I say it was a combination of canned food and a greatly reduced need for calories due to the disease itself, which slowed their metabolisms as drastically as it slowed their aging process.
Here's some content from the Final Draft script of "Miri" by Adrian Spies, dated August 12, 1966:
(after a pause)
Now tell me – how long ago did it happen, how long ago did all the grups die?
Three – hundred – years…the onlies mark down all the hots and colds at a secret place… it’s not much fun…
(quickly, childishly, trying to recover )
But it wasn’t like that!... We just went on – being onlies…
(frantic not to seem old to him)
We just had – foolies…
(getting up, forcing a big grin)
If we were hungry, we just took
something… there are lots of mommies and cans.
Can opener things.
Can opener is a dumb word. We found a kind of fresh word, maybe…
(another beat, a little giggle, also sly)
Mommies… that’s what we call the can openers, mommies…
(to all of them, a smile that is a very sad thing)
You get it?
(they all nod. They get it)
But mostly – mostly it was fun, foolies…
(the smile more frantic)
After all, with no grups to say no, what else would onlies do, anyway?