Why Exercise Is Not Optional
A plain language guide for men on hormone suppression or chemotherapy
Exercise changes your biology. It protects your muscles from wasting. It reduces the fatigue that treatment causes, not by resting through it, but by working against its causes. It improves the way your body handles insulin and energy. In some contexts, it may even influence how the tumour itself behaves.
If you have been told to exercise during prostate cancer treatment, you have probably been told it will help with energy and mood. That is true. But it is only part of the story, and not even the most important part.
Exercise changes your biology. It protects your muscles from wasting. It reduces the fatigue that treatment causes, not by resting through it, but by working against its causes. It improves the way your body handles insulin and energy. In some contexts, it may even influence how the tumour itself behaves.
This guide explains what exercise does, why it matters specifically for the treatment you are on, and what kind of exercise is most relevant for your situation.
Exercise does not just make treatment more bearable. It changes what is happening in your body at a cellular level, in ways that matter for your health now and for your resilience later.
What Exercise Actually Does in Your Body
Your body has a remarkable ability to adapt to physical demand. When you exercise, several important things happen at a molecular level.
It improves how your body handles sugar and insulin
Most men with prostate cancer have some degree of insulin resistance — their cells have become less responsive to insulin, so their bodies produce more of it. High insulin levels are a problem in prostate cancer because they activate a chain of signals that help cancer cells survive and grow, while suppressing p53, one of the body's main tumour-suppressing systems.
Exercise directly improves insulin sensitivity. It activates a protein called AMPK (think of this as the body's energy regulator), which makes cells take up glucose without needing extra insulin. Over time, regular exercise reduces the chronic high-insulin state that drives some of the biological problems associated with prostate cancer progression.
It keeps your mitochondria healthy
Mitochondria are the energy-producing structures inside your cells. Both hormone treatment and chemotherapy damage mitochondria in muscle cells, which is one reason why fatigue under these treatments is so deep and persistent. It is not just tiredness; it is a genuine reduction in your cells' ability to produce energy.
Exercise, especially aerobic exercise, stimulates the production of new, healthy mitochondria and triggers the clearance of damaged ones. This is one of the most important things exercise does that no drug can replicate. It is why men who exercise during treatment generally have more energy than those who do not, even though exercise itself requires energy.
It may influence the tumour environment
There is emerging evidence — not yet confirmed in a peer-reviewed primary paper and should be treated as promising but preliminary — that exercise produces proteins from muscle (called myokines) that can reach the tumour and affect its surrounding tissue. One pathway under investigation involves a protein called CNTF, which appears to prevent the formation of a kind of scar tissue around tumours that normally stops immune cells from getting in. If this holds in human tissue, exercise would have a direct role in making prostate tumours more visible and accessible to the immune system.
What is already established is that exercise reduces chronic inflammation — and inflammation is a major driver of the tissue changes that support tumour growth.
If You Are on Hormone Treatment (ADT)
Hormone treatment, whether LHRH injections (like Zoladex or Prostap), or tablets like enzalutamide or abiraterone, works by dramatically reducing testosterone. Testosterone is not just a sex hormone. It is a signal that maintains muscle mass, bone density, energy, mood, and metabolic health. Removing it has consequences throughout the body.
You probably already know some of these consequences. Here is what exercise can do about each of them.
Fatigue
ADT-related fatigue is partly caused by inflammation and partly by changes in how your cells produce energy. Exercise, especially higher intensity exercise, reduces the inflammatory signals that drive this fatigue and rebuilds your energy production capacity. The evidence is clear and strong: men who exercise on ADT have significantly less fatigue than those who do not.
Muscle loss
Testosterone is your body's primary signal for building and maintaining muscle. Without it, muscle breaks down unless you give it another reason to stay. That reason is mechanical load, resistance training. Lifting weights tells your muscles to maintain themselves through a completely different signalling pathway that does not depend on testosterone. Without resistance training on ADT, muscle loss is essentially inevitable over time.
Weight gain
ADT shifts the body toward fat storage, especially around the abdomen. Aerobic exercise burns fat directly and improves the metabolic signals that govern how your body distributes energy. Resistance training increases muscle mass, which raises your resting metabolic rate. Both together address weight gain more effectively than either alone.
