In vitro digestibility - reducing animal use whilst meeting the demand to evaluate alternative proteins

There is increasing awareness of the need for foods to be both healthy and sustainable, with a shift toward more plant-based foods, with reductions in red meat (e.g. beef and lamb) but continued consumption of white meat (e.g. chicken), eggs and dairy products. Hence poultry meat is and will remain an important part of the diet in many parts of the world, but its production is currently very dependent upon feeding chickens significant volumes of wheat, maize and soya, with the latter providing the main protein source. Clearly each of these crops are human edible, so there is a real push to try and identify so-called alternative sources, particularly protein sources.

There is wide interest in a range of alternative (under-utilised) crops, such as bambara groundnut and winged bean, as well as novel protein sources such as insects, bacteria and fungi. Although these are being investigated for human consumption, it is more likely that they will be more readily accepted (particularly in western countries) as alternatives to wheat, maize and soya in animal feeds. Before any of these protein sources can be used to feed animals (or humans for that matter), it is important to evaluate both their nutritional value (i.e. does it contain the nutrients required in the right amounts) and any possible negative impacts. In terms of dietary protein, the quality of a protein source is determined by its amino acid composition, particularly the essential or indispensible amino acids (those that must be provided in the diet), as well as their digestibility.

Digestibility relates to the ability of an animal to digest its food and extract all the nutrients it needs. Soya is a good example because it contains enzyme inhibitors (called anti-nutritional factors) that reduce its digestibility when it is not processed (e.g. heating), meaning that animals are unable to extract a large proportion of the nutrients present and therefore produce more waste, in the form of faeces and greenhouse gases. Hence, poor digestibility can have environmental impacts, but it also has commercial implications, making food production inefficient. It can, in some cases, also lead to negative impacts on health, due to failure to grow adequately or damage to the digestive system. Historically the determination of digestibility of foods has been done using animals, such as poultry, cattle, pigs or rodents (as models for humans). To evaluate novel foods therefore requires large numbers of animals, with the possibility that some foods might lead to reduced digestibility (and therefore poor growth) or even result in health problems. This project will compare two in vitro (i.e. test-tube) models of digestion, to evaluate their ability to determine digestibility and sensitivity to the presence of anti-nutritional factors, before comparing to in vivo digestibility determined in chickens. The overall aim is to develop an accurate, sensitive and robust in vitro protein digestion method with which to screen novel protein sources prior to any evaluation in animals. This should then partially replace and reduce the numbers of animals being used in the future, as well as reducing the incidence of negative impacts on those animals.

Protein quality of any feedstuff is determined by its essential amino acid (AA) content and its digestibility. The latter is normally determined in vivo by assessing ileal digesta amino acid content, which is invasive and expensive if done in live animals, so is often done after death. The impact of reduced AA digestibility is particularly detrimental in animals with high AA requirements, such as rapidly growing broiler chickens. Due to the projected global increase in human population and associated demand for animal products, particularly chicken meat, alternative economically and environmentally sustainable protein sources need to be identified for animal use to replace human edible feed ingredients such as soya. Their protein quality will need evaluating, which is thus set to increase animal use.
In vitro digestibility methods could reduce the number of animals used for in vivo assessment, but there is no validated in vitro digestion model for any species. Ideally an in vitro protein digestion model should be rapid (compared to in vivo), reproducible, can discriminate between samples and is predictive of in vivo protein digestibility. Hence, our aim is to identify an appropriate in vitro digestion model, establish its sensitivity/ limitations and how it compares to measurements of apparent ileal protein/AA digestibility in broiler chickens.
This will be via 3 objectives:
1. Comparison of the INFOGEST in vitro digestion and chicken specific in vitro digestion models using a range of protein sources.
2. Determining the sensitivity of the selected in vitro digestion model to determine protein quality via incremental increases in anti-nutritional factor contents.
3. Comparison of in vitro protein/AA digestibility and in vivo apparent ileal protein/AA digestibility in broiler chickens.

This will provide a screening tool for future analyses of protein quality and thereby reduce and eventually avoid the need for in vivo digestibility studies.