Fan theories about the Marvel Cinematic Universe have only increased since the release of Avengers: Endgame. Some ideas have tried to suss out the future of the MCU, while others have explored explanations of smaller character beats. Given what's already happened in the MCU, even the most outlandish theories seem to have at least a kernel of plausibility.
One of the newest theories tries to explain just how Tony Stark was able to survive just as much as he did over the course of his MCU adventures, and it unexpectedly connects Iron Man even closer to Captain America.
Man In A Suit Of Armor
The theory from Reddit suggests that Tony Stark had a secret leg-up in life, even beyond the wealth and privileges he was afforded at birth. Howard Stark was always preparing for the future, throughout the history of the MCU. He helped found S.H.I.E.L.D. and even discovered a new element that he hid away to be used for the future. Howard Stark, like his son, always tried to get ahead of the curve, and this theory posits that Howard Stark would try to prepare his son with the greatest advantage of all: superpowers.
The theory starts by imagining that Howard could at least hypothesize that something like the fall of S.H.I.E.L.D. and the rise of Hydra was possible. This could be why he went to work trying to recreate the super-soldier serum first used on Steve Rogers and developed by Abraham Erskine. We know that by at least the time of his death, Howard had fully recreated the formula. This is why Hydra finally targeted him and his wife, to steal his work and unleash new super-soldiers onto the world. But Howard could have been secretly dosing his son with at least precursors to the serum, all to help him in the future. Seeing a dangerous world that his son was going to inherit, it might have been Howard's way of preparing him for anything.
The Invincible Iron Man
Across the events of the Marvel Cinematic Universe, Iron Man has suffered some pretty ridiculous injuries. A quick breakdown of his worst injuries including having a bomb exploded right in front of his chest, crash-landing in the desert while only wearing the remains of a damaged prototype suit, being blasted by a massive arc reactor, almost dying of palladium poisoning, being tossed around a helicarrier turbine, blasted out of space to fall miles back down to the earth, having his house dropped on him, getting beat up by Captain America, being telekinetically thrown around by Ebony Maw, and having a planet thrown at him before being promptly being stabbed in the gut by Thanos.
Even compared to other superheroes, Tony Stark has survived a ridiculous amount of injuries. Nominally, he's just a man wearing armor, but even with all of his inventions and advancements, it's hard to imagine him surviving all of that. Having some semblance of the super-soldier serum in his body could explain exactly how Stark was able to survive all these injuries, giving him more strength and endurance than what any other human could reasonably have. The serum, which brings out the potential in someone, could even be partly responsible for his advanced intellect. The serum amplifies the goodness in Captain America and the darkness in Red Skull. It could have been responsible for bringing out the potential in Tony if it was given to him when he was young enough, explaining how he got so much smarter than everyone else.
Proof Tony Stark Has A Heart
However, there is just as much proof that Tony Stark really is just a mortal man wearing a fancy suit. Every single one of those absurd injuries had an effect on Tony, leading to both physical and psychological scars. Unlike almost any other Avenger, Tony specifically takes time to recover from many of his worst injuries. Whenever he's out of his suit, Tony takes extreme care not to be hurt. The original Iron Man even shows Tony being tortured and interrogated by the Ten Rings with much the same results as it would have on any other human.
Even when he's created suits strong enough to stand up to gods and titans, he can be seriously wounded by a single good stab. That suggests that Tony really is just as human as anyone else. Taking the Infinity Stones started killing him even before he activated them, with his nanotechnology being the only thing that probably kept him from being killed immediately, such as when the Collector's servant grabbed the Power Stone in Guardians of the Galaxy. This means he has no natural resistance like Peter Quill or the Hulk, which is how they survived and interacted with the Stones. If the super-soldier serum really was in Tony, then it would have probably at least helped him withstand some of that power.
Perhaps the biggest piece of proof though may come from Tony's most human weakness. The character is frequently seen drinking, especially in Iron Man 2 when he believes the pallidum poisoning is going to finally do him in. He spends most of the second act of the film incredibly drunk, indicating just how dangerously negligent he's becoming. But if he had the super-soldier serum in him, he wouldn't be capable of going down this particular drain.
As a despondent Steve Rogers tells Peggy Carter when he thinks Bucky has been killed in Captain America: The First Avenger, the super-soldier serum keeps metabolizing the alcohol before it can affect him. Someone with these kinds of advancements wouldn't be able to enjoy alcohol as much as someone like Tony Stark does.
While it's a fun theory to speculate on, the fact that Tony Stark is only human is often the driving factor behind his popularity. It's what makes his eventual death all the more meaningful. It's unlikely that his story would be undermined by having him turn out to have been super-human this entire time, so don't expect to ever see Marvel hinting at this one being true. However, it is still a fun way to consider just how tough Iron Man is.