Negative Digestibility: A Theoretical Perspective

Digestibility is a fundamental concept in animal nutrition and is widely used to evaluate feed ingredients. Although digestibility values are generally expected to be positive, negative values are occasionally reported. Negative values are not analytical or calculation errors; rather, they often convey critical information about feed ingredients. Negative digestibility is most commonly observed for minerals, which readily interact with other components in compound feeds and within the gut lumen. A proper understanding of negative digestibility is therefore essential for formulating nutritionally complete diets. This paper introduces Mellanby’s toxamin theory to illustrate the mechanisms underlying negative digestibility. It also explains the rationale behind digestibility calculations, which sometimes yield mathematically extreme and biologically unrealistic negative values. Finally, the paper proposes in vivo mineral-binding capacity as an alternative metric for expressing ingredient interactions, as negative digestibility values themselves can lead to confusion and misinterpretation. By reinterpreting the biological significance of negative digestibility, this paper seeks to translate an often-confusing observation into a more practical and informative nutritional index.