It doesn’t really matter whether the limit is 5 or 10 nanomoles per liter because most trans women are going to be under 2 anyway.
Hemoglobin levels are higher in men, which allows more oxygen to get into the muscles. Is the main advantage of that greater endurance because muscles don’t fatigue as quickly?
Harper: Yes, the single most important physiological factor for endurance athletes is hemoglobin. In sports it’s often measured as hematocrit levels. Hematocrit is the percentage of blood that’s red blood cells.
That’s the reason that endurance athletes go to altitude to train, or [those who cheat] use blood doping or EPO. There are a lot of things that go into making an endurance athlete, but the single most important factor would be hemoglobin levels.
Hemoglobin levels also drop as testosterone levels decline, right?
Harper: Hemoglobin levels follow the testosterone. Within weeks of starting testosterone suppression, testosterone levels will be within female norms, but it takes a little longer for hemoglobin to get the female norms, probably in the 3- to 4-month range. That’s roughly the life cycle of red blood cells.
As trans women replace their red blood cells, with their new lower testosterone levels, they will have fewer red blood cells and lower hemoglobin, but it does take 3 to 4 months.
What distances are we typically talking about where endurance is an advantage — is it an 800-meter track event, or is it more in the 1,500 meters and above range?
Harper: The 800 meters is likely more speed than endurance. You’re probably talking 1,500 meters and above, but that’s just track. There’s [also] distance swimming, distance cycling, the triathlon. There are any number of endurance type sports.
How are other potential advantages such as lean body mass and strength affected by medical transition?
Harper: Lean body mass and strength are less affected than hemoglobin, but we don’t really have studies on trans athletes. Our review and another one looked at studies on non-athletes. One of the important things to note is that even before starting hormone therapy, the trans women in these studies were substantially less strong than cis men. I put it colloquially that as a population group, trans women would rather starve themselves to look like models, than put on muscle to be athletes.