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Tryptophan – what you need to know

By Tracy Meyer, ADVIT Animal Nutrition
Looking at a standard formulation for a pig ration, you will notice that synthetic amino acids comprise less than one percent of the total diet. This seems like a small figure but with the cost of protein sources increasing, the need for synthetic amino acids makes economical sense.  The more familiar amino acids supplemented in a diet are lysine, methionine and threonine. These are by order of importance the first, second and third limiting amino acids respectively. Tryptophan lies in fourth place. As I discussed in my previous article “Understanding Micro Nutrients”, nutrient supply follows the principle of Liebigs’ barrel. Any amino acid (or nutrient for that matter) can be represented by the orange stave – this is the first limiting amino acid (or nutrient).  This requirement must be satisfied for optimum growth. As stated by Mitchell et al. 1946), “if the rate of protein synthesis is lowered due to an inadequate supply of a limiting amino acid then increasing the limiting amino acid should increase protein synthesis”.
As the economy changes and raw material costs rise, diet formulation is adjusted to cost-effectively meet the nutrient requirements of the pig. Previously rations were formulated on a crude protein estimation, but this has now changed with the increase in feed costs. It is now possible to lower the crude protein level in the diet, and to supplement the amino acid requirement with synthetic amino acids, rather than relying on the protein source to supply them.
This change in ration formulation can have very beneficial cost savings.  Lysine, methionine and threonine are essential amino acids, which mean that the pig cannot synthesise these in the body, and relies on the food to supply them. Tryptophan is also an essential amino acid, and until recently has been adequately supplied by the diet. The effects of tryptophan seem to justify it to be included in the diet as an additional amino acid.
Tryptophan, together with the other essential amino acids, is an important substrate for protein synthesis. This however is not its only function.
•  Tryptophan is the precursor for the neuro-transmitter serotonin, synthesised in the brain and gastro-intestinal tract, which plays a very important role in feed intake. Tryptophan also stimulates the release of the hormone ghrelin from cells in the stomach and pancreas. Ghrelin stimulates the feeling of hunger. Zhang et al. (2007) concluded that increased tryptophan ingestion increased the ghrelin expression in the stomach and ultimately the ghrelin level in the plasma.  This causes an increased feed intake by the pig to satisfy the hunger feeling. An increased feed intake leads to increased growth and improved feed conversion.Applications for tryptophan in the commercial unit could be in the piglet and lactating sow compartments where feed intake limitations do occur (Dapoza, 2009).
•  Seratonin is often referred to as the “happy hormone” although it is not a hormone at all. Seratonin has a marked effect on the mental well-being of an animal. In the 1980’s tryptophan was prescribed to patients suffering from depression. Poletto et al. (2009) studied the effect of additional tryptophan on the behaviour and aggressiveness in grower gilts. They found that supplying a diet with an increased level of tryptophan reduced the behavioural activity and the aggressiveness of grower gilts. Most likely this was due to the increased production of serotonin. Diets supplemented with additional tryptophan could be fed at weaning where pigs are mixed to reduce the aggression. The appearance of pork products influences the consumer. In 1994 Jeremiah performed a survey among 1 115 shoppers. He found that consumers were 2.5 times more likely to decline purchasing fresh pork loin chops if they exhibited PSE (pale, soft and exudative) characteristic versus those that didn’t. Adeola and Ball, 1992) found that pigs given 5g tryptophan/kg for five days prior to slaughter exhibited significantly higher brain serotonin concentrations and generally produced pork that had a lower incidence of PSE.
•  When a disease challenge occurs and the health status of the herd is compromised there is a greater demand for tryptophan. One can see from the graph that the tryptophan levels in the plasma decrease when disease challenged (Le Floc’h, 2009). This challenge causes an increased demand for tryptophan for the synthesis of proteins required for an immune response. To counteract the greater demand for tryptophan for non-productive functions, tryptophan levels in the diet can be increased, especially in low health conditions (Dapoza, 2009).
Feed intake is what dictates whether your farm is successful or not. The addition of tryptophan will have a positive effect on the feed intake on the farm – with results being even greater should there be a slight deficiency. With feed costs rising every day, we need to turn back to the basics of nutrition and make sure we optimise the feed quality by including all nutrients required by the pig to maximise the growth potential.
Dapoza, C. 2009. Tryptophan in swine nutrition.  Evonik Degussa International AG. Spain
Poletto R., R.L. Meisel, B.T. Richert, H-W. Cheng and J.N. Marchant-Forde, 2009. Aggression in replacement grower and finisher gilts fed a short-term high-tryptophan diet and the effect of long-term human-animal interaction. Applied Animal Behaviour Science (in press). doi:10.1016/j.applanim.2009.11.015
Zhang, H., Yin, J., Zhou, X., and Li, X.  2007. Tryptophan enhances ghrelin expression and secretion associated with increased food intake and weigh gain in weanling pigs.  Domestic Animal Endocrinology.  33:  47-61.
Jeremiah, L.E.  1994.  Consumer responses to pork loin chops with different degrees of muscle quality in two western Canadian cities.  Can.  J.  Anim.  Sci. 74:  425.
Adeola, O and R.O. Ball.  1992.  Hypothalamic neurotransmitter concentrations and meat quality in stressed pigs offered excess dietary tryptophan and tyrosine.  J.  Anim.  Sci.  70: 1888.
Le Floc’h, N.  2009.  Why tryptophan is an important amino acid for piglet nutrition.

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