Elsevier

Aquaculture

Volume 273, Issue 1, 30 November 2007, Pages 96-107
Aquaculture

Nutrient digestibilities and gut function in Atlantic salmon (Salmo salar) fed diets with cellulose or non-starch polysaccharides from soy

https://doi.org/10.1016/j.aquaculture.2007.09.013Get rights and content

Abstract

Duplicate groups of Atlantic salmon (Salmo salar), kept in saltwater, were fed fish meal based cold-pelleted diets where graded levels of native or extruded non-starch polysaccharides (NSP) from soybeans replaced cellulose, at a total NSP level of 100 g kg 1 for 28 days. The study also included a diet where defatted soybean meal (SBM) constituted the NSP at a level of 100 g kg 1 and a reference diet without NSP supplementation.

The SBM diet resulted in a significant reduction in faecal dry matter content, apparent digestibilities of starch and organic material, and growth, and an increased faecal output of several elements (K, Na, Zn), compared to all the other diets. Morphological changes were only seen in the intestine of the fish fed the SBM, i.e. the diets holding purified soy-NSP did not induce enteritis. When compared to the diet without NSP, cellulose addition increased faecal dry matter, while inclusion of native soy-NSP reduced it. Dry matter in faeces and apparent digestibility of crude protein decreased in a linear manner, while the digestibility of starch and faecal output of K and Na increased linearly when native soy-NSP replaced graded levels of cellulose from 0 to 100 g kg 1 feed. When diets with 75 or 100 g kg 1 of native NSP and extruded soy-NSP were compared, fish fed native soy-NSP had reduced faecal dry matter, higher digestibility of starch, and increased faecal output of Cu, Fe, and K. Dry matter in faeces and faecal output of Cu was lower for the highest inclusion level, while digestibility of starch and faecal output of Mn and K were higher.

In conclusion, soy-NSP was inert compared to the fish meal reference, with respect to nutrient digestibilities and intestinal pathologies, but affected faecal mineral excretion in Atlantic salmon.

Introduction

The increasing level of plant ingredients, in which fibre or non-starch polysaccharides (NSP) are inherent, in today's fish feed demands more knowledge on the physical and nutritional consequences of NSP in the feed. Soybean meal (SBM) is a frequently used source of protein in salmonid diets and contains a wide variety of antinutrients in addition to NSP (Storebakken et al., 2000).

Effects in salmonids such as increased faecal water content have been ascribed to indigestible oligosaccharides in SBM (van den Ingh et al., 1996), but the specific effects of the NSP moiety of SBM remain to be elucidated. Dietary SBM may also result in “soybean meal induced enteritis” in the posterior intestine of both Atlantic salmon (van den Ingh et al., 1991, Baeverfjord and Krogdahl, 1996) and rainbow trout (Refstie et al., 2000, Ostaszewska et al., 2005), without that a unique causing agent was identified. Refstie et al. (1999) and Romarheim et al. (2006) reported that SBM caused reduced blood cholesterol, while Storebakken et al. (2000) and Romarheim et al. (2006) reported reduction in lipid digestibility. The latter has tentatively been ascribed to the high content of NSP in SBM (Refstie et al., 1999). Diets with SBM have also been associated with lower feed intake, and a reduced carbohydrate level in the SBM has been associated with improved digestibility of main nutrients (Refstie et al., 1998). Carbohydrates from other sources than SBM have also been ascribed to reduce digestibility of macronutrients, such as oligosaccharides in lupines given to rainbow trout (Glencross et al., 2003).

Studies have been performed where semi-purified and soluble NSP, as guar gum and alginate (the latter has negatively charged carboxyl groups), have been added to the feed. This resulted in decreased level of dry matter (DM) of the faeces in both fish (Storebakken, 1985, Amirkolaie et al., 2005, Leenhouwers et al., 2006) and mammals (Zentek, 1996). Few, or no, positive effects were seen regarding digestibilities of main nutrients. Common for many of these polysaccharides is that they are widely used as thickeners, gelling agents and binders in nutritional products, since their hydroxyl groups are able to structure nearby located water molecules. Leenhouwers et al. (2006) found increased intestinal viscosity in African catfish, leading to decreased digestibility of main nutrients when including guar gum, at 4% and 8%, to the diets. Generally, no clear effect on intestinal viscosity by the presence of soluble NSP in the diet has been observed (Amirkolaie et al., 2005, Leenhouwers et al., 2006). Leenhouwers et al. (2006) explained that reduced digestibility of main nutrient was related to reduced distribution of digestive enzymes in a viscous solution and a lowered flow at the mucosal layer.

