Digestibility

Digestibility is the proportion of ingested nutrients that is absorbed and available for use by the body. In nutrition science it is typically expressed as a digestibility coefficient, derived from comparisons of intake and excretion.

Apparent digestibility measures intake minus fecal output divided by intake, but it does not account for endogenous losses such as digestive secretions and sloughed cells. True or standardized digestibility attempts to correct for these losses, providing a more accurate estimate of what the diet contributes to the body's stores and metabolism.

Measurement methods vary by species and purpose. In humans and many animals, digestibility is estimated from controlled feeding trials with fecal collection or indirect markers. In monogastric animals (such as pigs and poultry), ileal digestibility is sometimes reported to reflect nutrient absorption before microbial fermentation. In ruminants, total-tract digestibility is common due to extensive fermentation in the digestive system.

Digestibility is influenced by many factors, including species, age, health, and gut microbiota; the chemical form of the nutrient; particle size and processing (grinding, heating, extrusion); fiber content and cell-wall structure; anti-nutrients that reduce mineral or protein availability; and interactions among diet components.

In human nutrition, digestibility affects energy yield and nutrient availability. Certain foods are less digestible due to lactose intolerance, enzyme deficiencies, or complex carbohydrates; processing can improve or reduce digestibility depending on the context.

Understanding digestibility supports diet formulation for animals and humans, guides labeling and dietary recommendations, and underpins nutritional research on energy balance and nutrient provisioning.