The effect of dietary protein source (fishmeal vs. plant protein) and non‐starch polysaccharide level on fat digestibility and faecal bile acid loss in rainbow trout (Oncorhynchus mykiss)

Abstract

This study investigated in rainbow trout (Oncorhynchus mykiss) if diet composition and feeding level affect faecal bile acid loss, and whether this reflects on the apparent digestibility coefficient (ADC) of fat. Six diets were formulated with either fishmeal or plant protein as main protein source. This created a contrast in the supply of bile acids, the bile acid precursor cholesterol, taurine and the taurine precursors (methionine + cysteine) involved in bile acid conjugation. For both protein sources, three diets were formulated with increasing inclusion of a non-starch polysaccharide (NSP)-rich ingredient mixture (0.0, 82.0 and 164.2 g/kg diet). This aimed at enhancing faecal bile acid loss. Fish were fed both restrictively (1.2% BW/day) and to satiation.A similar fat ADC was found when substituting fishmeal with a plant protein mixture, suggesting that the lower content of bile acids, cholesterol, taurine, methionine and cysteine in the plant-based diets did not limit fat digestion. Faecal bile acid loss increased alongside dietary NSP level, however, only during satiation feeding and most strongly for fish fed the fishmeal-based diets. Enhanced faecal bile acid loss was not caused by NSP-bile acid binding/entrapment, but by an increase in faeces production. During satiation feeding, fat ADC negatively correlated with faecal bile acid loss.From this it is concluded that bile acid availability/synthesis can become limiting for fat digestion in rainbow trout under conditions that enhance faecal bile acid loss (i.e. dietary NSP level and feeding level). K E Y W O R D Sbile acids, cholesterol, fat digestibility, non-starch polysaccharides, Oncorhynchus mykiss, taurine | 1171 STAESSEN ET Al.

Time‐related changes in nutrient digestibility and faecal bile acid loss of rainbow trout ( Oncorhynchus mykiss ) as affected by dietary fat level and non‐starch polysaccharide level

“…Possibly, the lower feed intake (and thus faecal bile acid loss) of fish fed the high‐fat diets compared to the low‐fat diets could explain the presence of an NSP effect on fat ADC in the low‐fat diets while absent for the high‐fat diets. The same explanation holds when comparing the effects of NSP level on fat ADC and faecal bile acid loss of the current study with those of our previous studies (Staessen, Verdegem, Koletsi, et al (2020); Staessen, Verdegem, Weththasinghe, et al, 2020). The overall NSP effects in the current study were weaker compared to those previous studies, and most likely these NSP effects were dampened by the strong time‐related decrease in feed intake (i.e., faecal bile acid loss).…”

“…Our previous studies showed that conditions that stimulate enhanced faecal bile acid loss (i.e., NSP level and feed intake) coincide with a reduction of fat ADC in rainbow trout (Staessen, Verdegem, Koletsi, et al (2020); Staessen, Verdegem, Weththasinghe, et al, 2020). A reduction of bile acid availability in the intestine was proposed to cause this reduction in fat ADC.…”

“…This lack of bile acids is hypothesized to have occurred either instantly through formation of insoluble NSP‐bile acid complexes (Ebihara & Schneeman, 1989; Sinha et al., 2011; Vahouny et al., 1981), or to have occurred over time as enhanced bile acid loss gradually depleted the bile acid pool (Dowling et al., 1970). Unfortunately, the studies by Staessen, Verdegem, Weththasinghe, et al (2020) and Staessen, Verdegem, Koletsi, et al (2020) measured fat ADC using faecal samples that were collected during the last week of the experimental period, and therefore, information on the time‐related relationship between faecal bile acid loss and fat ADC is not available.…”

“…Under homeostasis, faecal bile acid loss is compensated by de novo bile acid synthesis in the liver (Hofmann, 2009). Staessen, Verdegem, Weththasinghe, et al (2020) and Staessen, Verdegem, Koletsi, and Schrama (2020) observed enhanced faecal bile acid loss in rainbow trout (Oncorhynchus mykiss) that was fed NSP-rich diets to satiation. Enhanced faecal bile acid loss correlated negatively with fat ADC in those studies, and this inverse relationship disappeared with dietary bile acid supplementation (Staessen, Verdegem, Koletsi, et al, 2020).…”

“…Staessen, Verdegem, Weththasinghe, et al (2020) and Staessen, Verdegem, Koletsi, and Schrama (2020) observed enhanced faecal bile acid loss in rainbow trout (Oncorhynchus mykiss) that was fed NSP-rich diets to satiation. Enhanced faecal bile acid loss correlated negatively with fat ADC in those studies, and this inverse relationship disappeared with dietary bile acid supplementation (Staessen, Verdegem, Koletsi, et al, 2020). The outcome of these 2 studies suggests that enhanced faecal bile acid loss of rainbow trout results in a lack of bile acids that are available for fat digestion in the intestine.…”