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Besides being a nutrient that is fundamental to allow maximum growth of the pigs, tryptophan also has other physiological functions of great interest.
Physiological functions
a) Regulation of voluntary feed intake
Tryptophan has positive effects on voluntary feed intake. This characteristic makes it a very important nutrient in feeds for animals with low appetite and limited feed intake capacity, as it is the case for lactating sows and weaned piglets.
This greater feed intake leads to increased growth performance and improved feed conversion ratio in piglets, and to lower body weight losses in sows during lactation. The effect can be very noticeable when extreme levels of tryptophan are used, but even in commercial ranges (22% vs 18% of Trp/Lys) significant improvements can be obtained in piglets: +6% in feed intake, +9% in weight gain and -3% in feed conversion ratio, according to average data from literature.
Although recent studies suggest the potential mediation of hormones such as melatonin (pineal gland), insulin (pancreas) or ghrelin (stomach), the effect of tryptophan on feed intake seems to be fundamentally related to its role as a precursor of serotonin, a neurotransmitter synthesized in the brain and in the gastrointestinal tract, which plays a role in regulating feed intake.
It has been observed that the amount of dietary tryptophan needed to achieve a certain level of feed intake is greater in diets with a higher protein content. High protein diets contain a greater quantity of large neutral amino acids (valine, isoleucine, leucine, phenylalanine and tyrosine), which compete with tryptophan for the same transporter during the intestinal absorption and also to pass the blood-brain-barrier, which reduces the quantity of tryptophan available for the synthesis of serotonin. Some authors recommend a tryptophan to large neutral amino acids ratio of at least 4% in swine diets.
b) Control of stress response
Tryptophan can modulate aggressive behaviors and improve the stress responses in swine. It has been observed in various species, including humans, for which tryptophan has been used as an anti-depressant in the 1980s. It has also been reported that tryptophan may reduce the negative effects on meat quality caused by stress during transport and before slaughter.
The mechanism of coping stress is not well described, but it is most likely due to tryptophan being served as precursor for serotonin synthesis.
However, clear effects have been observed only when using levels of tryptophan that are either much lower or much higher than the usual levels found in commercial feeds.
c) Control of immune response
The level of tryptophan in the blood and the quantity of tryptophan available for body protein synthesis are decreased during inflammatory or disease challenge conditions. This is due to the lower level of feed intake during inflammatory states, the increased demand for tryptophan for the synthesis of acute-phase proteins, which are rich in tryptophan, and also due to the greater catabolism of tryptophan via kynurenine pathway caused by the action of the enzyme IDO (indoleamine 2,3 dioxygenase), which is stimulated in the presence of cytokines (inflammatory states).
An adequate inclusion of tryptophan is especially advisable in low health conditions due to its effect on ingestion capacity and also to counteract the greater demand for tryptophan for non-productive functions. Moreover, in some studies it has been observed that the inclusion of tryptophan in the diet is capable of modifying the inflammatory response. According to these studies, a diet with an adequate level of tryptophan produces a lower inflammatory response than a diet that is deficient in tryptophan, with lower levels of plasma haptoglobin (acute phase protein) and a lower activity of enzyme IDO.
Recommended levels
In feeds for piglets of up to 30 kg, the contribution of tryptophan from raw materials usually is not enough to cover the animal’s requirements and therefore a supplement of industrial tryptophan is required. Traditionally, it is for these body weight ranges that most effort has been made to define the level of tryptophan correctly.
In the literature, recommendations are usually expressed in digestible basis and in relation to lysine, following the concept of ideal protein. In table 1 the values recommended for piglets by different institutions are shown. Traditionally Trp:Lys ratios of 18 to 20% have been recommended, but there is currently a great deal of evidence that suggests that higher levels (≥ 22%) can be more profitable, especially in low health conditions.
Table 1. Optimum digestible Trp:Lys ratios recommended for 5 to 30 kg pigs
| Source |
Recommended Trp:Lys (%)
|
| BSAS (United Kingdom) |
18.8
|
| CVB (The Netherlands) |
19
|
| Evonik Degussa |
221
|
| FEDNA (Spain) |
192
|
| GfE (Germany) |
183
|
| ITP (France) |
19
|
| NCPP (Denmark) |
19.52
|
| NRC (USA) |
18
|
1. They have updated lately their recommendations to 22% based on the analysis of recent studies.
2. They note that according to recent European studies improvements can be obtained with higher Trp/Lys levels.
3. They do not take into account the effect on ingestion capacity.
In the same way, the levels traditionally recommended for lactating sows oscillate between 18 and 20 % according to the sources, but there are also studies that have shown improvements in feed intake and reduced weight loss with a Trp:Lys ratio of at least 22%.
Tryptophan is an amino acid that is thriving in swine diet formulations due to its positive effect on feed intake and growth performance. The moderation on the price of industrial tryptophan combined with the reduction in protein levels of the formulas observed recently is turning tryptophan into an even more interesting nutrient.
