Tryptophan is an indispensable amino acid that is often limiting for growth in pigs fed corn-soybean meal-based diets. Tryptophan may act as a regulator of feed intake by enhancing serotonin signaling in the brain, because tryptophan is a precursor for serotonin. High tryptophan intake increases feed intake, and this is partly attributed to increased serotonin synthesis. Availability of dietary tryptophan in the brain is considered the rate-limiting step in hypothalamic serotonin synthesis.
However, to be transported into the brain, tryptophan competes with other large neutral amino acids such as valine, leucine, isoleucine, tyrosine and phenylalanine for a common transporter (L-type amino acid transporter 1) to cross the blood-brain barrier.
Diets based on corn and corn co-products and sorghum and sorghum co-products often contain much more leucine than required by the pigs because of the high concentration of leucine in corn and sorghum protein. Because leucine is one of the amino acids that competes with tryptophan for transport into the brain, excess dietary leucine has been shown to reduce synthesis of serotonin and thereby reduce feed intake of pigs.
It is, however, possible that increased dietary tryptophan can overcome these effects. Therefore, an experiment was conducted to test the hypothesis that increased dietary tryptophan is needed in high-leucine diets for growing pigs to prevent drop in hypothalamic serotonin concentration and to maintain feed intake and growth performance of pigs.
A total of 144 growing pigs (initial body weight: 28.2 ± 1.9 kilograms) were assigned to nine dietary treatments with two pigs per pen and eight replicate pens per treatment. Three basal diets based on corn, soybean meal, wheat and barley were formulated to contain 100, 200 or 300% of the requirement for standardized ileal digestible leucine. These three diets were formulated to have a SID tryptophan:lysine ratio of 18%, which is assumed to be the requirement for growing pigs. Six additional diets were formulated by adding either 0.05% or 0.10% crystalline tryptophan to each of the three basal diets to provide diets with SID tryptophan:lysine ratios of 23% or 28%, respectively. Thus a total of nine diets were used.
Individual pig body weights were recorded at the conclusion of the 21-day experiment, and on the last day of the experiment, one pig per pen was sacrificed and the hypothalamus was collected to measure hypothalamic serotonin concentrations.
Results confirmed that average daily feed intake was negatively affected by excess dietary leucine in the diet (Figure 1), and this resulted in negative effects on average daily gain (Figure 2).
However, the negative effect of excess leucine on feed intake and gain was partially ameliorated by increasing dietary tryptophan. Hypothalamic serotonin was also negatively affected by dietary leucine indicating that excess leucine will restrict uptake of tryptophan in the brain (Figure 3). But with increased dietary tryptophan, the negative effect of excess leucine was partially overcome. These data confirm the negative effect of leucine on serotonin synthesis, which is likely because excess leucine reduces tryptophan uptake in the brain due to competition for the shared transporter from blood to brain.
It is, therefore, likely that the reduced feed intake of pigs fed excess leucine, with subsequent reductions in average daily gain, is partially a result of the reduced synthesis of serotonin in the brain. However, it is also clear that even with the greatest concentrations of tryptophan used in this experiment, it was not possible to fully restore growth performance for pigs fed a diet containing 300% of the leucine requirement to that of pigs fed the diet with 100% of the leucine requirement.
This observation indicates that the reduced synthesis of serotonin is not the only negative effect of excess leucine in the diets.
In summary, increased dietary leucine reduced synthesis of serotonin in the brain due to restricted uptake of tryptophan in the brain. Because of reduced synthesis of serotonin, feed intake was reduced which then resulted in reduced average daily gain. The implication of these results is that the negative effects of excess dietary leucine can be partially overcome by adding more tryptophan to the diet.