Weekly, Seasonal, and Festive Period Weight Gain Among Australian Adults

IMPORTANCE: Obesity is a major global health concern. A better understanding of temporal patterns of weight gain will enable the design and implementation of interventions with potential to alter obesity trajectories. OBJECTIVE: To describe changes in daily weight across 12 months among Australian adults. DESIGN, SETTING, AND PARTICIPANTS: This cohort study conducted between December 1, 2019, and December 31, 2021 in Adelaide, South Australia, involved 375 community-dwelling adults aged 18 to 65 years. Participants wore a fitness tracker and were encouraged to weigh themselves, preferably daily but at least weekly, using a body weight scale. Data were remotely gathered using custom-developed software. EXPOSURE: Time assessed weekly, seasonally, and at Christmas/New Year and Easter. MAIN OUTCOMES AND MEASURES: Data were visually inspected to assess the overall yearly pattern in weight change. Data were detrended (to remove systematic bias from intraindividual gradual increases or decreases in weight) by calculating a line of best fit for each individual’s annual weight change relative to baseline and subtracting this from each participant’s weight data. Multilevel mixed-effects linear regression analysis was used to compare weight across days of the week and seasons and at Christmas/New Year and Easter. RESULTS: Of 375 participants recruited, 368 (mean [SD] age, 40.2 [5.9] years; 209 [56.8%] female; mean [SD] baseline weight, 84.0 [20.5] kg) provided at least 7 days of weight data for inclusion in analyses. Across the 12-month period, participants gained a median of 0.26% body weight (218 g) (range, −29.4% to 24.0%). Weight fluctuated by approximately 0.3% (252 g) each week, with Mondays and Tuesdays being the heaviest days of the week. Relative to Monday, participants’ weight gradually decreased from Tuesday, although not significantly so (mean [SE] weight change, 0.01% [0.03%]; P = .83), to Friday (mean [SE] weight change, −0.18% [0.03%]; P < .001) and increased across the weekend to Monday (mean [SE] weight change for Saturday, −0.16% [0.03%]; P < .001; mean [SE] weight change for Sunday, −0.10% [0.03%]; P < .001). Participants’ weight increased sharply at Christmas/New Year (mean [SE] increase, 0.65% [0.03%]; z score, 25.30; P < .001) and Easter (mean [SE] weight change, 0.29% [0.02%], z score, 11.51; P < .001). Overall, participants were heaviest in summer (significantly heavier than in all other seasons), were lightest in autumn (mean [SE] weight change relative to summer, −0.47% [0.07%]; P < .001), regained some weight in winter (mean [SE] weight change relative to summer, −0.23% [0.07%]; P = .001), and became lighter in spring (mean [SE] weight change relative to summer, −0.27% [0.07%]; P < .001). CONCLUSIONS AND RELEVANCE: In this cohort study of Australian adults with weekly and yearly patterns in weight gain observed across 12 months, high-risk times for weight gain were Christmas/New Year, weekends, and winter, suggesting that temporally targeted weight gain prevention interventions may be warranted.