1.1. Metabolic Association
Obesity is a condition of chronic dysfunction characterized by a low-grade, systemic inflammatory state. This pathological condition predisposes to the onset of some diseases, such as diabetes, hyperlipidemia, and hypertension. MetS (metabolic syndrome) is mainly characterized by obesity, hyperglycemia, hyperlipidemia, and hypertension, even if its definition has undergone changes over time
[1][2][3]. Adipose tissue must be considered an endocrine organ, which regulates many body functions and has a critical role in energy homeostasis, producing, for example, several biologically active substances, like adipokines. Just as adipose tissue can be considered a real organ that acts on the body’s metabolism, bone tissue can also represent an organ that exerts an action on many other organs.
Leptin: Leptin is secreted by the white adipose tissue. Hyperleptinemia found in the obese subject seems to be one of the causes of bone weakening. In fact, leptin has a dual effect on bone. One of these effects is positive: in vitro, leptin stimulates stromal cells to differentiate into osteoblasts, stimulates the proliferation of the latter and inhibits the formation of osteoclasts
[4][5][6]. It has also been shown that a knockout of the leptin gene causes a reduction in BMD and bone volume
[7]. The negative effect seems to prevail over the positive one
[8][9]. This negative effect would be exerted via the central nervous system. Leptin would cause decreased production of serotonin in the hypothalamic neurons, resulting in decreased bone formation
[10][11]. In mice lacking leptin or leptin receptors, several authors found a decrease of femur bone mass and an increase of femur bone marrow fat
[10][11]. Jansson et al.
[12] based on their animal study in rats and mice, hypothesize that, in addition to the action of Leptin, which would have the ability to reduce body weight, also through a reduced food intake
[13][14] there would be a homeostat, called “gravitostat” by the authors, located in the weight-bearing lower extremities, which would be activated with the increase in body weight, producing a decrease in fat mass regardless of leptin. The same group of authors, in a more recent paper
[15] found that the gravitostat regulates fat mass in obese mice, while leptin regulates fat mass in lean mice, concluding that the gravitostat protects against obesity, whereas undernutrition induces low levels of leptin, with subsequent weight gain. The findings of these two studies lead to an interesting conclusion: obesity has an effect on bone, but bone also has an effect on body weight.
Adiponectine: Adiponectine, secreted by white adipose tissue, is an adipokine that has been proven to stimulate bone formation. It has been shown that adiponectin stimulates osteoblastic proliferation, with an increase in the activity of alkaline phosphatase, and the formation of type I collagen and osteocalcin, all markers of differentiation and maturation of osteoblasts. The osteogenesis of mesenchymal stem cells stimulated by adiponectin is mediated by the adipoR1 phosphorylation of P38 MAPK, which enhances COX-2 (cyclooxygenase2) and BMP2 expression (bone morphogenetic protein 2), a cytokine with considerable osteogenic potential
[16][17][18]. In obesity, a low concentration of adiponectin is usually present
[19]. This condition induces the reduction in osteoblastogenesis and the increase in osteoclastogenesis
[20], through the mechanisms described above and, overall, through a mechanism mediated by inflammation markers. The chronic inflammatory condition present in obesity is likely to be partly linked to the lack of adiponectin. Adiponectin deficiency is also found in insulin-resistant diabetes
[19]. This could be one of the links between obesity and diabetes.
The concentration of adiponectin is inversely proportional to that of numerous inflammatory cytokines, such as C-reactive protein (CRP), IL-6, and TNF-α. It is therefore presumable that the chronic inflammatory state present in obesity expresses a high concentration of these inflammation markers, which are potent inhibitors of adiponectin expression
[21].
TNF-α: As already mentioned, in the obese subject there is a greater expression of TNF-α (GK57). TNF-α, through multiple mechanisms, leads to an increase in RANKL (RANK-Ligand)
[22][23][24]. The latter promotes an osteoclastic bone resorption process. TNF-α also stimulates the production of osteoprotogerin
[25].
