Obesity and hypovitaminosis D: causality or casualty?

Abstract

Epidemiological studies reported that vitamin D deficiency represents an increasingly widespread phenomenon in various populations. Vitamin D deficiency is considered a clinical syndrome determined by low circulating levels of 25hydroxyvitamin D (25(OH)D), which is the biologically-inactive intermediate and represents the predominant circulating form. Different mechanisms have been hypothesized to explain the association between hypovitaminosis D and obesity, including lower dietary intake of vitamin D, lesser skin exposure to sunlight, due to less outdoor physical activity, decreased intestinal absorption, impaired hydroxylation in adipose tissue and 25(OH)D accumulation in fat. However, several studies speculated that vitamin D deficiency itself could cause obesity or prevent weight loss. The fat-solubility of vitamin D leads to the hypothesis that a sequestration process occurs in body fat depots, resulting in a lower bioavailability in the obese state. After investigating the clinical aspects of vitamin D deficiency and the proposed mechanisms for low 25 (OH)D in obesity, in this manuscript we discuss the possible role of vitamin D replacement treatment, with different formulations, to restore normal levels in individuals affected by obesity, and evaluate potential positive effects on obesity itself and its metabolic consequences. Food-based prevention strategies for enhancement of vitamin D status and, therefore, lowering skeletal and extra-skeletal diseases risk have been widely proposed in the past decades; however pharmacological supplementation, namely cholecalciferol and calcifediol, is required in the treatment of vitamin D insufficiency and its comorbidities. In individuals affected by obesity, high doses of vitamin D are required to normalize serum vitamin D levels, but the different liposolubility of different supplements should be taken into account. Although the results are inconsistent, some studies reported that vitamin D supplementation may have some beneficial effects in people with obesity.

Vitamin D Supplementation and Blood Pressure in Children and Adolescents: A Systematic Review and Meta-Analysis

“…Most notably, it plays an integral function in the regulation of calcium and phosphorus homeostasis, and thus has a vital role in bone health. Emerging evidence suggests further extra-skeletal physiological actions, but clinical consequences are still debatable [1][2][3][4].…”

Serum 25-hydroxyvitamin D levels and its relationship with sex hormones, puberty and obesity degree in children and adolescents

“…On the other hand, the relationship between vitamin D deficiency and obesity is still unclear. Several human studies have shown that adipose tissue retains vitamin D, compromising its bioavailability to perform its actions (Wortsman et al 2000;González-Gross et al 2012;Valtueña et al 2013;Migliaccio et al 2019). Moreover, it has been suggested that adipose tissue metabolizes vitamin D, and this mechanism is altered in obesity (Wamberg et al 2013).…”

Molecular and Lifestyle Factors Modulating Obesity Disease

“…Exposure to sunlight and diet are the only sources of inactive vitamin D [58]. Its activation involves complex processes such as the conversion by hepatic vitamin D-25-hydroxylase to 25-hydroxyvitamin D [25(OH)D], which in turn undergoes an hydroxylation process to become the biologically active form 1,25-dihydroxyvitamin D [1,25(OH)D] in the kidneys [58].…”

“…Different factors or mechanisms have been proposed to contribute to vitamin D deficiency in obesity. Limited outdoor activity together with a poor dietary vitamin D intake have been suggested as causes [58]. However, studies in humans and in animal models have suggested fat deposits to induce vitamin D abduction in individuals with obesity [60,61].…”