The skin synthesis of vitamin D represents the first step of a metabolic pathway whose features have been extensively studied and clarified in the last decades. In particular, the production of active and inactive forms of the hormone and the actions of the corresponding enzymes have offered new insights into the knowledge of vitamin D metabolism. Additionally, the description of the different organs and tissues expressing the vitamin D receptor and its possible functions, as well as its genetic determinants, have allowed focusing on the interrelationship between vitamin D and many physiological and pathological functions. In this context, many studies reported the association between vitamin D and adipose tissue metabolism, as well as the possible role of the hormone in obesity, weight, and fat mass distribution. Finally, many reports focused on the vitamin D-related effects on skeletal muscle, particularly on the mechanisms by which vitamin D could directly affect muscle mass and strength. This paper is mainly aimed to review vitamin D metabolism and its relationship with obesity and skeletal muscle function.

Summary: The possible relationships between biochemical measurements and both densitometric and radiographic indexes of skeletal fragility were evaluated in 65 postmenopausal women with monoclonal gammopathy of undetermined significance (MGUS). There was a significantly higher prevalence of vertebral fractures in the MGUS group compared with a control population (P ≤ 0·001). The MGUS patients were then grouped according to the presence or absence of at least one mild vertebral fracture. Patients with fractures (Fx, n = 34) were older (62·8 ± 6·1 years), with long‐standing disease (8·8 ± 7·1 years) when compared with those without fractures (NFx, n = 31; 59·7 ± 5·0 years, P ≤ 0·05 and 5·8 ± 4·1 years, P ≤ 0·05). The receptor activator of nuclear factor kappa‐B ligand/osteoprotegerin ratio was higher in Fx compared with NFx (0·092 ± 0·018 vs. 0·082 ± 0·020; P ≤ 0·05). Lumbar spine (0·811 ± 0·14 vs. 0·956 ± 0·12 g/cm²), femoral neck (0·660 ± 0·09 vs. 0·747 ± 0·10 g/cm²) and total bone mineral density (BMD) (0·788 ± 0·11 vs. 0·884 ± 0·11 g/cm²) were lower (all P ≤ 0·001) in Fx‐MGUS compared with Nfx patients. Receiver operating characteristic curves identified lumbar BMD as the variable that best predicted vertebral fractures (area under the curve 0·817; 95% confidence interval, 0·713–0·921). This study provides an indication for the measurement of BMD in MGUS patients, as a means of predicting vertebral fractures, especially in those that are asymptomatic. Patients with prevalent fractures should undergo pharmacological treatment to prevent further fractures.