Aerobic, hematological, and lipid changes after five months of endurance training in distance runners.

Comparative longitudinal data across competitive levels in distance running remain limited, particularly in North African cohorts. Interpreting changes in aerobic and hematological markers is especially important when maximal aerobic capacity is estimated rather than directly measured and when plasma volume is not assessed. To compare baseline anthropometric, aerobic, hematological, and lipid variables among elite runners, competitive runners, and sedentary men, and to describe changes observed across a five-month supervised endurance-training period in the athlete groups. Twenty-one healthy men participated (elite runners, n = 7; competitive runners, n = 7; sedentary controls, n = 7). Sedentary controls completed pre- and post-assessments but did not participate in the supervised endurance training program. Athletes were evaluated before and after a five-month training period. Estimated VO2max (VO2max_est) was derived from the Université de Montréal Track Test. Fasting venous blood samples were analyzed for red blood cell count (RBC), hemoglobin (Hb), hematocrit (Hct), total cholesterol (TC), triglycerides (TG), HDL-C, and LDL-C. Baseline differences were examined with one-way ANOVA. Longitudinal athlete changes were summarized using mean differences, conservative 95% confidence intervals, percent change, and Cohen's d_av. Longitudinal patterns were additionally examined using linear mixed-effects models. At baseline, elite runners were leaner than sedentary controls and displayed markedly higher VO2max_est than both competitive runners and sedentary men. Across the training period, observed VO2max_est increases were modest in both athlete groups (elite: +1.66 mL·kg⁻¹·min⁻¹, conservative 95% CI 0.87 to 2.44; +2.2%; d_av = 2.21; competitive: +1.77 mL·kg⁻¹·min⁻¹, conservative 95% CI 1.49 to 2.05; +3.4%; d_av = 6.64). RBC count remained essentially stable. Hematocrit declined in both groups, while hemoglobin showed a small increase in elite runners and a decline in competitive runners. These concentration-based hematological changes are compatible with, but do not demonstrate, training-related hemodilution. Lipid changes were small in magnitude and mixed in direction. The pre-post findings therefore reflect within-athlete changes in the trained groups, with sedentary controls serving as a longitudinal non-training comparison group. Across a five-month training period, elite and competitive runners showed small increases in VO2max_est and concentration-based hematological changes that are compatible with, but do not demonstrate, training-related hemodilution. Given the small sample, the hybrid non-randomized design with sedentary men serving as longitudinal non-training comparators, the lack of objective training-load and dietary data, and the use of a field estimate rather than direct gas-exchange measurement, the findings should be interpreted as preliminary.