Energy availability

Energy availability is energy intake minus exercise expenditure, expressed per kilogram of lean mass, to describe what’s left for normal body functions.^{1Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 123.} A client with 89.8 kg of lean mass who eats 3,000 kcal and burns 400 in exercise has 2,600 kcal left over, or 29 kcal/kg.^{2Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 124.}

A client can maintain energy balance and a stable weight yet still be in relative energy deficiency, with reproductive and metabolic function turned down to keep that balance.^{3Helms et al., The Muscle & Strength Pyramid: Nutrition, 2nd ed., p. 68.} The International Olympic Committee calls this relative energy deficiency in sport (RED-S).^{1Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 123.} It often shows up in aesthetic, endurance and weight-class athletes, and in clients trying to lose weight on a low intake while exercising.^{1Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 123.}

Where sources differ

The NSCA cites the IOC’s threshold of 30 kcal/kg of lean mass but notes that the point where RED-S occurs varies from person to person.^{2Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 124.} Helms calls a hard cutoff at 30 kcal/kg scientifically problematic.^{4Helms et al., The Muscle & Strength Pyramid: Nutrition, 2nd ed., p. 70.} The original research used sedentary women; symptoms in women can appear anywhere from 30 to 45 kcal/kg, and in men the threshold may be closer to 20 to 25.^{4Helms et al., The Muscle & Strength Pyramid: Nutrition, 2nd ed., p. 70.}

In practice

• Risk factors include disordered eating, extreme weight loss methods, losing 10% or more of body mass in a month, menstrual changes and a BMI of 17.5 or lower.^{2Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 124.}
• A calculation alone isn’t a screening tool.^{2Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 124.} Only health care professionals should diagnose or treat RED-S, so refer a client to a registered dietitian or physician if you’re concerned.^{2Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 124.}