Red Blood Cells, Ketones, and the Endurance Hype: What Cyclists Actually Need to Understand

In cycling, we love to chase the thing behind the thing. We look at power numbers, heart rate, lactate, glucose, heat adaptation, altitude camps, ketones, sodium bicarbonate, beet juice, caffeine, recovery drinks, sleep scores, HRV, and every new bottle, patch, powder, or metric that promises to make us faster. But underneath all of that, there is one very old, very basic, very powerful truth: your body has to move oxygen.

One of the most important players in that entire system is the red blood cell. For cyclists, red blood cells are not just a lab value that shows up on blood work. They are part of the engine. They influence how well oxygen gets from the lungs into the working muscles, and they are tied to endurance, fatigue resistance, altitude adaptation, recovery, iron status, hydration, and the long history of why athletes have always been obsessed with “more oxygen.”

Once we understand oxygen, we can naturally move into another hyped area of endurance performance: ketones. Ketones, like red blood cells, sit in that fascinating space between real science and over-marketed athlete obsession. They can matter. They can help in the right context. But they are not magic, and they do not replace the actual foundation of endurance: training, oxygen delivery, carbohydrates, sleep, recovery, and consistency.

What Red Blood Cells Actually Are

Red blood cells, also called erythrocytes, are tiny, flexible, disc-shaped cells that move through your bloodstream carrying oxygen. Their main job is simple but absolutely essential. They pick up oxygen in the lungs, transport it through the blood, and deliver it to the tissues that need it. For cyclists, those tissues are often the working muscles in your legs, along with your heart, lungs, and brain.

Inside each red blood cell is hemoglobin, the iron-containing protein that binds oxygen. This is why iron matters so much. Iron is not just something you take because someone told you endurance athletes are often low. Iron is literally part of the hemoglobin structure that allows oxygen to be carried from your lungs to your muscles. Without enough healthy hemoglobin, the body has a harder time moving oxygen efficiently, and for an endurance athlete, that can show up as fatigue, breathlessness, poor recovery, heavy legs, and a frustrating loss of power.

When you are riding endurance pace, climbing for an hour, racing gravel, doing a long bikepacking push, or trying to stay steady deep into a fatigue state, your body is constantly asking the same questions. Can I keep delivering oxygen? Can I clear carbon dioxide? Can I keep producing ATP? Can I maintain enough blood flow to the working muscles? Red blood cells are part of that answer. They do not create fitness by themselves, but they help support the internal environment where aerobic fitness can actually show up.

How Red Blood Cells Are Made

Red blood cells are made through a process called erythropoiesis. Most red blood cell production happens in the bone marrow, where stem cells gradually mature into red blood cells. As they develop, they build hemoglobin, lose their nucleus, and eventually enter the bloodstream as functioning oxygen-carrying cells.

The hormone that tells the body to make more red blood cells is erythropoietin, better known as EPO. EPO is mainly produced by the kidneys. When the kidneys sense that blood oxygen is low, they increase EPO production. EPO then signals the bone marrow to make more red blood cells.

This is one of the reasons altitude training became so famous. At altitude, the air contains less oxygen pressure. Your body senses that lower oxygen availability and responds by increasing EPO, which can stimulate red blood cell production over time. That is the natural version of what athletes have tried to manipulate for decades, and this is where the science becomes both fascinating and complicated.

Why Cyclists Care So Much About Red Blood Cells

Cycling is brutally dependent on oxygen delivery. Yes, there are moments where the sport becomes highly anaerobic. A sprint, a hard attack, a steep punchy climb, or a VO2 max interval all require high-intensity energy production. But even then, your ability to recover between hard efforts still depends heavily on aerobic metabolism.

The stronger your oxygen delivery system, the better you can sustain work. The better you can clear byproducts. The better you can keep your muscles fueled. The better you can resist that deep, whole-body fatigue that shows up after hours of riding. This is why hemoglobin, hematocrit, ferritin, red blood cell volume, plasma volume, and oxygen-carrying capacity all matter in endurance sport.

But here is the key nuance: more is not always better. A healthy, well-supported blood system is good. A manipulated, overly concentrated, or artificially boosted blood system can become dangerous. The goal is not to make your blood “thicker.” The goal is to support healthy oxygen transport. That difference matters.