Bone density
ADT reduces bone density because testosterone (converted to oestrogen in bone tissue) normally protects bone. Weight-bearing and resistance exercise provides mechanical stress to bone that stimulates it to maintain its density. This is important, not just for osteoporosis risk, but because bone health matters greatly if the cancer ever involves bone.
Mood and cognition
Exercise stimulates BDNF, a growth factor that supports brain cell function and mood regulation. It also releases endorphins and endocannabinoids that genuinely improve mood and reduce anxiety. The cognitive effects of ADT (sometimes called brain fog, though the mechanism is different from chemotherapy) are partially offset by regular aerobic exercise.
Hot flushes
Aerobic training improves the body's thermoregulatory systems, which reduces the frequency and severity of hot flushes in many men. This is not universal, but it is a consistent finding in exercise oncology studies.
On ADT, resistance training is not a nice-to-have. It is the primary defence against muscle loss. No supplement, no protein powder, and no drug does what progressive resistance training does for muscle preservation in the absence of testosterone.
If You Are on Enzalutamide or Abiraterone
Enzalutamide and abiraterone extend hormone suppression further. Enzalutamide blocks the androgen receptor directly even at very low testosterone levels, while abiraterone stops the body making residual testosterone in the adrenal glands and tumour tissue itself.
The effects on muscle and energy are the same as ADT but often more pronounced. Abiraterone requires a steroid (usually prednisone) which itself causes muscle breakdown over time, making resistance training even more important, not less.
Enzalutamide can cause a particular kind of fatigue that feels mental as well as physical, a cognitive slowing and heaviness that some men find more difficult than physical tiredness. Exercise still helps, but the intensity may need to be calibrated carefully. Starting at moderate intensity and building gradually is sensible. Pushing through genuine cognitive fatigue is not the goal.
If You Are on Chemotherapy (Docetaxel or Cabazitaxel)
Chemotherapy presents a different challenge. Docetaxel and cabazitaxel work by interfering with cell division, which means they affect not just cancer cells but also blood cells, gut lining, and nerve endings. The side effects reflect this: fatigue, nausea, low blood counts, and often peripheral neuropathy (numbness or tingling in the hands and feet).
The instinct when feeling this unwell is to rest. This instinct is understandable, and rest is important. But complete inactivity during chemotherapy allows a deterioration, in muscle mass, in cardiovascular fitness, in energy production, that compounds the treatment's side effects and makes each cycle harder than the last.
The approach that works is cycle-synchronised exercise.
In the week after chemotherapy, when your blood counts are at their lowest (usually days 7 to 14), keep activity gentle. Walking is fine. Avoid gyms or crowded spaces if your neutrophil count is low.
From around day 14 onward, as counts recover, increase intensity gradually. This is the time for resistance training and aerobic work.
In the week before your next infusion, you should be able to exercise at close to normal intensity.
Muscle wasting under chemotherapy
Chemotherapy drives muscle breakdown through inflammatory signals, the same pathways involved in cancer cachexia. Resistance training directly suppresses the molecular switches that trigger this breakdown. In mouse studies of prostate cancer, exercising animals maintained significantly greater muscle mass and grip strength over the course of treatment. The same principle applies in humans.
Peripheral neuropathy and balance
If you have numbness or tingling in your feet, standard weight-bearing exercise may feel unsteady or uncomfortable. Aquatic exercise, resistance work in water, removes the ground impact while preserving the muscle stimulus. Balance-specific training (standing on one leg, stability work) should be added as a regular component, both to manage falls risk and to partially compensate for the proprioceptive loss that neuropathy causes.
Fatigue under chemotherapy
This is counterintuitive but well established: exercise during chemotherapy reduces fatigue, even though it requires energy. The reason is that the fatigue is partly caused by inflammation and partly by deconditioning, and exercise addresses both. Rest alone allows deconditioning to progress, which makes the next cycle feel harder. Moderate exercise breaks this spiral.
What Kind of Exercise, and How Much
The short answer is: a combination of resistance training and aerobic exercise, most weeks of the year, adjusted for how you are feeling and where you are in your treatment cycle.
