2.1 Composition and origin of SCFAs
SCFAs, including a class of acid metabolites (C2-C5) such as acetic acid, propionic acid, butyric acid, valeric acid, isobutyl and isovaleric acid, are produced by bacteria in the gut of humans and animals by the fermentation of indigestible carbohydrates and proteins in food [23]. SCFAs are derived from the soluble dietary fiber found in foods such as oligosaccharides (bananas, Onions and asparagus), pectin (apples, apricots, carrots, oranges), kidney beans, oat bran, corn starch, milk, yogurt, and sprouted barley. Resistant starches such as barley, rice, beans, green bananas and potatoes are also important sources of SCFAs[24–27]. These indigestible fibers are not digested and absorbed in the small intestine and are subsequently fermented by microbiota in the cecum and large intestine; SCFAs are mostly produced in the cecum aswell asproximal colon and less in the distal colon[28]. Among them, acetic acid, propionic acid and butyric acid account for 90%−95% of the total amount of SCFAs produced by gut microbiota and are the main components of intestinal SCFAs[29]. Therefore, the above-mentioned three kinds of SCFAs have been studied extensively, especially butyric acid.
2.2 Biological functions of SCFAs
SCFAs are closely related to intestinal microecology and are involved in a wide-range of biological processes, including signal transduction, regulation of cytokines, immune cells and intestinal mucosal barrier function[30–31]. Furthermore, SCFAs increase bone formation and improve bone quality through regulating immunity, intestinal barrier function and immune cell activity.
SCFAs can be directly transported throughout the body by the host cells at the intestinal endothelial barrier, thus affecting distant tissues. G protein-coupled receptors (GPRs) are capable of binding to SCFAs, including GPR43, GPR41 and GPR109a[32]. These membrane-bound receptors are expressed on various immune cell types, including monocyte macrophages, and on other non-immune cells such as intestinal epithelial cells, adipocytes and enteroendocrine cells[33]. When these receptors bind to SCFA, it leads to intracellular Ca2+ release and activation of different downstream signaling pathways, such as ERK/MAPK, p38 or Akt/PI3K signaling cascades, thus regulating cellular activity and function[34]. In addition, some SCFAs, such as butyrate, serve as the main source of energy for the vital activities of intestinal epithelial cells, which directly affect the growth and development of these cells. Nonetheless, a certain number of SCFAs reach the bloodstream through transport systems (SMCT1/Slc5a8 pathway, MCT1/Slc16a1 pathway or passive diffusion) and are transported to the whole body through the transport system. Once SCFAs enter the circulatory system, they will affect the metabolism and function of peripheral tissues (adipose tissue, skeletal muscle, bone, etc.) by activating GPRs[35–36].
2.2.1 Butyric acid (C4) and bone health
However, butyric acid exhibits the most extensive biological activity among SCFAs, including regulating inflammation, maintaining immune homeostasis, and reducing bone loss from inflammation. Butyric acid can downregulate the pro-inflammatory mediators NO, IL-6 and IL-12 produced by lipopolysaccharide (LPS)-induced macrophages and can also promote the production of IL-22 through GPR41 and HDAC inhibition[37]. Butyric acid inhibits LPS-induced maturation and biological activity of monocyte-derived dendritic cells and promotes the polarization of early CD4 + T cells into IL-10-producing Tregs[38]. While, butyric acid regulates the inflammatory state of the body by activating GPRs in the intestinal mucosal epithelium, reducing the synthesis and secretion of pro-inflammatory factors such as Tumor necrosis factor-α(TNF-α) and Cyclooxygenase-2, thereby reducing the bone loss brought about by inflammation[10, 39].
Furthermore, butyric acid alleviates intestinal inflammation and reduces osteoclast differentiation by attenuating TNF-α-mediated immune responses and reducing inflammatory vesicles such as NLRP3[40]. Butyrate has also been reported to regulate Claudin-2 expression, reduce intestinal permeability through an IL-10 receptor-dependent mechanism, and strengthen intestinal barrier function by increasing colonic mucin and tight junction protein production[1, 41], enhancing immune response system function thus reducing bone loss[42]. Kaisar et al. found that butyrate inhibited the expression of the pro-inflammatory factor IFN-γ caused by the overactivation of the IFN-γ/ STAT1 signaling pathway[43]. Moreover, butyrate can also modulate the immune effect and relieve osteoarthritis by inhibiting the activity of inflammation-related pathways NF-κB and JAK/ STAT and IL-12p70, IL-23, and preventing the polarization of early CD4+ T lymphocytes into Th1(T helper cells 1) and Th17[44]. In short, Butyric acid has a positive effect on the maintenance of bone health by regulating the immune function of the animal body and alleviating bone destruction and loss.
