Excess leucine in pig diets leads to multiple problems, Illinois study shows

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Recent changes in corn and sorghum processing for ethanol have led to changes in the composition of grain byproducts, including distiller’s dried grains with solubles (DDGS), a major component of swine feed. As a result, today’s DDGS is typically higher in the amino acid leucine than is necessary to meet nutritional requirements of pigs. According to research from the University of Illinois, excess leucine in pig diets can decrease protein synthesis and reduce feed intake.

“If you feed diets high in corn or sorghum protein in the form of DDGS, you could easily get to 200% of the leucine requirement. We are seeing more and more of these high-protein products in the market, so it’s going to be something we have to deal with in the future,” says Hans H. Stein, professor in the Department of Animal Sciences and the Division of Nutritional Sciences at the University of Illinois. Stein is co-author on a recent study published in the Journal of Animal Science.

Proteins are made up of amino acids, so why would a high amount of one amino acid lead to decreased protein synthesis and reduced feed intake? To find the answer, Stein, along with graduate student Woong B. Kwon and others, fed diets with varying levels of leucine to growing pigs over 15 days.

The experimental diets were formulated with identical quantities of corn, soybean meal, wheat, and barley, but varied in the amount of added leucine: 0, 150, 200, 250, and 300% of the daily requirement.

“We saw linear reductions in feed intake, gain-to-feed ratios, and other measurements as leucine increased. So clearly, excess leucine was detrimental to growth performance of the pigs, particularly when we got to 200% or more of the daily requirement,” Stein says.

Stein’s study shows pigs fed excess leucine eat less because their brains are not making enough serotonin, the hormone that controls appetite. Before serotonin can be made, tryptophan, an amino acid precursor, needs to get into the brain through membrane transporters in the blood-brain barrier. Leucine also fits into those transporters. With excess leucine swamping the transporters, tryptophan can’t get into the brain.

“We saw a reduction in serotonin in the brain as we increased leucine in the diet,” Stein says.

The research team also determined that leucine decreased protein synthesis by breaking down related amino acids isoleucine and valine. The key is that the three amino acids are all in the same molecular family – the branched-chain amino acids – which means the body treats them similarly.

“If you have excess leucine in the diet, the body makes more of the enzyme needed to break down leucine. But that enzyme also breaks down valine and isoleucine. Even if you had enough valine and isoleucine in the diet, you basically create a deficiency of those amino acids; it’s not enough for protein synthesis,” Stein explains. “Our data clearly support that hypothesis.”

The reality is that many pig diets currently deliver 200% of the leucine requirement, so producers may already be seeing these consequences. Stein is currently conducting follow-up studies to test amino acid supplements that could counteract the problems related to high-leucine diets.

“We have demonstrated the negative effects of excess leucine. Now we have to focus on finding out how we can prevent this from reducing pig performance. We may have to add more valine, isoleucine, and tryptophan in the diet,” Stein says.

The article, “Excess dietary leucine in diets for growing pigs reduces growth performance, biological value of protein, protein retention, and serotonin synthesis,” is published in the Journal of Animal Science [DOI: 10.1093/jas/skz259]. Authors include Woong Kwon, Kevin Touchette, Aude Simongiovanni, Kostas Syriopoulos, Anna Wessels, and Hans Stein. The research was funded by Ajinomoto Animal Nutrition North America.

The Department of Animal Sciences and the Division of Nutritional Sciences are in the College of Agricultural, Consumer and Environmental Sciences at the University of Illinois.

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