This happens in pigs and other livestock. Animals are specifically bred for their Feed Conversion Efficiency (https://en.wikipedia.org/wiki/Feed_conversion_ratio). I don't see why something similar couldn't happen in people as well.
Thankfully the nobility never became a genetic isolated population - they are all just jumped-up peasants who think they are better than everyone else.
who wouldn't want to be born with excellent immune system, strong and tall, smart and attractive?
You could argue that this would be a solution to many discrimination issues today if everyone is beautiful, strong, tall, ... . Mhh. Interesting thought.
However, I could imagine in the past feed efficiency was an evolutionary advantage.