Background/Objectives: High-altitude hypoxia may affect ECG readings, but it is unclear whether the "live-low-train-high" approach prevents these changes in winter sports athletes. Methods: This cross-sectional study assessed cardiovascular parameters in 102 winter-sport athletes (mean age 20 ± 4 y; 57% women), divided by training altitude into a high-altitude (HA) group (2500-3300 m, n = 70; skiers/snowboarders) and a low-altitude (LA) group (738 m, n = 32; ice hockey/figure skaters). Mid-season assessments included resting ECG, blood pressure, blood biochemistry, and three 24 h dietary recalls. Results: All ECG parameters were physiological, and no significant differences (p < 0.05) were observed in heart rate, PR interval, or QTc between groups. However, HA group exhibited higher systolic blood pressure and a short QT interval. Lactate was significantly higher in HA (p = 0.028). The HA diet contained more saturated fat (p < 0.001), cholesterol (p = 0.013), magnesium (p = 0.003) and potassium (p = 0.001), whereas LA athletes consumed more glucose (p = 0.024). In HA, total energy expenditure correlated positively (p = 0.05) with QRS (p = 0.52) and QT (p = 0.56), while heart rate correlated inversely with vitamin D (p = -0.59). In LA, QTc showed strong inverse correlations with zinc (p = -0.62) and selenium (p = -0.85). Conclusions: This finding suggests that intermittent high-altitude training did not alter ECG patterns when nutrient intake was adequate. High lactate level and specific nutrient correlations point to a residual physiological load and a modulatory role of electrolytes, B-vitamins, and vitamin D on cardiac repolarisation.
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