Matt on Beach doing HIIT

Researchers Uncover New Reasons Not to Overdo HIIT

There’s a good chance you came across the following headline, or another one like it, a couple of weeks back: “Too Much High-Intensity Exercise May Be Bad for Your Health.” These click-baiting newsflashes referred to a new study out of Sweden’s famed Karolinska Institute that looked at the molecular and metabolic effects of a HIIT program in previously casual adult exercisers. Eleven unlucky volunteers were subjected to an utterly brutal regimen that ramped up to five HIIT sessions per week within three weeks, each session comprising a mix of four- and eight-minute intervals performed at maximum intensity. The consequences of this diabolic torture program included severely compromised mitochondrial function and erratic blood sugar levels.

My first thought on reading the above-referenced headlines was duh. In more than 20 years of serious endurance training I never attempted anything approaching the savagery of what those poor, unsuspecting guinea pigs underwent in a state of woeful unpreparedness. I know with 100 percent certainty that the same routine would have utterly steamrolled me even if I’d gone into it at my absolute lifetime peak of fitness. Hilariously, though, the negative effects seen in this study’s subjects were referred to in one article as “unexpected.” Is the popular media really that clueless? I guess so!

Coincidentally, these shockingly predictable fresh findings from the Karolinska Institute came fast on the heels of a highly complementary recent study by scientists at the University of Guelph. For this experiment, 23 overweight, sedentary men were separated into groups, one of which did three HIIT workouts per week on stationary bikes while the other did five longer, low-intensity workouts. Versions of this format had been used in many prior studies, but what was different this time was that the two exercise programs were not matched for total workload (i.e., total energy expenditure).

The purported rationale for matching workloads in past research was fairness. The scientists conducting these experiments wanted to see whether low-intensity or high-intensity exercise was more “effective,” and in their minds this required that total energy expenditure be held equal. In my mind, however, the format unfairly disadvantaged low-intensity exercise, for the thing about HIIT is that a little goes a long way, so of course it’s going to seem more effective if a little HIIT is compared against a little low-intensity exercise, which only has a chance to really shine in large amounts, which (unlike large amounts of high intensity) are well tolerated by the human body.

Anyway, in the new Guelph study, common sense prevailed at long last, and the low-intensity and high-intensity programs were balanced in a more realistic way. And wouldn’t you know, the low-intensity program kicked the HIIT program’s ass in terms of health benefits, yielding bigger improvements in body composition, lipid metabolism, blood pressure, and blood sugar regulation.

I’m now waiting for it to finally cross the minds of exercise scientists to investigate the health effects of mixing together workouts of different intensities as we endurance athletes do. Up to this point, researchers interested in the health effects of aerobic exercise have focused entirely on trying to figure out which intensity is “best,” but to me that’s a little like trying to determine which of a baseball pitcher’s five pitches is his best pitch with a view toward having him throw only that pitch going forward. What would happen in this scenario, of course, is that hitters would know exactly what was coming their way every time the pitcher wound up, and all of a sudden his best pitch wouldn’t be so effective anymore!

Okay, that’s a poor analogy, but you get my point. Sometimes things work better in combination than they do in isolation. And we already know this is true of exercise intensities when they are assessed according to their effects on fitness. Specifically, we know that an exercise program comprising an 80/20 balance of low intensity and moderate/high intensity yields greater gains in endurance fitness and performance than a program consisting entirely (or even mostly) of work at either low intensity or moderate/high intensity.

Would an 80/20 program also yield greater health benefits than the alternatives? It seems likely to me that it would. It’s been shown, for example, that 80/20 training yields greater improvements in aerobic capacity than does any other way of balancing intensities, and we know that aerobic capacity is a strong predictor of health in old age and longevity.

What are the chances, though, that 80/20’s factual superiority will result in its popularization beyond the endurance community? Rather slim, I would imagine. One of the reasons HIIT became so popular is that interval sessions are sexier than slow-and-steady aerobic workouts. Can you picture a bunch of folks gathering for a group fitness class where the instructor pedals nice and easy for 80 minutes straight and invites everyone else to do the same? Yeah, me neither. But one can dream.