Skeletal muscle is a heterogeneous tissue made up of different fiber types, in which glycolysis and mitochondrial oxidative phosphorylation for energy production takes place [1]. Skeletal muscle oxidative capacity is determined by mitochondrial biogenesis [2], which involves both proliferation and differentiation processes [3]. Mitochondrial dysfunction involved in alteration of oxidative metabolism is thought to play a crucial role ‎ in insulin resistance [4]. ‎However, the cause-and-effect of this relationship between mitochondrial dysfunction and the development of diabetes remains unclear [4,5,6].

Obesity may be characterized by fat accumulation in skeletal muscle, and this changes likely leads to long-term metabolic derangements including type 2 diabetes mellitus (T2DM) [7]. Insulin resistance, a whole mark of T2DM, is caused by the ‎‎inability of insulin-target tissues to respond properly to insulin [8], and in whose ‎‎etiology mitochondrial dysfunction is thought to play a crucial role [4,5]. ‎In addition, visceral fat depots have direct portal access, and thus, a greater potential to harm the liver, impaired glucose uptake by skeletal muscle and increased basal lipolytic rate and free fatty acid release.

Vitamin D deficiency has been associated with ‎diabetes mellitus and its ‎correction ‎contributes to decreased diabetes risk and cardiovascular ‎diseases, underlying the ‎importance of ‎its role in the prevention of several non-communicable ‎diseases [9]. ‎‎Increasing evidence also indicates that vitamin D plays an essential role in many tissues including skeletal muscle. Early clinical descriptions of a myopathy associated with severe vitamin D deficiency indicated a potential association between vitamin D and muscle [10]. Indeed, skeletal symptoms were found to be responsive to treatment with vitamin D; however, the underlying mechanisms remained undefined [11]. Taken together, the present study sought to elucidate the effect of vitamin D on histopathological and ultrastructural changes in skeletal muscle in a rodent model fed on high and low fat diet.

Exposure ‎to HFD lead to significant body weight gain as compared with that of LFD groups by weeks 8 and 12 where the mean percentage weight gain were ‎‎26.4% vs. 6.4% (p < 0.05) and 30.2% vs. 8.4% (p < 0.01), respectively. The body weight of HFD ‎fed animals became different as the weight of mice gradually but noticeably decreased in vitamin D ‎group compared with its control group after 12 weeks of treatment (11.4% vs. 30.2%, p < 0.05). At the ‎end of the study, those animals which received HFD with vitamin D had the smallest percentage weight ‎gain as opposed to controls (6.8% vs. 28.7%, p < 0.01).

Skeletal muscle of mice fed on LFD showed a disturbance in sarcosome, sarcosomal membrane and sarcosomal cristae. There was accumulation of some fussy materials. The remaining structural is intact with other normal features (Figure 1A and Figure 2A). Vitamin D treatment was associated with normalization of the LFD induced skeletal muscle and is normal as in controls (Figure 1B and Figure 2B). However, HFD treatment induced a great disturbance in sarcosome region, the whole sarcosome along with its membrane and cristae are severely damaged. The mitochondria were on the verge of degenerations. There was also accumulation of fussy materials in sarcoplasm; whereas sarcolemma and sarcomere were well disturbed with scattered myocytes. The bands are diminished with loss of connections among myofibrils (Figure 1C and Figure 2C). Vitamin D treatment of mice feed on HFD was associated with normalization of skeletal muscle features (Figure 1D and Figure 2D), which were similar to that of untreated controls (Figure 1E and Figure 2E).

This study was undertaken to evaluate the effect of vitamin D treatment on muscle histological and ultrastructural changes associated with weight gain in a C57BL/6J mouse model. This mouse strain has especially been used as a human obesity model because it develops obesity, insulin resistance and hyperlipidemia when raised on a high-fat and high-sucrose diet; however, it remains lean if the fat content of the diet is limited [12,13,14,15].

Skeletal muscle oxidative capacity is determined by mitochondrial function and biogenesis [2]. Indeed, it is well established that mitochondria takes active part in aerobic biogenesis and oxidation of skeletal muscles [16]. Mitochondrial biogenesis is involved in the proliferation and differentiation of mitochondrial number and an improvement of the functional capabilities of pre-existing mitochondria [3]. Therefore, mitochondrial dysfunction has been proposed to be involved in the alteration of oxidative metabolism associated to insulin resistance [17]. However, the cause-and-effect relationship between mitochondrial dysfunction and the development of insulin resistance remains unclear [4,5,6]. In the present study, a large number of mitochondria were in the process of degeneration, which was observed in mice fed on either LFD or HFD. This in turn can affect the oxidation capacity and the biogenesis process.

Even though vitamin D deficiency has long been associated with muscle weakness [18,19], until recently no etiological mechanism has been described. Vitamin D treatment has been shown to be protective against the development of insulin resistance in mice [20]. The evident effect of vitamin D treatment on the histological abnormalities of the muscles in the LFD and HFD group also accentuates its role as a muscular-protective agent [21]. To our knowledge, the current data is the first to determine a causal relationship between vitamin D intake and mitochondrial degeneration in myocytes under high fat fed conditions using an animal model. This effect is possibly mediated through a modulation in the circulating levels of adipokines. For example, adiponectin is a hormone secreted by adipocytes that plays an important role in the regulation of mitochondrial biogenesis and insulin sensitivity [1,22,23]. Adiponectin binds to its receptors (AdipoR1, the most abundantly expressed in skeletal muscle, and AdipoR2) activating 5'-AMP-activated protein kinase (AMPK), which finally leads to the stimulation of glucose uptake and fatty acid oxidation [22,23].

In summary, this study demonstrated that in mice, the skeletal muscle mitochondrial ‎biogenesis and oxidative metabolism can be severely affected by HFD, and vitamin D treatment ‎restored associated histological and ultrastructural abnormalities. Additional studies are ‎warranted to elucidate the molecular mechanism(s) by which vitamin D emulates mitochondrial ‎degeneration under various dietary conditions.‎