What does it mean?

Learning English Forum - Ask a Tutor
1

It was a concession, with penalties-not the least of which was, most probably, a dramatically decreased life expectancy

I guess it means you are given a permisson to eat meat, but you could not live longer. What does ‘not the least of which was’ mean?

2

‘not the least of which was’ means that it was not the least important thing. The author is saying that it was actually a very important thing. In this case, it was a very important ‘penalty’.