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Characterization of Complex Systems Using the Design of Experiments Approach: Transient Protein Expression in Tobacco as a Case Study
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Energy requirements: the case for the factorial model.

D Joe Millward1, James A Betts2, James Lj Bilzon2

  • 1Nutrition, Exercise, Chronobiology & Sleep Discipline, School of Biosciences, Faculty of Health and Medical Sciences, University of Surrey, Guildford, United Kingdom.

The American Journal of Clinical Nutrition
|July 1, 2026
PubMed
Summary

The factorial model for human energy requirements is re-examined. New analysis shows high energy expenditure in traditional populations, challenging the idea that total energy expenditure is constrained and highlighting the need to consider physical activity energy expenditure variation.

Keywords:
PALdoubly labeled waterenergy metabolism and expenditurephysical activity energy expenditureregression equations for energy expenditure

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Published on: November 11, 2021

Area of Science:

  • Human physiology
  • Nutritional science
  • Metabolic research

Background:

  • Previous methodologies for human energy requirements (ERs) are questioned by the Food and Agriculture Organization and International Atomic Energy Agency (IAEA).
  • IAEA proposes predictive equations using doubly labeled water (DLW) data to estimate population ERs, deeming the factorial approach unnecessary.
  • Concerns exist that predictive equations may not adequately account for physical activity energy expenditure (PAEE) variations.

Purpose of the Study:

  • To re-examine the factorial model for human energy expenditure and requirements.
  • To investigate whether total energy expenditure (TEE) is constrained within a narrow physiological range, irrespective of physical activity.
  • To analyze interindividual variation in PAEE within different population groups.

Main Methods:

  • Re-examination of the factorial model for energy expenditure.
  • Derivation of weight-adjusted metrics for physical activity energy expenditure (PAEE).
  • Analysis of published energy expenditure data from traditional populations and comparison with a DLW dataset for UK dietary reference values.

Main Results:

  • Analysis revealed very high rates of energy expenditure in traditional populations.
  • These findings suggest lifestyles with no obvious evidence of energy constraint.
  • The results challenge the notion that TEE is a constrained product of physiology and does not increase with physical activity.

Conclusions:

  • Any update to human energy requirements must account for interindividual variation in PAEE.
  • The factorial model, when properly accounting for PAEE, provides valuable insights into energy expenditure.
  • The assumption of a constrained TEE may be inaccurate, particularly for diverse populations with varying activity levels.