Factors That Can Change Red Blood Cell Count

Red blood cell count can shift for many reasons. Some are normal. Some are training-related. Some are nutrition-related. Some are medical. Some are environmental. For cyclists, the biggest factors include altitude, iron status, training load, hydration, menstrual health, inflammation, illness, stress, and kidney function.

Altitude is one of the most well-known natural triggers for red blood cell adaptation. When oxygen availability drops, the body increases EPO signaling. Over time, this can support increases in hemoglobin mass and red cell volume, although the response varies from athlete to athlete. This is why altitude camps are not automatically magic. Some athletes respond beautifully. Some feel flat. Some get sick. Some train too hard and never absorb the adaptation. Some go to altitude without enough iron stores and cannot fully support red blood cell production. Altitude is a stress, and it can be useful, but only if the body has the resources to adapt.

Iron status is another major factor. Iron is one of the most important nutrition-related pieces for cyclists because if iron stores are low, the body may struggle to produce enough hemoglobin. You may be training hard, doing all the workouts, fueling well, and sleeping enough, but if iron availability is low, oxygen transport can still suffer. This does not mean every cyclist should blindly take iron. Iron supplementation should be based on blood work and professional guidance because too much iron can be harmful. The smart approach is to check ferritin, hemoglobin, hematocrit, transferrin saturation, and related markers with a qualified medical provider. For athletes, especially menstruating athletes, vegan or vegetarian athletes, ultra-endurance athletes, and those training at altitude, this is not a luxury. It is part of performance health.

Endurance training itself also changes the blood. One of the most interesting things about endurance athletes is that they often develop expanded plasma volume. This means the liquid portion of the blood increases. That can actually make hemoglobin concentration or hematocrit look lower on a standard blood test, even when total blood volume and performance capacity are strong. This is sometimes referred to as “sports anemia,” although it is not always true anemia. It can simply be dilution from expanded plasma volume. This is why interpreting labs in athletes requires context. A cyclist who trains 15 hours a week may not look the same on paper as a sedentary person. The numbers need to be interpreted with the athlete’s training load, symptoms, nutrition, hydration, menstrual status, altitude exposure, and recent fatigue in mind.

Hydration also changes blood concentration. If you are dehydrated, plasma volume drops and your blood becomes more concentrated. That can make hematocrit appear higher. If you are well hydrated or have expanded plasma volume from training, blood values may look lower by concentration. This is another reason one blood test never tells the whole story. Blood values are a snapshot. They are not the entire athlete.

For female athletes, menstrual blood loss can influence iron status and, over time, red blood cell health. Heavy menstrual bleeding can increase the risk of iron deficiency, and this matters deeply in cycling because the symptoms can be subtle at first. You might not feel “sick.” You might just feel like your top end disappeared. Your legs feel heavy. Your heart rate feels strange. You are more breathless than normal. You cannot recover between efforts. You feel flat on climbs that normally feel manageable. This is why female endurance athletes need better education around blood work, iron, ferritin, fueling, and menstrual health. Not as an afterthought, but as performance physiology.

Illness, inflammation, and stress can also affect red blood cell health. Inflammation can alter iron metabolism, and when the body is under stress, sick, or fighting infection, it may change how iron is stored and used. This can make performance feel off even before obvious illness hits. Chronic inflammation, poor recovery, underfueling, and excessive training stress can all disrupt the normal rhythm of adaptation. You cannot bully your blood into adapting. You have to give it the raw materials and recovery environment to build.

Because the kidneys produce most of the body’s EPO, kidney function also matters for red blood cell production. For most healthy athletes, this is not something to obsess over, but it is a reminder that red blood cells are not just about iron. They are part of a whole-body system involving the kidneys, bone marrow, hormones, oxygen sensing, nutrition, and recovery.

The Athlete Hype: Why Blood Became So Famous in Cycling

Cycling has a complicated history with blood. That is the honest truth. The reason red blood cells became so hyped is because they are directly connected to oxygen delivery, and oxygen delivery is directly connected to endurance performance. If you can move more oxygen to working muscles, you can often sustain higher aerobic power.

