We all know a sprint feels different than a longer, take-it-slow kind of workout.
Your heart is pounding. You gasp for air. Maybe your muscles burn.
Now, new research reveals this kind of high-intensity training provokes a remarkably robust response throughout the bloodstream. The training substantially mobilizes more proteins and other molecules in the hours after exercising.
The findings were published by a team of scientists in the U.S., Australia and Sweden. The results do not mean that sprint workouts and other forms of high-intensity interval training, or HIIT, are better than moderate exercise where your heart rate remains at a steady level.
But the data does suggest that pushing yourself in these all-out, short bursts of effort may yield unique benefits for health and fitness, according to Dr. Paul Cohen, whose lab at Rockefeller University led the study, which recently appeared in the journal Cell Reports Medicine.
In a series of experiments, Cohen and his colleagues collected blood from young, healthy adults before and after they exercised, and combed their blood for thousands of proteins and other metabolites.
For example, participants were divided into two opposing camps: One completed six sets of a 30-second maximum effort on the bike, each spaced by 4 minutes of rest.
The others did a 90-minute moderate-paced ride.
When they ran their analysis, it was clear the sprint group had "many, many more changes," explains Cohen. "What we can conclude is that sprinting does have a greater effect on remodeling the molecules in the bloodstream."
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The practical implications of those changes need to be teased apart in future trials; however, the study did find some initial clues. They overlaid the results from their study participants with a massive database of 50,000 people, collected from the U.K. Biobank.
That evaluation showed the proteins that were altered in the sprinters were associated with improved cardiovascular and metabolic health.
In another part of the study, the research team saw that fat cells behaved differently when drenched with blood collected from the HIIT folks, including changes involved in many genes that regulate fat and whole-body metabolism.
"That's further evidence and much more causal evidence that something in the blood following sprint training can remodel the molecular landscape of other tissues, like fat tissue," Cohen says.
This latest work reflects a broader effort in the field of precision medicine to unpack all the ways molecules — in blood vessels, muscles, the liver and other organs — respond to exercise, including different intensities.
"We know now there's very orchestrated events happening under the hood during exercise," says Scott Trappe, director of the Human Performance Lab at Ball State University. "All these organs are communicating, it's like a super highway and the circulation is that highway."
Trappe is also an investigator in a National Institutes of Health initiative to uncover the molecular inner workings of physical activity. He says this new data clearly shows that "there are some potential benefits you're leaving on the table if you don't do high-intensity exercise."
Though he and other experts emphasize that HIIT may not be practical, or even safe, for everyone.
"I don't view this in the light of high intensity versus moderate," he says. "There's a continuum and all of this stuff is good."
In the study, the participants were young, physically fit, and predominantly males, and that limits how much you can extrapolate to other groups, especially those who are older and less conditioned.
The cycling protocol itself — six sets of 30-second maximum sprints — could prove enormously taxing for the average person, he notes.
Overall, the human studies comparing high- and moderate-intensity exercise suggest they're largely comparable to each other when you look at measures of fitness and other areas of health, says Todd Astorino, an exercise physiologist at California State University, San Marcos. He has spent almost two decades studying HIIT.
The real upside with high-intensity training — assuming you're healthy enough to do it — is how much you benefit from a small investment of time, he explains.
"Sometimes, I think people worry too much about time, or frequency," he says. "If you amp up the intensity, you can do a hell of a lot less of that and still get meaningful adaptations and responses."
For example, Astorino says new data from a grant he's involved in shows two sprints a day totaling less than 40 seconds can lead to meaningful improvement in VO₂ max – a measure of cardiorespiratory fitness – within just 6 to 8 weeks.
The work from Cohen's lab and others is all driving at the goal of eventually developing nuanced exercise prescriptions — and possibly drugs — that can be tailored to the individual.
"Perhaps some individuals have a greater response to sprint training, whereas others might do better with moderate intensity training," he says. "This is just the first step towards providing those kinds of recommendations."
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