Physical activity energy expenditure (PAEE), the most variable component of total energy expenditure, is influenced by both volume and intensity. While previous research often examined these factors separately, their combined effect on health outcomes remains less understood. This study aimed to investigate the integrated joint associations between objectively measured PAEE volume, intensity, and body fatness in a large sample of free-living adults. The research questions focused on determining the individual and combined effects of PAEE volume and intensity on body fat percentage, considering different intensity profiles (sedentary/sleep, light, moderate, vigorous physical activity). This integrated approach was inspired by similar analyses in nutritional epidemiology, where total energy intake and macronutrient composition are studied simultaneously. The study's importance lies in advancing our understanding of the relationship between physical activity and obesity, informing public health recommendations and interventions. Using objective measures of PAEE and body fatness in a large population would allow for a more robust analysis of this complex relationship than has previously been possible.

Existing literature demonstrates a strong link between physical activity (PA) and various metabolic conditions, including obesity and mortality. Studies typically examined overall PA volume and intensity in parallel, providing limited insight into the role of intensity beyond the volume effect. Isotemporal substitution studies, which examine the associations between reallocating time from one activity to another, have been conducted; however, these studies don't account for the increase in PAEE that occurs when time is reallocated to a more intense activity. To address these gaps, this study employed a novel methodological approach to integrate the analysis of PAEE volume and intensity simultaneously. This integrated approach resembles methodologies used in nutritional epidemiology where total energy intake and macronutrient composition are examined jointly.

The Fenland study, an ongoing population-based observational study of 12,435 young and middle-aged adults (29–64 years old), provided the data. After exclusions (pregnancy, diabetes, inability to walk unaided, psychosis, terminal illness, missing data), 11,468 participants (6148 women, 5320 men) were included. Anthropometric measurements (height, weight, body fat percentage using DXA) were collected. Physical activity was objectively measured using a combined heart rate and movement sensor (Actiheart) worn for six days. Data were pre-processed, individually calibrated, and combined to estimate instantaneous PAEE. PAEE was categorized into sedentary/sleep (<1.5 METs), light (1.5–3 METs), moderate (3–6 METs), and vigorous (>6 METs) activity. Covariates included age, sex, socioeconomic status, dietary factors (energy intake, macronutrients, vitamin C), and smoking status. Analyses were sex-stratified. Linear regression was used to examine associations between PAEE (total and intensity fractions) and body fatness (percentage and fat mass index). Isocaloric substitution analysis was employed to simulate the effect of reallocating energy expenditure between intensity categories on body adiposity. Compositional data analysis was used to further investigate the volume-intensity interaction.

Higher PAEE levels were significantly associated with lower body fat percentage (BF%) and fat mass index (FMI) in both sexes. In women, each additional 1 kJ day⁻¹ kg⁻¹ of PAEE was associated with a 0.16% decrease in BF%, and in men, a 0.09% decrease. Intensity composition played a significant role. Reallocation of energy to vigorous physical activity (>6 METs) from other intensities was associated with lower body fatness, while reallocation to light activity was associated with higher body fatness. However, the overall effect of PAEE volume was far greater than the effect of intensity composition. Stratifying by PAEE tertile, the difference in BF% between the first and second tertiles in women was associated with a 3 percentage-point difference, which is larger than the 1 percentage-point difference observed when reallocating all energy to the maximum observed VPA for this group. Isocaloric substitution analyses consistently showed that higher PAEE was associated with lower BF%, regardless of intensity distribution. This suggests that overall energy expenditure is a critical factor in body fat regulation. The benefits of higher intensity PA were secondary to volume.

This study confirms the strong inverse association between overall PAEE volume and body fatness, highlighting its paramount importance in maintaining healthy body weight. The marginal contribution of intensity composition, when considered alongside overall PAEE, suggests that public health messaging should emphasize the benefits of any physical activity, rather than prescribing specific intensity or duration targets. The findings underscore the importance of increasing overall PAEE, which seems to be more impactful than optimizing intensity alone. The paradoxical findings regarding reallocation of energy to sedentary behavior in highly active men warrant further research but are unlikely to be practically relevant for most people. The observed sex differences in body fatness and PAEE may reflect biological factors or social influences impacting activity and adiposity in women vs. men. Further research is needed to clarify these potential mechanistic pathways.

This large, population-based study provides robust evidence supporting the strong inverse association between total physical activity energy expenditure and body fatness. While increased proportions of vigorous activity are associated with lower fatness, this effect is comparatively less pronounced than the primary impact of overall activity volume. These findings reinforce the current guidelines advocating for increased physical activity of any kind for better health outcomes. Future research should focus on longitudinal studies and randomized controlled trials to establish causality and explore the underlying biological mechanisms contributing to these associations.

The cross-sectional design of this study precludes causal inferences. While objective measures of PAEE and body fatness were employed, reliance on self-reported dietary information might introduce some measurement error. Although the large sample size strengthens the findings, generalizability may be limited to similar populations. Future longitudinal studies and randomized controlled trials are needed to further elucidate the causal relationships and fully understand the dynamic interplay between PAEE volume, intensity, and body composition.