In April 2019 researchers from the USA published their review on whether endurance exercise is safe. The researchers stated that endurance exercise produces acute but not chronic increases in blood urea nitrogen, creatinine, and urine albumin. However, iron-deficiency anemia has been observed in female athletes. Whilst an increase in upper respiratory illness has been noted in elite athletes it appears to be decreased in recreational athletes. Exercising in the presence of air pollution is known to have an adverse effect on the respiratory system, although the benefits outweigh the risks. In addition there is no convincing evidence that endurance athletes have an increased risk of osteoarthritis. Common gastrointestinal symptoms can also occur and there have been a few reports of gastrointestinal bleeding. However, there are many reports which relate to the cardiovascular system. There is an increased risk of heart attack during exercise and a tendency for atrial fibrillation. Troponin (a protein involved in muscle contraction) is elevated after exercise. The researchers concluded by stating that although there are some health concerns concerning endurance exercise, it is generally regarded as safe.
Rutlen C, Rutlen DL. Is Endurance Exercise Safe? The Myth of Pheidippides. South Med J. 2019 Apr;112(4):210-214.