Blood Volume Expansion in Athletes: Dilutional Pseudoanemia (Sports Anemia) Explained

Introduction

When I see athletic patients, they are often highly aware of anemia and iron deficiency, and may become concerned when they notice that their hemoglobin is unexpectedly low on routine blood tests. However, in a well-trained endurance athlete, there is a high probability that this reflects something fundamentally different: dilutional pseudoanemia, often referred to as sports anemia, a direct consequence of one of the most elegant cardiovascular adaptations in human physiology.

Understanding blood volume expansion — what drives it, what it does to standard blood markers, and how to distinguish it from true pathology — is one of the more clinically important competencies in sports medicine. Coaches and clinicians who misread these results intervene when they shouldn’t, or miss the real problem entirely.

In this article, I explain the physiology of blood volume expansion in trained athletes, why it causes hemoglobin and hematocrit values to fall, why that fall is beneficial rather than harmful, and what the clinical assessment should actually look like.

Why Athletes Have More Blood

Endurance training produces a well-documented and reproducible increase in total blood volume — a state of physiological hypervolemia [1]. This is not incidental. It is one of the primary cardiovascular adaptations driving improvements in VO₂max, stroke volume, cardiac output, and thermoregulatory capacity [2].

The expansion occurs in two components. In the first two to four weeks of intensified training, plasma volume accounts for virtually all of the increase in total blood volume [1]. Beyond that initial window, the expansion distributes more equally between plasma volume and red cell volume, as sustained exercise also stimulates erythropoiesis. The net result, across months of consistent endurance training, is a substantially expanded total blood volume — in one study of male endurance athletes, plasma volumes were 37.5% higher than those of sedentary controls [3].

This can often be misleading for clinicians, as the increase in plasma volume makes hemoglobin appear lower — even though total hemoglobin mass has not decreased. In routine clinical practice, we do not have practical ways to measure hemoglobin mass directly; instead, we rely on indirect markers. More precise measurement methods are generally limited to research settings and specialized care.

The Mechanism: Hormones, Albumin, and Sodium Retention

The plasma expansion is not passive. It is driven by a coordinated hormonal response that begins with each training session. Exercise acutely elevates plasma renin activity — in the Convertino et al. study, renin activity rose approximately ninefold during training sessions [4]. This renin-angiotensin-aldosterone cascade stimulates renal tubular sodium reabsorption, drawing water into the intravascular space and expanding plasma volume.

Simultaneously, plasma albumin content rises progressively across training days [4]. Albumin accounts for 60–80% of the colloid osmotic pressure in the intravascular compartment [5], so increased albumin effectively anchors additional fluid inside the blood vessels. Arginine vasopressin (AVP) and aldosterone act in concert with these albumin shifts to sustain and consolidate the expansion [6].

The scale of this expansion matters. Research demonstrates that plasma volume can increase by 9 to 25% with sustained endurance training, corresponding to an additional 300 to 700 mL of circulating plasma [6]. A single intense session can begin shifting plasma volume upward within 24 hours; chronic training consolidates and amplifies this effect over weeks [7].

In some cases, plasma volume is also manipulated artificially. In blood doping, erythropoietin (EPO) may be used to stimulate erythropoiesis and increase red cell mass. In certain situations, this can be combined with plasma expanders such as hydroxyethyl starch (HES), which dilute the blood and lower hemoglobin concentration and hematocrit, potentially making abnormal values less apparent. Unfortunately, this phenomenon is well known in Finland due to past doping scandals in cross-country skiing, where similar mechanisms became familiar to the public.

What Is Sports Anemia?

Sports anemia is a commonly used term to describe the dilutional reduction in hemoglobin concentration seen in endurance athletes. Despite the name, it does not represent true anemia, but rather a physiological adaptation driven by plasma volume expansion. In clinical practice, this distinction is essential, as sports anemia does not require treatment unless true iron deficiency or another underlying cause is present.

