INTERNATIONAL JOURNAL OF AGRICULTURE & BIOLOGY ISSN Print: 1560–8530; ISSN Online: 1814–9596 13–1308/2015/17–1–1–8 http://www.fspublishers.org Review Article To cite this paper: Shah, S.Z.H., M. Afzal, S.Y. Khan, S.M. Hussain and R.Z. Habib, 2015. Prospects of using citric acid as fish feed supplement. Int. J. Agric. Biol., 17: 1-8 Prospects of Using Citric Acid as Fish Feed Supplement Syed Zakir Hussain Shah 1* , Muhammad Afzal 1 , Shafaat Yar Khan 2 , Syed Makhdoom Hussain 3 and Rana Zeeshan Habib 1 1 Fish Nutrition Laboratory, Department of Zoology and Fisheries, University of Agriculture, Faisalabad, Pakistan 2 Department of Biological Sciences, University of Sargodha, Sargodha, Pakistan 3 Department of Zoology, Wildlife and Fisheries, Government College University, Faisalabad, Pakistan * For correspondence: zakiruaf@gmail.com Abstract Fishes usually have low levels of acid secretion in the gut as compared to mammals. Inclusion of organic acids in their diet reduces the pH in the gut. This lowering of pH increases the phytate hydrolysis, kills the pathogens, decreases the rate of gastric emptying and improves mineralization and nutrient absorption. Among the organic acids, citric acid (CA) has been used extensively for diet acidification due to its unique flavor and high buffering capacity. It has great potential to replace fish meal (up to 70%) with plant based protein sources. Being a strong chelator of calcium and phosphorus, CA enhances the phytate hydrolysis. It improves the bioavailability of minerals by solubilizing the bones and competing with other chelators. It also increases the endogenous as well as exogenous phytases efficiency by providing an optimum pH in gut. Besides, it acts as antimicrobial agent and stimulates feeding in fish. Purpose of this review is to appraise the applications of citric acid supplementations in fish feed and to highlight its role in improving the growth performance, nutrient digestibility, minerals availability and phytase efficacy. © 2015 Friends Science Publishers Keywords: Fish nutrition; Citric acid; Diet acidification; Phytase; Antibiotics Introduction Use of the antibiotic growth promoters in fish feed improves their growth, feed conversion and survival rate. However, these antibiotics produce resistance in micro-biota of fish that may lead to cross–resistance among human. These public concerns led to a worldwide ban on the use of these antibiotic growth promoters in fish feed. Consequently, researchers focused on alternative additives such as organic acids, probiotics, herbs, enzymes and essential oils. Among them, short-chain organic acids are of special interest due to their beneficial effects in preservation of feed (Luckstadt, 2006; Atapattu and Senevirathne, 2013; Sing et al., 2014). Many studies have been conducted on broilers (Brenes et al., 2003; Ali et al., 2013; Ahmad et al., 2013), pigs (Li et al., 1998) and rabbits (Debi et al., 2010) to investigate the effects of organic acids in diet (. However, a little information is available concerning the fish nutrition. Few available studies showed improved production of Rainbow trout (Onchorynchus mykiss) (Sugiura et al., 1998), Red sea bream (Pagrus major) (Sarker et al., 2005) and Rohu (Labeo rohita) (Baruah et al., 2005) in response to organic acids. Sugiura et al. (2006) reported two specialized types of cells in mammalian stomach (parietal and peptic) which secrete HCl and pepsinogen, for the acidification of lumen and digestion of protein. In contrast, stomach of other non- mammalian vertebrates including fish has only one type of cells called oxynticopeptic cells which are responsible for the secretion of both HCl and pepsinogen. However, the acid secretion of these cells is not as efficient as in mammalian parietal cells (Koelz, 1992). Therefore, rainbow trout has high postprandial gastric pH i.e. ~4.0 (Sugiura and Ferraris, 2004) as compared to pH 2 or less in mammalian stomach (Berne and Levy, 1990). This problem can be tackled by adding organic acids in fish feed (Eidelsburger, 1997). Inclusion of these organic acids in diet lowers the pH of feed and intestinal digesta (Baruah et al., 2005). This lowering in pH increases the nutrient absorption (Boling- Frankenbach et al., 2001) and phytate solubility (Jongbloed, 1987). Besides this, rate of gastric emptying is also being reduced by acidification of diet (Mayer, 1994). In addition, due to their antimicrobial effect, they also improve the gut health of the animal (Ravindran and Kornegay, 1993; Partanen and Mroz, 1999). Bioavailability of dietary minerals is also being greatly influenced by acidification through organic acids in several ways. Firstly, they modify the mineral transport mechanism by altering the gastric acidity. Secondly, chelating and complex forming ability of elements (Cross et al., 1990; Ravindran and Kornegay, 1993) is also affected by inclusion of these organic acids in diet. Chelation of these acids with calcium (Ca) ions reduces the antagonistic interactions, including precipitation and co-precipitation between Ca and phosphates or trace elements at the