Okay, yes, obviously when you take the gut parasites out of people, they tend to gain weight immediately after. That’s not exactly what I’m talking about.
First, let’s assume you come from a place where humans and hookworms have co-existed for a long, long time. The hookworms that just about everybody in the American South used to have appear to have come from Africa, so I think it safe to assume that hookworms have probably been infecting a lot of people in Africa for a long time. I don’t know how long–could be anywhere from a few hundred years, if they’d come from somewhere else or recently mutated or something, or could be tens of thousands or hundreds of thousands of years, if they’ve just always been hanging around. Let’s just go with tens of thousands, because if it wasn’t them, it was probably something else.
Over a few thousand years of constant infection, you’d expect to develop some sort of biological response to minimize the chances of death–that is, your ancestors would have evolved over time to be less susceptible to the parasite. Obviously not getting the parasite is one great way to avoid getting killed by it, but let’s assume that’s not an option.
Another solution would be to just absorb food differently–faster, say, or in a manner that circumvents the parts of the gut that are normally infected. Over time, humans and parasites might tend toward an equilibrium–humans stepping up their digestion to make up for what’s lost to the parasite.
Remove the parasite, and equilibrium is lost: suddenly the human starts gaining a lot of weight, especially compared to people from populations that did not adapt to the parasite.
That functional a gut isn’t needed anymore, but it might persist for a while if there are no counter-evolutionary pressures.