Hard to lose, easy to gain; Trends in obesity and weight change across the life course in four British Birth cohorts

Background: As global obesity rates have increased, so too have efforts to manage obesity. This work estimates how many people maintain a lifelong healthy weight, have weight loss potential, and who successfully lose weight without medical support. Methods: Using the 1946 National Survey of Health and Development (1946NSHD; n=4,423), the 1958 National Child Development Study (1958NCDS, n=16,749) and the 1970 British Cohort Study (1970BCS; n=15,612), we quantify the prevalence of lifetime healthy weight, overweight and obesity by ages 51-55, and compare prevalence in early adulthood to the 2000-02 Millenium Cohort Study (2001MCS; n=9,675). We identified those who maintained, lost, gained, or cycled weight up to ages 50-55 (1958NCDS) and 46-54 (1970BCS), relative to their highest body mass index (BMI) before age 42, and examined predictors of group membership using multinomial regression models. Findings: In 1970BCS one-in-five people maintained a healthy BMI into their fifties, whilst 43% experienced obesity at least once, up from 25% in 1946NSHD. In 2001MCS 19% already had obesity by age 23, compared to 1-2% in the oldest cohorts. Across cohorts, those who maintained a healthy BMI were more socioeconomically advantaged, while those who experienced obesity were the most disadvantaged. Among those with obesity, a similar proportion lost weight in both cohorts (~13%), whilst 33-39% continued weight gain. Few potential drivers were associated with weight loss after adjusting for peak BMI, whilst socioeconomic disadvantage predicted further weight gain, as did the intention to lose weight. Interpretation: Weight loss from obesity is rare and the rate has remained consistent over time, whilst weight gain into obesity is common, and prevalence of lifetime obesity has increased.