Changes in body composition and their association with erythrocyte mass in regular exercisers: A cross-sectional study

Abstract

Most erythrocytosis cases are acquired and result from conditions that cause low oxygen levels, such as smoking; living at high altitudes; and certain heart, lung, or kidney diseases. Intense exercises aimed at changing body composition are being increasingly practiced. These exercises reduce body fat and increase the muscle mass. An increase in muscle mass is expected to increase the oxygen demand, which could lead to erythrocytosis. Therefore, this study aimed to investigate the effects of changes in body composition on erythrocyte mass. This study included 56 male volunteers who engaged in regular exercise to alter body composition and 51 male volunteers in the control group who did not engage in regular exercise. Height, weight, body fat, and muscle mass were measured, and laboratory studies were conducted. A total of 107 male participants with a median age of 24 (range 18–59), were included in the study. The body fat percentage was significantly lower in the athletes (18.8 ± 6.0 vs 24.9 ± 6.4, P < .001). Hemoglobin (Hb) and hematocrit (HCT) levels were significantly higher in the exercise group (Hb 15.9 ± 0.8 vs 15.3 ± 0.8; HCT 46.8 ± 2.6 vs 45.2 ± 2.4). The total testosterone levels were also higher in the exercise group (430 ± 174 vs 368 ± 138). Changes in body composition during exercise increased erythrocyte mass.

1. Introduction

Erythrocytosis (polycythemia) is characterized by an abnormal increase in hemoglobin (Hb) and/or hematocrit (Hct) levels in the peripheral blood. It is defined as Hb levels > 16.5 g/dL or Hct > 49% in men, and Hb > 16.0 g/dL or Hct > 48% in women.^[1] Erythrocytosis can result from either an absolute or relative increase in red blood cell mass (RCM). Absolute erythrocytosis is associated with excessive red blood cell production and may be due to clonal primary erythrocytosis or increased serum erythropoietin (sEPO) levels. In contrast, relative erythrocytosis is characterized by elevated Hct levels without an actual increase in RCM, usually due to a reduction in plasma volume.^[2]

Primary erythrocytosis typically refers to autonomous erythrocyte production caused by myeloproliferative neoplasms, such as polycythemia vera. Secondary erythrocytosis (SE) arises as a physiological response to increased sEPO levels, often due to tissue hypoxia. The most common acquired primary erythrocytosis is polycythemia vera, which is frequently associated with JAK2 mutations. SE is a heterogeneous group of conditions driven by abnormal sEPO elevation in response to insufficient tissue oxygenation and can be triggered by cardiopulmonary diseases, smoking, or hydronephrosis. Hypoxia-independent SE may also result from autonomous sEPO production, such as in the context of certain diuretic medications, post-renal transplantation, or specific tumors.^[3–5] A previous study examining patients with polycythemia lacking JAK2 mutations reported the most common causes to be obstructive sleep apnea (19%), followed by nicotine use (17%), elevated HbCO > 5% (5%), respiratory diseases (4%), nonmalignant renal disorders (4%), androgen therapy (3%), relative erythrocytosis with normal RCM (3%), cancers (3%) (kidney, adrenal, seminoma, lung, brain), and renal transplantation (2%). No identifiable cause was found in 30% of the cases.^[6]

In recent years, body composition modifications through exercise have become increasingly common, particularly among young individuals. These training regimens aim to increase muscle mass while reducing body fat percentage. Given that an increase in muscle mass likely elevates tissue oxygen demand, it is hypothesized that this may stimulate sEPO production and contribute to physiological erythrocytosis.

This study was designed as a cross-sectional observational study to evaluate the impact of changes in body composition on erythrocyte mass. Specifically, we aimed to compare hemoglobin and hematocrit levels between individuals who engage in regular exercise and those who do not, in order to explore whether alterations in body composition are associated with SE.

2. Methods

2.1. Study design and setting

This was a cross-sectional observational study designed to assess the impact of body composition changes on erythrocyte mass. The study was conducted at Necmettin Erbakan University between June and December 2024, and was approved by the university’s Ethics Committee. In the study, instead of including only individuals who are professionally engaged in bodybuilding, volunteer participants with varying ages, genders, and levels of physical activity were included to ensure sample diversity and minimize selection bias.

In this study, the sample size was determined to include a total of 155 healthy male participants. Forty-eight participants were excluded from the study because they did not meet the inclusion criteria (Fig. 1). The participants were randomly assigned to the exercise group (n = 56) and the control group (n = 51). The median age was similar between groups (24 years in both; P = .625), ensuring the homogeneity and comparability of the groups. The sample size was chosen to provide adequate statistical power to detect meaningful differences between the groups. Additionally, the broad age range (18–59 years) increases the generalizability of the findings to a wider adult male population. Data from all participants were fully obtained; therefore, there were no participants with missing data.

