Two back-to-back days of hard workouts can lower levels of anti-inflammatory compounds in the body.

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CrossFit enthusiasts may be smart to take a break after two consecutive days of hard workouts, suggests a new study, especially if they’re new to the sport. Otherwise, they may experience a temporary drop in anti-inflammatory immune system proteins.

Study author Ramires Tibana, PhD, is a CrossFitter himself. Writing in the journal Frontiers in Physiology, he notes that the fitness phenomenon—which has more than 13,000 affiliates around the world—clearly has a lot of benefits.

CrossFit has been shown to improve muscle strength, cardiovascular fitness, and body composition, he says. And its model—different high-intensity workouts every day, in a group environment with lots of community support—keeps members coming back and helps them commit to regular workouts.

But CrossFit has also been criticized for focusing on results rather than technique, which some say can fatigue muscles and raise injury risk. And Tibana, a professor at the Catholic University of Brasilia in Brazil, was curious about research that suggests that repeated bouts of high-intensity exercise can put stress on the body, temporarily impairing immunity.

So he recruited a group of nine male CrossFit participants, all of whom had been following the program for at least six months. The men did intense CrossFit-style workouts two days in a row—including Olympic lifting, power lifting, strength-training moves, and aerobic drills—aiming to finish them as quickly as possible without compromising their technique.

During and after each workout, Tibana and his colleagues measured the participants’ muscle power, as well as levels of inflammatory cytokines and metabolic markers in their blood.

The good news? Two days of intense exercise didn’t compromise the CrossFitters’ muscular strength. The bad? After day 2, participants had reduced levels of anti-inflammatory cytokines—proteins produced by white blood cells that fight off threats to the body, like illness or infection.

The study was small, only included male CrossFitters, and didn't show that the workouts definitely increased vulnerability to illness. And the results don’t mean CrossFit isn’t safe, says Tibana, but they do suggest that following the same workout schedule may not be best for everyone. Beginners, especially, may need more rest days than people with higher fitness levels and more experience.

"For non-athlete subjects who want to improve their health and quality of life through Crossfit training, we recommend that they decrease their training volume after two consecutive days of high intensity training to prevent possible immunosuppression," he says.

This is particularly important for people recovering from an illness or who already have compromised immune systems, or during times of the year when viral illnesses are prevalent. (Healthy, well-trained athletes, Tibana adds, can likely tolerate a higher workout volume without negative effects.)

If you’re new to CrossFit or want to give it a try, Tibana recommends finding a facility with trained professionals that encourages gradual progression. He also suggests taking rest days after exhaustive workout sessions, and making time for alternative recovery techniques such as massage, and gentle, restorative exercise.

Still, there’s no hard and fast rule as to how often you should sit out a workout, he says—it depends on a lot of factors, including how hard you push yourself every day. “The main concern is to control training volume and intensity,” he says. If health and safety is your main goal, he adds, aim for a combination of high- and low-intensity sessions.