There was a news paper article about this very epidemic among Sri Lankan monks I saw online a few years ago. It confirmed what I’ve witnessed as a layperson and as a yogi living in a monastery for several years in Asia and in the west.
I don’t know how to say this nicely, so I’ll be blunt. Lay people offer an overabundance of snack type of foods, sweets with refined sugar, baked goods high in saturated fats, potato chip type of foods really high in salt and boiled in vats of industrial cooking oil that makes the oil become carcinogenic.
And on the monastic side of things, the monastics are often uneducated or unaware of the health consequences of eating such foods in high quantity, and/or they can not curb their urges for snacky foods.
There are other sensitive aspects which makes things more complex, but basically it boils down to the two points above if you want the root causes of the diabetes, obesity, and unhealthy monks.
According to the vinaya, the monastics are supposed to not complain about the food and appreciate whats offered, and if this is taken to the extreme, then the monastics pretty much are doomed to suffer ill health stemming from a poor diet the older they get. I’ve noticed the more famous monks get, the more they seem likely to have this problem. And once they start taking various medications for diabetes, etc, then the problem compounds because the medications makes them bloated, weaker, perpetuating a negative downward spiral.
Perhaps the vinaya allows monastics, once they start to become ill, to then refuse to eat the more egregious unhealthy foods and only eat the healthier fare (without complaint of course, but observant lay people may ask and find out, and then start to increase the number of healthier items offered, on their own volition).
The solution to the problem is first of all, monastics and laypeople need some rudimentary knowledge about eating a healthy balanced diet.
There is a supply and demand kind of cycle that goes on as well that the monastics have to be accountable for. If they’re noticably happy eating proportionally large amounts of unhealthy food, it signals to the laypeople that they like it and should keep offering it. If they only take a token amount of the unhealthy fare, to show appreciation, but eat the bulk of their food on the healthier fare, then observant lay people will readjust the amount of healthy fare they offer.
That’s just one kind of typical scenario, and other circumstances call for different measures. If you’re going for alms round in a poor village and all they can offer are frog legs and white rice, then you eat that happily with appreciation of course.
I really feel for the monastics who are not robust or strong in health. It’s great to have the ideal of eating purely according to the vinaya, but when you find it’s hard to carry out spiritual practice from having a weak body and groggy mind because of the diet, it’s not an easy life.