Yes, not only just "protein" as a whole, but one or two specific amino acids, apparently. Reducing intake of methionine is being targeted in a lot of longevity research these days. Most animal sources of protein are pretty high in methionine, and some are very high, like egg whites. Vegetable sources of protein tend to be low. There are some exceptions, like animal collagen/gelatin, which I recall is quite low in methionine.

Just saw these posts. Thank you, Lisa, for your very thorough reply to my question about the thyroid!