Concurrent Changes in Diet Quality and Physical Activity and Association With Adiposity in Adults.

Importance: Diet and physical activity (PA) are both associated with body weight, but less is understood about how long-term changes in these behaviors may be associated with changes in adiposity in the general population. Objective: To evaluate associations of concurrent changes in diet quality and PA with different body composition indices. Design, Setting, and Participants: This cohort study was conducted among participants in the UK population-based Fenland study, with repeated measurements of health behaviors and adiposity. Data collection occurred in 2005 to 2015 and 2014 to 2020, with a mean (SD) follow-up of 7.2 (2.0) years. Data were analyzed from January 2024 through April 2025. Exposures: Mediterranean diet score (MDS), a measure of adherence to the Mediterranean diet (range, 0-15 points), was assessed with a food frequency questionnaire. PA energy expenditure (PAEE), measured in kilojoules per kilogram per day, was assessed and calibrated with heart rate and movement sensing. Change variables were derived. Main Outcomes and Measures: Indices of total adiposity (weight, body mass index and body fat) and regional adiposity (waist circumference, visceral adipose tissue [VAT], and subcutaneous adipose tissue) were assessed with anthropometry and dual-energy x-ray absorptiometry. Hepatic steatosis was ascertained by abdominal ultrasonography. Results: The analysis included 7256 participants (mean [SD; range] age at recruitment, 48.8 [7.4; 29-65] years; 3748 female [51.7%]). In multivariable linear regression models after adjustment for potential confounders and baseline values, changes in both exposures were negatively associated with changes in all adiposity indices. For example, each 1-SD (1.27 points) increase in the change in MDS was negatively associated with changes in adiposity measures (change in body fat: β = -0.47 kg; 95% CI, -0.58 to -0.36 kg; change in VAT: β = -45 g; 95% CI, -55 to -35 g), and each 1-SD (19.0 kJ/kg/d) in the change in PAEE was similarly negatively associated with changes in adiposity measures (change in body fat: β = -1.40 kg; 95% CI, -1.51 to -1.26 kg; change in VAT: β = -108 g; 95% CI, -118 to -98 g) . Despite no evidence for a diet-PA interaction, simultaneous increases in MDS and PAEE were associated with a greater magnitude of decrease in adiposity, with a β of -149 g (95% CI, -187 to -111 g) for change in VAT among participants in higher joint change in MDS and change in PAEE tertiles. Associations had higher β values among individuals with overweight or obesity or who were physically inactive at baseline; for example, for each 1-SD increase in the change in PAEE, the β for change in body fat was -0.96 kg (95% CI, -1.10 to -0.81 kg) when baseline BMI was less than 25 and -1.74 kg (95% CI, -1.91 to -1.57 kg) when baseline BMI was 25 or greater (P for interaction < .001). Conclusions and Relevance: In this study, improved diet quality and increased PA were independently associated with weight loss or less weight gain and a healthier adiposity profile. Combining these health behaviors was associated with the greatest adiposity-related benefits.