Congenital liver shunts are quite common in some breeds of dog - in the U.S., this is notably the yorkie. The proper terminology is "Portosystemic shunt" (PSS), and it denotes a situation where blood flow from the GI tract bypasses processing by the liver. This critical mistake means that protein particles absorbed from the GI tract are left untended by the gentle ministrations of the liver. The liver specializes in converting ammonia, a by-product of protein breakdown, into urea, which is less toxic and easily eliminated by the kidneys.
When ammonia accumulates in the blood, changes in neurotransmitters lead to brain dysfunction and the clinical manifestation ranges from simple lethargy to outright seizures. This condition is called hepatic encephalopathy or HE. For dogs with PSS, this is the limiting factor on quality of life and life expectancy, although the disease can be managed more or less with surgery or diet change (and often both).
It helps to restrict the protein in the diet, because that will lower overall ammonia levels. Some evidence also suggests that the *type* of protein influences the types of neurotransmitters orchestrating brain function. If aromatic amino acids predominate (more common in meat protein), the neurotransmitter balance will be tipped towards brain dysfunction.
This study compared the clinical results of feeding a low protein diet composed primarily of soy protein vs a low protein diet containing primarily poultry protein.
Dogs with PSS were fed first one of these diets, then the other, in blinded fashion so that neither the investigators nor the owners were aware of the diet being fed.
The study showed that both diets reduced signs of encephalopathy, which might be expected when reducing overall protein content. Interestingly, when dogs were eating the soy-based diet, ammonia levels were significantly reduced. In addition, these dogs had better blood coagulation capacity. [When the liver fails, the clotting factors normally produced are decreased, leading to possible bleeding episodes). This improved clotting capacity, along with an increase in another protein made by the liver, suggests that the soy-based diet actually enhances the liver's capacity to function.
The results of this study may be particularly important to owners considering shunt surgery for their dogs. Elevated ammonia levels (with the associated changes in brain neurotransmitter levels) mean that anesthesia is a bigger risk in these dogs than in normal dogs of the same age. And if clotting is immpaired, surgery could lead to uncontrolled bleeding.
So the study is *really* interesting because it suggests that feeding a low protein soy based diet BEFORE surgery could reduce the risk surgery presents. Nice.
Proot S, Biourge V, Teske E, Rothuizen J. Soy protein isolate vs meat-based low protein diet for dogs congenital portosystemic shunts. J Vet Intern Med 2009;23:794