Chemical oxygen demand (COD) measures the mass of oxygen required to completely oxidize organic carbon by microbiota, a term used in the field of environmental biotechnology.  COD is a direct measure of the number of electron equivalents in a substrate. 

In contrast, the field of human metabolism reports the energy content of foods as measured by the complete combustion (enthalpy) of a food and is reported in calories.  Because of the different units of measure it is not currently possible to fully understand the role of gut microbiota in energy availability to human hosts.  A study by Davis and colleagues reported in the February 2021 issue of The Journal of Nutrition explores the relationship between COD and gross energy of foods.

The theoretical gross energy and COD of over 100 foods were calculated.  A subset of foods was used to measure gross energy and COD, which allowed the authors to validate models between the two measures for the whole food and for the carbohydrate, protein and fat components of the foods.

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The regression models developed for carbohydrate, protein and fat, as well as the overall food accurately converted between COD and gross energy.  The smallest model errors were obtained for the food components.  The results led the authors to conclude that it is possible to analyze both human and microbial energy metabolism using a single unit of measure.

In a commentary on this article, Edwards describes the different methods used to measure energy metabolism in wastewater treatment plants and that used to measure energy metabolism in humans.  The difference in approach and units of measure are noted as limiting our ability to fully understand the impact of diet on microbiota and health.  Edwards concludes that the work of Davis and colleagues will benefit both human metabolism research and the field of wastewater treatment.


Davis TL, Dirks B, Carnero EA, Corbin KD, Krakoff J, Parrington S, et al.  Chemical oxygen demand can be converted to gross energy for food items using a linear regression mode. The Journal of Nutrition, Volume 151, Issue 2, February 2021, Pages 445–453,

Edwards EA. Electron balances for nutrients and health. The Journal of Nutrition, Volume 151, Issue 2, February 2021, Page 277,

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