2.2.2 Acetic acid (C2) and bone health
Acetic acid is one of the most abundant SCFAs produced by the gut microbiota[45]. It positively impacts bone health by maintaining the integrity of the intestinal mucosal barrier, preventing the invasion of pathogenic bacteria and enhancing the host's immune function. In animals, acetic acid is mainly present as free acid in tissues, excreta and blood[46]. It was found that acetic acid increases the production of IgA in the colon, alters the ability of IgA to bind to specific intestinal bacteria, and alters the colonization of these bacteria, enhancing the immune barrier function of the intestinal mucosa, thereby indirectly reducing the release of pro-inflammatory factors (TNF-α, IL-1β, etc.), inhibiting osteoclast activity and reducing bone loss[47, 48]. In addition, acetic acid can activate the GPR41 receptor on the surface of immune cells, enhancing the immune effect and facilitating the maintenance of bone health[49–50]. Maslowski et al. displayed that acetic acid significantly improved intestinal function, reduced DNA-dependent activator of IFN-regulatory factors and inflammatory mediator myeloperoxidase levels and TNF-α, thus facilitating the remission of the inflammatory response and reducing osteoclast production and differentiation[51]. The above facts indicated that acetic acid enhances immune function and reduces the release of pro-inflammatory factors, thereby inhibiting the activation of osteolytic effects.
2.2.3 Propionic acid (C3) and bone health
Propionic acid is an organic acid that occurs naturally as a result of the kind of bacterial action found on the skin or in the GIT. Propionic acid in the gut plays a variety of roles after passing through the bloodstream, including affecting hepatic cholesterol metabolism, promoting calcium absorption, increasing calcium deposition, and facilitating bone production[52–53]. In addition, it was found that propionic acid not only activates NLRP3 inflammatory vesicles in intestinal epithelial cells, induces IL-18 secretion, and improves the integrity of the intestinal mucosal epithelial barrier, but also inhibits histone deacetylase and decreases NF-κB activity, thereby reducing the release of inflammatory factors TNF-α, IL-6, and IL-8 and affecting the structure and function of the intestinal mucosal barrier[54]. Osteoporosis is closely associated with the cellular imbalance of the immune system and immune-mediated effects on bone structure through the intestine have been demonstrated[55]. Interestingly, SCFAs intake resulted in increased bone mass in mice accompanied by a decrease in inflammation-induced bone loss. In a study of the effects of propionic acid supplementation on human bone metabolism, Duscha et al. found that propionic acid intake induced a significant increase in serum osteocalcin (a marker of bone formation) levels and a significant decrease in β-CrossLaps (a marker of bone resorption) levels; suggesting that propionic acid induces increased bone formation and decreased bone resorption[56].
2.2.4 Valeric acid (C5) and bone health
The GIT displayed high propionate and butyrate levels and low valeric acid levels[57]. However, Yuille's study confirmed that total specific inhibition of HDAC and promotion of differentiation of Tregs into T cells by increasing the ability of intestinal flora to produce butyric and valeric acid could enhance bone immunity indirectly[58]. Studies have shown that although valeric acid levels are low in animals, dietary fiber intake helps to increase valeric acid levels and that higher valeric acid levels reduce the release of pro-inflammatory factors and mitigate bone destruction[58, 59]; suggesting that the immunomodulatory ability of valeric acid and its potential therapeutic value for inflammation-induced bone diseases.
There are similarities in the functions of the different SCFAs. For example, they can regulate the structure of the gut microbiota (regulating pH in the gut and reducing the colonization of pathogenic bacteria), which facilitates the establishment of the intestinal immune barrier, and inhibit the release of inflammation-related signaling molecules (IL-6, IL-7, Receptor activator for nuclear factor-κB ligand, (RANKL)), which reduces osteoclast differentiation and thus facilitates bone health. The above facts show that intestinal health is closely related to bone health.