Eight in ten recreational functional fitness athletes show low energy availability, yet short-term health checks stay normal

A two-week monitoring study of 30 recreational high-intensity functional training athletes has found that 80 percent fell below the energy availability threshold commonly used to flag risk of Relative Energy Deficiency in Sport, yet the researchers could not detect any accompanying differences in recovery, sleep quality, perceived exertion, or bone mineral density between those classified as low and those who were not. The findings, published in Frontiers in Nutrition, highlight both how common under-fueling may be in this rapidly growing fitness community and how difficult it is to capture its physiological consequences over a short observational window.

High-intensity functional training, often called HIFT, blends resistance training, Olympic lifting, bodyweight exercise, and metabolic conditioning into constantly varied, high-intensity sessions. Because this style of training produces substantial exercise energy expenditure while demanding multiple domains of fitness, researchers have suspected that participants may be vulnerable to low energy availability, defined as the dietary energy remaining for normal physiological function after exercise costs are subtracted, expressed relative to fat-free mass. When energy availability drops below roughly 30 kilocalories per kilogram of fat-free mass per day, controlled studies show disruptions in endocrine and metabolic function, and chronic exposure is considered the primary driver of the syndrome the International Olympic Committee labels RED-S.

Recreational HIFT athletes may face particular risks. Unlike elite competitors, they typically juggle demanding training schedules of five or more weekly sessions alongside jobs, studies, and family responsibilities, often without access to professional nutrition support. The culture surrounding functional fitness, with its emphasis on leanness and performance, may also encourage intentional or unintentional dietary restriction. Despite these concerns, most existing research on energy availability has focused on endurance, aesthetic, weight-sensitive, or elite athlete populations, leaving a gap in knowledge about ordinary gym-goers who train hard several times per week.

To address this gap, researchers at the University of South Carolina recruited 30 recreational HIFT athletes, 18 women and 12 men with an average age of about 31 years, from local CrossFit-style facilities. Participants trained five days per week for two weeks on a standardized program supervised by certified coaches. They kept detailed dietary logs on every training day, wore a multi-sensor armband that estimated exercise energy expenditure, and underwent dual-energy X-ray absorptiometry scans to measure fat-free mass and bone mineral density. The team also measured resting metabolic rate through indirect calorimetry, and participants rated their perceived exertion after each workout, their recovery before each session, and completed a validated sleep quality questionnaire.

The headline number was striking: mean energy availability across the two weeks was 24.9 kilocalories per kilogram of fat-free mass per day, well below the 30 kilocalorie threshold, and 24 of the 30 participants, including 83 percent of the women and 75 percent of the men, met the criteria for low energy availability. Yet the physiological picture told a more complicated story. The average ratio of measured to predicted resting metabolic rate was 1.10, comfortably above the value below 0.90 that suggests metabolic suppression from chronic energy deficiency, and only four participants, all of whom were also classified as low energy availability, showed this metabolic signature. The association between energy availability and metabolic ratio was positive but fell just short of statistical significance.

When the researchers compared the 24 athletes flagged as low energy availability with the six who were not, they found no statistically significant differences in any measured outcome. Perceived exertion during training averaged 15.9 on the standard Borg scale, perceived recovery sat in the moderate range, and sleep duration averaged 6.8 hours per night in both groups. Bone mineral density Z-scores, available for 29 participants, averaged 1.8, actually above population norms, and did not differ between groups. Continuous energy availability showed only weak, non-significant correlations with every performance and health measure examined, from recovery ratings to sleep quality scores.

The one notable positive finding emerged from an exploratory analysis: sleep duration was moderately associated with total quality recovery scores, with a Spearman correlation of 0.46. Athletes who slept longer tended to report better overall recovery, a pattern consistent with a large body of literature linking sleep to muscle repair, hormonal regulation, and readiness to train. The authors caution that this association was exploratory, was not adjusted for multiple comparisons, and cannot establish that sleep causally improves recovery, but it suggests that sleep may influence how recreational athletes feel from day to day more immediately than their calculated energy status does.

Why did such a high prevalence of low energy availability produce no measurable consequences? The researchers offer several explanations, beginning with measurement uncertainty. Self-reported food logs are known to underestimate intake, particularly among physically active people, and the reported energy intakes in this sample were low, with men reporting roughly 2,200 kilocalories per day despite measured resting metabolic rates near 2,300 kilocalories. Underreporting would inflate the apparent prevalence of low energy availability. Similarly, the armband used to estimate exercise expenditure is less accurate during vigorous, intermittent exercise of the kind HIFT involves, and the authors note its error rates may grow at higher intensities.

Timing and study design also matter. Bone mineral density changes slowly, typically over months to years, so a two-week window cannot detect skeletal effects even if they exist; the null bone findings should not be read as evidence that low energy availability is harmless to bone, especially since resistance and impact loading in HIFT may independently support skeletal health. The study was also small, with an imbalanced 24-to-6 group split, and a sensitivity analysis showed it was powered only to detect very large between-group differences, meaning modest but real effects could have been missed. Subjective scales like perceived exertion and recovery ratings may likewise lack sensitivity to early, subclinical physiological changes, and HIFT performance may be buffered in the short term by glycogen stores, motivation, and pacing strategies.

The authors conclude that short-term low energy availability in recreational HIFT athletes may be common yet clinically silent, with consequences more likely to emerge over longer exposure or in people with additional risk factors. They recommend that a single calculated energy availability value below the 30 kilocalorie threshold should not serve as a stand-alone diagnostic or screening tool in this population, but instead be interpreted alongside reproductive health, injury history, eating behaviors, and metabolic indicators. Longer longitudinal studies tracking hormones, bone turnover markers, and objective performance are needed to determine whether sustained under-fueling in recreational functional fitness athletes produces the health and performance impairments predicted by the RED-S framework.

Subject of Research: Low energy availability and its health and performance associations in recreational high-intensity functional training athletes

Article Title: Low energy availability in recreational high-intensity functional training athletes: associations with health and performance outcomes

Article References: Vlattas, G., Wang, X., Ortaglia, A., Hong, S., & Torres-McGehee, T. (2026). Low energy availability in recreational high-intensity functional training athletes: associations with health and performance outcomes. Frontiers in Nutrition, 13, Article 1953641. https://doi.org/10.3389/fnut.2026.1953641

Image Credits: AI Generated

DOI: 10.3389/fnut.2026.1953641

Keywords: low energy availability, RED-S, high-intensity functional training, sports nutrition, exercise recovery, bone mineral density, sleep quality, energy expenditure, resting metabolic rate, recreational athletes, energy, availability