What RED-S is
Relative Energy Deficiency in Sport describes the consequences of low energy availability, when the energy left over after training is not enough to support normal bodily functions. In a growing adolescent, that shortfall has to cover growth as well.
The body responds by down-regulating processes it can afford to reduce: reproductive function, bone remodelling, immune function, metabolic rate and, eventually, performance.
How it happens
It is worth emphasising that RED-S is often unintentional. Many affected athletes are simply not eating enough for what they are doing.
- Training load increasing without a matching increase in food.
- Busy schedules leaving no time to eat properly between school and training.
- Deliberate restriction, sometimes driven by body image or sport culture.
- Growth spurts sharply increasing requirements that nobody adjusts for.
- Fussy or limited diets that fall short on total energy and key nutrients.
What it affects
- Bone: reduced bone mineral density and stress fractures, with lasting consequences.
- Menstrual function: irregular or absent periods.
- Growth: potential impact on the adolescent growth trajectory.
- Immunity: frequent illness and slow recovery.
- Mood and cognition: low mood, irritability, poor concentration.
- Performance: reduced endurance, strength, speed and training response.
Warning signs for parents
- Periods becoming irregular or stopping.
- Recurrent bone stress injuries.
- Weight loss or failure to gain weight during a growth phase.
- Fatigue disproportionate to training.
- Performance declining despite training more.
- Avoidance of meals or whole food groups, or anxiety around eating.
- Frequent illness and slow healing.
What to do
The first step is increasing energy intake - more meals, more carbohydrate around training, regular snacks, and reviewing training load. In many mild cases those two changes resolve the picture.
Where periods have stopped, there are bone stress injuries, or there is any sign of disordered eating, involve a GP and ideally a sports dietitian. This is not something to manage informally.
Preventing it
- Eat before and after training as a routine, not an afterthought.
- Increase food when training increases, the schedule change should trigger it.
- Include carbohydrate at every meal for a training adolescent.
- Ensure adequate calcium, vitamin D and iron intake.
- Talk about food as fuel for performance rather than as weight control.
- Treat a missing period as a signal to investigate, not something to ignore.