The assumption is that the kilt simply came back into fashion. Fashion was different three hundred years ago, why will it not be different three hundred years hence? Like Heinlein, Lazarus Long is a Missouri boy who was born in the last half of the Nineteenth Century; basic pioneer stock. He's going to be rugged and self-reliant. The Methuselahs are different from the rest of humanity, they may just need more calories as a matter of course. Also, if you're really working, 3000 calories at a sitting may not be bad. I'm a Minnesota farm boy, and I know from stories handed down that people routinely ate that during the horse-drawn days of agriculture and didn't gain an ounce. Methuselah's Children was written for Astounding Stories during the 1940s, you're not going to see a lot of sex in it. Sorry.