Whenever the situation gets out of control, Popeye eats spinach to gain superpowers. It's been a while, but I remember him sailing his ship out of the sea on the land for miles using hand paddles. He also gets a rocket engine when Olive is far away as a hostage of Bluto and such.
Has it ever been addressed in-universe or out-of-universe why spinach gives Popeye superpowers? Is Popeye an alien or does he have a mutated gene for that?
In-universe, it was mentioned on several occasions that Popeye's desire for spinach (and the enormous strength it gives him) were linked to spinach's high levels of Vitamin A.
Out of universe, his love of spinach (and in particular fresh and later tinned spinach) were part of an extended marketing campaign by the US government to sell more spinach, particularly to impressionable young boys, hence why he didn't start eating spinach until several years after the comic was launched, during the depression-era food crisis and why he later switched to canned spinach during the war years.
Oh, and purely apropos of nothing, the idea that spinach was the inadvertant beneficiary of a misplaced decimal place was debunked a few years back. Myth busted.
The decimal point fallacy
Now, Popeye is unlikely to have known that because he was created around 1929, featuring in his own cartoon in December 1930. A British Medical Journal paper from 1981 claimed that the fallacy of spinach being high in iron came about because the decimal point was placed in the wrong spot in the original analysis back in the 19th century, giving spinach an iron content ten times more than it was in reality.
Earlier this year, Dr Mike Sutton from Nottingham Trent University in the UK, refutes the whole argument stating that there is no evidence that a decimal point was ever placed in the wrong spot. He has even written to author of the BMJ article and says the response from the author provided no proof for the claim.
Spinach is healthy. Popeye eats a lot of spinach; it's his favorite food. So he is incredibly healthy and strong, especially right after he eats some.
It's as simple as that. Popeye explains in his song:
I'm strong to the fin-ich,
'Cause I eats me spinch.
As to whether he's an alien or mutant or something:
I yam what I yam, and that's all I yam.
When I was a kid, I read that he gained the power by petting a magic bird and spinach reactivated his strength. I couldn't find a whole lot of support but Mental Floss does cover this (at the bottom).
However, in the animated version, Wimpy gained super strength through spinach at least once so it probably depends on the writer.
http://mentalfloss.com/article/54568/15-things-know-about-popeye-his-85th-anniversary
