Exercise has long been recommended for people going through cancer treatment to manage fatigue, preserve fitness, and support quality of life. But new research from the Fred Hutchinson Cancer Center suggests that the way exercise is commonly prescribed may be leaving one of its more powerful benefits on the table: namely its ability to sharpen the immune system's capacity to fight cancer.
The scoping review, published in JNCI Cancer Spectrum, analyzed 21 clinical trials examining the relationship between exercise training intensity and immune cell counts and function in patients with cancer. The picture that emerged was that vigorous exercise produced effects that gentler exercise often did not. "When it comes to exercise and immunity in cancer, how hard patients exercise may matter as much as whether they exercise," said lead author Dong-Woo Kang. "Across 21 trials, we saw a fairly consistent pattern: vigorous-intensity training was more likely to yield systemic immunomodulation, most prominently natural killer (NK) cell activity, while lighter intensities often produced no or minimal measurable change, especially in patients undergoing active treatment. That may have implications for how we prescribe exercise for cancer patients, rather than treating it as a one-size-fits-all recommendation."
To understand why intensity might matter, it helps to understand what exercise does to the immune system. Physical activity triggers a cascade of biological signals, including the release of stress hormones called catecholamines and muscle-derived proteins called myokines, that mobilize immune cells and alter their behavior. A single session of aerobic exercise at moderate or vigorous intensity floods the bloodstream with immune cells, particularly those with immediate capacity to detect and destroy threats. With repeated training over weeks or months, these acute responses can consolidate into lasting changes in how the immune system is calibrated.
Among the trials using vigorous-intensity exercise, four out of six reported enhanced immune activity. In men with prostate cancer awaiting surgery, a high-intensity interval training (HIIT) program increased natural killer cells within the tumor itself; and the more sessions patients completed, the more NK cells were found. In patients with chronic lymphocytic leukemia, a vigorous combined aerobic and resistance training program boosted circulating NK cell numbers and markedly increased their capacity to destroy cancer cells. And in postmenopausal breast cancer survivors, fifteen weeks of vigorous aerobic training increased NK cell cytotoxic activity compared with usual care. In patients with multiple myeloma on maintenance therapy, a five-month vigorous training program increased the proportion of activated T cells in the bone marrow, and at the end of the study, none of the exercising patients had detectable residual disease, compared to about 12% of the control group. And in patients undergoing surgery for suspected non-small cell lung cancer, a brief preoperative HIIT program shortened hospital stays and cut post-operative complications – one of the most clinically tangible signals in the review.
Lighter intensities told more variable stories. Among patients undergoing active chemotherapy or radiation, light-to-moderate exercise frequently produced no significant changes in circulating immune cells. One important exception: in esophageal cancer patients receiving neoadjuvant chemotherapy, moderate-intensity training increased CD8⁺ T cell infiltration into tumors and promoted more mature lymphoid structures around them – suggesting that lower intensities may still reshape the local tumor environment even when the bloodstream looks unchanged. The blood compartment, in particular, may require a stronger stimulus to shift.
There were important nuances. Where patients were in their treatment (before, during, or after) clearly influenced outcomes, as did cancer type. Chemotherapy and radiation suppress the immune system, which may blunt the response to exercise even at higher intensities. And some studies using moderate-to-vigorous exercise found no immune changes at all, a reminder that the evidence is still heterogeneous and is an incomplete picture.
Perhaps one of the important findings in this study is what the literature lacks. Despite decades of research on exercise and immunity in cancer, "Not a single trial has directly compared two different exercise intensities in the same group of cancer patients," Kang said. "Until we do that, we're working from indirect clues. The field needs head-to-head studies with immune outcomes prespecified and tied to clinical endpoints like recurrence or treatment response before we can confidently tell a patient that one exercise prescription is better than another." This gap matters because the trials reviewed here differed not just in exercise regimen but in cancer type, treatment regimen, immune outcomes measured, as well as patient age, comorbidities, and other host variables. Drawing firm conclusions across such heterogeneous studies is inherently limited.
The timing of this research is not incidental. Immunotherapies, drugs that unleash or amplify the immune system's own capacity to recognize and destroy tumors, have transformed cancer treatment over the past decade. If vigorous exercise can independently boost the same immune mechanisms that immunotherapy targets, the combination could be more powerful than either approach alone. "With immunotherapies becoming more widely used in cancer care, figuring out if exercise and which exercise prescriptions could amplify the immune response is the question driving our group's current work," Kang said.
His team is currently running three trials designed to answer it directly: the BOOST and ENHANCE trials (NCI) in patients with lung cancer receiving immunotherapy, and the DUO trial (World Cancer Research Fund / American Institute for Cancer Research) in melanoma, which also examines how high-intensity exercise interacts with a high-fiber diet to modulate the gut microbiome–immune axis. "Each trial is designed to ask not just whether exercise changes the immune system during cancer treatment, but whether those changes ultimately translate into better outcomes for patients."
That translation from immune marker to clinical outcome remains the field's most pressing unanswered question. What this study makes clear is that exercise is no longer just a supportive care recommendation. It is a biological intervention with a dose; and getting the dose right may prove as important as getting patients moving in the first place.
This research was supported by the Cancer Prevention Research Institute of Texas.
Fred Hutch/University of Washington/Seattle Children’s Cancer Consortium Member Dr. Dong-Woo Kang contributed to this research.
Kang, D. W., Courneya, K. S., Swartz, M. C., Maleki Vareki, S., Gordon, N. B., Rosa Neto, J. C., Simpson, R. J., Baker, K. S., Schadler, K. L., & LaVoy, E. C. (2026). Chronic exercise training intensity, immune cells, and cancer outcomes: a scoping review. JNCI cancer spectrum, 10(2), pkag021.
Science Spotlight writer Darya Moosavi is a postdoctoral research fellow within Johanna Lampe's research group at Fred Hutch. Darya studies the nuanced connections between diet, gut epithelium, and gut microbiome in relation to colorectal cancer using high-dimensional approaches.