Tryptophan: feed intake
and immune functions

Tryptophan presents the particularity to be involved in various biologic functions other than protein synthesis and one of the most important is the regulation of appetite. In addition, tryptophan has a role in the control of the immune response, health maintenance and animal behaviour.

Tryptophan: a key nutrient in the regulation of feed intake

It has been clearly demonstrated in piglets that an adequate dietary Trp:Lys ratio, compared to a deficient one, enhances feed intake. It is known that the appetite regulation is a complex mechanism orchestrated by numerous pathway in which tryptophan has an important role (Figure 1).

Figure 1. Summary of possible implications of tryptophan in appetite regulation.

The effect of tryptophan on appetite regulation could be mediated through the regulation of the central production of serotonin which is involved in the regulation of satiation and appetite. Tryptophan being the precursor of this neurotransmitter serotonin, a decrease in brain tryptophan leads to a decrease in the production of serotonin. Because Large Neutral Amino Acids (LNAA, namely phenylalanine, tyrosine, isoleucine, valine and leucine) and tryptophan share the same transport route through the blood brain barrier, the production of serotonin in the brain depends not only on dietary tryptophan levels, but also on dietary LNAA supply. Consequently, reducing dietary crude protein content and ensure a minimum of 22% SID Trp:Lys ratio in piglets diet is a guarantee to optimise feed intake regulation.

Tryptophan may also have a role in the regulation of appetite and feed intake through the peripheral control. Melatonin which is produced from tryptophan in the gastrointestinal tract, may serve as a signal for the synchronization of the feeding and digestion processes.

The effect of tryptophan on the regulation of appetite could also be explained by its effect on the gene expression and the synthesis of the ghrelin hormone. Ghrelin is an appetite stimulating hormone produced and secreted by the stomach and the duodenum. It is also a Growth Hormone secretagogue, which is involved in numerous functions such as protein synthesis, bone growth, muscle growth… Dietary tryptophan content improving ghrelin gene expression, it increases feed intake and weight gain.

Tryptophan and immune function

The decrease in plasma tryptophan concentration in pigs under low sanitary conditions reflects the specific use of this amino acid to satisfy the immune functions of the animal. When health is impacted, the decrease in plasma tryptophan could be explained at least by two hypotheses:

  • by the synthesis of acute phase proteins which are tryptophan rich proteins; during inflammation, the synthesis of these protein is increased,
  • by a higher degradation of tryptophan into kynurenine through the induction of indoleamine 2,3 dioxygenase (IDO) activity. IDO has a wide localisation in the body and is specifically induced by immune system mediators. In such a situation, it is responsible for an increased in the catabolism of tryptophan into the kynurenine pathway.

Thus, under low sanitary conditions, competition could occur between the different tryptophan pathways, and consequently could impact upon the animal response to tryptophan.

For instance, dietary L-Tryptophan supplementation limits the decrease in piglet feed intake and weight gain after E. coli challenge (Figure 2).

In another experiment, the effect of tryptophan on growth of piglets reared in different environmental conditions has been tested (Figure 3).

Figure 2. Effect of the challenge to E. coli and the L-Tryptophan addition on average daily gain (ADG) and feed intake (expressed in the % of the performance of the piglet control response) during the first four days post-challenge.

Figure 3. Effect of dietary tryptophan and depressing sanitary status on weight gain and feed intake of pigs from 8 to 27 kg, AB means antibiotics in the diet

A deficiency in tryptophan affects more severely the growth of piglets in poor health environment. Additional dietary tryptophan partly prevents the negative consequence of poor health status on growth rate.

These results have important practical implications. First, in case of immune challenge, the feed intake is decreased. Then, when the immune system is activated even moderately, the disappearance of tryptophan from plasma limits its availability for growth. Consequently, tryptophan supply should be adequate to cover the total requirement for the immune system and also to maintain growth performance.

For further information, please read our technical bulletins: numerous experimental results are reported.