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A new paper in the Journal of Sports Sciences, in setting up what turns out to be a highly unusual and interesting experiment, casually drops this little fact-bomb in its opening sentence: “The cardiac phenotype of a substantial fraction of the population, i.e., mature women, is mainly unresponsive to endurance training.”
Wow. The hearts of mature women are “mainly unresponsive”?! That seems like kind of a big deal, since the health-promoting effects of endurance training are an article of faith in this column. So it’s worth starting out by acknowledging the chronic underrepresentation of women in exercise science studies. Exactly how women respond to a given training program, and how that changes with age, remains uncertain because it hasn’t been studied enough.
In fact, there’s some history to this claim. Back in 2019, two of the authors of the new study, Candela Diaz-Canestro and David Montero of the University of Calgary, published a meta-analysis looking at the extent to which men and women can raise their VO2 max, a key marker of aerobic fitness, through endurance training. Their conclusion: for a given dose of endurance training, men got a bigger VO2 max boost than women by about 2 ml/min/kg—a difference that corresponds to a 7 to 9 percent reduction in premature death.
Following that study, a group of researchers from the Mayo Clinic wrote a letter to the editor of the journal suggesting that the difference was mainly present in older women, not all women. Diaz-Canestro and Montero didn’t agree, but in 2020 they published another meta-analysis, focusing this time on how the structure and function of the heart changes in response to endurance training. Once again, they saw bigger adaptations in men than women—but this time, they did see evidence that the difference showed up primarily in older women. Something, perhaps related to the hormonal changes that accompany menopause, seems to alter how women respond to training later in life.
That’s the background that sets up the new study, from Diaz-Canestro, Montero, and Christoph Siebenmann. Their goal is to figure out a way to enable postmenopausal women to reap the full benefits of endurance training—and their suggestion is to donate blood. A typical blood donation takes about 10 percent of your blood, or 500 milliliters. This is pretty much the opposite of altitude training: you lose 10 percent of your oxygen-carrying red blood cells, and consequently your aerobic fitness drops immediately. But the additional stress on your heart, in combination with vigorous endurance training, could provide the stimulus needed to improve fitness even in postmenopausal women.
When you withdraw 500 mL of blood, about 60 percent of it is plasma, and 40 percent is red blood cells. The plasma, which is the liquid part of the blood, is easy to restore. Within 24 to 48 hours of a blood donation, you’ll have added enough plasma to bring your total blood volume pretty much back up to normal. But now your blood is diluted, because there are still fewer red blood cells to carry oxygen in any given volume of blood pumped by your heart. In women, red blood cells may not return to normal for 9 to 12 weeks. During that time, your heart will have to pump a little harder to supply oxygen for a given level of exertion—and perhaps, as a result, it will get correspondingly stronger.
For the study, 15 moderately active women between the ages of 52 and 75, all postmenopausal, agreed to have 10 percent of their blood volume removed. Then, after three weeks to allow partial recovery of blood values, they completed an eight-week endurance training protocol that involved two to five interval sessions per week on a stationary bike.
Here’s what the oxygen use (VO2) looked like at different heart rates. For each exercise intensity, the white bar shows the baseline value, the striped bar shows immediately after the blood withdrawal, and the black bar shows after eight weeks of training:
You can see there’s a consistent pattern: the amount of oxygen consumed at a given heart rate drops after blood withdrawal, then increases to a higher-than-baseline value after training. The increase from baseline is only significant at 80 and 90 percent heart rates, so we can’t say that this protocol increases VO2 max, but the data is certainly suggestive of a possible effect. Measurements of heart structure and function—the outcomes that, in the researchers’ earlier meta-analysis, didn’t respond to training in older women—also showed improvement in some but not all cases.
So is this a good reason to rush out and donate blood? Endurance athletes are famously (and justifiably) reluctant to part with their hard-earned hemoglobin. The idea that, rather than trashing your training, a well-time blood donation might actually give you an extra training stimulus is interesting. If this research tempts you into donating a few weeks before the start of your next big training build-up, that seems like a win-win situation for you and for society.
As for the hypothesis that blood donation will overcome the reduced training response in older women, there are a bunch of caveats to consider. First, there was no control group that did the same training without donating blood. The authors emphasize that other studies with similar training protocols in similar populations haven’t produced big changes like this, which suggests that it’s the blood donation that sparked the magic. But still, without a direct comparison, how can we know that this particular program of intervals wasn’t simply better than the training used in previous studies?
More generally, it’s hard to interpret the nuances of heart adaptations. To be convincing, you want to know that a protocol helps women stay healthier or live longer, not that, say, it subtly changes the dimensions of one chamber of the heart. That, as the researchers acknowledge, will take more and longer-term research.
Still, whether or not their proposed answer is correct, the question posed by these researchers is an important one. Those of us who dispense exercise advice tend to glibly assume that exercise is a universal prescription that benefits everyone equally. If some groups such as postmenopausal women don’t benefit as much from the usual advice—and that’s still a big if that deserves more study—then we need to explore alternative approaches.
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