Editor's note: This is the part of a series highlighting the 2021 Olympic Games with a specific emphasis on health and wellness.
July 15, 2021 -- The participation of transgender women in female sports will garner even more attention when the New Zealand weightlifter, Laurel Hubbard, becomes the first openly trans athlete to participate in the Olympic games.
WebMD/Medscape interviewed sports physicist Joanna Harper, who has advised the International Olympic Committee (IOC) and other sporting bodies on gender and sports. She pivoted from working in cancer research to becoming a PhD student at Loughborough University in London, one of the world’s top institutes for sports research, after publishing on race times of nonelite athletes -- including herself -- before and after transition.
This interview has been edited for length and clarity.
This interview focuses on transgender athletes – primarily those assigned male at birth who hormonally transition to female. It does not address athletes, such as Caster Semenya, who have disorders of sexual development.
Most guidelines on the participation of transgender women in sports use testosterone levels in the prior 12 months with cutoffs of 5 nanomoles per liter or 10 nanomoles per liter. Is that correct?
Harper: In 2016, the IOC adopted a guideline where trans women could compete after 1 year of having testosterone levels below 10 nanomoles per liter. World athletics subsequently put in a rule that required 5 nanomoles per liter. But it's important to note that there are different tests used for those two rules.
The IOC rule is based on immunoassay testing, and world athletics was based on liquid chromatography mass spectrometry, or LCMS, for short. Basically, the 10 nanomoles on an immunoassay is equivalent to approximately 7.5 nanomoles by LCMS. The idea that world athletics has half the testosterone limit is absolutely not true, because you're talking about different testing methodologies.
The NCAA does not actually have a testosterone limit; they require one year of hormone therapy, but there's no testosterone limit with that. And there's little to no testosterone testing done by the NCAA.
Testosterone levels drop quickly after hormone treatment. Is it true that the testosterone levels of trans women are similar to cis female levels?
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.
But that's the population these studies looked at -- they weren't looking at trans women who are active in sports. We found that the trans women didn't lose very much strength or lean body mass. But they probably didn't build up very much before they started hormone therapy. How applicable that finding is to trans athletes is somewhat debatable.
There's absolutely no question in my mind that trans women will maintain strength advantages over cis women, even after hormone therapy. That's based on my clinical experience, rather than published data, but I would say there's zero doubt in my mind.
Your review found that lean body mass and muscle strength decline but remain above cis female levels up to 3 years after hormone transition. Do you think that represents a plateau or that there will be further declines over time?
Harper: It is probably true that the greatest changes in the strength of trans women on hormone therapy occur in the first 12 months, but there aren't enough data to make a definitive statement. It is possible that there are further declines in strength after 36 months, and a recent small study out of Brazil suggests that the entire strength advantage might go away. But they just looked at grip strength in eight trans women, compared with eight cis women matched for BMI, so I think the matter is far from settled.
A recent paper quantified the male advantage across various sports using data from cis men and women. Track and swimming were at the lower end with about a 12% differential, weightlifting was on the high end, and for throwing a baseball pitch that was a 50% difference. Do you think these comparisons are valid?
Harper: I don't dispute their data on cis people at all. I think that they did a good job compiling that.
Given the range, would you assume that there might be different advantages for trans women in different sports?
Harper: Absolutely. I've been saying since 2018 that we should be looking sport-by-sport on regulations for trans athletes.
The strength differences between cis men and women are higher for upper body strength than lower body, so can we assume that trans women will have more of an advantage in sports that require more upper body strength?
Harper: It’s not just strength, it’s height sports. Trans woman will lose strength with medical transition, but they won’t lose height at all. In sports, like basketball and volleyball, that advantage isn't going to be mitigated; the strength advantage will be mitigated, but that the height advantage isn't going to go away at all.
So far, we’ve been talking about trans women who transitioned after puberty. Presumably, there is no advantage if a trans girl never went through male puberty?
Harper: I suspect that trans girls would still, on average, be taller. I don't know that for certain. There may be very minimal advantages. When the IOC first put rules in place in 2004, they said these rules only apply to trans women who experience male puberty.
So, the IOC has long said that any advantages that trans women who don't go through male puberty are small enough that they're not going to worry about it. Most sporting organizations would say the same thing. Not everyone would agree, but most sporting bodies agree on that.
For trans women’s participation in female sports, is your goal to enable fair competition?
Harper: I like to use the term “meaningful competition.” We always focus on advantages; well we actually allow advantages in sport. And we allow advantages for one group over another. For instance, in baseball, the configuration of the diamond lends many advantages to left-handed players over right-handed players. And yet, that's an advantage we allow.
On the other hand, we don’t let heavyweight boxers get into the ring with flyweight boxers. And here's the important difference. We can have meaningful competition between left-handed baseball players, and right-handed baseball players, despite the advantages. In fact, many would say that the lefty/righty combination is one of the most important factors in baseball. But there's no meaningful competition between big boxers and little boxers; the big boxer wins every time.
The important question isn't, “Do trans women have advantages?” What really matters is can trans women and cis women compete against one another in meaningful competition. There's no indication that trans women are anywhere close to taking over women's sport, it's not happening now. It's not likely to happen anytime in the future.
