Exercise is increasingly being recognized not just for its role in fitness, but as a systemic therapy with the potential to influence cancer biology. 
 

A recent study in breast cancer survivors adds weight to this idea by showing that a single workout—whether resistance training or high-intensity intervals—can acutely alter circulating blood factors and directly suppress the growth of cancer cells in the lab.

It adds yet another piece of mechanistic evidence to the case for physical activity (any type!) as a critical part of both cancer prevention and care. In today's email, we'll break down the study and discuss its implications for cancer prevention and treatment, as well as the broader role of exercise at every stage in the cancer continuum.

Fighting Cancer Cells with Strength Training and HIIT

The researchers recruited 32 women who had previously been diagnosed with stage I–III breast cancer (on average 29 months earlier) and who had completed treatment at least four months prior. Participants were randomly assigned to perform one of two exercise protocols: resistance exercise or high-intensity aerobic exercise (HIIT). The resistance training group completed 5 sets of 8 repetitions of 8 standard exercises (chest press, seated row, shoulder press, lat pulldown, leg press, leg extension, leg curl, and lunges) at about 80% of their one-repetition maximum, corresponding to an exertion level of 7–9 out of 10. 

The HIIT group performed seven bouts of 30 seconds of high-intensity exercise (30 seconds recovery) for 4 sets, using at least three different machines such as the treadmill, rowing machine, or stationary bike. Intensity was matched at a 7–9 out of 10 effort, or 70–90% of age-predicted maximum heart rate.

Blood samples were collected before exercise, immediately after, and 30 minutes later to measure concentrations of four myokines with known or suspected anti-cancer properties: interleukin-6 (IL-6), decorin, oncostatin M (OSM), and SPARC. Serum from these samples (at all 3 timepoints) was then applied to a triple-negative breast cancer cell line (MDA-MB-231) for 72 hours to assess cell growth.

Both exercise modes triggered measurable increases in myokines. After resistance training, decorin rose by 23%, IL-6 by 9%, and SPARC by 15% immediately post-exercise, with further increases in IL-6 and OSM seen after 30 minutes (a 7% and 23% increase, respectively). 

HIIT induced even larger acute shifts: decorin increased by 30%, IL-6 by 47%, and SPARC by 26%, with IL-6 still elevated by 13% 30 minutes later. In head-to-head comparison, HIIT produced a stronger rise in IL-6 than resistance training, but the other differences, while numerically larger for HIIT, weren't statistically significantly different.

Why Exercise is So Strongly Anti-Cancer

This isn’t the first study to show that exercise has anti-cancer effects, but it is one of the few in breast cancer—the most common cancer among women. And it's the first to directly compared aerobic and resistance training. It adds yet another piece of mechanistic evidence to the case for physical activity (any type!) as a critical part of both cancer prevention and care. Broadly, exercise exerts its anti-cancer effects through three main routes.

The first is myokines. Exercise turns skeletal muscle into an endocrine organ—a small molecular pharmacy that dispenses dozens, if not hundreds, of signaling molecules known as myokines (and other “exerkines”). These messengers are often packaged in extracellular vesicles and carried through the bloodstream to tumors and immune organs. There, they can:

• Activate anti-tumor immunity
• Directly stress or reprogram cancer cells
• Remodel the tumor microenvironment
• Shift the body toward anti-inflammatory, anti-growth signaling

This anti-tumor activity appears immediately after exercise and strengthens with regular training. For example, in one 12-week training study in men with prostate cancer, serum collected after exercise showed elevated levels of the myokine OSM and directly suppressed prostate cancer cell growth in vitro.

Exercise also exerts a direct physical force on cancer cells. As blood flow increases during exercise, so does hemodynamic shear stress—the frictional force of moving blood. Circulating tumor cells (or CTCs for short) are unusually fragile compared to normal blood cells and can be pushed into apoptosis or necrosis (cell death) by these forces. Experiments simulating exercise-level shear stress have killed up to 90% of CTCs, including those from breast, lung, and ovarian cancers.

At the same time, exercise mobilizes natural killer (NK) cells and other cytotoxic immune cells into the bloodstream. These cells can destroy CTCs directly, and the high-shear environment seems to make them even more effective. Importantly, not all shear is lethal—vigorous, variable blood flow produces the most hostile conditions for CTCs, underscoring the potential added value of higher-intensity exercise.

Finally, exercise provides protection across the entire cancer continuum—before, during, and after treatment. Roughly 40% of cancers could be prevented (according to Dr. Kerry Courneya who appeared on episode #99 of the FoundMyFitness podcast) if people adhered to optimal lifestyle behaviors. Exercise consistently ranks just behind smoking cessation, maintaining a healthy weight, and moderating alcohol in importance. Epidemiological studies show that regular physical activity reduces the risk of several cancers—including colon, breast, and endometrial—by 20–30%.

For those who do develop cancer, years of activity serve as prehabilitation, building cardiovascular fitness, muscle mass, and resilience that improve tolerance for surgery, chemotherapy, or radiation. A lifetime of movement may not always prevent cancer, but it can delay its onset, reduce its aggressiveness, or improve early detection. Entering treatment fit is like carrying reserves into battle. 

During treatment, exercise is one of the most effective tools patients have. It consistently reduces fatigue, improves sleep, and helps people complete more of their chemotherapy or radiation on schedule and at full dose—a critical determinant of long-term survival. Aerobic training enhances blood flow and tumor oxygenation, improving delivery of chemotherapy and sensitivity to radiation, while resistance training preserves lean mass and may even aid drug metabolism. Far from undermining treatment, exercise often makes it more effective—a direct challenge to the outdated advice to “rest and take it easy,” which paradoxically worsens fatigue and weakens outcomes. After treatment, exercise lowers recurrence risk, bolsters immune surveillance, and counters long-term side effects such as cardiovascular disease, osteoporosis, and metabolic dysfunction—conditions often accelerated by cancer therapies themselves.

Final thoughts

We're far from being able to claim that "exercise cures cancer" or even that it can prevent it from occurring. Genetics, environment, and several other factors interact to determine each individual's susceptibility to cancer.

But there's no doubt that being physically active vastly lowers your risk of developing any of several cancer types. That means no matter your baseline risk, it will be lower if you exercise. And as we've seen today, every single bout of exercise contributes toward lowering cancer risk.

I think that's a powerful take away, and just one of the many reasons to regularly engage in exercise. The next time you hit the gym or go for a run, try to imagine the powerful myokines releasing from your muscles, circulating throughout the body to wage war on cancer cells.

Warm regards

— Rhonda and the FoundMyFitness team