There seems to be a view these days that one person's opinion is as good as another's, regardless of whether the opinion is an informed opinion or an uninformed opinion. This may well not do too much harm in itself, but the boundaries have become smudged and too many people now confuse opinion with facts that are well supported by evidence. My especial concern is in the matter of whether more exercise reduces obesity in people in general and in overweight children in particular.
We are in the happy position of being able to check out research on this matter. Researchers from the University of Glasgow found no evidence that exercise either prevented obesity in children or promoted weight loss in children who were already overweight. See http://www.guardian.co.uk/society/2006/oct/06/health.food where we read that the research was lead by John Reilly, a professor in paediatric energy metabolism at Glasgow University. His "team set out to establish whether greater physical activity would prevent children from becoming overweight. They recruited 545 children in their last year at 36 nursery schools. Half the schools instituted three extra half-hour sessions of physical play and activity every week, and parents were given information packs encouraging them to give their children more activity and less television. The other half had no extra activity or information. All the children were regularly weighed and measured and their body mass index (BMI - the relationship between weight and height used to check for obesity) was calculated, and there was no difference between the groups. "Despite rigorous implementation, we found no significant effect of the intervention on physical activity, sedentary behaviour or body mass index," wrote the researchers. The BMJ article about the research is here: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1647320/
Dr John Briffa is a UK medical doctor with a particular interest in nutrition and he also has an interest in weight loss for people who are overweight/obese. Here is one of his blog articles, entitled Does exercise promote weight loss? No. He examines the evidence and gives references to relevant scholarly studies. His conclusion is, "There are plenty of good reasons for taking regular exercise. Weight loss isn’t one of them."
There is evidence that obesity is 'set before age of five'. Professor Terry Wilkin, of the Peninsula Medical School, Plymouth, the lead researcher in this research, said, "When they reach the age of five the die seems to be cast, at least until the age of puberty. What is causing it is very difficult to know." He said there must be a factor now that was not there 25 years ago which is making today's children obese. And, given the young age, this is likely to be in a child's home rather than school environment and linked to upbringing rather than schooling. Rather than lack of physical exercise, he believes diet could be to blame. "