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Figure 2. Posterior intestine of rainbow trout fed with a diet containing only fish meal (FMC), 50% hydrolysed wheat gluten, 50% vital wheat gluten, or a blend of hydrolysed and vital wheat gluten, on a crude protein basis (electronic microscopy, Ă— 900). No large areas of necrosis whatever the treatment. Enterocytes appear uniform, regular and healthy.

92 and 98.5 percent (Apper-Bossard et al., 2013). In rainbow trout, VWG successfully substitutes more than 50 percent FM providing diets supplemented with lysine without affecting protein and lipid composition of the carcasses (Davies et al., 1997). Furthermore, the inclusion of 14.5 percent VWG in diets does not adversely affect the flavour of fillets (Skonberg et al., 1998). In Atlantic salmon, the replacement of 35 percent FM with VWG without supplementing by lysine results in similar final body weight and growth (Storebakken et al., 2000). These authors estimate the replacement of FM with VWG without amino acid supplementation can go up to 50 percent based on the amount and the availability of lysine in VWG and on the requirement of fish. In European sea bass, substituting more than 50 percent FM with VWG does not impair palatability, growth performance, and nitrogenenergy retention (Tibaldi et al., 2003). In gilthead sea bream, the use of 88 percent CP from VWG not only successfully replaced FM but also produced better growth and feed conversion ratio, probably related to higher protein and energy intake of fish (Allan et al., 2000). In Nile tilapia fed with diets differing in their protein sources, the highest growth is reported for VWG, FM, and soybean extract diet. In shrimp, results are scarce but the replacement of up to 20 percent marine protein with VWG does not significantly affect feed efficiency and growth performance (Molina-Povida et al., 2004; Tereos i nternal data). HWG also seems promising in aquaculture feeds as it results in high growth performance. CP digestibility of HWG has recently been measured. It is very high and further increases with higher HWG inclusion rate in FM-diet for rainbow trout (figure 1; Apper et al., 2014). Recent studies on juvenile hybrid sturgeon show that the replacement of 1 to 5 percent of soy protein concentrate by 1 to 5 percent of HWG in a diet containing animal and plant proteins significantly increases growth performance (Qiyou, 2011). The use of 12.5 to 50 percent CP of HWG to replace high quality FM does not modify growth performance and feed efficiency in rainbow trout (Apper et al., 2014). In the same experiment, authors compared energy and nitrogen retention obtained with either VWG or HWG. Nitrogen retention was similar for the 2 protein sources, with very low metabolic losses of nitrogen (35.1 and 43.4 kg/ton of fish produced for HWG and VWG respectively). Energy retention was higher with HWG than with VWG.

Figure 3. Microvilli of posterior intestine of rainbow trout fed with a diet containing only fish meal (FMC), 50% hydrolysed wheat gluteTable 1Formulation and compositions of experimental diets (%).tein basis (electronic microscopy, Ă— 20 000). Microvilli are uniform and densely packed for all treatments.

Effects of wheat proteins on gut health and microbiota

Compared to a fishmeal-based diet, the use of Wheat Proteins does not damage gut morphology and microbiota. In Rainbow trout, the replacement of up to 50 percent high quality FM by either VWG or HWG results in no modification of gut structure (figures 2 and 3; Apper et al., 2014). Indeed, no areas of necrosis were observed in enterocytes, all appearing uniform, regular, and healthy. Microvilli observations confirmed such results, all microvilli being packed and showing the same density. Similarly, in the same study, microbiota was not significantly different between FM, HWG and VWG diets, with a predominance of Firmicutes. Richness, OTUs, and diversity of microbiota were not different across treatments, suggesting that replacing up to 50 percent of high quality FM by WP is without consequence for gut health. Such results are typical of wheat proteins as soy-proteins or pea proteins have been demonstrated to impair gut morphology or microbiota at high inclusion rates (Mc Kellep Bakke et al., 2007; Penn et al., 2011). In soy-based diets, the inclusion of hydrolysed wheat proteins modulates gut function and morphology, the anti-oxidative system, and the non-specific immune system. The replacement from 1 to 5 percent of soy protein concentrate in a diet based on 20 percent FM, 20 percent soybean meal, eight percent corn gluten meal and 10 percent blood meal increased digestive enzyme activities and fold heights, modulated non-specific immune response and stimulated anti-oxidative status (Qiyou et al., 2011; Zhu et al., 2011). Interestingly, in these studies, the results of 3 percent of HWG inclusion were equivalent to results obtained when authors added one percent free glutamine in the soy protein concentrate diet. Such results suggest that HWG may have a bioactive role, by acting on highly proliferative cells or by saving energy as a glucose precursor. Due to their technological and nutritional properties, Vital and Hydrolysed Wheat Gluten already appear as high value protein sources for fish feeds. Furthermore, new insights on gut morphology, microbiota and health highlight a potential functional role of these proteins on the antioxidative system and on digestive enzyme activity and reveal that wheat proteins do not disturb carnivorous fish microbiota significantly. Further research is needed to confirm these functional benefits and to fully understand the underlying mechanisms.

16 | INTERNATIONAL AQUAFEED | March-April 2015

Mar | Apr 2015 - International Aquafeed magazine  
Mar | Apr 2015 - International Aquafeed magazine  

The March - April 2015 edition of International Aquafeed magazine