"Mothers today need around 200 fewer calories a day than those of previous generations because they spend more time watching TV," the Daily Mail has reported. Its story is based on research looking at the physical activity levels of American mothers over the last 45 years.
Among a host of results, the study found that in 2010 mothers with younger children spent nearly 14 less hours a week on physical activity than in 1965, and expended an average of nearly 1,600 fewer calories a week. Instead, mothers spent more time on sedentary activities such as "screen-based media use", which includes time spent watching TV and using smartphones.
The findings correlate with a wider trend among all population groups towards less exercise and more sedentary behaviour, which is thought to be driving up rates of chronic non-infectious diseases such as type 2 diabetes and non-alcoholic fatty liver disease. The trend is associated with many factors, such as car and TV ownership, less manual employment and greater use of gadgets in the home.
But the Mail's claim that today's mothers should therefore eat fewer calories than their 1960s counterparts is unsupported as the study did not look at mothers' diets.
However, it's hard to fault the study's authors' overall conclusion. Greater physical activity is essential for health and needs to be encouraged, especially in those who are role models for children.
The study was carried out by researchers from the Arnold School of Public Health at the University of South Carolina, Montclair State University, the University of Texas at Austin and Tarleton State University, all in the US, and the University of Queensland School of Medicine, Australia. It was funded by the Coca-Cola Company. There do not appear to be any conflicts of interest in terms of funding.
The Daily Mail's reporting of the study is reasonably accurate, but its claim that mothers should be eating 200 fewer calories a day appears to be its own interpretation of the study. Many of us could do with eating a little less, but advising all mothers to eat less without considering their individual circumstances is irresponsible.
The study only made recommendations about how much exercise women should get. It did not discuss how many calories women should consume. As the study did not compare the women's energy intake with their energy expenditure, it is uncertain if the former was greater than the latter.
This study was based on data from the American Heritage Time Use Study, a nationally representative database on trends in time use, which consists of more than 50,000 diary days spanning from 1965 to 2010.
The authors say that in the last 50 years people's physical activity has fallen significantly, with obesity and many chronic diseases in women and children, such as type 2 diabetes, rising in step.
The evidence increasingly suggests that maternal behaviours may also play a role in determining how children develop, as well as their risk of obesity and chronic disease.
The researchers point out that although it is known that women are significantly less active and more sedentary than they were 50 years ago, these trends have not been addressed systematically.
The authors obtained data on the amount of time mothers spent doing physical activity and sedentary behaviour (spending time doing things sat down, such as office work or watching TV) from the American Heritage Time Use Study.
The number of weighted diaries from mothers with children up to the age of 18 available for analysis was:
Physically active behaviours included the total amount of time spent:
Sedentary behaviour was the total time spent:
The researchers measured the amount of time mothers spent on physical activity relative to the amount of time spent on sedentary behaviour. They calculated this as either a positive value, which meant the woman spent more time in physical activity than sedentary behaviour, or a negative value, which indicated the opposite.
Women were analysed in two groups, depending on whether they had any young children (aged five or under) or only had older children. Mothers were also categorised as employed or unemployed based on self-reported work (in hours per week).
To calculate the women's physical activity energy expenditure, the researchers assigned each of the physically active tasks a metabolic equivalent based on international guidelines.
Because women of childbearing age were heavier in 2010 than in 1965, the researchers also took account of increments in body weight used for each survey period to estimate their physical activity energy expenditure. As body weights were not included in the study data, the researchers calculated the increments based on two representative national surveys.
The researchers found that from 1965 to 2010:
The researchers say there was a significant reallocation of time by mothers from physical activity to sedentary behaviour between 1965 and 2010.
They say that it is known that physical activity is crucial for health, and that mothers can potentially "transmit" obesity and behaviours likely to promote obesity to their children. For this reason, they say, maternal inactivity could be an important way to prevent obesity and other chronic diseases.
The researchers state that maternal behaviour has a profound influence on children's development, and may influence children's subsequent risk of obesity and chronic disease.
Children raised by inactive, sedentary – and therefore unhealthy – caregivers may have an increased risk of being inactive, sedentary and unhealthy as adults, the researchers conclude.
They add that physical activity recommendations for Americans may need to be revised in light of the decline in physical activity levels necessary for daily life, as revealed by the study. Policies targeting pre-conception activity levels of potential mothers should also be introduced, they advise.
Are mums really lazier than they were 50 years ago? And is this making their kids unhealthy? Unfortunately, there are few answers in this study to these rather tabloid questions.
The study has several limitations, especially the fact that the women participating did not have their weight recorded. This is crucial in the calculation of energy expenditure. The complexity of combining different data sets in the way the researchers did means the results are open to error. Further, the self-reported data may also be misleading.
This is a US study and its results may not be applicable to other populations, although it is fair to say that where the US leads, the UK usually follows.
The widely recognised reductions in physical activity and increases in screen time in all sectors of the population have been acknowledged as a risk to health and a contributing factor to obesity.
The importance of parents as active role models and the need for them to encourage their children to lead healthy lifestyles has also been recognised. It's a moot point as to whether mothers should be regarded as any more responsible for this than fathers.
Rather than seeking to blame anyone for making their children unhealthy, it would be best to look for ways to reduce the risk of this happening.
If you are too busy juggling a career and childcare to regularly go the gym, there are quick and simple home-based exercises you could try. Why not try the NHS Choices 10-minute cardiovascular workout?