Waldman, HS, Krings, BM, Basham, SA, Smith, JW, Fountain, BJ, and McAllister, MJ. Effects of a 15-day low carbohydrate, high-fat diet in resistance-trained men. J Strength Cond Res 32(11): 3103–3111, 2018—This study examined the effects of a 15-day isocaloric low carbohydrate (<25% E), high-fat (>50% E) (LCHF) diet on physiological and metabolic alterations in resistance-trained (RT) men. College-aged RT men (n = 11) completed 4 V[Combining Dot Above]O2max tests using treadmill every 5 days during the 15-day trial. Blood was drawn intravenously pre-exercise across each experimental trial for insulin, cortisol, and glucose. Pulmonary data were collected and substrate oxidation (OXI) was calculated during exercise. Body mass decreased (p < 0.04) with no further changes in anthropometric measures. Time to exhaustion was not affected across each day. Insulin dropped below baseline values (p < 0.0005). Cortisol increased from baseline to day 5 (p < 0.004) but returned back to near baseline at day 10, whereas glucose remained within normal range throughout the duration of the study. Carbohydrate (CHO) OXI dropped (p < 0.001) from baseline to day 5, and fat OXI increased from baseline to day 5 (p < 0.0001). Heart rate decreased from baseline to day 5 (p < 0.001) and again from day 10 to 15 (p < 0.02). Oxygen uptake (V[Combining Dot Above]O2) decreased from day 5 to 10 (p < 0.0001). A nonketo LCHF diet appears to favor RT men by altering metabolic markers without decrements in aerobic performance and be a potential diet intervention used by coaches. However, the reported cardiorespiratory responses should be interpreted reasonably because of the possibility the subjects running economy improved over experimental trials.