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Increasing attention is currently being paid to the protective role of polyphenols in health and oxidative status in fish. For this reason, the potential use of different natural sources of such compounds, like wine by products, is under study. One key step required to gain a better understanding on the biological roles of polyphenols for a given species is to assess the different factors affecting their digestive bioaccessibility, and a great number of such studies is based in the use of in vitro digestion models. In the present study the potential digestive bioavailability of the phenolic compounds present in wine bagasse and lees was evaluated for two fish species showing great differences in their digestive phisyiology: the omnivorous gilthead sea bream (Sparus aurata) and the herbivorous flathead grey mullet (Mugil cephalus). The study was developed using in vitro models adapted to simulate their digestion and a factorial experimental design that simultaneously evaluated the effects of the ingredient used as source of polyphenols, presence or absence of feed matrix, fish species and digestion time. The release of the phenolic compounds was evaluated using ultra-high performance liquid chromatography (UHPLC) coupled to high resolution mass spectrometry (HRMS) detection. Both the presence of feed matrix and the type of wine by-product showed a significant effect on the digestive release of both total and specific types of polyphenols while fish species showed to be significant only for some specific compounds, like eriodyctiol or syringic acid. The time of digestion was not identified as a statistically significant factor in the release of phenolic compounds due to the great variability in the patterns observed that were classified as early, sustained and late. The observed great variations in the patterns of release of different types of phenolic compounds with time suggest an important effect of gut transit rates on the net bioavailability of a given phenolic compound in the live fish. The present study is, to our knowledge, the first one on which an in vitro approach was applied to assess to what extent the possible complexation of wine polyphenols present in wine by-products with either digestive enzymes or components of the feed matrix could limit their bioaccessibility if included in diets of two different fish species.

Understanding the daily digestive function could lead to more efficient feeding practices in the farmed fish species. In this work, the effect of several daily feeding protocols was assessed on the daily expression of the genes related to the circadian system (arntl1, clock, per1, and cry1), appetite (cck), nutrients hydrolysis (try1, ctrb1, pga, atp4a, cel, and amy2a), peptides transport (slc15a1), pH, and activity of key digestive (trypsin, chymotrypsin, amylase, 4C- and 7C-like lipases) and absorptive (alkaline phosphatase) enzymes in the gastrointestinal tract (GIT) of Senegalese sole (Solea senegalensis) juveniles. Some of the tested feeding protocols demonstrated a significant influence on the entrainment of the peripheral oscillator. The synchrony of slc15a1 and some digestive proenzymes with per1 in some of the experimental treatments could underline the role of clock genes in the regulation of these biological functions. Unlike gene expression of the digestive proenzymes wherein feeding protocol occasionally affected the temporal organization, the influence of this factor was determinant on the daily average production and/or circadian pattern of their activities. Daily pH was maintained at approximately neutral levels in all the GIT segments, and no pepsin activity was detected. Fish fed a single diurnal meal showed the lowest activity for all the studied enzymes, except for amylase, suggesting an insufficient utilization of the ingested nutrients. Higher amylase activity in this group, on the other hand, implies the possible activation of alternative compensatory metabolic pathways. Overall, our results pointed to the suitability of higher feeding frequencies, especially during dark hours, for the aquaculture production of this species., This research was funded by the Spanish Ministry of Economic Affairs and Competitiveness (MINECO) by projects EFISHDIGEST (AGL2014-52888-R) with FEDER/ERDF contribution granted to MY and WISEFEED funded by the European Union's H2020 Program (Marie Skłodowska-Curie Grant No. 691150). NG was supported by a doctoral fellowship (BES-2015-071662) from MINECO (Spain).