows. Pig femur VEGF expression was reduced to 0·75-fold in pigs produced by ++D sows. MMP9 and VEGF mRNA expression offer potential markers for the initiation of bone lesions in the hypovitaminosis D kyphotic pig model.

The effect of chronic oral vitamin D supplementation on adiposity and insulin secretion in hypothalamic obese rats

Reduced plasma vitamin D (VD) levels may contribute to excessive white adipose tissue, insulin resistance (IR) and dyslipidaemia. We evaluated the effect of chronic oral VD supplementation on adiposity and insulin secretion in monosodium glutamate (MSG)-treated rats. During their first 5 d of life, male neonate rats received subcutaneous injections of MSG (4 g/kg), while the control (CON) group received saline solution. After weaning, groups were randomly distributed into VD supplemented (12 µg/kg; three times/week) and non-supplemented (NS) rats, forming four experimental groups (n 15 rats/group): CON-NS, CON-VD, MSG-NS and MSG-VD. At 76 d of life, rats were submitted to an oral glucose tolerance test (OGTT; 2 g/kg), and at 86 d, obesity, IR and plasma metabolic parameters were evaluated. Pancreatic islets were isolated for glucose-induced insulin secretion (GIIS), cholinergic insulinotropic response and muscarinic 3 receptor (M3R), protein kinase C (PKC) and protein kinase A (PKA) expressions. Pancreas was submitted to histological analyses. VD supplementation decreased hyperinsulinaemia (86 %), hypertriacylglycerolaemia (50 %) and restored insulin sensibility (89 %) in MSG-VD rats, without modifying adiposity, OGTT or GIIS, compared with the MSG-NS group. The cholinergic action was reduced (57 %) in islets from MSG-VD rats, without any change in M3R, PKA or PKC expression. In conclusion, chronic oral VD supplementation of MSG-obese rats was able to prevent hyperinsulinaemia and IR, improving triacylglycerolaemia without modifying adiposity. A reduced cholinergic pancreatic effect, in response to VD, could be involved in the normalisation of plasma insulin levels, an event that appears to be independent of M3R and its downstream pathways.