I have recently begun to read the book by Tim Spector called the 'The Diet Myth'. Spector is a Professor at Kings College, London and described as in the top 1% of cited scientists world wide.
He has been studying the natural biome, mainly the over 2lbs of bacteria etc that lives in our guts and the contribution it makes to our health and weight. There is a close connection between the range of bacteria present in the biome and the level of variety in a person's diet and the extent that they eat processed food. Those with the poorest diets and the least number of different species in their biome have the most weight problems. Weight gain is not necessarily a function of how much you put your mouth, as what you put in your mouth. Generally the lower family income, the more likely they are to be relient on cheap over-processed food.
I understand aprilrose's argument, but would modify it. She is obviously quite young and the rate of childhood obesity has been growing long term. I started school in 1948 and today's children are significantly better nourished and larger than children in school with me.
The other thing that affects is childhood obesity is income. The higher the family income, the fewer children are overweight. I live opposite the primary school in an affluent commuter village in south Oxfordshire. Obesity among children there is unobservable.
My DGC go to a school in York. Their school catchment area covers, in about even quantities, a large council estate, still mainly in public ownership with many families there having every difficulty from drug use, poverty and disability and half from a prosperous, but not affluent, area of pleasant interwar semis and new estates. When I collect DGS from school, I do see a significant number of visibly overweight children.