As science starts to unravel the whole story behind short chain fatty acids, it’s becoming clear that fatty acid metabolism is complex and the effects can be far reaching. But what are SCFAs, and what are the potentials they have in animal diets?
Short chain fatty acids (SCFAs) are defined as aliphatic monocarboxylic acids with a chain length of less than C6. The group includes formic, acetic, propionic, lactic, isobutyric, butyric, isovaleric and valeric acid. It’s acetic (C2), propionic (C3) and butyric (C4) that are the stand out stars in biological terms. End products of the microbial fermentation of fibre, SCFAs are a source of energy, contributing around 10% in humans and other monogastrics but nearer 80% in ruminants where rumen bacteria utilise most of the glucose ingested1. In poultry, due to limitations in the ability to ferment fibre, it has been suggested that the products of fermentation may account for 3 to 4% of metabolisable energy intake2.
It is well established that butyrate is the preferred source of energy for colonocytes and the vast majority is used by these cells, with the remainder oxidised in the liver. Although the effects in the gut have been the focus for much research, a proportion of SCFAs also enter the bloodstream and act as signal molecules or substrates. In the liver, acetate can be used for energy production and acts as a substrate in the synthesis of glutamate, glutamine, long chain fatty acids and cholesterol.
Acetate is also used by a number of other organs including the heart, kidneys, muscle and adipose tissue. Glutamine – one of the products of acetate metabolism – is utilised by many actively proliferating tissues, such as the intestinal mucosa.
Propionate is mainly utilised in gluconeogenesis in the liver. The amount and proportions of SCFA produced depends largely on fibre type and amount in the diet and the composition of the microbiota in the gut, which means that dietary manipulation or supplementation can have very profound effects on animal health.
For nutritionists and producers the emphasis tends to be on SCFAs in biological systems but feed quality is a perennial issue – both in terms of the ingredients selected and in ensuring long term nutrient stability. In purely feed additive terms, formic acid (C1) and propionic (C3) are the SCFAs that are most relevant in today’s marketplace. Most of us will have experienced an inflammatory response as a result of contact with formic acid in the form of a nettle sting. However, formic acid is also an antibacterial, inhibiting Salmonella, E. coli and Staphylococcus. Propionic acid is especially effective in the inhibition of yeasts and moulds. Both act as acid preservatives, reducing decay of nutrients. Both formic and propionic are sources of energy and are completely metabolised by the animal – resulting in improvements in growth and production. Handling difficulties and compatibility with equipment with corrosive organic acids has limited their practical use in the past but the advent of new buffered formulations is set to change that picture significantly over the next few years.
Butyrate is of primary importance when it comes to gut health and not only because it provides a nutrient fuel for colonocytes. In fact, the effects within the colonocyte are much more extensive and butyrate has been shown to influence intracellular pH and gene expression, regulate proliferation and differentiation, and increase cell volume1. In other words: bigger, plumper, more robust cells, producing a stronger, more stable barrier against pathogens and insults. The gut has a very high rate of metabolic activity, accounting for up to 35% of energy expenditure at rest in sheep and pigs3. As well as providing a source of energy, SCFAs are reported to result in increased mucosal surface area and mucosal hypertrophy, indicating increased absorptive capacity4. Cell turnover is also increased. The composition and/or enzyme activity of intestinal bacteria is affected by SCFAs, which shift the microbiota balance in favour of beneficial bacteria. Beneficial bacteria synthesise vitamins, help in the absorption of food and inhibit pathogenic bacteria. There are also positive effects on gut motility4. While acetate is less readily absorbed in the colon, it has also been shown to increase colonic blood flow and enhance illeal motility1, suggesting some impact on digestive processes. Bacteria in the gut may also be able to use acetate to further enhance butyrate production1.
In ruminants, an increase in the absorptive surface area also occurs in the presence of SCFAs5. Gene expression with regard to fatty acid metabolism, ion transport, and intracellular homeostasis is known to alter during the period of adaptation to a diet with higher SCFA production. Genes involved in the maintenance of cellular tight junction integrity and induction of inflammation are also differentially expressed5, suggesting that SCFA supply or metabolism may play a role in diseases associated with compromised rumen function, such as subacute ruminal acidosis. Supplementation with butyrate across a variety of species including ruminants, pigs and poultry has been shown to improve nutrient digestibility, increase mineral absorption, optimise the profile of intestinal microbiota, down regulate bacterial virulence and support tissue development and repair1,6. SCFAs have also been demonstrated to positively influence the treatment of gut conditions such as antibiotic induced diarrhoea and ulcerative colitis1. Effects on the immunogenicity of cancer cells, control of apoptosis and antiproliferative effects have been suggested as mechanisms by which butyrate may have an anti-carcinogenic action within the colon7; while this is less important in production species, it perhaps underlines the extent to which butyrate is the ‘master controller’ within the colonocyte.
At its simplest, adding SCFAs or increasing SCFA production through diet, provides animals with an additional source of energy. But SCFAs also regulate the balance between fatty acid synthesis, fatty-acid oxidation and lipolysis in the body – essentially how energy is used in the body. Taken alongside the positive effects on the gut, it seems logical that animals might perform better in a production setting as a result of an increase in SCFA. This is confirmed through recorded improvements in feed conversion rates and weight gain8. A recent interesting development is in the use of SCFAs to prevent and treat metabolic syndrome which is associated with high energy diets and low levels of activity and characterised by obesity and loss of glycemic control.
Mice given SCFAs showed a reduction in diet induced obesity associated with normalised plasma glucose levels, reduced fat storage and increased fat oxidation1. While definitive proof may be some way off, there is a suggestion here that SCFAs might not only improve delivery of energy but also optimise lean tissue production, reflecting consumer demand for healthier low fat meat.
Dietary manipulation is not an easy solution to increasing delivery of SCFAs, due to concerns over cost, sourcing, bioavailability and palatability resulting from even the smallest changes in ingredients. There are also limitations in a species’ ability to ferment fibre to generate sufficient SCFAs for optimal effects. Supplementation has been held back by the need to release fatty acids at the point in the gut where they could exert a beneficial effect. For some time, coated preparations seemed to be the answer. Now newer esterified formulations can be digested by lipases in the small intestine, releasing butyrate. For the animal that means a slower rate of release in line with natural digestive processes. In use this 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. With this major technological hurdle overcome, the way lies open for feed companies and producers to look very closely at SCFAs as a very promising option to help them take the next big step forwards in improving animal health and creating sustainable competitive advantage.