It is a widely held assumption among policymakers that the way to tackle childhood obesity - particularly among working class children - is to get them all doing lots of school sport.

And after the success of the London 2012 Olympics, there were understandably voluble calls for increased physical education (PE) in the school curriculum. There are lots of good reasons for doing sports at school, of course, not least for character development. Physical activity has all sorts of beneficial effects. But it is unlikely to reduce levels of obesity in children.

To see why, we can look at the results of EarlyBird, a cohort study monitoring the causes and behaviour of insulin resistance in contemporary children. Insulin resistance, largely the result of obesity, is believed to underpin the high prevalence of diabetes and cardiovascular disease that characterises modern society.

EarlyBird is distinctive in combining objective measures of physical activity and body composition from the age of 5 with annual fasting blood samples, which reach beyond simple body composition (BMI and body fat) to metabolic health (glucose control, insulin sensitivity, blood fats, cholesterol, blood pressure). The study is generating some novel and sometimes counter-intuitive findings, which have nevertheless withstood the rigours of peer review (over 60 papers are now in print). Here are some of the headline findings:

  • Obesity starts early: All children gain weight during growth, but EarlyBird is interested in the excess gained. It finds that over 90% of the excess weight in girls, and over 70% in boys, is gained before the child ever gets to school age. These findings support a need to re-direct public health initiatives towards an earlier period in childhood.
  • Obese parents, in particular those of the same gender, are key to childhood obesity: Daughters of obese mothers are 10 times more likely to be obese than the daughters of normal weight mothers, and the sons of obese fathers six times. Gender assortment of this kind suggests rolemodelling. It may therefore be more effective to target the obese parent than the obese child.
  • Inactivity does not lead to obesity, rather obesity leads to inactivity: Using time-lagged correlation to imply direction of causality, weight gain appears to precede inactivity, rather than inactivity to weight gain. This is crucially important because it suggests that calorie reduction, rather than increased physical activity, may be the key to weight reduction.
  • Taller children really are fatter:EarlyBird has found that taller children really are fatter than their shorter peers, have higher leptin levels and are more insulin resistant. Attempts to render indices of body mass or fat mass independent of height in childhood seem inappropriate if the object of the index is to convey health risk.
  • Parents are unaware and unconcerned: Being overweight is now perceived as the norm, and today's parents are no longer aware of their own or their children's weight. Parents are essential partners in the struggle yet, crucially, they do not acknowledge the problem.
  • Social inequality is not associated with physical inactivity: Despite clear evidence of a socioeconomic gradient in sports club attendance among the children, EarlyBird can find no evidence for corresponding differences in physical activity. The assumption that children of lower socioeconomic status suffer from their lack of structured opportunity for physical activity is not reflected in the evidence. Indeed, analysis suggests that poorer boys may be marginally more, rather than less, active than those who are wealthier.
  • Social inequality is not associated with metabolic risk: Lifestyle interventions to improve health in young children tend to target areas of relative deprivation, but the evidence for so doing is largely historical. EarlyBird researchers have re-examined the link between deprivation, obesity and metabolic risk in contemporary UK children. The data does not appear to support the assumption that obesity, metabolic disturbance and thus risk of type 2 diabetes are more prevalent among less affluent children. In today's increasingly obesogenic environment, youngsters from all backgrounds appear to be vulnerable, with population-wide implications for public health spending, and the prevention of diabetes in contemporary youth.
  • Dietary habits appear to be established early in life and are retained throughout childhood: Notwithstanding, more children appear to experience a deterioration in their diet over childhood than an improvement. Dietary choice is moderately systematic, but quality diminishes from 5 years of age to 13 years of age in 30% of the children and improves in only a few. Early assessment of dietary habits seems important to disease prevention.
  • Children who keep active gain no less weight, but they do become metabolically healthier: The UK and US governments advise at least 60 minutes of moderate physical activity every day. Only 42% of the EarlyBird boys and 11% of the girls met this guideline consistently over the three-year period from 5-8 years old. Governments use BMI as their outcome measure, but there were no differences in the trend for BMI in either sex, while the more active children became metabolically healthier. The study questions the utility of BMI as the outcome measure of physical activity programmes in children, and whether the bar for girls should be lowered because girls systematically record less physical activity than boys.
  • Children's activity is not determined by environmental provision: Those who do less in school do more out of school, and end up doing the same overall. Less than 1% of the four-fold variation in physical activity among children can be explained by the five-fold variation in PE provided at school. The original report was based on a single school term but EarlyBird has recently extended the analysis to four consecutive school terms. The result was replicated, and it leads the researchers to question government policy of linking physical activity to recreational facilities.
  • Being driven to school may not be eco-friendly, but it does not reduce a child's overall activity: The activity cost at the age of 7 of being driven to and from school during the hours 8am-4pm is 16%, but is nil (<0. 1%) over an entire 24 hours. Those who lack the opportunity for physical activity at one period of the day appear to compensate for it at another.

The evidence EarlyBird is accumulating suggests the activity of children is 'programmed' - either genetically or as a result of very early experience. There is little evidence from EarlyBird studies that the physical activity of free-living children is linked to recreation or environmental opportunity. They appear instead to have an 'activity stat'.

Rising levels of obesity are one of the biggest health challenges facing Britain as we look to the future. If we cannot reduce obesity among children, we will be left facing major economic and social costs into the future. The EarlyBird study is therefore hugely important and its conclusions highly surprising and significant. None of this should be taken as implying that we should not attempt to promote school sports or provide young people from disadvantaged backgrounds with similar opportunities to participate in sport as those from wealthier backgrounds.

However it does mean that if obesity is the problem we are seeking to solve we need to start acting much earlier in a child's life and focus as much on the parents. On obesity the science has moved on - it is time for the politicians to catch up.