Figure 1.

Study flow diagram.

2.2. Participants

A total of 107 healthy male volunteers aged 18 to 59 years were recruited and categorized into 2 groups:

• The exercise group included 56 males who had been engaging in bodybuilding training at least twice per week for a minimum of 6 months.

• The control group included 51 males who did not engage in any form of regular physical activity and had no known comorbidities.

All participants provided written informed consent prior to any measurements or blood collection.

2.3. Exercise assessment

Participants categorized as engaging in “regular exercise” had been performing bodybuilding-style resistance training at least twice per week for the past 6 months. All individuals reported following consistent, moderate-to-high intensity strength-based exercise routines. This homogeneity in exercise modality, frequency, and intensity contributed to a relatively uniform exercise profile within the study group.

2.4. Exclusion criteria

Participants were excluded if they met any of the following criteria:

• Presence of chronic comorbidities

• History of autologous blood transfusion, recombinant erythropoietin (EPO) use, or androgen/anabolic steroid use for muscle growth or performance enhancement

• Use of tobacco products (cigarettes, hookah, e-cigarettes) within the past 6 months

• History of cardiovascular or pulmonary disease

• Oxygen saturation (SaO₂) < 92% on room air

• Blood donation in the last 6 months

• Use of iron supplements

• History of bleeding within the last 6 months

• To live at high altitude

2.5. Measurements and data collection

Anthropometric and body composition measurements were obtained using the InBody 120 bioelectrical impedance device, and included:

• Height, body weight, body mass index (BMI)

• Skeletal muscle mass, body fat mass, body fat percentage

• Visceral fat level, regional muscle mass, and regional fat distribution

SaO₂ was measured via pulse oximetry.

Blood samples were analyzed at the hospital biochemistry laboratory for the following parameters:

• Complete blood count (Hb, HCT, platelet count, white blood cell count)

• Liver enzymes (SGPT, SGOT)

• Ferritin, sEPO, serum creatinine

• Total testosterone levels

Participants’ body measurements and blood tests were conducted before exercise, in a fasting state, and between 8:00 and 10:00 am

2.6. Variables

The primary outcome variables were Hb and HCT levels. Secondary outcomes included body composition parameters, serum erythropoietin, and testosterone levels. The exposure variable was participation in regular resistance training.

2.7. Statistical analysis

Statistical analyses were performed using SPSS IBM version 23 (Chicago). Continuous numerical variables were analyzed for distribution characteristics using the Kolmogorov-Smirnov test. Descriptive statistics are presented as mean ± standard deviation for normally distributed data and median (range) for non-normally distributed data. Group comparisons were performed using the independent samples t-test and Mann–Whitney U test. Categorical variables were expressed as percentages (%) and compared using the chi-square test. Statistical significance was set at P < .05.

3. Results

A total of 107 healthy male participants with a median age of 24 years (range: 18–59) were included in the study. The exercise group comprised 56 individuals (52.3%), and the control group included 51 individuals (47.7%). The median age was similar between the groups: 24 years (range: 18–59) in the exercise group and 24 years (range: 19–51) in the control group (P = .625).

3.1. Body composition and anthropometric measurements

Total body weight and skeletal muscle mass were comparable between the 2 groups. However, the exercise group had significantly lower fat mass (15.5 ± 6.7 kg vs 22.1 ± 9.0 kg, P < .001) and lower body fat percentage (18.8 ± 6.0% vs 24.9 ± 6.4%, P < .001). BMI and waist-to-hip ratio were similar between groups, while the control group was significantly taller than the exercise group (178 cm vs 174 cm, P = .030). Detailed comparisons of anthropometric and metabolic parameters are provided in Table 1.

Table 1.

Comparison of metabolic parameters between the exercise and control group.

	Exercise group (n:56)	Control group (n:51)	P
Age	24 (18–59)	24 (19–51)	.625*
SpO2 (%)	96.5 (93–99)	97 (93–98)	.878*
Pulse (/dk)	80 (59–158)	81 (54–132)	.837*
Body weight (kg)	80.2 ± 10.5	84.8 ± 14.3	.061†
Skeletal weight (kg)	36.8 ± 3.9	35.7 ± 4.5	.194†
Fat mass (kg)	15.5 ± 6.7	22.1 ± 9	<.001 †
Body fat percentage (%)	18.8 ± 6.0	24.9 ± 6.4	<.001 †
Height (cm)	175 (159–186)	178 (166–194)	.030 *
BMI (kg/m2)	25.6 (18–35)	26.6 (17.3–38.3)	.179*
Waist-to-hip ratio	0.92 (0.82–1.0)	0.92 (0.82–1.0)	.776*

Statistically significant results are presented in bold.

