Right Frank International Nutrition and Genetics Technologies
News on the Use of Acids in Fish Nutrition
In monogastric animals, including a wide variety of different fish species (ranging from salmon and trout, tilapia, sea bass to pangasius), the chemical breakdown is next to others achieved in the stomach through acidification. After being swallowed, the food passes from the esophagus into the stomach, where stomach acid and enzymes help to break down the food. Bile salts stored in the gall bladder empty the contents of the stomach into the small intestines where most fats are broken down.’
| The ‘stomach acid’ working in all monogastric animals is hydrochloric acid, a very strong inorganic acid, which is produced by gastric glands (parietal cells). This acid is able to lower the pH in the stomach to levels between pH 1-3. The hydrochloric acid production at birth is negligible, but it will increase with the age of the animal. The more the acid is produced in the stomach, the lower is the pH. The present pH is involved in the activation of pepsin, which is a proteolytic enzyme. This is needed in the digestion of protein. Pepsin is secreted as an inactive zymogen, called pepsinogen (inactive in order to not ‘digest’ the stomach itself when no food is available). Its conversion into the active form is catalysed by the action of the acid. Similar to every enzyme, pepsin has certain optimal conditions in which it is works best. The optimal pH for pepsin activity is 2.0. At higher pH levels the activity is severely reduced. Monogastric aquaculture species What are the implications of this for monogastric aquaculture species, which are heavily depending on the high protein inputs and on proper digestion of these expensive ingredients? One of the possible answers may come from feed additives. The of organic acids or acid salts has been studied and reported in publications over the past half-century in animal nutrition (Cole et 1968). Supplementing diets with organic acids reduces the pH in stomach, it stimulates thereby the activation of pepsinogen to pepsin thus may improve protein digestibility and decrease the rate of emptying. It further improves protein digestion by increasing the rate proteolysis of large protein molecules (Theobald and Lückstädt, 2011). The reduction of pH in the feed and stomach largely depends on buffering capacity of feed ingredients. Animal protein (e.g. extensively used in aquaculture diets, has a 15 fold higher capacity compared to cereals. These effects are especially important view of the low hydrochloric acid output in young animals, as before (Freitag, 2007). Most of these data are however stem monogastric livestock, such as pig. | Its investigation in diets however has been done only very Bucking and Wood (2009) looked into the effect of feeding stomach pH. The authors fed rainbow trout (mean weight 350g) commercial trout feed with 41% crude protein in a single-meal (body weight ration) and monitored the resulting pH in the stomach. Just before feeding the stomach pH was at ~2.7, whereas the pH hour after feeding went significantly up to pH 4.9. It remained for at least 8 hours, thereby being far above the optimum for activity. The chyme was released into the duodenum 8 hours feeding, at far too high pH-levels. The authors speculated that the buffering capacity of the feed was a major contributor to the pH of gastric fluids. It took the fish more than 24 hours to reach ‘low’ initial pH of the stomach (Figure 1a and The effect of diet buffering capacity on the gastric in juvenile fish was proven as well by Marquez et al. (2011a). found that fishmeal diets had a 10-fold higher buffering capacity therefore needed more energy for acid secretion per digestion than test-diets without animal protein meals. DEVELOPMENTS Similar observations on feeding and gastric pH have been made Yufera et al. (2011). This Spanish research group fed juvenile seabream in three different ways: once, twice or continuously monitored the resulting pH-levels in the stomach. Feeding was either at 9:00 or at 9:00 and 17:00, or continuously between 9:00 21:00. The gastric pH exhibited significant daily rhythms under three tested feeding regimes (Figure 2). Only in the feeding regime continuous offer of feed the stomach pH remained for a while in region of the pepsin optima. This may be a reason for the greater weight of the fish from that feeding regime. It could be explained by higher gastric activity in fish with gastric pH-levels 4.5, as documented by Marquez et al. ( A subsequent study from Yufera et al. (2012) took this a step This time, the authors looked at the connection between stomach and pepsin activity in juvenile marine fish. Fish were again fed a single-meal, twice or continuously the same diet at the same as mentioned above. In comparison, fish fed only once again had levels in the stomach around 4.5, while the highest pepsin activity actually reported before the feeding with 30 pepsin activity units fish. Contrary to that, continuously fed fish reached a minimum pH in stomach of 52.5 and had a resulting pepsin activity of almost 280 per fish in the late afternoon. This clearly demonstrates the impact of pH on pepsin activation. |
