In another case–control study, the fecal metabolites composition and the role of metabolites were investigated in 30 IBS patients with diarrhea (IBS-D) and 15 healthy controls. Data showed that isobutyric acid levels were higher in the stools of patients with IBS, whereas isovalerate levels positively correlated with the severity or frequency of abdominal pain. Both isovaleric and isobutyric acids were associated with visceral hypersensitivity and contributed to abdominal pain. Furthermore, isohexanoate was significantly related to the severity or frequency of abdominal pain in IBS-D patients
[30][49].
Another team studied the changes in fecal fatty acids after fecal microbiota transplantation (FMT) in IBS (all subtypes) patients. One hundred and forty-two IBS patients were divided into three groups: placebo (own feces), 30 g (superdonor feces), and 60 g (superdonor feces). In responders of the 60 g FMT group, fecal levels of isovaleric and isobutyric rose overall. Isobutyric levels increased in IBS-D and IBS with constipation (IBS-C), but not in IBS mixed (IBS-M) patients.
Moreover, higher concentrations of BCFAs were positively associated with longer colonic transit time (CTT), and longer CTT was also associated with increased proteolytic fermentation in healthy adults, while higher levels of SCFA were related to shorter CTT
[31][11]. A possible hypothesis is that LFD and higher levels of BCFA could be indicated for IBS-D management, while a high-FODMAP diet and higher SCFA levels could be suitable for IBS-C. Certainly, CTT is affected by a plethora of factors, such as sex, age, stress, body mass index, colonic anatomy, treatment, and gut hormones, among others
[31][11].
2.2. BCFAs, Inflammation, and Low-FODMAP Diet
2.2.1. Preclinical Trials
In 2019, Tuck et al.
[32][51] performed experiments in mice with dextran sodium sulfate (DSS)-induced colitis during the inflammatory phase and after treatment. Animals were divided into three groups: two control treatments (“negative-control” and “positive-control”; with and without inflammation, respectively) and a “post-inflammatory” treatment group that mimicked quiescent IBD with IBS-like symptoms. After the recovery, mice were randomized to 2-week low-(0.51 g) or high-FODMAP (4.10 g) diets, respectively. In the positive-control and post-inflammatory treatment groups following the LFD, total levels of stool BCFAs were higher compared with those of the negative controls; statistical significance was reached for isobutyric and isovaleric acid. The results suggested that the higher proteolytic fermentation occurred in the LFD group. Considering the alterations of inflammatory markers, Myeloperoxidase (MPO) activity was lower in the negative-control group, in contrast to its higher levels in the positive-control group, regardless of the FODMAPs content. The research team created a histological score to evaluate the inflammatory status in the colon. The negative controls scored 0, but positive controls had higher scores only in the high-FODMAP diet group. Even if compared with the post-inflammatory group, positive control animals fed with a high-FODMAP diet had significantly higher histological scores. However, no significant differences were observed between low- and high-FODMAP diets. There were no significant changes regarding TNF-α, Granulocyte-macrophage colony-stimulating factor (GM-CSF), IL-10, and IL-4.
The same research team evaluated commercially available rodent diets across research institutions. Forty mice were randomized into four groups to assess the dietary impact on gut microbiota, SCFAs, and BCFAs profiles. Animals in Group A were euthanized at baseline (controls), mice in Group B received the breeding institution chow, mice in Group C received a low-gluten and LFD, and Group D consumed a high-gluten and FODMAP diet. In the LFD group, the BCFA stool levels were higher compared with those in the high-FODMAP group. More specifically, isovalerate and isobutyric acids levels were highest in the LFD group. The explanation is based on protein fermentation and reflects a trend for increased protein metabolism in low-carbohydrate diets.
2.2.2. Clinical Trials
Halmos et al. [33][53] assessed the effects of an LFD versus a typical Australian diet on fecal biomarkers. This study included 33 participants (27 with IBS and 6 healthy controls). Volunteers followed two diets differing in FODMAP content (LFD contained 3.05 g, whereas the typical Australian diet contained 23.7 g of FODMAPs, respectively). At baseline, isovaleric and isobutyric levels were lower in IBS patients compared to controls, though no difference was noticed during the dietary intervention [33][53].
