RED-S Warning: Why Under-Fuelling Can Undermine Hybrid Training

Persistent fatigue and stalled progress may deserve a closer look at your food intake, not another harder session. In a Men’s Health report, nutrition and training specialists have highlighted the risks of relative energy deficiency in sport, known as RED-S, and the pressures that can leave active people under-fuelled.

The report brings together advice from Albert Matheny, a registered dietitian, certified strength and conditioning specialist and co-founder of SoHo Strength Lab, and Dani Singer, a certified personal trainer and founder of Fit2Go Personal Training. For athletes preparing for HYROX or hybrid racing, their central message is relevant: training demands need adequate nutritional support.

What RED-S means for training

RED-S develops when energy intake persistently falls short of the demands of exercise and everyday life. The report distinguishes this from a modest calorie deficit used for weight loss: the concern is sustained under-fuelling while continuing to ask the body for high levels of output.

Matheny explains that, with insufficient energy available, the body gives priority to essential functions rather than muscle growth and sporting performance. Singer similarly stresses that food must support more than workouts. Breathing, hormone production and cellular repair also require energy.

For a hybrid athlete, the implication is straightforward. Food is not simply something to reduce while pursuing a race goal. It supports the recovery and physical development that training is intended to produce.

• Deep, persistent fatigue, alongside soreness that lasts for days after a basic workout.
• Reduced motivation to train and a plateau in fitness progress.
• Feeling cold frequently, weakened immunity and hormonal disruption.
• Greater susceptibility to overuse injuries and stress fractures, with reduced bone mineral density among the concerns.

Singer links these problems to the body having too little energy available for recovery. He also explains that building muscle and developing physical skills become harder when the necessary nutritional resources are missing.

That matters when assessing a disappointing training block. The report’s advice points towards reviewing whether your intake supports your workload, rather than treating persistent exhaustion or stalled performance as a reason to demand still more from yourself.

Diet pressure, GLP-1s and a practical response

The report identifies a combination of restrictive diet messaging and demanding exercise routines as a concern. Singer describes the tension between pursuing maximum physical output and trying to keep calorie intake as low as possible.

Appetite-suppressing GLP-1 medications add another consideration. Singer explains that someone taking these medicines may feel full despite eating very little. Pairing that reduced intake with a rigorous new training routine can create a shortfall they do not recognise.

The issue described is the mismatch between food intake and energy demands. For athletes pursuing weight loss alongside race preparation, that makes an honest review of eating habits particularly relevant.

The practical starting point is to assess what you actually eat against the activity you expect your body to sustain. Singer recommends paying attention to persistent exhaustion, prolonged soreness and falling motivation rather than dismissing them as unavoidable parts of training.

He also advises prioritising protein to support recovery and eating enough carbohydrate to fuel the work being undertaken. The emphasis is on providing resources for the capabilities you want to build, rather than judging food only by how much you can cut.

Energy needs differ between individuals. If you are struggling to balance fuelling and training, the report recommends working with a registered dietitian who understands sport. For HYROX and hybrid racing, the useful takeaway is to treat adequate fuelling as part of preparation, not something separate from it.

This article draws on Korin Miller’s reporting and the expert contributions from Matheny and Singer in Men’s Health, covering RED-S symptoms, contributing factors and approaches to supporting training through adequate nutrition.

This story is based on reporting from the original source.