Studies to determine what about the absorption? The old line of thought about complete vs. incomplete protein has mostly fallen by the wayside, and there are certainly some fairly compelling general studies on the health overall[1] (disclaimer, I am not a vegan, but I have been vegetarian for 30+ years, and I bike, lift, and climb without any trouble).

1: https://jamanetwork.com/journals/jamainternalmedicine/fullar...