Is There a True Altruistic Act?

I think most people will intuitively accept if they think about it hard enough that there's no truly altruistic act, you're always going to benefit in some way from an action that you commit. I'm thinking of, probably like a soldier throwing himself in a grenade. This is the sort of example I was thinking. You can still see, even if it's totally irrational, somebody is doing that because either to maximize the intense pleasure of virtue that they feel in the moment of doing so,. or the converse, which is the avoidance of the pain of the guilt of having not done so for the rest of their life. It's easy to take the high ground and say, no