Organic Acids as Gut Environment Modifiers (GEM)

Bradley Lawrence, Ph.D.

Organic Acids Defined
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Organic acids (OA) are molecules containing a carbon chain [R-] and a terminal carboxyl group [COOH]. By contrast, inorganic acids such as hydrochloric, sulfuric and phosphoric, do not contain carbon in their chemical structure. The most common OAs used world-wide in livestock diets are supplied either in a liquid (formic, acetic, propionic, butyric and lactic acids) or a dry (fumaric, citric, benzoic, and sorbic acids) form, or in the form of their Ca, Na, K, or ammonium salts. Additionally, the liquid acids may be adsorbed onto a carrier resulting in a maximum of 60 - 70% of the liquid acid in the final product. The OA differ in the following ways: 1) their acid strength (ability to lower pH), 2) solubility in water, 3) molecular weight, 4) energy content, 5) pKa or pH at which 50% of the acid molecules are in the dissociated form, and 6) antimicrobial activity.

Functional Role of Organic Acids
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Lactic, formic, propionic, and butyric are among the OA naturally produced in the gastrointestinal tract (GIT) as a result of microbial fermentation. Their relative levels vary depending on location (stomach, small intestine, large intestine) and diet composition (dietary lactose level, fiber level, etc). Lactic acid is in the greatest concentration in the stomach and small intestine while another OA, acetic, along with propionic and butyric are predominant in the large intestine. The main roles of OA in the GIT are to: 1) increase effectiveness of the barrier function of the stomach against pathogens by helping to reduce stomach pH, particularly in young animals, 2) regulate intestinal cell proliferation through direct provision of energy, altering blood flow and therefore nutrient delivery to intestinal tissue, and mediation of various gastrointestinal hormones and growth factors which affect intestinal cell proliferation, 3) increase pancreatic secretions, 4) diffuse across cell membranes of pathogens in the intestine and either destroy the pathogens cytoplasm or interfere with cellular metabolism of the pathogen, and 5) serve as a precursor for synthesis of non-essential amino acids, DNA, and some lipids required for intestinal growth. With these functional roles, OA are clearly gut environment modifiers (GEMs). OA are also widely used to improve food, feed, and water hygiene as well as preserve silages.

Factors Affecting OA Response
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The response to OA can be affected by: 1) type and level of OA fed, 2) type of diet and its buffering capacity (BC), 3) pig age, 4) presence/absence of receptors for bacterial colonization on the intestinal epithelium, 5) influence of maternal antibody protection, 6) hygiene and environmental factors, and 7) endogenous production of OA as a result of microbial fermentation of feedstuffs. The performance response to OA is anticipated to be lower when good hygiene practices are in place and when the pigs environment such as temperature, humidity and ventilation are nearer to optimum. Pig genetic lines lacking intestinal receptors for pathogens such as those exhibiting the Type I fimbria (E. coli for instance) are likely to see less of a response to OA supplementation. Pigs from farms implementing sow vaccination for enteric pathogens resulting in elevated levels of maternal antibody protection at weaning are also likely to see a lower response to OA. The response to OA is typically best in newly weaned pigs due to immature acid secreting capacity, and therefore a compromised stomach barrier function. In the U. S., pigs are typically weaned to high protein diets (20 - 24%) sometimes containing milk protein from whey, animal protein including but not limited to plasma, fishmeal, and pet-food grade poultry by-product meal resulting in high dietary Ca levels (0.9 - 1.0%). Pharmacological levels of Zn (2,500 - 3,000 ppm) and copper (175 - 250 ppm) are also fed. These factors all contribute to a high BC of the diet. The high BC of the weaner diet is compounded by the normally low acid secretory capacity of the weaned pig. Therefore, adequate levels of OA are needed to decrease the stomach pH sufficiently to provide a barrier function against pathogens finding their way to the small intestine. Weaner diets are also composed of more complex protein sources than the dam's milk. Therefore, OA induced stimulation of pancreatic enzyme secretion should increase piglet growth and feed utilization efficiency.

Alimet as an OA
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ALIMET^® feed supplement (DL - 2 Hydroxy-4-(MethylThio)-Butanoic acid, aka HMTBa) is generally accepted as a methionine source but has the dual functionality of being structurally similar to lactic acid. In culture broth studies, ALIMET has been found to have antibacterial properties against E. coli, Salmonella spp and Campylobacter spp. In in vitro studies, ALIMET alone at 0.5% has been shown to lead to a reduction in levels of Salmonella enteritidis similar to that observed with 0.5% formic acid. ALIMET at 0.25% or 0.375% in combination with 0.25% or 0.125% formic respectively appeared to act synergistically with respect to Salmonella kill. This apparent synergy on Salmonella kill between ALIMET and other OA has also been demonstrated. The mode of action of this synergy is as yet undefined.

Historical Response to OA
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Partanen (2001) summarized numerous (36) trials where various OA (formic, fumaric, citric and potassium diformate) at varying levels (0.3 - 2.5%) were fed to weaned pigs. Overall, OA addition improved gain 7.6% and feed efficiency 3.2%. The response was greatest for formic acid and its salt potassium diformate (11.2% improvement in ADG - Average Daily Gain - and 2.8% improvement in feed conversion). Jongbloed and Jongbloed (1996) evaluated levels of lactic acid ranging from 0.8% to 3.2% and observed and improvement in growth rate of 8.3 - 10.2% (avg 9.1%) and feed efficiency of 3.6 - 8.4% (avg 5.6%). Maribo (1999) saw somewhat lower response to 2% lactic acid for two weeks followed by 1% for two additional weeks observing an improvement of 5.5% in ADG and 2.8% in feed efficiency. Average improvements in ADG of 16.7% and 18.2% have been observed by Maribo and Callesen (1998) and Van der Ploeg (1998) respectively. From these historical data, depending on the factors affecting OA response, an improvement in gain of 5 - 10% and an improvement in feed conversion of 2 - 5% may be anticipated.

Activate Starter DA and Associated Performance
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ACTIVATE STARTER DA^® is a blend of ALIMET, benzoic, and fumaric acids formulated to enhance stomach barrier function against pathogenic bacteria such as E. coli and Salmonella. By reducing the intestinal damage associated with these pathogens, and by the other functional properties outlined above, a performance enhancement when included at 0.5% in weaning pig diets is anticipated. Results of ten trials confirm a 5.1% improvement in ADG and a 3.2% improvement in feed conversion when ACTIVATE STARTER DA is added to non-medicated diets for approximately 40 days. Furthermore, an ACTIVATE STARTER DA program through 40-d post-weaning achieved 87% of the ADG response and 107% of the feed efficiency response of a Carbadox medication program. These results confirm the effectiveness of ACTIVATE STARTER DA as a means of weaning pig performance.

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