Here is where the clinical misinterpretation occurs. Standard laboratory reports measure hemoglobin concentration and hematocrit as ratios — grams per deciliter and percentage of volume occupied by red cells, respectively. When plasma volume expands substantially but red cell mass has not yet caught up, those ratios fall — even though the absolute number of red cells and the total hemoglobin mass in circulation may be entirely normal or even increased[8]. This is exactly why interpreting the RBC count in isolation is similarly misleading in endurance athletes, as discussed in detail in the article on RBC count in endurance athletes.

This is dilutional pseudoanemia. The blood has been diluted — not depleted. The condition has also been called “sports anemia,” which is an acknowledged misnomer in the peer-reviewed literature [9]. As Eichner noted in the foundational description of this phenomenon, sports anemia is a false anemia and a beneficial adaptation to aerobic exercise, caused by an expanded plasma volume that dilutes red blood cells [9].

The absolute hemoglobin mass, meanwhile, is stimulated upward by endurance training. Physical effort drives erythropoiesis [8]. The problem, in the early and middle phases of training adaptation, is that plasma expansion is faster — it outpaces the erythropoietic response [10]. The concentration values in the blood test therefore drop, even though the athlete is physiologically healthier than their reference-range-normal sedentary counterparts.

Although my athletic patients are often concerned about their hemoglobin levels, it’s best to explain to them that the concentration itself is not the most important factor — total hemoglobin mass is what truly matters. In fact, their total hemoglobin mass is often higher than that of less-trained individuals. Unfortunately, in routine clinical practice we are limited to these simple concentration-based laboratory tests. As a result, we rely on assessing iron parameters, and if those are within normal range, we can reasonably conclude that there is no iron deficiency and that the apparent anemia is most likely not a true pathological anemia.

Why This Is Actually an Advantage

The expanded plasma volume — and the consequent mild reduction in hematocrit — confers several performance benefits. These are not side effects of training; they are among the primary mechanisms of endurance adaptation.

First, cardiac preload is enhanced. Greater intravascular volume increases ventricular end-diastolic filling, amplifying stroke volume through the Frank-Starling mechanism [2]. Endurance-trained athletes show total blood volumes of approximately 76 mL/kg compared to 64 mL/kg in sedentary peers, and this difference is directly associated with higher peak stroke volumes and higher VO₂max values [2].

Second, hemorheological properties improve. Blood viscosity decreases with hemodilution, reducing resistance to flow in the microcirculation and improving oxygen delivery to working muscle [8]. Research in aerobically trained athletes demonstrates that this training-associated hemorheological profile promotes superior oxygen transport compared to resistance-trained athletes with higher hematocrits [11].

Third, thermoregulatory capacity is improved. A larger plasma volume means more body fluid is available for heat dissipation through sweating, maintaining cardiovascular stability during prolonged exercise in the heat [1].

Taken together, often mildly low hemoglobin concentration and hematocrit are actually markers of superior cardiovascular conditioning — not evidence of disease. This is one of the core reasons why standard laboratory reference ranges routinely misclassify well-trained athletes, a problem explored more broadly in the article on endurance athlete blood ranges.

It is important to remember that the dilutional effect is usually relatively modest. Plasma volume typically expands by about 9–25% with sustained endurance training[6]. Therefore, if a patient is clearly anemic, it is rarely appropriate to attribute the findings to dilution alone. A useful parallel can be seen in pregnancy, a setting where physiological hemodilution is well recognized among clinicians. In that context, we do not attribute clearly abnormal hemoglobin values to dilution alone, but instead evaluate for underlying causes such as iron deficiency. The same principle applies here — dilution may explain mild changes, but significant anemia should always prompt further assessment.

Distinguishing Pseudoanemia from True Pathology

The clinical challenge is that the same superficial blood picture — low hemoglobin, low hematocrit — can arise from genuine iron deficiency anemia, which does impair performance and does require intervention. The distinction requires assessing the full picture, not just concentration values.