Resistance training means working muscles against load, whether weights, resistance bands, or body weight. The goal is progressive overload: gradually increasing the challenge over time so the muscle continues to adapt. Two to three sessions per week, covering the major muscle groups (legs, back, chest, shoulders), is the target.
Aerobic exercise means sustained cardiovascular work, whether walking briskly, cycling, swimming, running, or rowing. Higher intensity produces larger benefits for fatigue and cardiovascular health, but any aerobic exercise is better than none. The ERASE trial found that high-intensity interval training (short bursts of hard effort with recovery periods) was more effective for fatigue reduction than moderate continuous exercise in men on hormone treatment.
If you are currently doing very little, the most important thing is to start, not to start perfectly. Walking daily and adding some bodyweight resistance work (squats, press-ups, resistance bands) is a meaningful beginning. Build from there.
The body adapts to what it is asked to do. If it is asked to do nothing, it reduces its capacity. If it is asked to do something consistently, it maintains and builds. The goal is not to train like an athlete. The goal is to give your body a reason to stay strong.
Protein: what you may have heard and what the evidence shows
Exercise builds and maintains muscle only if the raw materials are available. Protein provides those materials. On hormone treatment or chemotherapy, your protein requirements are higher than normal because your body's anabolic signals are suppressed and muscle breakdown is accelerated.
You may have been told, or read, that protein promotes prostate cancer by raising a growth signal called IGF-1, which activates a growth pathway called mTOR. This concern is understandable, because the biological chain it describes is real. But the conclusion drawn from it is not correct.
Why the protein-cancer argument does not hold
There are two different kinds of mTOR activity in the body. One is the chronic, dysregulated kind found in cancer cells. It is driven by persistent insulin resistance and long-term hormonal signalling, and it runs continuously regardless of what you eat. This is what hormone therapy and the broader Quiet Biology protocol address.
The other kind is the normal, short-lived activity that happens in your muscles after you eat protein or complete a resistance training session. It rises, drives muscle repair and growth over the next few hours, then returns to baseline. This is the mechanism you are trying to activate. It is not the same biology as cancer-driving mTOR activity, and eating protein does not tip one into the other.
The study most often cited to support protein restriction in cancer patients found that in adults over 65, eating more protein was associated with lower cancer death rates and lower overall death rates. The same study is routinely cited to argue for protein restriction in a population it actually argues for protein adequacy.
A 2024 review of all available systematic evidence, conducted for national nutritional guidelines, found no association between protein intake and cancer risk across multiple cancer types including prostate cancer. Neither animal protein nor plant protein was associated with increased cancer risk at any level of evidence.
If there is a source worth being cautious about, it is processed meat specifically (bacon, ham, salami, processed sausages), for reasons related to preservation compounds rather than protein content. Unprocessed meat, fish, eggs, dairy, legumes, and protein supplements carry no such concern.
How much and when
A practical target is 1.6 grams of protein per kilogram of body weight per day as a minimum. Under active hormone treatment or chemotherapy, 1.8 to 2.0 grams per kilogram is more appropriate, because catabolic pressure is higher. For an 80 kg man at the lower target, that is around 128 grams per day.
This is higher than most people eat habitually. Spreading protein across meals rather than concentrating it in one sitting is more effective, and having a protein-containing meal or snack within a few hours of resistance training makes the most of the anabolic window that exercise opens.
If appetite is reduced under treatment, protein supplementation (whey, casein, or plant-based) is a practical way to meet targets without forcing large meals.
The Bottom Line
You are being asked to do something difficult under difficult circumstances. The treatments that keep prostate cancer under control also impose a significant biological cost, on muscle, on energy, on metabolism, on bone, on mood.
Exercise is not a cure. It does not replace any part of your medical treatment. But it is the most powerful tool available to you to preserve the physical substrate on which your treatment and your life depend. Muscle lost to ADT without countermeasure does not come back easily. Fitness lost to deconditioning during chemotherapy takes months to rebuild. The investment in exercise during treatment is an investment in your capacity, now, and for whatever comes next.
The biology supports this. The clinical evidence supports this. The experience of men who have done it supports this.
It is not optional. It is medicine.