Although I have no evidence for it, I suspect that what we define as 'normal weight' is determined less by health considerations than by the sexual aesthetics of western societies. Put simply we view slim people as more attractive ergo being slim is healthier.
Except this isn't true, the evidence - sometimes (and misleadingly) call the 'obesity paradox' - suggests that the reverse is true and being overweight is more healthy than being normal weight:
...dozens of studies have confirmed the existence of the paradox. Being overweight is now believed to help protect patients with an increasingly long list of medical problems, including pneumonia, burns, stroke, cancer, hypertension, and heart disease. Researchers who have tried to show that the paradox is based on faulty data or reasoning have largely come up short. And while scientists do not yet agree on what the paradox means for health, most accept the evidence behind it.
Despite this evidence - and there are literally hundreds of studies that confirm the 'paradox' - public health persists in conflating overweight with obesity to create a massive scare story ('two-thirds obese or overweight' or similar) that justifies whole population interventions such as sugar taxes, fast food shop bans, ad restrictions and Jamie Oliver. We have well-funded 'obesity strategies' produced by every English local council that feature a raft of activities and interventions predicated on the idea that there's something called an 'obesogenic environment' filled will temptations that make folk fat.
The evidence here suggests that, instead of making nannying interventions that demonise individual macronutrients (sugar, fat and so on), we need to focus our effort and resources on the very fat and the very thin. And on healthy behaviours:
Paul McAuley, a health education researcher at Winston-Salem State University in North Carolina, has been studying fitness for close to 20 years. He says most studies on weight and health fail to take it into account. “Or they ask one question about it,” he says, and don’t bother to go further. When McAuley collects data on fitness, he finds that it predicts health and longevity much more strongly than fatness.
It's time - just as with drinking - that we listened to the actual evidence on risk and harm associated with excess weight. And recognised that our definition of 'normal' weight do no match that evidence - in fact seems (if the findings that overweight people have lower mortality rates is true) to suggest that that 'normal' is actually unhealthy.