Low Energy Availability and Increased Risk of Relative Energy Deficiency in Sport (RED-S) During a 3767-km Thru-Hike...

Long-distance "thru-hiking" has extraordinary physical demands and has become increasingly popular. This report describes a man (55 y) who thru-hiked the Pacific Crest Trail in 2021 and was at risk of developing the relative energy deficiency in sport (RED-S) syndrome. Hiking distance was 3767 km over 128 d. Eighty-eight days (69%) were full days of hiking, covering 38±8 km/d (mean±SD) in 7.9±1.6 h/d. Exercise energy expenditure above rest (heart rate vs indirect calorimetry regression method) was 2834±1518 kcal/d, total energy expenditure was 5702±1323 kcal/d, and energy intake was 4141 kcal/d. Body mass decreased by 9%, and fat mass (dual-energy X-ray absorptiometry) decreased by 46%. Energy availability (energy intake minus exercise energy expenditure) was 19.3 kcal/d/kg fat-free mass, indicating low energy availability (defined as <30 kcal/d/kg). Dual-energy X-ray absorptiometry-measured spine bone mineral density (BMD) decreased by 8.6%, with little to no decrease in total hip (-1.0%) and femoral neck (-1.5%) BMD. Total cholesterol, low-density lipoprotein cholesterol, and triglycerides increased by 24, 39, and 57%, respectively. Within 8 mo after the hike, BMD and serum lipids nearly or fully returned to baseline. No changes in high-density lipoprotein cholesterol, glycemia, or blood pressure were observed. According to guidelines, these observations are consistent with a moderate risk of RED-S, and a medical evaluation and treatment plan are advisable in order to avoid clinical manifestations (eg, stress fractures, anemia, psychological disturbances). To minimize RED-S risk, it may be prudent for thru-hikers to optimize energy availability by moderating daily hiking distances and/or increasing food intake.