Hard to lose, easy to gain; Trends in obesity and weight change across the life course in four British Birth cohorts
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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.
Funding
BRC UCLH, ESRC
Research in context
Evidence before this study
A systematic review of evidence on generational differences in health from the British birth cohort studies showed that more recently born cohorts were more likely to be overweight or living with obesity than their predecessors at the same age, that they experienced an earlier onset of overweight or obesity, and that they were more likely to experience rapid increases in BMI across the life course (Seach terms were for six UK cohort studies from 1946 through to 2000-02, identifying pairwise combinations of the six cohort studies and for terms indicating the inclusion of two or more British birth cohorts. Included papers were those comparing health, including obesity, in two or more studies). While intervention studies and randomised controlled trials provide valuable evidence on short-term weight loss including its predictors, often within clinical populations, there is limited understanding of how healthy weight maintenance and sustained weight loss occur at a nationally representative, population level over longer time periods.
Added value of this study
This work highlights the increasing size of the population who could potentially benefit from weight loss across successive cohorts, with only 19% of people in their early 50s today having never experienced overweight or obesity. Comparatively, the same number continue to gain weight, either into or within, obesity in their early 50s. We also show that those individuals who would be most likely to benefit from weight loss tend to be the most socioeconomically disadvantaged, and that this socioeconomic gradient has remained deeply entrenched across cohorts.
Our findings highlight the rarity of sustained weight loss from obesity in midlife in the era before obesity management medications were widely available, when looking at the full cohort (2.6-3.8%), and it remains uncommon even when restricting to only those with obesity at their highest BMI before there early 40s (13.1-13.7%). Any increase in the proportion of people who lost weight from obesity across cohorts, appears to be driven by the greater proportion of cohort members living with obesity rather than an increase in the propensity of those with obesity to lose weight. The majority of those with obesity and overweight peak BMIs did not just maintain overweight or obesity but continued to gain significant amounts of weight (33-39% who experienced obesity by age 42 continued to gain >5% of their bodyweight). Few indicators we tested were associated with weight loss, once peak weight was accounted for. The reported intention to lose weight was associated with continued weight gain, rather than weight loss, in fully adjusted models. Whilst there are likely other factors or unobserved drivers of weight loss not included in our analysis, one of the strongest drivers we identified of losing weight, is gaining large amounts of weight in the first place. Comparatively, being in disadvantages socioeconomic circumstances, at multiple different points across life, was related to obesity and weight gain in later mid-life.
Implications of all the available evidence
Despite more public health policies to help manage obesity rates at the population level, there appears to be little change in the number of people who successfully lose weight once they have developed obesity, whilst large numbers – over a third – continue to gain weight. Importantly, the intention to lose weight had little bearing on successful weight loss, suggesting that for the majority of people, will-power alone (and by implication interventions and policy that are high-agency driven), is inadequate to achieve successful weight loss.
Looking forward, it appears that a large and growing proportion of the population will experience obesity at some point across adulthood - in keeping with a progressively more obesogenic environment - and therefore more may benefit from weight-loss treatment. While the emergence of GLP-1 medications is likely to disrupt historic trends, the scale of need suggests that potential demand (and costs) will be considerable, without continued efforts on preventing the development of obesity in the first place.
