A research agenda for obesity research:
We don't have a clear understanding of the connection between being somewhat overweight and being heathy. We are more certain that being obese (very overweight) results in poorer health. Long term, as our weight has increased, our longevity has improved. We have grown fatter but now live longer.
We are unsure that losing weight - perhaps even lots of weight - improves health for those who were originally very overweight. Losing lots of weight may worsen mortality. Most studies do not support the notion that weight loss among overweight people improves their mortality though it may reduce their mortality specifically due to weight-related disorders. If this is true then the modern passion for weight loss is sensibly motivated by aesthetics not health.
We are unsure that people can sustainably lose weight. Diets and exercise programs have short-run effects but invariably seem to fail longer-term. Is weight-gain a hard-wired trait?
As a particular case of this proposition: it is unclear that even 'well-designed' public health programs can lead to weight loss even if this is a good idea.
We don't know exactly why so many people are getting fat. Exercise or diet? Yes, certainly some mix of these two - by a 'law of nature' it must be so - but, exactly what, no-one knows. Or perhaps everyone knows and that's the problem. There are a myriad of theories explaining modern propensities to bulge but none works really well. We are not obviously getting lazier for example.
We don't know why poor people in developed countries are eating unhealthy foods when they are not time-poor and when healthy foods are cheaper alternatives?
This is an index of my modesty. On obesity we know little.