Short chain fatty acids (SCFAs) have been successfully used by the animal feed industry for decades but it’s clear that their full potential is still being unravelled.
For nutritionists, SCFAs are proving to be one of the most cost-effective ways to help farmers gain the competitive advantage – something which is becoming ever more important as profit margins tighten and emphasis on reducing antimicrobial usage increases1. From improving nutrient digestibility to enhancing feed preservation, SCFAs have an increasingly important role to play in optimising feed quality and maximising livestock health and production2.
SCFAs are organic acids that consist of one to six carbon atoms, of which formic (C1), acetic (C2), propionic (C3) and butyric (C4) acid stand out in terms of biological function. Natural by-products of anaerobic microbial fermentation of fibre, SCFAs have numerous functions within the gut as well as a number of other organs. In addition to the varying properties of different acids, there are also multiple forms of SCFAs on the market. Overall, their convenience and versatility makes them ideal for a multitude of purposes.
In today’s market, SCFAs are commonly applied directly to feedstuffs and compound feed as well as silages and drinking water either as a liquid formulation spray or as solid acids or their salts. With such a wide remit of functions, it may be surprising that a relatively simple mechanism of action underpins many beneficial effects of SCFAs. SCFAs are classified as ‘weak acids’, which means that a certain proportion of their molecules remain ‘undissociated’ at particular pH values. These uncharged molecules pass more easily across the lipid cell membrane of microorganisms. Once internalised into the neutral pH of the cell cytoplasm, the acid dissociates, releasing its protons (H+) in the less acidic environment. This decreases the intracellular pH which can disrupt cell metabolism, preventing growth and multiplication of harmful microorganisms.3 Their simple structure also means that SCFAs are easily accessible by many organs of the body, facilitating involvement in a wide range of metabolic processes.2,3
An acidic environment in the stomach promotes the action of pepsin, hence protein digestion. This is especially the case in young animals.
One of the earliest uses of organic acids was for the preservation of feed quality; something which is still key in the animal feed industry today. In fact, the European feed acidifiers market is thought to be the fastest growing feed additives market in the world.4,5 As microorganisms function at an optimum pH range, altering this can disturb their growth. SCFAs make good dietary acidifiers which have been shown to reduce harmful microorganisms in both the feed and subsequently in the gut of the animal.6 This has been particularly important since the ban on antibiotic growth promoters in Europe, with SCFAs tackling a wide range of pathogens that are known to have detrimental effects on animal health and welfare, such as Salmonella and E. coli.5,6,7
When it comes to efficient acidification, SCFAs offer a practically ideal solution. Acids with low molecular weight have the highest number of molecules per weight, resulting in the highest number of available ions to release. This is why even at low doses SCFAs such as formic acid are particularly effective at acidification contributing to inhibition of bacteria and prevention of nutrient losses that occur through processes such as silage making and grain drying. In addition to altering the feed environment, SCFAs are able to cross the bacterial cell wall and disrupt the metabolism of the microorganism itself, resulting in a two-pronged attack on microorganisms. An acidic environment in the stomach also promotes the action of pepsin, the enzyme which breaks down protein.7 Therefore, protein digestion is enhanced, especially in young animals. Lowering stomach pH also stimulates more pancreatic juices to be produced.8 These contain a variety of digestive enzymes so digestion of carbohydrates and fats is also likely to be improved by adding SCFAs to feed.
Another important aspect of feed preservation is mould prevention. Not only do moulds cause deterioration of feed ingredient quality and decrease palatability, they can also produce mycotoxins which are potentially extremely harmful to livestock and human health.9 Mycotoxins can even cause sub-clinical diseases, resulting in decreased performance. Once moulds start growing, they increase the moisture and temperature of feed allowing other harmful pathogens to grow. Spores from which moulds grow are very difficult to kill so clearly prevention is very important.10 SCFAs are strong mould inhibitors, of which propionic acid is considered to be one of the most effective due to its lipophilic nature (ability to combine with fats).10 This property enables it to cross the complex wall structure of moulds, which is why propionic acid tends to form the main component of the majority of commercially available feed preservatives. By suppressing the growth of bacterial pathogens and mould, SCFAs allow better use of feed resources, promote livestock productivity as well as safeguard animal and human health.
