Combination of adaptations: increased heart volume and decreased basic metabolic rate?
So, more efficient heart from increased heart volume (good) and decreased basal metabolic rate due to what factors? Increased lactic acid in body from anaerobic respiration during marathons, leading to high blood acidity, to lower CO2 content in blood, leading to lower tissue oxygenation? Or increased keto acid in body from fatty acid oxidation during marathons, leading to high blood acidity, to lower CO2 content in blood, to lower as well tissue oxygenation? I would think it's the latter, since lactic acid buildup would lead to fatigue and not being able to finish the marathon.