Energy availability, not body fatness, regulates reproductive function in women

Summary

Anne Loucks's 2003 review consolidates a foundational principle for women's exercise and nutrition science: it is energy availability — calories left over after subtracting exercise expenditure from intake — that regulates reproductive function, not body fatness. Through a series of careful in-laboratory studies measuring LH (luteinizing hormone) pulsatility as a surrogate for menstrual cycle integrity, Loucks and colleagues found that reproductive disruption begins when energy availability falls below a threshold between 20 and 30 kcal per kilogram of lean body mass per day. Above the threshold, women maintain normal reproductive endocrine function; below it, even with adequate body fat, LH pulsatility breaks down and menstrual disruption follows. The implication is that "thinness" itself does not cause amenorrhea; sustained energy deficit does. The framework gave rise to the modern Female Athlete Triad and RED-S (Relative Energy Deficiency in Sport) clinical concepts, which are now standard in sports medicine. The 30 kcal/kg LBM/day threshold remains the most-cited clinical cutoff for evaluating energy-availability risk in active women.