Recently, Nordin and her team investigated the effects of different dietary patterns on fecal microbiota, fecal fatty acids, and plasma metabolome in IBS symptoms. One hundred and three IBS patients were randomized into three groups in this double-blinded, placebo-controlled trial. Each group followed all the dietary plans (placebo, gluten, and FODMAP) but in a different sequence. Placebo consisted of 18 g of sucrose, gluten intervention contained 17.3 g of gluten, and the daily dose of FODMAP intake was 50 g. During the intervention, patients filled in questionnaires, and they underwent blood and fecal analyses and anthropometric measurements periodically. Results showed a reduction in plasma levels of isobutyrate in the FODMAP group compared to the placebo, while in feces, a decrease was observed in isovalerate after the gluten diet.
2.3. BCFAs as Potentially Harmful Metabolites
BCFAs have been proposed as markers of colonic protein fermentation, a process that leads to the production of nitrogenous metabolites of protein and amino acid fermentation, such as amines, hydrogen sulfide, p-cresol, phenols, and ammonia. These metabolites are toxic for colonocytes and they are associated with the development inflammatory conditions
[17][34][29,57]. Windey et al. supported, with evidence, the hypothesis that these luminal protein end-products affect not only epithelial cell metabolism but also intestinal barrier function
[35][58]. In a cross-sectional study, scientists compared the concentrations of BCFAs, ammonia, and fecal pH between vegans and omnivores. Results showed that protein intake was higher in omnivores than in vegans and there was a trend of lower BCFAs concentrations in vegans compared to omnivores, though no significant correlation with protein intake was detected
[34][57]. Moreover, in an in vitro study
[36][59], BCFA levels were lower in anaerobic incubation of vegetarians stools compared to incubation of fecal samples of omnivorous. Bacteria from vegetarian donors grew faster on soy protein as substrate, while in omnivorous donors, meat protein and casein were the preferred growth substrates. Fermentation patterns on different substrates were observed between the gut microbiota of vegetarians and omnivore donors. Differences were focused on BCFAs, ammonia, and total bacteria, with lower BCFA levels to be found in vegetarians; this suggests that these donors may have lower branched-chain amino acids intake
[36][59]. The ability of protein end-products to have harmful effects on the GI is likely related to luminal concentrations of these metabolites. Metabolites’ concentrations may depend upon the balance between the rates of production, detoxification by the colonic epithelium, and absorption or excretion from the large intestine
[14][25]. The protein source (animal- or plant-based) and the effect of food processing may play a role in altering its digestibility. The proportion of protein consumed and the presence of other nutrients may share a similar efficiency in digestibility and absorption
[15][26].
3. Conclusions
IBS is a difficult-to-manage disorder, and it is associated with poor health quality of life
[1]. Diet and dietary end-products have been related to the development or the management of the syndrome. While most of the literature focuses on SCFAs, less is known about the way that BCFAs influence the development or dietary management of IBS. LFD is an established dietary intervention for amelioration of IBS symptoms; it has been associated with changes in of the gut microbiome, and due to its composition, it leads to higher protein and amino acid fermentation. Both animal studies and human clinical trials show that changes in the diet of fermentable, indigestible carbohydrates, like in LFD intervention, may lead to production of BCFAs, the byproduct of this fermentation, which, under certain circumstances, might produce large amounts of SCFAs that eventually may modulate and decrease intestinal inflammation
[37][38].
Although evidence suggests that BCFAs might play a protective role in gut inflammation, other nitrogenous metabolites of protein fermentation, such as amines, hydrogen sulfide, p-cresol, phenols, and ammonia, have detrimental effects on colonocytes and they are associated with gut inflammation, a condition that has been pathogenetically associated with IBS
[1]. Dietary modifications in protein intake by changing, for example, red meat consumption to white meat such as chicken and fish, or plant-based proteins, may reduce the availability of nitrites in the colon
[14][25].