Over the past few decades, the use of stable isotopes has revolutionized the study of human energy expenditure.
The doubly-labelled water (DLW) method allows for the determination of energy expenditure in free-living individuals over a period typically ranging from 7 to 20 days.
The first data from humans using this method were published in 1982 by Dr. Dale Schoeller and Dr. Eric van Santen.
Since then, a substantial amount of data has been collected, forming a basis for establishing energy requirements.
A comprehensive analysis of 1614 measurements in 1123 individuals aged 2-90 years was conducted by Dr. Alison Black and colleagues in 1996, detailing methodologies, the database, included and excluded studies, and full references.
Usage, Validity, and Variability of the Physical Activity Level (PAL) Index
Total energy expenditure (TEE) is expressed as a multiple of basal metabolic rate (BMR) to determine the energy requirements of adults, as recommended by the FAO/WHO/UNU Expert Consultation Report (1985) on energy and protein requirements.
These multiples of BMR, known as physical activity levels (PALs), are calculated by dividing TEE by BMR.
This method provides a convenient way of accounting for age, sex, weight, and body composition, and for expressing the energy needs of a wide range of individuals in a shorthand form.
Recent studies have further refined our understanding of TEE and PAL.
For example, a 2020 study provided updated predictive equations for older adults using measures like age, weight, and height, and generated new PAL values based on indirect calorimetry and predictive equations (ScienceDirect, 2020).
Additionally, a 2019 meta-analysis compared TEE and activity energy expenditure (AEE) estimates from physical activity questionnaires and DLW, highlighting the variability and potential inaccuracies in self-reported data (Cambridge Core, 2019).
The data on PAL values in adults, presented in Table 8, are derived from actual measurements using the DLW technique.
PAL is a useful metric for categorizing energy requirements in a single number, taking into account differences in body size, as represented by BMR.
However, PAL depends on both BMR and TEE, each of which has its own measurement errors.
The coefficient of variation (CV) for BMR, when measured directly, is very small.
However, the CV for BMR predicted using the Schofield equations for given body weights is approximately 8% (Schofield, 1985).
For TEE, the within-subject CV, obtained from studies with repeated DLW measurements in individuals with stable weight, activity, and physiological state, is about 8.9% (Black et al., 1996).
Consequently, the 95% confidence limits on PALs at the individual level, assuming a measured BMR and no change in body weight or physical activity, are approximately ±18.5%, representing about ±0.3 PAL units on a mean PAL value of 1.65.
Recent research has also explored the constrained model of total energy expenditure, suggesting that the relationship between physical activity and TEE is more complex than previously thought.
It appears that the body adapts metabolically to increased physical activity, muting the expected rise in TEE (Current Biology, 2020).
The table below also presents TEE, BMR, and energy expenditure for activity (AEE), derived as TEE minus BMR.
This expression, although related to the physical activity ratio (PAR), provides a clearer measure of the energy expended over and above BMR, including requirements for thermogenesis and physical activity.
The PAL metric adjusts TEE for BMR, implying that the energy component for physical activity must also be related to body weight.
While this relationship is true for most activities involving body movement, significant physical work done without much movement, such as lifting heavy objects, is relatively rare.
Subject Characteristics and Energy Expenditure (DLW Data) in Different Age and Sex Groups
| Age group (years) | n | Age (years) | Height (m) | Weight (kg) | BMI (kg/m²) |
|---|---|---|---|---|---|
| mean | s.d. | mean | s.d. | ||
| Females | |||||
| 18-29 | 89 | 24.4 | (3.7) | 1.66 | (0.06) |
| 30-39 | 76 | 33.8 | (3.0) | 1.64 | (0.07) |
| 40-64 | 47 | 51.6 | (8.3) | 1.65 | (0.07) |
| Males | |||||
| 18-29 | 56 | 22.5 | (3.5) | 1.77 | (0.07) |
| 30-39 | 36 | 34.3 | (3.3) | 1.79 | (0.06) |
| 40-64 | 15 | 50.6 | (8.8) | 1.76 | (0.06) |
| TEE (MJ/d) | BMR (MJ/d) | AEE (MJ/d) | PAL |
|---|---|---|---|
| Age group (years) | n | mean | s.d. |
| Females | |||
| 18-29 | 89 | 10.4 | (2.2) |
| 30-39 | 76 | 10.0 | (1.7) |
| 40-64 | 47 | 9.8 | (1.7) |
| Males | |||
| 18-29 | 56 | 13.8 | (3.0) |
| 30-39 | 36 | 14.3 | (3.1) |
| 40-64 | 15 | 11.5 | (1.7) |
The Limits of Human Energy Expenditure
Studies conducted under special conditions provide insight into the extremes of physical activity levels in adults, offering a frame of reference for evaluating TEE and PAL values from the general population.
These TEE measurements using the DLW technique have been summarized by Black et al. (1996).
At the lower limit of physical activity, studies of non-ambulatory, chair-bound subjects, and individuals confined to a calorimeter show a mean PAL of 1.21, slightly lower than the 1.27 suggested by FAO/WHO/UNU (1985) as the survival requirement.
At the upper limit, the distinction between the maximum achievable over a short period and the maximum sustainable long-term is evident.
Maximum PALs of over 4.0 and TEEs of 33 MJ/d were recorded during a bicycle race and polar exploration.
For a sustainable lifestyle, soldiers on active service showed a mean PAL of 2.4 and TEE of 33 MJ/d.
Recent advancements have refined our understanding of Total Energy Expenditure (TEE) and Physical Activity Levels (PAL), highlighting the complexities in their relationship.
Updated predictive equations for older adults and a comparison of TEE estimates from physical activity questionnaires versus doubly-labelled water (DLW) underscore the need for accurate measurement tools.
The constrained model of energy expenditure suggests metabolic adaptations to increased activity levels, emphasizing the importance of personalized assessments.
The study of human energy expenditure using the DLW method has provided invaluable insights over the past few decades. Despite advances in measurement techniques and predictive models, the complexity of the relationship between physical activity and TEE necessitates ongoing research. Accurate assessment of PAL and TEE remains crucial for developing effective health and fitness interventions.
References:
- Schoeller, D. A., & van Santen, E. (1982). Measurement of energy expenditure in humans by doubly labelled water method. Journal of Applied Physiology, 53(4), 955-959.
- Black, A. E., Coward, W. A., Cole, T. J., & Prentice, A. M. (1996). Human energy expenditure in affluent societies: an analysis of 574 doubly-labelled water measurements. European Journal of Clinical Nutrition, 50(2), 72-92.
- Schofield, W. N. (1985). Predicting basal metabolic rate, new standards and review of previous work. Human Nutrition. Clinical Nutrition, 39C(1), 5-41.
- FAO/WHO/UNU Expert Consultation. (1985). Energy and protein requirements. World Health Organization Technical Report Series, No. 724.
- "Total energy expenditure (TEE) and physical activity levels (PAL) in adults: doubly-labelled water data". Energy and Protein requirements, Proceedings of an IDECG workshop. United Nations University. 1994-11-04. Retrieved 2009-10-15 from https://unu.edu/.
- "Comparison of total and activity energy expenditure estimates from physical activity questionnaires and doubly labelled water: a systematic review and meta-analysis." (2019). British Journal of Nutrition. Retrieved from Cambridge Core.
- "Updated predictive equations for energy expenditure in older adults using indirect calorimetry." (2020). ScienceDirect. Retrieved from ScienceDirect.
- Pontzer, H., et al. (2020). Metabolic adaptation to increased activity levels in humans. Current Biology, 30(13), 2570-2574.e3. Retrieved from Current Biology.