^*Mann–Whitney U test.

^†Independent samples t-test.

3.2. Biochemical parameters

sEPO levels did not differ significantly between the groups. However, ferritin levels were significantly higher in the control group (99.4 ng/mL vs 81.5 ng/mL, P = .036), although values remained within the normal reference range in both groups.

Total testosterone levels were significantly higher in the exercise group (430 ± 174 ng/dL vs 368 ± 138 ng/dL, P = .047), again within the physiological range for all participants.

Serum creatinine and liver transaminases (SGPT, SGOT) showed no significant differences between groups. The results of biochemical analyses are summarized in Table 2.

Table 2.

Comparison of biochemical parameters between the exercise and control group.

	Exercise group (n:56)	Control group (n:51)	P
Erythropoietin	7.4 (2.3–18.8)	8.5 (2.7–26.3)	.166*
Ferritin (ng/mL)	81.5 (11.9–284)	99.4 (9.8–324)	.036 *
Total testosterone (ng/dL)	430 ± 174	368 ± 138	.047 †
Creatinine (mg/dL)	1.0 ± 0.15	1.0 ± 0.17	.820†
SGPT (IU)	20.7 (5.8–43.4)	21.3 (6.5–73.9)	.556*
SGOT (IU)	21 (9.4–41)	19.2 (12.2–52)	.359*

Statistically significant results are presented in bold.

SGOT = aspartate aminotransferase, SGPT = alanine aminotransferase.

^*Mann–Whitney U test.

^†Independent samples t-test.

3.3. Hematological parameters

All hematologic parameters were within normal reference ranges. No significant differences were observed in RBC count, white blood cell count, or platelet count (PLT) between groups. However, the exercise group showed significantly higher values for:

• Hb: 15.9 ± 0.8 g/dL versus 15.3 ± 0.8 g/dL (P = .001)

• HCT: 46.8 ± 2.6% vs. 45.2 ± 2.4% (P = .002)

• Mean corpuscular volume: 86.9 ± 3.7 fL versus 85.5 ± 3.7 fL (P = .033)

A complete comparison of hematologic findings is presented in Table 3.

Table 3.

Comparison of hemogram parameters between the exercise and control group.

	Exercise group (n:56)	Control group (n:51)	P *
RBC (/million)	5.39 ± 0.38	5.26 ± 0.47	.130
Hb (g/dL)	15.9 ± 0.8	15.3 ± 0.8	.001
HCT (%)	46.8 ± 2.6	45.2 ± 2.4	.002
MCV (FL)	86.9 ± 3.7	85.5 ± 3.7	.033
WBC (/L)	7.4 ± 1.5	7.2 ± 1.4	.417
PLT (/L)	264 ± 48	264 ± 50	.991

Statistically significant results are presented in bold.

Hb = hemoglobin, HCT = hematocrit, MCV = mean corpuscular volume, PLT = platelet count, RBC = red blood cell, WBC = white blood cell count.

^*Independent samples t-test.

4. Discussion

In our study, we observed that the average Hb level was 0.6 g/dL higher, and the average HCT level was 1.6% higher in the exercise group compared to the control group. Additionally, while ferritin levels were significantly lower in the exercise group, total testosterone levels were significantly higher. These findings suggest an increase in erythrocyte mass in individuals engaged in regular exercise. Low ferritin levels, in particular, have been associated with increased erythropoiesis, as ferritin serves as a primary storage form of iron, which is essential for red blood cell production.

The production of RBC is regulated by EPO, a hormone that is primarily produced by the kidneys in response to low oxygen levels (hypoxia). Under hypoxic conditions, the concentration of EPO increases in the bloodstream, stimulating the proliferation and differentiation of erythroid precursors in the bone marrow, which leads to an increase in RBC production.^[7,8] Conversely, excess oxygen inhibits EPO production and delays the release of new RBCs into circulation.^[9] In our study, however, EPO levels were within normal limits in both groups, and no significant differences were observed between the exercise and control groups. This may suggest that, despite the increased erythrocyte mass observed in the exercise group, the increase was not sufficient to cause polycythemia or abnormal EPO levels.

