Becoming a parent and leaving school linked to weight gain

"Going to university and becoming a parent are two key life events that 'cause weight gain'" reports the Metro.

The headlines are prompted by 2 related reviews looking at the impact of 'significant life events' on people's body mass index (BMI).

One review focused on the effects of parenthood. Researchers gathered the results from 11 studies that looked at weight changes in women before pregnancy and after having a baby. They found that the body mass index (BMI) for women without children increased by an average of 2.8 over the course of 5 years. But those who were mothers added an extra 0.47 to their BMI on top of that. There was little change in physical activity, and minimal evidence to say if parenthood affects men's BMI.

The second review examined studies that looked at changes in diet or exercise following life transitions, such as leaving high school to start higher education or employment. 7 studies found that young people spent 7 fewer minutes in moderate-to-vigorous activity after leaving high school, while another 3 studies suggested they gained weight.

These results are all based on observational studies, which cannot prove that these single changes – becoming a mother or leaving school – are the direct cause of the weight gain or reduced activity. Many health and lifestyle factors could be having an influence. If they are having a direct effect, the studies cannot tell us why this is, or how much of a difference these life changes might make.

But in the search to stem rising overweight and obesity rates, these studies highlight key life moments where there may be a need to emphasise diet and activity advice and support people in living healthier lifestyles.

So, if you are starting a new chapter in your life in terms of education, work or parenting, maybe you could complement these life changes with positive changes to your lifestyle.

Read more about eating a balanced diet and the benefits of exercise.

Where did the story come from?

The reviews were carried out by researchers from the University of Cambridge. Both reviews received funding from the Centre for Diet and Activity Research (CEDAR), a UKCRC Public Health Centre of Excellence, the UK Clinical Research Collaboration, the Medical Research Council, a Gates Cambridge Scholarship, and NIHR School for Public Health.

Both were published in the peer-reviewed Obesity Reviews journal and are freely available to access online. Read the study into parenthood and the study into school and work.

The UK media's reporting of both reviews was largely accurate.

What kind of research was this?

These were 2 systematic reviews. They aimed to gather published studies that had looked at whether becoming a parent or changes in education and employment had an impact on BMI, diet or physical activity.

A systematic review is the best way of identifying the evidence available to date on a topic. However, the quality and strength of findings from a review are only as good as the underlying studies. As these were all observational studies, they can identify links but are not going to be able to prove direct cause and effect.

What did the research involve?

Review into the impact of becoming a parent

This review identified follow-up studies that had collected data from people between the ages of 15 and 35 before and after having their first child, with the same comparison period for those who did not have children. Their BMI or weight could be self-reported or directly measured, while any measures could be made of diet or activity levels.

11 studies were included and the results from 6 studies in women could be pooled in meta-analysis. These 6 studies were mostly large (over 1,000 people), came predominantly from North America, had 55% white ethnicity, included women with a baseline age of 23 years and followed them for on average 5.6 years. Most studies were assessed to be of weak quality.

Review into the impact of leaving school or work

The second review identified studies that reported change in weight, diet or activity associated with entering or leaving school or work. Studies similarly had to follow people and take 2 assessments between the ages of 15 and 35 years.

19 studies were included, with most coming from the US or Australia. Only 6 studies took objective measurements, with others based on self-reporting. 7 studies were small, including fewer than 250 people. 10 of the 19 papers were assessed to be of high quality.

What were the basic results?

Review into the impact of becoming a parent

The pooled results showed that young women who did not have children gained an average 2.8kg/m2 (+/-1.3kg) in BMI over 5.6 years. Those who had become mothers gained an extra 0.47kg/m2 on top of this (+/-0.26). The BMI increase was calculated to be 17% greater in women who were parents compared with those who were not parents.

Results on diet or exercise could not be combined because studies had assessed this in different ways. Results were inconclusive. Two studies found no difference in diet between parents and non-parents, while 1 study found differences in fat intake. One study suggested mothers were more likely to become inactive than those who were not mothers, another found that both groups had a similar decline in activity levels over the years, while another 2 had mixed results.

Review into the impact of a change to schooling or work

12 studies reported change in activity after leaving high school (8) or upon entering university (4). 9 studies had results that could be pooled in meta-analysis and showed that leaving high school is associated with a decrease in moderate-to-vigorous activity of 7.04 minutes/day (95% confidence interval [CI] -2.82 to -11.26).

3 studies looked at change in body weight after leaving high school, which generally showed a gain in body weight, regardless of whether the person went to university or not. Results for dietary intake were mixed: 2 studies showed decreases in fruit and dairy consumption (1 of the studies also with vegetables), 1 study showed decreased overall energy intake after going to university, and 1 found decreased snacking.

3 studies found no clear evidence that starting a new job was associated with any change.

How did the researchers interpret the results?

The review into the impact of becoming a parent concludes a 'need for obesity prevention among all young women, including mothers'. The second review also highlights leaving high school as another 'important time to support individuals to prevent decreases in physical activity and gains in body weight'.

Conclusion

The increasing number of people becoming overweight is a global health problem. Extensive research has looked at ways to tackle this, such as finding lifestyle changes that may be linked, like in this case.

These reviews find some evidence that becoming a parent (or a mother specifically in this case) or leaving school may be associated with slight increases in BMI or decreased physical activity. For some, the findings may be more or less as expected. For example, it could be expected that some new parents may find less opportunity for exercise or find less time to prepare healthy meals. Similarly, it may be the possibly clichéd idea that young people leaving home for the first time and having to cook for themselves could make poorer dietary choices.

But these reviews cannot prove that these lifestyle changes were directly responsible for the changes in weight or activity. The studies were observational and of variable quality. Many unmeasured health and lifestyle factors could be having an influence on the outcomes.

If there is a direct link, the studies have not carried out detailed enough assessment of the individuals (or the review has not focused on this) to tell us why this may be.

The studies were also highly variable in terms of their design, recruited populations and methods of assessment and follow-up. For example, the review into the impact of becoming a parent included a mix of ethnic groups across North America, Australia and Europe and focused mostly on women who had become mothers at a relatively young age in their early to mid-20s. So, we do not know if the findings apply to everyone or to some groups more than others. Also, although most studies in the reviews were published within the past 10 years, some were older and carried out in the early 2000s or 1990s. These studies may be even less representative of people today when we have become more aware of the importance of healthy diet and daily physical activity.

The estimated changes in BMI and physical activity are also fairly small. We do not know if they could have an impact on health. Also, as the studies have only looked at short-term effects, we do not know if they could be reversed with time (for example, as children get older).

Nevertheless, the reviews reinforce the need to promote healthy weight, diet and activity advice to all age groups and highlights particular times when more support and better training may be needed, such as during changes in education status or during the post-natal stage.

Read more about healthy living.

Page last reviewed: Tue Feb 2020 Next review due: Tue Feb 2020