To show any biological effects, bioactive peptides should be absorbed from the gastrointestinal tract. Different mechanisms are in place to achieve uptake of these peptides. These mechanims can be listed as: Paracellular transport through intercellular tight junctions, direct penetration of the epithelial cell­membranes, endocytosis/phagocytosis by cells, and active transport by specific carrier proteins. Studies on bioactive peptides reported inadequate evidence on ability of the dietary bioactive peptides to enter the hepatic portal system in physiologically relevant concentrations whereas there is enough evidence on bioavailability of the dipeptides and tripeptides (Chakrabarti et al., 2018).

As bioactive peptides can be produced by microbial fermentation. Using Lactic acid bacteria is ac­cepted as an effective way of producing bioactive peptides in fermentaton media or food fermentation processes. It is reported that around 13 g of protein and peptides/day was estimated to reach human colon (Louis & Flint, 2017). It is known that many peptides have the capacity to modulate human gut microbiota (Wu et al., 2021). Human gut microbiota should have capacity to produce bioactive peptides from ingested proteins reached colon. However, there is limited information on this subject, especially on plant derived ones. A study investigated the milk peptides released in the gastrointestinal tract of mini­pigs. Six different dairy food matrices were given to the pigs, later duodenal effluents were obtained over a 5 h-period and analyzed by tandem mass-spectrometry. Production of bioactive peptides were also reported and that approves bioactive peptide production by gut microbiota as suggested in in vivo experiments (Barbé et al., 2014).

Glucosinolates are sulphur containing glycosides found in cruciferous vegetables mainly belong to the Brassicaceae, Capparaceae and Caricaceae genera. These include broccoli, radish, cabbage, Brussels sprouts, cauliflower and turnip of the different Brassica species. Glucosinolates have little biological activity but their degradation products serve as a defence against herbivores including insects, birds, aphids and mammals. Glucosinolates are also responsible for the characteristic flavour and odour of these vegetables. It was reported that they have antifungal and antibacterial properties (Fahey et al., 2001). Glucosinolate research started with toxicological aspects of glucosinolates and methods for removing them from dietary sources and animal feed but nowadays, the studies are mainly focused on their potential health promoting effects (Narbad & Rossiter, 2018).

Luang-In et al investigated the metabolism of glucoerucin, glucoiberin and glucoraphanin by gut

iberin and sulforaphane in low concentrations. Escherichia coli VL8 showed 80-90% degradation of glucoiberin and glucoraphanin. The metabolism by E. coli VL8 produced glucoiberverin from glucoi­berin and glucoerucin from glucoraphanin. Other metabolites were erucin, erucin nitrile and iberverin, iberverin nitrile respectively which points a reductase activity. This reductase activity was proposed to

degradation of glucoraphanin by rat cecal microbiota and reported evidence for glucoraphanin hydrolysis to sulforaphane and its absorption across the cecal walls of rats. Glucoraphanin (150 µmol/kg BW) was directly introduced to cecum and ITC were detected in the mesenteric plasma by 120 min and plasma levels keeping the stability for an hour (Lai et al., 2010). A rat model study investigated glucosinolate hydrolysis in gnotobiotic rats. Rats harbouring a whole human faecal flora (Flora+) was compared with that in germ-free rats (Flora-). To test the effect of plant myrosinase, myrosinase was either active (Myro+) or inactive (Myro-) in the diets given to the rats. Urinary mercapturic acids which are end products of isothiocyanate metabolism were used o estimate isothiocyanate formation. The study con­cluded that highest excretion of urinary mercapturic acids was found when plant myrosinase was active (Myro+) in grem-free rats (Flora-). The excretion was lower in rats with whole human feacal microbiota (Flora+) and given active myrosinase (Myro+). Excretion of urinary mercapturic acids was low in rats harbouring a whole human faecal flora (Flora+) but Flora+ treatments also show promising capacity to break down glucosinolates as no intact glucosinolates were detected in the faeces of rats (Rouzaud et al., 2003a). These results emphasize the importance of an active and efficient myrosinase to confer health benefits from glucosinolates.

Health claim on glucosinolates depends on ITC formation and its concentration but glucosinolate deg­radation not always end up by formation of ITCs. To determine the amounts of glucosinolate metabolite, a dynamic in vitro large-intestinal model was used. The study investigated sinigrin and its metabolites using human gut microbiota. Allyl isothiocyanate (from sinigrin) peak levels were observed between 9 and 12 h after the addition of sinigrin. The study reported that only 10% to 30% (mean 19%) of the sinigrin was converted into allyl isothiocyanate so rest of the sinigrin was supposed to be converted into unknown metabolites (Krul et al., 2002). Another in vitro fermentation assay using rat intestinal micro­biota reported degradation of up to 64% of the initial sinigrin amount during 12h. The study showed that major products were allyl isothiocyanate (15% of initial sinigrin) and allyl cyanide (20% of initial sinigrin) after 12h. 1-cyano-2,3-epithiopropane was also detected in trace amounts in culture medium (Lu et al., 2011). This result raises the question about rest of the degraded sinigrin. Similar results were reported in another study testing the effect of human gut microbiota on glucoraphanin. The amounts of the metabolites detected by combined HPLC and LC–MS/MS methods did not account for the initial amount of the glucoraphanin in the media.

Clarke et al. (2011) investigated the bioavailability and excretion of the mercapturic acid pathway metabolites of isothiocyanates after human consumption of fresh broccoli sprouts or broccoli supple­ment. The study was designed as a cross-over study in which 12 subjects consumed 40 grams of fresh broccoli sprouts followed by a 1 month washout period and then the same 12 subjects consumed 6 pills of a broccoli supplement. Control group was given alfalfa sprouts during the first phase and placebo pills during the second phase. The bioavailability of sulforaphane and erucin, main ITC products of brocoli, is siginificantly higher in fresh broccoli sprout consuming group. Interconversion of sulforaphane and erucin was also observed within each subject but this conversion was variable among the subjects. This study also confirms the importance of an active myrosinase to produce ITCs as stated in previous rat model study (Rouzaud et al., 2003a).

Some of the studies reporting the effect of gut microbiota on glucosinolate metabolism are listed in Table 4. Briefly, many studies report the glucosinolate degradation by community based microbiota or pure bacterial cultures and glycoside hydrolases are suggested to be the responsible enzyme. Many attempts were also made to identify the gut microbial myrosinases (Liou et al., 2020; Luang-In et al.,

other compounds change how microorganisms act on phytochemicals and which metabolites are formed. Briefly, more studies are needed to adress food matrix effect on bioavailability.

The studies reveal that many microorganisms are involved in transformation of pytochemicals into their metabolites. Sometimes, these metabolites are more bioactive than parent compounds and in some cases they can be toxic to host as well. For certain phytochemicals, it was known that which enzymes in the gut are involved in biotansformation. For instance, α-rhamnosidase, exo-β-glucosidase, endo-β­glucosidase and/or β-glucuronidase are suggested to be involved in flavonoid degradation and gut bacterial myrosinases (possibly glycoside hydrolases), reductases (such as methionine reductases), sulfatases in glucosinolate degradation. However, further studies are needed. Various studies emphasize that there is inter-individual diffences among people in phytochemical metabolism. The knowledge we have about pytochemical bioavailability shoud enable us to develop better strategies. By doing so, it would be pos­sible to enhance the biavailability of these compounds and make the most of them.