Another important aspect of feed preservation is mould prevention, such as in the feed silo. Not only do moulds cause deterioration of feed ingredient quality and decrease palatability, they can also produce mycotoxins which are potentially extremely harmful to livestock and human health.
As mentioned, SCFAs are produced naturally by the body as a result of the fermentation of fibre in the gut by microogansims. A primary role of these SCFAs is the provision of energy, with butyrate being the preferred metabolite for colonocytes.2 In ruminants, SCFAs contribute almost 80% of total energy as rumen bacteria utilise most of the glucose ingested.2 SCFAs that escape the colonocytes join the hepatic portal blood going to the liver. In the liver, acetic acid is used for gluconeogenesis as well as lipolysis while propionic acid may work to lower plasma cholesterol levels as well as being involved in energy metabolism.2
As well as simply providing energy, SCFAs have also been shown to affect how the energy is used. This means that not only is more energy available to the animal overall, it’s also used more efficiently for growth and production. What’s more, SCFAs regulate proliferation and differentiation of cells and increase cell volume and promote healthy gut development.2, 12, 13 The result is a stronger barrier to protect against invading pathogens and intestinal diseases as well as an increased absorptive surface area for improved nutrient uptake.
In most production systems, feed is by far the greatest cost so it makes sense to maximise conversion of nutrients into growth and production. Gut health optimisation, largely through regulation of intestinal microbiota is key to achieving this. Butyrate is of primary importance when it comes to SCFAs with gut health benefits. Supplementation with butyrate in a variety of species including poultry, pigs and ruminants has been shown to have many functions over and above providing an additional source of energy.2,14,15 This includes improving nutrient digestibility, increasing mineral absorption, optimising the profile of intestinal microbiota, down regulating bacterial virulence and supporting tissue development and repair.2
SCFAs have also been shown to influence expression of genes involved in the maintenance of tight junctions between gut cells and to modulate inflammation12,17 as well as positively influencing the treatment of gut conditions such as antibiotic induced diarrhoea and ulcerative colitis.2 Its anti-carcinogenic action within the colon may be less important in production species but it perhaps highlights the extent to which SCFAs can positively influence gut health and function.2 It’s likely that all of these factors combine to give SCFAs like butyric acid their beneficial effects and when considered in conjunction with their function as potent fuel providers, perhaps it’s not surprising that animals supplemented with SCFAs perform better in a production setting.15-18 This is demonstrated by improvements in feed conversion rates and weight gain, resulting in reduced times to market weight and better economic returns.13-15
Despite the vast advantages of supplementation with SCFAs dietary manipulation is not a completely straightforward solution to increasing delivery of SCFAs as cost, sourcing, bioavailability and palatability can all provide potential challenges. For example, ensuring that fatty acids occur at the correct point in the gut to be beneficial has proved tricky for the feed industry. For some time, coated preparations seemed to be the answer. However, the coating often forms up to 70% of the product, reducing the concentration of SCFA it can contain for its weight.
Newer esterified formulations do not need such coatings and therefore release more SCFA per kilogram. Tributyrins have become the most popular form of esterified SCFA product in the marketplace today, offering a concentrated form of butyric acid which has significant benefits for gut health and production performance. Esterification involves combining the short chain acid with a glycerol molecule; the powerful bonds between them only being broken down by specific enzymes.
This means that in the stomach the tributyrin stays intact, but in the intestine pancreatic lipase is present to break the bond and the butyric acid is released where it is needed. For the animal that means a slower rate of release in line with natural digestive processes; for the farmer, a more cost-effective and efficient solution; and for the nutritionist, more opportunities to get the most from SCFAs. The esterified form is more stable, has no odour and is easier to handle. The end result is positive, reliable and predictable at every stage of the production process.
As understanding of short chain fatty acid mechanisms progress and technical hurdles relating to their practical application are overcome, nutritionists can begin to look even more closely at SCFAs as an extremely promising sustainable option for improving animal health and performance.
References are available on request.