Karstoft, K, Solomon, TP, Laye, MJ, and Pedersen, BK. Daily marathon running for a week—The biochemical and body compositional effects of participation. J Strength Cond Res 27(11): 2927–2933, 2013—Although long-distance running, such as ultramarathons and multistage races, is increasingly popular, it maybe potentially harmful to health, despite sparse evidence. We studied 8 experienced recreational runners participating in a multiple-marathon running event in which 7 marathons were completed on consecutive days. Fasting blood chemistry and body composition were assessed before and 20–24 hours after the race. The total finish time for the 7 marathons ranged between 23:25:42 and 34:25:21 (hours:minutes:seconds). Only minor increases in circulating skeletal muscle cell damage markers, liver cell damage markers, and inflammatory markers occurred after the race. No other significant adverse biochemical effects were observed. The homeostasis model assessment of insulin resistance decreased markedly, and an improved lipid profile was found. A decrease in fat mass and increase in lean body mass was observed, resulting in no overall weight changes. In summary, the race did not cause any major adverse effects, whereas some traditional markers of cardiovascular disease improved acutely after the race.