If there were any justice in the world, running 60 miles or cycling 300 miles a week would entitle you to sit at your desk (or, better yet, lie on the sofa) for as long you felt like it. But about a decade ago, we started seeing more and more research about the negative health effects of prolonged sedentary behavior. Crucially, the data suggested that these effects were independent of your exercise habits: even the fittest people raised their risk of conditions like heart disease if they spent big chunks of their day sitting at their desks or in front of the TV.
I took this research to heart: I’m typing this from a sit-stand desk. But I’m nonetheless very happy to see a new study in Medicine & Science in Sports & Exercise, from Takuma Morishima and his colleagues at Hosei University in Japan, suggesting that endurance training might have some protective effects after all. The key, according to the new results, is the subtle but unmistakable difference between training and training.
There are a bunch of different theories about what makes prolonged sitting so bad, but one of them relates to the associated reduction in blood flow in your legs. Your blood vessels sense the frictional drag of blood rushing past the vessel walls, and respond by producing molecules such as nitric oxide that help keep the vessels supple and responsive. If you spend too much time sitting, this signal is reduced, and you end up with blood vessels that are stiffer and less capable of dilating and contracting in response to changes in blood flow. Over time, that leaves you more likely to develop atherosclerosis, a hardening and narrowing of the arteries, and ultimately heart disease.
You can test how responsive your blood vessels are with a technique called flow-mediated dilation. Basically, you temporarily restrict blood flow with an inflatable cuff like the ones doctors use to measure your blood pressure, then release the cuff and see how much the vessels dilate in response. If you take this measurement before and after a three-hour bout of sitting, you find that the amount of dilation is dramatically reduced after sitting—a bad sign for the health of your arteries.
That’s the protocol used in the new study, which compared 10 male cyclists from the university’s racing team with matched controls who didn’t do any regular endurance training. The graph below shows the percentage increase in blood flow through the lower leg’s popliteal artery when the cuff is released. On the left, you can see that even before sitting, the trained cyclists (black) have a somewhat bigger response than the control group (white), which is expected since endurance training enhances baseline levels of nitric oxide. But the starkest difference, on the right, emerges after three hours of sitting.
The bout of sitting almost wipes out the flow-mediated dilation response in the control group, but it barely changes in the cyclists. Hooray! I can leave my desk in the sitting position for another hour!
There are a few wrinkles, though. As Morishima points out in his discussion, the idea that regular endurance training might protect you from the ravages of sitting has been tested a few times before. And both of those previous studies failed to find any protection for the trained athletes. Morishima’s explanation is that the difference is a function of training level. In the previous studies, the “trained” group had an average VO2 max of about 50 ml/kg/min, which is above average for young adults but not exceptional. The competitive cyclists in the new study, on the other hand, had an average of 61 ml/kg/min, which is considered excellent. They’d been training for at least five years, and were averaging more than 1,500 miles of training per month.
I like that suggestion, because it raises the possibility that the same distinction applies in other situations where exercise supposedly doesn’t help—like, say, weight loss. Perhaps the big benefits only show up when you’re really pushing your limits. But when I reached out to Ryan Garten, the Virginia Commonwealth University exercise physiologist who was the lead researcher on the two earlier studies, he pointed out some other important differences between his studies and Morishima’s new study.
Morishima’s measurement of flow-mediated dilation assesses macrovascular function, which refers to the function of the big arteries pumping blood from the heart to various parts of the body. Garten’s first study used a different technique that assesses microvascular function, which refers to the function of the smaller arteries that branch off the big ones and thread into the muscles. So it may be that high fitness protects you from problems with the former but not the latter during prolonged sitting, and the differences in training level in the studies aren’t relevant after all.
Both forms of vascular function are important, but in different ways, Garten points out. The responsiveness of the big arteries is a great predictor of your risk of atherosclerosis. The responsiveness of the small arteries, on the other hand, dictates how quickly and effectively you can ramp up the delivery of oxygen-rich blood to your muscles. In fact, another new study, from researchers at Brigham Young University, recently demonstrated that microvascular function at rest predicts how much blood flows to your muscle during high-intensity exercise—an important consideration for any athlete.
For now, the data’s too sparse to draw any firm conclusions. We can tentatively suggest that endurance training at a serious competitive level might offer some protection against the negative effects of sitting on your macrovascular function, which would be good news for your long-term health. And we can suggest that it might not protect your microvascular function, which would be bad news for your athletic performance. But both those conclusions are provisional, so until further notice I’d suggest hedging your bets: train hard and try not to sit around all day.
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