Short-chain fatty acids (SCFAs)

Formate holds a unique position among the SCFAs since has a critical role in one-carbon metabolism, primarily as a source of one-carbon groups for the synthesis of 10-formyl-tetrahydrofolate (10-fTHF). 10-fTHF is a substrate for purine synthesis, and after further reduction of the one-carbon group, may serve as a substrate for thymidylate synthesis and for homocysteine remethylation. Serum formate shows a positive association with serum folate and with potential metabolic precursors (serine, methionine, tryptophan, choline) and an inverse association with vitamin B12 (1).
Acetate, propionate and butyrate are SCFAs produced from non-digestible complex carbohydrates (dietary fibre) in the gut during microbial fermentation, whereas the branched-chain SCFAs, isobutyrate , isovalerate and α-methylbutyrate, are bacterial-produced metabolites of amino acids. Acetate is the most abundant (50-70 %) followed by propionate and butyrate. They are absorbed into the systemic circulation, and each SCFA exerts specific metabolic effects, and may mediate beneficial but also adverse health effects.
SCFAs, in particular butyrate, are anti-inflammatory, expand the pool of intestinal regulatory T cells, protect against allergic sensitization, mitigate production of reactive oxygen species, are essential for gut integrity, and exert antiproliferative effects on cancer cells. Butyrate’s effects on the immune system are mediated through the inhibition of class I histone deacetylases and activation of G-protein coupled receptors: GPR109A, GPR41 and GPR43. SCFAs increase insulin secretion (via GPR41/43), and low gut-derived SCFAs have been suggested to be associated with type II diabetes, insulin resistance, obesity and NAFLD (2).
In a recent seminal review article, the authors recommend measurement of circulating SCFAs in human studies, since the SCFAs in faeces may not be an accurate measure of SCFA production or exposure (3).