Leaving school and getting a job both lead to a drop in the amount of physical activity, while becoming a mother is linked to increased weight gain, researchers have found.
Many people tend to put on weight as they leave adolescence and move into adulthood and this is the age when the levels of obesity increase the fastest, the study said.
This weight gain is related to changes in diet and physical activity behaviour across the life events of early adulthood, including the move from school to further education and employment, starting new relationships and having children.
"This evidence suggests that the pressures of university, employment and childcare drive changes in behaviour which are likely to be bad for long-term health," said study researcher Eleanor Winpenny from University of Cambridge in the US.
For the study, published in the journal Obesity Reviews, researchers looked at changes in physical activity, diet and body weight as young adults move from education into employment and to becoming a parent.
To do this, they carried out systematic reviews and meta-analyses of existing scientific literature.
In the first of the two studies, the research team looked at the evidence relating to the transition from high school into higher education or employment and how this affects body weight, diet and physical activity.
In total, they found 19 studies covering ages 15-35 years, of which 17 assessed changes in physical activity, three body weight, and five diet or eating behaviours.
The team found that leaving high school was associated with a decrease of seven minutes per day of moderate-to-vigorous physical activity.
The decrease was larger for males than it was for females (a decrease of 16.4 minutes per day for men compared to 6.7 minutes per day for women).
More detailed analysis revealed that the change is largest when people go to university, with overall levels of moderate-to-vigorous physical activity falling by 11.4 minutes per day.
In the second study, the team looked at the impact of becoming a parent on weight, diet and physical activity.
A meta-analysis of six studies found the difference in change in body mass index (BMI) between remaining without children and becoming a parent was 17 per cent: a woman of average height (164 cm) who had no children gained around 7.5 kg over five to six years, while a mother of the same height would gain an additional 1.3 kg.
These equate to increases in BMI of 2.8 versus 3.3.
According to the researchers, most studies including physical activity showed a greater decline in parents versus non-parents.
The research team found limited evidence for diet, which did not seem to differ between parents and non-parents.