Effects of dietary lysine levels on the concentrations of selected nutrient metabolites in blood plasma of late‐stage finishing pigs

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Sustainable swine production with less wastage of dietary nutrients requires a thorough understanding of nutrient metabolism. All nutrients, including AA, are directly or indirectly associated with each other via a complex network of intermediary metabolic pathways (Salway, 2004), and the metabolite profile of blood plasma reflects the dynamics of nutrient metabolism in response to dietary nutrient supply. The apparent changes in growth performance such as average daily gain (Wang et al., 2015) should be reflected in the changes in blood metabolites. Studying the alteration of plasma concentrations of metabolite following administration of one particular dietary nutrient can help animal nutritionists to understand the physiological roles of the given particular nutrient and its interactions with others, and fine‐tune the inclusion rates of that nutrient to finally improve the feed efficiency.
Lysine, the first‐limiting AA in typical swine diets, plays very important roles in exerting many metabolic and physiological functions in pigs (Liao, Wang, & Regmi, 2015). In the experiments with growing and finishing pigs (Fuller, Reeds, Cadenhead, Seve, & Preston, 1987; Roy, Lapierre, & Bernier, 2000; Shelton et al., 2011), crystalline lysine supplementation increased nitrogen retention and protein accretion and improved animal growth performance. It has been shown that dietary lysine affects plasma concentrations of nutrient metabolites including total protein, albumin (Kamalakar et al., 2009; Yang et al., 2008), urea nitrogen (UN) (Fernández‐Fígares, Lachica, Nieto, Rivera‐Ferre, & Aguilera, 2007; Zeng et al., 2013), glucose (Zhang et al., 2008), triglycerides, and cholesterol (Bouyeh & Gevorgyan, 2011; Skiba, 2005). However, results from these previous studies were rather inconsistent, and in many cases the dietary concentrations of other AAs were adjusted to constant ratios to lysine based on the ideal protein concept (Baker, Hahn, Chung, & Hahn, 1993; NRC, 2012). The ratio adjustment is necessary to apply the ideal protein concept in swine feeding practice, but the adjustment makes it difficult to attribute the observed effects on nutrient metabolism to the level change in dietary lysine only.
Dietary lysine requirement of pigs at late finishing stage is lower than that at growing stage, yet it is the last but not least stage when producers can use nutritional measures to maximize pigs’ growth performance and carcass characteristics (Wang et al., 2015). To understand how a single dietary AA could affect nutrient utilization or metabolism in marketing pigs, this study was conducted to investigate the effects of lysine, which is the only variable in the diet, on six selected metabolic parameters (UN, albumin, total protein, glucose, triglyceride, and total cholesterol) of the blood plasma in late‐stage finishing pigs. These parameters are indicators of energy, protein, and lipid metabolism of pigs (Roy et al., 2000; Zeng et al., 2013). To formulate a truly lysine‐deficient or lysine‐excess diet, no effort was made to generate isonitrogenous diets, although the diets were isocaloric for this study.
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