In dilutional pseudoanemia, total hemoglobin mass is normal or elevated, red cell morphology is normal (normocytic, normochromic), and iron stores are adequate. Ferritin, serum iron, and transferrin saturation are within normal range. Performance is typically not impaired. MCV behavior in trained athletes adds another layer of nuance here — for a detailed breakdown, see the dedicated article on MCV changes in athletes.

In true iron deficiency anemia, ferritin is depleted, erythrocyte distribution width (RDW) rises, and mean corpuscular volume (MCV) falls as microcytic cells appear [10]. Iron-deficiency anemia is the most common true anemia in athletes and does demand treatment [9].

A Swiss Society of Sports Medicine consensus statement provides useful clinical reference points: in adult athletes, ferritin below 15 µg/L indicates empty iron stores, values between 15 and 30 µg/L indicate low stores, and in adult elite athletes preparing for altitude training, a ferritin threshold of 50 µg/L is recommended as a practical target[10]. These thresholds are higher than general population references precisely because athlete physiology demands more — the full reasoning behind them is covered in the article on ferritin levels for athletes.

It is also worth noting that ferritin itself is an acute-phase reactant — it rises with inflammation and can appear falsely normal or even elevated in an iron-deficient athlete recovering from intense training. If the clinical picture doesn’t fit, additional markers such as soluble transferrin receptor (sTfR) can help clarify iron supply to the bone marrow regardless of inflammatory status. The role of sTfR in athletic iron assessment is covered in detail in the article on soluble transferrin receptor in athletes.

In patients who are clearly anemic — whether athletes or not — I typically start with a basic iron panel. In Finland, a standard initial workup often includes a complete blood count and ferritin, while more comprehensive panels may also include transferrin or total iron-binding capacity (TIBC), transferrin saturation, and soluble transferrin receptor (sTfR). In selected cases, additional markers such as haptoglobin and bilirubin are useful when hemolysis is suspected. Folate and vitamin B12 levels are also commonly assessed, and in some cases, active B12 (holotranscobalamin, B12-TC2) may provide additional information.

Beyond this initial workup, further investigations often move into specialist care. If blood loss anemia or hemolysis is suspected, gastrointestinal bleeding is typically excluded first. If no clear source is identified, patients are often referred to hematology, where evaluation may include bone marrow studies and more advanced or specialized laboratory testing.

Summary

Endurance training leads to a significant expansion of plasma volume, which lowers hemoglobin concentration and hematocrit through a dilutional effect. This so-called dilutional pseudoanemia, often referred to as sports anemia, is not a true pathological state — total hemoglobin mass is typically normal or increased, and the change reflects a beneficial cardiovascular adaptation rather than disease.

The key clinical task is to distinguish this physiological adaptation from true anemia, particularly iron deficiency. Because routine laboratory tests measure concentration rather than total hemoglobin mass, interpretation must always be contextual. Mild reductions in hemoglobin are common in well-trained athletes, but the effect is usually modest, and clearly abnormal values should not be attributed to dilution alone.

In practice, clinicians should assess iron status and the overall clinical picture rather than relying on isolated laboratory values. When interpreted correctly, a mildly low hemoglobin in an endurance athlete is often a marker of adaptation — not pathology.

References

1. https://pubmed.ncbi.nlm.nih.gov/1798375/
2. https://journals.physiology.org/doi/full/10.1152/jappl.1998.85.2.484
3. https://pubmed.ncbi.nlm.nih.gov/1874243/
4. https://pubmed.ncbi.nlm.nih.gov/6991463/
5. https://pmc.ncbi.nlm.nih.gov/articles/PMC10836693/
6. https://pubmed.ncbi.nlm.nih.gov/1553454/
7. https://www.ncbi.nlm.nih.gov/books/NBK572066/
8. https://pmc.ncbi.nlm.nih.gov/articles/PMC3824146/
9. https://pubmed.ncbi.nlm.nih.gov/1406203/
10. https://pmc.ncbi.nlm.nih.gov/articles/PMC8472039/
11. https://pubmed.ncbi.nlm.nih.gov/23514971/