V SC 497A Pathology of Nutritional and Metabolic Diseases
Dr. Robert J. Van Saun
Lecture 15: Protein and Amino Acid Deficiency and Toxicity Diseases
There is no storage of protein consumed in excess of requirements, therefore, excess amino acids must be catabolized for energy or converted to fat. In catabolizing amino acids, the ammonia group must be removed and detoxified by the liver, at an energy cost, then excreted via the kidneys or recycled through saliva to the rumen. Besides the excessive costs associated with feeding excess protein, additional metabolic burden of ammonia have been associated with two possible disease concerns.
Renal disease - it has been proposed that excess protein feeding and resultant urea production would have detrimental effects on renal function. This was a great concern in the pet food industry and one particular company really promoted this issue. More recent research data has shown no detrimental effects of excess protein on renal function, if the kidneys have no underlying disease process. If there is some compromise of renal function, then feeding excess protein may not be warranted, given the increased metabolic and water demands on renal function.
Metabolic bone disease - a group of syndromes associated with abnormal bone and joint development. These problems are of greatest concern in growing foals and large breed puppies. Metabolic bone disease is a multifactorial disease problem, but in the past various nutrients have been implicated in its etiology. Previously, excess protein intake has been suggested as promoting various syndromes of metabolic bone disease. Again, recent research data in both puppies and foals has suggested no role of excess protein intake on the development of metabolic bone disease. In fact, it seems that protein deficiency may be more of a problem.
In feeding ruminant animals, we must consider two components of feeding protein; meeting the nitrogen needs of the rumen and the amino acid needs of the animal. Utilization of ruminal nitrogen is a complex, dynamic process related to available fermentable energy. When rumen available nitrogen sources are fed in excess of the microbial's ability to trap nitrogen, then the excess ammonia will diffuse across the rumen wall and enter the portal vein. Given the cellular toxicity potential of ammonia, the liver converts the ammonia to urea. Urea then can move throughout the body and be excreted in urine or recycled in the digestive tract through saliva.
Research over the past 20 years has tried to assess the role of protein nutrition on reproductive performance. A number of studies have suggested excess protein has a negative effect on conception rate and/or early embryo survival. Not all studies have shown a negative effect of excess protein on reproductive function. Some of the differences in results may be related to dietary protein fractions and available energy. It seems that excess soluble or total rumen degradable protein have the greatest negative effects on reproductive performance.
Work at Cornell University has shown a possible mechanism explaining the negative association observed between dietary protein and reproductive performance. Blood urea generated from excess dietary degradable protein can diffuse into all body water. Urea can also diffuse across the uterine mucosa into the uterine lumen. It was originally thought that high urea concentrations in the uterine lumen may be responsible for the negative effects. However, in vitro data does not support a role for a toxic effect of urea on sperm, fertility or the embryo. Recent research suggests that urea is cleaved in the uterine lumen producing ammonia and secondarily, ion exchange mechanisms to alter the pH of the uterine lumen. These Cornell data showed that the pH of uterine fluids remained low when animals were fed a diet with excess degradable protein. These researchers suggest that the lower pH in the week following ovulation has negative effects on fertilization and embryo survival. More recent work has shown negative effects of excess degradable protein diets on embryos viability when collected and cultured in vitro. It seems that embryos exposed to an uterine environment associated with excess protein feeding has a prolonged effect on future metabolic activity of the embryo.
Nonprotein nitrogen (NPN) toxicosis is primarily a disease of ruminant animals as a direct result of rumen microbial production of toxic compounds from excessive amounts of NPN consumption. Major NPN compounds of concern are urea, nitrates and nitrites. Urea is a common fertilizer and ruminant feed supplement. Nitrates and nitrites are NPN compounds found in plants and water. Certain plants, under the right environmental conditions like drought, can accumulate nitrates or nitrites and potentially pose a serious problem. Potential accumulating plants should be evaluated for nitrate or nitrite concentrations. All NPN sources are eventually reduced to ammonia, which can be used by rumen microbes for protein synthesis. This is a unique feature of the ruminant animal and is a primary reason for their ability to utilize poor quality feeds. This is another scenario where if a little is good, a lot is not better!
For rumen microbes to assimilate ammonia into microbial protein, a readily available energy source is required. As with all anabolic reactions, ATP-energy is necessary for protein synthesis. This energy is derived from microbial fermentation of dietary carbohydrate sources. Poor quality forages do not provide sufficient amounts of energy in a timely fashion to support NPN utilization by rumen microbes. Urea is rapidly cleaved into CO2 and two ammonia molecules. If ammonia is not utilized by the rumen microbes, it will diffuse across the rumen wall into portal blood circulation. Ammonia is a potent cellular toxin disrupting energy metabolism and potassium (K) homeostasis. The liver normally converts excess ammonia back into urea for recycling or excretion. However if the liver is overwhelmed with ammonia, blood concentrations will increase dramatically resulting in subsequent clinical signs of toxicity. Urea toxicosis occurs rapidly within 30-60 minutes of excess consumption. Clinical signs include frothy salivation, bruxism, colic, muscle tremors, incoordination and recumbency followed by death; similar to signs associated with hypomagnesemia and polioencephalomalacia. Treatment is unrewarding for the most part unless identified very early in toxicity. Diagnosis is based on history, smell of ammonia to the animal and quantification of ammonia in either blood or rumen contents.
Nitrates and nitrites induce their toxic effects through a different mechanism. Nitrate is reduced to nitrite, which is a very rapid reaction in the rumen. Nitrite is reduced to ammonia for microbial utilization, although presence of ammonia can inhibit this reaction. If nitrite accumulates as a result of either excess ammonia or nitrate presence, it is absorbed across the rumen wall into portal circulation. In the blood nitrite reacts with hemoglobin to produce methemaglobin and thus reducing oxygen carrying capacity. Clinical signs of nitrate or nitrite intoxication are similar to those seen with urea poisoning and include dyspnea, cyanosis and death depending upon the amount of methemoglobin formation. Abortions may also occur following exposure to nitrate intoxication. Diagnosis is based primarily on history of exposure and quantification of nitrate in vitreous humor. Classically nitrate intoxication cases are associated with "chocolate-colored" blood. Methylene blue is a specific antidote that can be used for treating early cases .
Prevention of inappropriate exposure to NPN sources is the best method of control. Animals must be adapted to NPN sources and have adequate amounts of readily fermentable carbohydrate sources in their diets. High forage, and subsequently high pH rumen fluids, and low concentrate diets predispose animals to NPN toxicosis. Unadapted or starved animals are the most susceptible to NPN toxicosis. Rumen microbes can be adapted to NPN sources by slowly increasing their incorporation rate in the diet and ensuring adequate fermentable carbohydrate sources. Forages that contain high concentrations of NPN sources should be diluted out with other feeds not containing NPN sources.