Decades, in most cases. And not necessarily self-neglect. Frequently it is neglect by the parents or by your environment. Currently, in many countries it is pretty difficult to maintain healthy diet if you have below average income.
The way obesity starts in most cases is that you eat the wrong things for a long time and this slowly damages your internal regulation. There isn't super firm consensus on what exactly causes this to develop, but you can bet on low protein, high sugar high carb paired with low amount of exercise and food intake spread throughout the entire day (snacking).
Hyperpalatable food that is currently majority of food items in stores is essentially designed to keep people eating.
But it typically takes a decade or two for most people before they visibly gaining weight. For example, I drank 2-4 liters of Coca Cola every day and for over a decade I was completely fine. Until I wasn't and started gaining weight dramatically.
As far as I understand most (yes, over half) of kids in developed countries are already progressing into obesity and have damaged metabolism to higher or lesser extent. That they are not classified as sick and in need of help is only due to our medical system which essentially waits until you require medical assistance until you get classified as sick.
Only some people get fat because they overeat a lot for whatever reason. Some unfortunate kids because parents let them eat whatever they want. Some unfortunate adults because maybe they are stressed and overeat to compensate.
But most people really take a long time to develop the underlying condition that causes them to gain weight.
> The advent of highly effective GLP-1 based obesity drugs like Wegovy and Zepbound have stoked excitement among many doctors as a new way to address rising obesity rates
> “These drugs are definitely an important tool, but they should not be seen as a solution to the problem,” Francesco Branca, director of nutrition and food safety at the World Health Organization, said in a press briefing discussing the study. “The solution still is in the transformation of food systems and the environment, such that obesity can be prevented.”
I bet we'll see evidence that giving them to parents (or parents-to-be) reduces obesity and obesity risk in the subsequent generations by altering the food system in the home and environment.
Maybe we'll need them every 3rd or 4th generation to reset things.
There is an epidemic aspect to it as well. From food deserts to unhealthy food advertising, there are plenty of external reasons for the obesity epidemic.
In Palestine 26.8% of the population is obese, 57%-67% of mothers in Gaza are obese or overweight, says Wikipedia, which cites a 2014 paper. It lists "decreased physical activity and greater than necessary food consumption" and "lack of sex-segregated facilities and cultural norms are prohibitive factors" as reasons. https://en.wikipedia.org/wiki/Health_in_the_State_of_Palesti...
Yes, just a bit jargon-y. "Middle East and North Africa" is one region for the purposes of the study, so it's proper that it has an "and" both in the middle and before it. And I haven't seen "underweight" used as a condition noun the same way "obesity" (rather than an adjective), but that tracks too, and doesn't seem to be too uncommon. Both of those constructions are also used in the linked Lancet paper.
Swapping out terms, a similar sentence could be "Lilliput and Blefuscu are also experiencing much more cooties than vampirism, the study said." and that sounds entirely reasonable to me.
As a mathematician we can easily confirm by going outside and waiting until we can see 19 people being visibly obese. Best see 19 obese people at the same time to eliminate possibility that some of them are the same person who just changed clothes and appearance to confuse the experiment.
Obesity (especially morbid obesity) takes months and years of self-neglect (or parental neglect in case of children).