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Applications of efficiency measurements in bioaccumulation studies: Definitions, clarifications, and a critique of methods

Measurements of digestion, absorption, and assimilation efficiencies play central roles in physiological and ecological studies of nutrition and in toxicological studies of bioaccumulation. There is, however, a pervasive problem with inconsistency in the definition and use of efficiency terms and in their measurement and interpretation. Digestion, absorption, and assimilation efficiencies are frequently confused, and the term “assimilation efficiency” is commonly and incorrectly used as a synonym for digestion and absorption efficiencies. To differentiate measurements that are truly assimilation efficiencies from measurements that are digestion efficiencies and absorption efficiencies, explicit definitions of terms and examination of methodologies are necessary. In general, comparisons of ingested food and feces are digestion efficiencies and comparisons of ingested food and tissues are assimilation efficiencies. Absorption efficiencies can be determined from comparisons of ingested food and feces when the food component of interest is directly absorbed without digestion (e.g., some particle‐associated toxicants) or is an absorbable tracer (e.g., 14C) that is not altered by digestion. Absorption efficiencies of toxicants are commonly measured using two tracer‐based methods, the dual tracer method and the selectivity index (SI) method. The assumptions of the dual tracer method can at least be tested and validated while the assumptions of the SI method are not valid. The SI method requires the assumption of a value for total organic carbon (TOC) absorption efficiency in order to calculate a toxicant absorption efficiency, and analysis of the equations associated with the SI method shows that calculating a toxicant absorption efficiency using an assumed TOC absorption efficiency is not any more justifiable than just assuming a toxicant absorption efficiency.