https://www.pnas.org/doi/full/10.1073/pnas.2420902122
Significance
Economic development is associated with increased prevalence of obesity and related health problems, but the relative importance of increased caloric intake and reduced energy expenditure remains unresolved. We show that daily energy expenditures are greater in developed populations, and activity energy expenditures are not reduced in more industrialized populations, challenging the hypothesis that decreased physical activity contributes to rises in obesity with economic development. Instead, our results suggest that dietary intake plays a far greater role than reduced expenditure in the elevated prevalence of obesity associated with economic development.
Abstract
Global economic development has been associated with an increased prevalence of obesity and related health problems. Increased caloric intake and reduced energy expenditure are both cited as development-related contributors to the obesity crisis, but their relative importance remains unresolved. Here, we examine energy expenditure and two measures of obesity (body fat percentage and body mass index, BMI) for 4,213 adults from 34 populations across six continents and a wide range of lifestyles and economies, including hunter-gatherer, pastoralist, farming, and industrialized populations. Economic development was positively associated with greater body mass, BMI, and body fat, but also with greater total, basal, and activity energy expenditure. Body size–adjusted total and basal energy expenditures both decreased approximately 6 to 11% with increasing economic development, but were highly variable among populations and did not correspond closely with lifestyle. Body size–adjusted total energy expenditure was negatively, but weakly, associated with measures of obesity, accounting for roughly one-tenth of the elevated body fat percentage and BMI associated with economic development. In contrast, estimated energy intake was greater in economically developed populations, and in populations with available data (n = 25), the percentage of ultraprocessed food in the diet was associated with body fat percentage, suggesting that dietary intake plays a far greater role than reduced energy expenditure in obesity related to economic development.
Media report:
https://www.science.org/content/article/new-study-blames-diet-not-physical-inactivity-obesity-crisis
O3 summary critique:
Quick take
A new PNAS paper led by evolutionary anthropologist Herman Pontzer and first‑author Amanda McGrosky pulls together the largest global dataset of human energy‑expenditure ever assembled—4,213 adults (18‑60 y) in 34 populations from hunter‑gatherers to office workers. Using gold‑standard doubly‑labelled‑water measurements plus basal metabolic data, the authors asked a deceptively simple question: Is the worldwide surge in obesity driven more by falling energy expenditure or by rising calorie intake? Their answer is blunt: total energy expenditure (TEE) explains only about 10 % of the excess obesity seen in wealthy nations; the rest is almost certainly dietary—especially the proliferation of ultra‑processed foods (UPFs) (Medical Xpress, The Washington Post).
What the study did
| Aspect | Details |
|---|---|
| Design | Meta‑analysis of existing doubly‑labelled‑water studies plus indirect calorimetry (n = 4,213; 34 populations; 6 continents). Populations were binned by UN Human Development Index (HDI) to capture economic gradient. |
| Key Measures | • TEE (all calories burned per day) |
| • Basal energy expenditure (BEE) | |
| • Activity energy expenditure (AEE = TEE – BEE) | |
| • Obesity: body‑fat % & BMI. | |
| Subset analyses included the share of calories from UPFs. | |
| Major findings | 1. Crude TEE, BEE and AEE are higher in industrialised cohorts—mainly because bodies are larger. |
- After adjusting for age, sex and body size, TEE and BEE are 6‑11 % lower in richer nations, while AEE is actually slightly higher.
- Variation in TEE explains only ≈10 % of the cross‑economic difference in body‑fat.
-
UPF share tracks body‑fat closely, implicating diet rather than inactivity (Medical Xpress, The Washington Post). |
| Interpretation | Supports Pontzer’s “constrained energy‑expenditure” model—our metabolisms re‑allocate rather than endlessly add calories burned (The Washington Post). |
Why it matters
Public‑health messaging still leans heavily on “move more”.
These data suggest that while exercise is indispensable for cardiometabolic health, curbing calorie intake—particularly from UPFs—must sit centre‑stage in obesity policy (Medical Xpress, The Washington Post).
Critique
| Strengths | Caveats & Limitations |
|---|---|
| • Unprecedented sample breadth—34 culturally distinct cohorts gives real global reach. | Cross‑sectional design: cannot prove that increasing UPF intake causes weight gain; it only correlates. |
| • Objective physiology—doubly‑labelled‑water is the metabolic gold standard, avoiding recall bias that plagues diet/exercise questionnaires. | Dietary data were sparse. UPF share was available only for a subset and came from country‑level food‑supply databases, not individual intake logs. |
| • Adjusted analyses—age, sex and body‑size covariates handled with mixed models, reducing simple confounding. | Temporal mismatch: Energy‑expenditure snapshots (≤14 days) may not reflect long‑term patterns, while obesity and diet are chronic phenomena. |
| • Consistent with prior constrained‑TEE work, lending theoretical coherence. | HDI as a proxy for “development” is blunt: it bundles healthcare, education and income with diet; residual confounding is inevitable. |
| No children <18 or adults >60—yet both groups are pivotal in the obesity pandemic. | |
| Population heterogeneity inside bins (e.g., US office workers vs. US agricultural labourers) could dilute signals. |
Methodological nit‑picks
- Energy‑intake was inferred, not measured. Stating that intake is “10× more important” relies on modelling assumptions; a longitudinal cohort with weighed‑food records would be stronger.
- Single‑time‑point metabolism may miss adaptive down‑regulation that happens during sustained exercise or dieting.
- BMI and body‑fat% were treated interchangeably in some regressions; BMI can misclassify muscular individuals.
Bottom line
The paper convincingly shows that most humans—whether Hadza hunter‑gatherers or London desk jockeys—burn roughly the same number of calories once body size is accounted for. The metabolic playing field is surprisingly level; what tilts the scales is what and how much we eat. Future work should pair longitudinal dietary tracking with repeat DLW measurements and include children and the elderly to nail down causality.
For clinicians and policy‑makers the practical takeaway is clear: exercise for health, but fix the food environment to fix obesity.