It is well-documented in the literature that exercise training – particularly endurance training, but also strength training – can stimulate erythropoiesis and increase blood volume by enhancing both red blood cell mass and plasma volume.^[10–13] Previous studies have demonstrated that exercise can increase Hb, HCT, platelet counts, circulating side population cells, and IL-6 levels in healthy individuals of various ages and fitness levels.^[14] Strength training, in particular, has been shown to increase the number of reticulocytes, which are immature RBCs released into circulation.^[15] Strength athletes often have higher Hb, HCT, and RBC levels than endurance athletes.^[16]

One study showed that strength training increases HCT and RBC while decreasing MCHC in physically inactive men aged 20 to 45.^[17] A study by McCarthy et al^[18] found that 12 weeks of strength training in sedentary young and middle-aged men increased erythrocyte and blood volumes, although the increase in plasma volume did not reach statistical significance. This suggests that plasma volume expansion may be less pronounced in strength training compared to endurance training. Additionally, the expansion of erythrocyte mass generally occurs gradually over weeks to months, in contrast to the rapid plasma volume expansion observed with endurance training.

In many cases, especially with intense endurance training, the increase in plasma volume may exceed the increase in RBC mass, leading to a decrease in Hb, HCT, or RBC levels, a phenomenon known as hemodilution. Other factors, such as iron deficiency^[19] and red blood cell destruction (hemolysis), may also contribute to changes in erythropoiesis during exercise training.^[20]

Sawka et al^[11] published a comprehensive review showing that endurance training significantly increases blood volume in most studies. Their findings suggest that the increase in plasma volume after endurance training contributes more significantly to blood volume expansion than an increase in RBC mass. In contrast, strength training appears to more directly enhance erythrocyte mass, with a smaller increase in plasma volume.

Studies also highlight that endurance athletes tend to have approximately 35% higher Hb mass and blood volume compared to the general population, with long-term endurance training leading to a 40% increase in RBC volüme.^[21–24] Similar findings have been observed in both cross-sectional^[16] and longitudinal studies^[15,17,18] stimulate erythropoiesis, though some studies have failed to show significant changes in Hb and HCT levels, especially in older adults.^[25–29] This highlights that individual responses to exercise, including changes in hematological parameters, may vary and may be influenced by factors such as age, genetics, and baseline fitness levels.

Our study’s findings align with these observations, as the exercise group demonstrated higher levels of Hb and HCT, suggesting an increase in erythrocyte mass due to regular exercise. However, it is important to note that the increase in erythropoiesis observed in the exercise group did not result in clinically significant polycythemia, as no group reached levels characteristic of SE.

Testosterone, the primary androgen in humans, has also been shown to play a role in stimulating erythropoiesis. Testosterone increases erythropoiesis through various mechanisms, often by modulating renal EPO release and bone marrow activity in a synergistic manner.^[30] In our study, testosterone levels were significantly higher in the exercise group compared to the control group, which may have contributed to the observed increase in erythrocyte mass. The relationship between higher testosterone levels and elevated Hb levels has been well-documented, particularly in the context of testosterone therapy, where erythrocytosis is often observed in elderly men treated with injectable testosterone preparations.^[31] In our study, however, EPO levels remained comparable between the groups, suggesting that the observed increase in erythrocyte mass in the exercise group may primarily be mediated by hormonal changes, particularly elevated testosterone levels, rather than a substantial increase in EPO production

5. Study limitations

One limitation of our study is the relatively small sample size. A larger sample size could enhance the generalizability of the findings and provide greater statistical power to detect subtle differences in erythropoiesis. The absence of objective measurements for exercise parameters such as intensity and duration may be considered a limitation of the study. The lack of assessment regarding dietary intake and hydration status was acknowledged as a limitation. Furthermore, the exclusion of smokers from the study made it challenging to recruit participants for the study group, which may have introduced selection bias.

6. Conclusion

In conclusion, our study demonstrates that exercise, particularly strength training, is associated with increased Hb and HCT levels, indicating an increase in erythrocyte mass. These findings suggest that exercise-induced changes in body composition may contribute to alterations in hematological parameters, including erythrocytosis. Therefore, when investigating the etiology of SE, exercise should be considered as a potential contributing factor. Further studies, particularly those that assess erythrocyte mass before and after exercise interventions, are warranted to better understand the physiological mechanisms underlying these changes.

Author contributions

Conceptualization: Sinan Demircioğlu, Mustafa Merter.

Data curation: Sinan Demircioğlu, Celalettin Korkmaz, Filiz Alkan Baylan, Mustafa Merter.

Formal analysis: Atakan Tekinalp, Filiz Alkan Baylan.

Investigation: Sinan Demircioğlu.

Methodology: Sinan Demircioğlu, Celalettin Korkmaz, Filiz Alkan Baylan.

Project administration: Sinan Demircioğlu.

Resources: Sinan Demircioğlu.

Writing – original draft: Sinan Demircioğlu.

Writing – review & editing: Sinan Demircioğlu, Atakan Tekinalp, Celalettin Korkmaz, Filiz Alkan Baylan, Mustafa Merter.