Mwachireya et al. (1999) in general found a negative influence on protein digestibility when feeding NSP rich diets to rainbow trout. Insoluble NSP seem to have less influence on apparent digestibility of main nutrients than soluble NSP. Studies with cellulose showed no or small effects on digestibility of main nutrients in fish fed diets of inclusion up to 20% cellulose (Dias et al., 1998, Amirkolaie et al., 2005). To the contrary, Aslaksen et al. (2007) reported decreased fat digestibility in salmon when cellulose levels in the diets increased. A majority of NSP in the relevant plant ingredients for use in fish feed are insoluble. Even though native NSP are not degraded during extrusion, their properties may change (Wang et al., 1993, Rinaldi et al., 2000). Extrusion of NSP rich diets may result in increased digestibility of main nutrients, although in most cases digestibility is not altered (Thiessen et al., 2003, Sklan et al., 2004, Barrows et al., 2007).

In a review by Storebakken et al. (2000), elevated faecal sodium excretion is pointed out as an effect of feeding soy products to Atlantic salmon. Increased sodium excretion was also observed when feeding protein rich plant ingredients, SBM among others, with inclusion level of ∼ 20% to Atlantic salmon. However, no correlation was seen between inherent NSP content and sodium excretion (Aslaksen et al., 2007), while sodium excretion has been found to increase proportionally with dietary cellulose level in rainbow trout (Øvrum Hansen and Storebakken, 2007). A negative influence on the faecal excretion of sodium and potassium by increasing the NSP levels in diets fed to monogastrics has been observed (Stanogias et al., 1994). Zentek (1996) registered an increase in sodium excretion in cellulose diets, compared to soluble NSP diets, given to beagles. Stanogias et al. (1994) found increased excretion of sodium and potassium when feeding increasing levels of complex NSP to pigs.

The aims of the present study were to investigate if soy-NSP could explain the differences in nutrient digestibilities and changes in intestinal morphology and function observed when feeding SBM to salmonids, which was done by comparing different NSP sources. In addition a digestibility study was performed in which graded levels of soy-NSP replaced cellulose. Furthermore it was an aim to find out whether extrusion of the soy-NSP affected the responses in the fish.

Section snippets

Dietary NSP sources, diets, and processing

Seven diets (Table 1), all with fish meal as the main protein source, had an inclusion rate of 100 g NSP kg 1, arising from cellulose, soy-NSP or both. A diet without any NSP addition, and a diet with SBM addition such that the soy-NSP level reached the target level at 100 g kg 1, were also made. The soy-NSP (FIBRIM 1260, The Solae Company, St. Louis, MO, USA) was either native (N) or extruded (E). The “E” was made by extruding an aliquot of “N” without prior conditioning on a laboratory-scale

Viscosity

Viscosity of the gut content was not significantly different among any diets for any of the three statistical analyses performed (Table 2, Table 7, Table 8).

Digestibility

When diets with 100 g NSP kg 1 from different sources (C100, N100, E100 and SBM) and the FM diet were compared, the fish fed the SBM diet responded with a reduction in faecal dry matter content, apparent digestibility of starch and organic material and a reduction in growth. The SBM diet gave increased faecal output of several elements (K,

Discussion

Analysis of the FM diet revealed a dietary fibre content of 29 g kg 1 DM, even though no NSP containing ingredients were used in this diet. Resistant starch may have been detected as dietary fibre (Goni et al., 1996). The measured levels of dietary NSP did not vary in amount and direction one could expect taking the inclusion of NSP ingredients into consideration. It is therefore likely that uncertainty in the analysis contributed to the observed differences in dietary fibre level.

The cold

Conclusions

Dietary inclusion of soy-NSP, up to 100 g kg 1, did not result in reduced digestibility of main nutrients or cause enteritis in the distal intestine of Atlantic salmon. However, inclusion of soy-NSP in the diets did result in decreased faecal DM and increased faecal excretion of sodium, with a significant correlation between these two parameters, possibly indicating that increased drinking was causing the high levels of sodium in faeces.

Extruding the soy-NSP did not alter the properties of the

Acknowledgements

The research was supported by grant # 145949 from the Research Council of Norway. Olav Fjeld Kraugerud was supported by a PhD-grant from The Norwegian University of Life Sciences, Ås, Norway.

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