Boxing brings up contact and collision sports. World Rugby recently came out against trans women's participation in women’s rugby, citing safety issues.
Harper: I absolutely agree that there's a safety issue because on average, trans women are taller and bigger than cis women. If you're looking at collision metrics, the two important factors are size and speed. The data I gathered from 20 distance runners and sprinters suggests that trans women aren't faster than cis women. A study from the U.S. Air Force does suggest that trans women are faster, but [it] didn't measure training in any way. They are still going to be bigger than cis women.
Now, of course, what you really want to know is how big are the trans women playing rugby versus the cis women playing rugby? We don't have any data on that. But as a population group, trans women are bigger than cis women, so there's definitely some reason for concern over a potential safety issue.
But you would prefer inclusion, and you think that the safety issue could be navigated?
Harper: I don't think that banning trans women is the right approach. World Rugby calculated a 20% to 30% increased risk when a ball carrier of typical female mass was tackled by a ball carrier of typical male mass. But, as I said earlier, if you look at the studies of non-athletic trans women, trans women don't have typical male body mass. Trans women are no more than halfway between female mass and male mass and maybe even closer to typical female mass.
The question is: How big are the women playing rugby? We don't know that. We've applied for a grant from World Rugby to study that question. Hopefully, they'll cough up some money because that's the important question.
By the way, World Rugby only looks at international-level rugby, and no openly trans woman has ever played an international rugby match. So, the total risk increase from trans women in international rugby is zero so far because there are no trans women. That may change -- there's a trans woman in France who might someday make the national team -- but that's a very small number.
I suggested to World Rugby that they put a limit on each national team of one trans woman per team. Because even if there was a 20% risk increase, when a trans woman tackles a cis woman, and I'm quite certain, it's not that big, if only one of 15 players on the field was trans, then only a very small percentage of the tackles would be made by a trans woman. In a given match, you're looking at a very small increase in risk.
That was my suggestion to World Rugby, but they didn't take it. No trans women are currently banned from rugby, because as it stands right now in the international game, there are none good enough. We'll see what develops in the future.
Are there any potential disadvantages for trans women athletes?
Harper: Absolutely. Trans women have larger frames, which are now being powered by reduced muscle mass and reduced aerobic capacity, and that can lead to disadvantages in things like quickness, recovery endurance, and things that might not be quite as obvious as seeing somebody bigger.
How do these disadvantages play out? We don’t know yet because the research is very early on. But the idea that trans women only have advantages just isn't true. The analogy I've used is that it's like a big car with a small engine competing against a small car with a small engine.
With trans men, is the assumption that there's no advantage?
Harper: Trans men are allowed to inject testosterone and no other athletes are allowed to do that. It's unclear right now, but even after taking testosterone, trans men are going to be shorter, they're going to be lighter, and they're probably not going to reach typical male strength. Those are fairly substantial disadvantages.
The study from the U.S. Air Force that I mentioned earlier looked at trans men and trans women in three fitness tests before and after their transition. The tests were a 1.5-mile run and the number of situps and pushups that can be done in a minute. After 2 years of hormone therapy, the trans men matched the cis men in the 1.5-mile run and in the number of pushups per minute, and they exceeded the cis men in the number of situps per minute. There may be some advantages that trans men have in sports, we’ll have to see how that all plays out. I'm inclined to think that trans men are less likely to be successful in men's sports than trans women are in women's sports.
There are some sports where you get subdivided by weight, and then the fact that trans men are smaller is not really a disadvantage, because you're usually the same size as somebody in your weight class. Those may be the sports where trans men have greater success. But again, it's very early days.
Do we know if there's any difference (assuming they've gone through male puberty) if a trans woman transitioned at say 21 versus 31 years of age?
Harper: I'm not sure there's a difference between 21 and 31. There's certainly a difference between 11 and 21. I would think that any additional testosterone gains from 21 to 31 that might not be undone by hormone therapy would be very small.
This is a very divisive topic, but you seem to be less dogmatic than some of the people on either side of this issue. Is that a fair representation of your point of view?
Harper: I have been reviled by people on either side of the chasm. I try to follow the data. I try to use logic and reason and rational thinking. I also try to talk about these things calmly -- as you probably noticed, that is somewhat in short supply. I may be one of the few to do that, but I'm certainly not the only one.
Most people that I've come across through the international sporting bodies want to navigate the somewhat treacherous middle ground in these debates.
You've worked with groups from both sides of the debate. How do you make a distinction between a group like the Women’s Sports Policy Working Group , that seem to be ‘good faith actors’, if I can use that term, versus those using fairness for women's sports as a fig leaf of respectability for a broader anti-trans agenda?
Harper: A number of states that have introduced bills to ban trans women in sports have said they're trying to save women’s sports. But in addition to these bans, they're also introducing legislation to try to limit the lives of trans people in other ways. They're showing their true colors.
The people who founded this Women's Sports Policy Working Group truly do care about women’s sports. People like Donna de Varona, Martina Navratilova, Nancy Hogshead-Makar, and Doriane Coleman have achieved on the sporting field and in life in general, and they care very much about women's sport. So, there's a huge chasm between these two groups.