IL-6: Just as in the case of TNF-α, obesity and insulin resistance cause an increase in interleukin 6 (IL-6)
[26], through its overproduction by adipocytes and fibroblasts. IL-6, like TNF-α, also induces osteoclastogenesis and bone resorption
[27][28][29].
Resistin: Resistin, a hormone of protein origin produced by visceral adipocytes and macrophages, has a controversial effect on bone. If it seems to favor the proliferation of osteoblasts, it also seems to favor osteoclastic proliferation and the release of inflammatory cytokines
[30]. A high concentration of resistin is found in obese people
[31].
Peroxisome proliferator-activated receptor gamma (PPARg): According to some authors, the peroxisome proliferator-activated receptor gamma (PPARg), together with its agonists, the thiazolidinediones, can act in the obese subject producing a negative effect on the bone. In fact, PPARgs have the property of promoting the differentiation of mesenchymal cells into adipocytes and blocking the transformation of mesenchymal cells into osteoblasts
[32].
Lipid metabolism: The review by Kim et al.
[33] carefully examines how the alterations in lipid metabolism present in the obese subject can negatively affect bone metabolism. The lipid alterations to which such consequences can be attributed are different and complex, with the involvement of SREBP, cholesterol, LXRs and RXRs, fatty acids, statins. The latter can impact the phenotype of osteoclasts and osteoblasts in pathological conditions.
Vitamin D: Vitamin D deficiency causes a reduction of calcium resorption and, consequently, osteoporosis and osteomalacia. In the obese, the serum levels of vitamin D are significantly lower than in non-obese
[34][35][36]. However, as researchers have previously reported, the obese patient’s BMD is higher. The incongruity of this phenomenon can be explained by the fact that a wide amount of vitamin D in the obese is stored in the largely represented adipose tissue, causing a serum hypovitaminosis D. This hypovitaminosis is only apparent though, since vitamin D in the adipose deposits is always available and, therefore, the obese subjects are not affected by the negative effects of the deficiency of this vitamin
[37]. In obese patients, it is very common (43% of the morbidly obese adults) to observe a secondary hyperparathyroidism, that can negatively impact skeletal health
[38][39].
Peptide YY: Not strictly related to metabolism, is the level of Peptide YY. Although it must be confirmed, the role of this peptide seems to have an influence both on obesity and bone mass. Peptide YY promotes satiety. PYY-deficient mice (Pyy(−/−)) have osteopenia with a reduction in trabecular bone mass and a deficit in bone strength. PYY levels are lower in obese adults and the elevation of PYY seen after a meal in lean subjects is blunted in obesity
[40][41].
1.2. Fat Bone Marrow
Bone marrow is an important deposit of fat, at the level of the “yellow” areas. Marrow adipose tissue (BAT) is estimated to occupy 70% of the marrow space by adulthood
[42] and accounts for about 8% of total fat mass
[43]. In the obese subject, bone marrow fat fraction (BMFF) was shown to be higher than in the normal weight subject
[44]. The adipocytes of the bone marrow are responsible for the secretion of adipokines, some of which induce the release of various inflammatory cytokines mentioned above, such as TNFα and IL-6.
The correlation between obesity and bone fragility can also originate from the adipose bone marrow, which has been shown to interfere with bone metabolism. It is not a coincidence that BMFF increases in obesity, in old age and in osteoporosis, especially in postmenopausal women
[45]. Actually, adipocytes and osteoblasts have a common origin, which are pluripotential, bone marrow-derived mesenchymal stem cells
[46]. It cannot be ruled out that the inability of the latter cells to differentiate into osteoblasts leads to an increased differentiation into adipocytes. The literature widely demonstrates that the presence of a greater amount of fat in the bone marrow induces osteoporosis
[47][48][49][50][51][52]. Indeed, an increase in marrow fat content has been demonstrated in obese women with low BMD, and Wehrli et al. reported that bone marrow adipose tissue in the lumbar spine is an independent predictive factor of fracture
[44][53][54][55].
It is also important to mention the importance of palmitate, reported by some authors
[56]. According to these authors, the lipotoxic effect of BAT is mainly due to the action of palmitate, which would have its toxic effect, especially on bone cells, mainly osteoblasts.