That is why athletes, coaches, scientists, and unfortunately dopers became obsessed with red blood cell mass, hematocrit, hemoglobin, EPO, transfusions, altitude tents, and every possible way to influence oxygen transport. The hype did not come from nowhere. It came from real physiology. Increasing oxygen-carrying capacity can improve endurance performance, but the illegal and unethical manipulation of that system comes with major risks and violates the foundation of fair sport.

Blood that becomes too concentrated can increase cardiovascular strain. It can increase clotting risk. It can damage athlete health. It also damages the trust that sport is built on. So when we talk about red blood cells, we need to separate two very different things: the healthy pursuit of better oxygen delivery through training, nutrition, recovery, and altitude exposure, and the dangerous, banned manipulation of blood through doping. One is performance health. The other is cheating and risk.

How Cyclists Can Support Red Blood Cells Naturally

The best way to support red blood cell health is not dramatic. It is actually very boring, and boring is usually where the good stuff lives. Eat enough. Recover enough. Sleep enough. Fuel enough. Check your blood work when needed. Support iron status intelligently. Do not chase hacks before you have built the foundation.

Underfueling is one of the fastest ways to sabotage adaptation. Your body cannot build red blood cells, repair muscle, support hormones, maintain immune function, and produce training adaptations if it is constantly in an energy deficit. Cyclists often think they need to be lighter, but sometimes the real performance gain is not losing weight. Sometimes it is finally giving the body enough energy to train, recover, and build.

Iron-rich foods are also important. These include red meat, poultry, fish, eggs, lentils, beans, tofu, spinach, pumpkin seeds, fortified grains, and other whole foods. Heme iron, found in animal foods, is generally more easily absorbed, but non-heme iron from plant foods can still be valuable, especially when paired with vitamin C. Lentils with bell peppers, spinach with citrus, beans with tomatoes, and tofu with broccoli are all simple examples of building meals that support iron absorption. Iron absorption can be reduced when taken with large amounts of calcium, coffee, or tea around the same meal, so timing can matter for athletes who are working to improve iron status.

It is also important to check ferritin, not just hemoglobin. This is huge. A person can have normal hemoglobin but low ferritin. Ferritin reflects stored iron, and low ferritin can show up before full anemia develops. That means an athlete may feel flat, heavy, tired, breathless, or unable to hit normal power even while being told their “blood work is normal.” For cyclists, this distinction matters, especially for female athletes, during big training blocks, at altitude, during pregnancy or postpartum phases, and in ultra-endurance training.

Altitude can help, but only when done correctly. You need enough time, enough recovery, enough fuel, enough iron availability, and enough patience to let the body adapt. You also need to avoid turning every ride into a survival effort. The “live high, train low” model became popular because it allows athletes to live in a hypoxic environment while still completing higher-quality training sessions at lower elevation. But again, individual response varies. Altitude should not be treated like a guaranteed upgrade. It is a tool.

Sleep is another major piece. Red blood cell production, hormone regulation, immune repair, and adaptation all depend on recovery. Sleep is not separate from performance. Sleep is where performance gets built. If you are chasing every supplement but sleeping five hours a night, you are skipping the biggest legal performance enhancer you have.

And finally, do not chase high hematocrit just for the sake of a higher number. You do not want to simply chase a higher red blood cell count. You want healthy oxygen delivery. You want good iron status, strong hemoglobin, adequate plasma volume, good vascular function, and the ability to train and recover. A higher number is not always a better athlete. The body is a system, not a spreadsheet.

How Better Red Blood Cell Health Helps You Become Stronger

When your red blood cell system is healthy, your aerobic engine can function more effectively. That means more oxygen delivery to working muscle, better support for mitochondrial energy production, better ability to sustain tempo, sweet spot, threshold, and long endurance efforts, better resilience during long climbs, better recovery between hard surges, better ability to stay mentally sharp late in a ride, and better durability when the ride goes from three hours to six hours to twelve hours and beyond.

But red blood cells do not replace training. They support training. You still need the workouts. You still need progressive overload. You still need fueling. You still need strength. You still need rest. Red blood cells are part of the delivery system. They bring oxygen to the fire, but you still have to build the fire.