I recall way back, must be like 1990, I was helping my buddy's brother out on a job. I was like 20, and my friends brother probably 25. His client was a cardiologist. As we were leaving, the physician caught a glimpse of his hands, and asked if his family had heart problems. His dad had died at 42 of a heart attack, and he had 4 uncles on fathers side, none of whom saw 50, so he of course said yes, how did you know? The guy mentioned something about his fingernails and the moon shape. Kind of shook him up given the family history. The physician told him to keep close tabs or something when he got older, and that was all I recall.
On the bright side, he's 49 now, and despite not living very cleanly, he's never had a heart issue. Always thin, but so was his father. The doctor was old then, so it may be some old homespun medicine that wouldn't hold up to any testing.
On the dark side, my friends brother is in jail for murder in California, and just started a 25-life sentence 2 years ago at 47, so I imagine he's probably half-hoping that old doctor was on to something.