Restrictive Eating and Running Performance
“Lighter = Faster.” On paper, it seems logical — and frankly, our weight impact calculator clearly shows that shedding a kilo can shave off a few seconds per kilometer. But be warned, this logic has its limits: below a certain threshold, losing weight will hinder your progress rather than help it. And personally, I think the damage extends far beyond your race times.
The RED-S Syndrome
RED-S (Relative Energy Deficiency in Sport) — formerly known as the “female athlete triad,” but it affects men too — encompasses a range of issues that arise when you don't consume enough calories relative to your energy expenditure (running + basal metabolism). Essentially, your body switches into “energy-saving mode”:
- Your hormones go haywire (amenorrhea in women, drop in testosterone in men)
- Your bones become fragile → hello stress fractures
- Your basal metabolic rate drops
- Your thyroid gland malfunctions
- You catch every bug (weakened immune system)
- Your mood and concentration take a hit
The Illusion of Short-Term Gains
A runner who drops 3 kg by restricting their diet might indeed shave 1-2 minutes off their marathon time initially. However, if this weight loss is accompanied by a chronic caloric deficit, the mid-term consequences completely negate any benefit: recurrent injuries, iron deficiency, overtraining, persistent fatigue, and more…
From my readings, BMI is a very crude indicator. A BMI of 18.5 might be perfectly fine for an elite athlete but dangerous for you or me. What truly matters is “energy availability” — essentially, the energy left after accounting for your training expenditure. Below 30 kcal/kg of lean mass/day, things start to get risky.
Healthy Weight Loss
- Small deficit (200-300 kcal/day max, no more)
- Outside of competitive periods
- Maintain your protein and micronutrient intake
- Gradual loss (0.3-0.5 kg/week)
- Medical supervision if you're losing significant weight
Signs of Energy Deficit
- Amenorrhea or irregular cycles
- Recurrent stress fractures
- Your performance is declining despite training
- You constantly think about food (or avoid it)
- You always feel cold
Optimizing Body Composition Without Deprivation
My take: The best approach for runners is to improve body composition through training — not through restriction. Strength training, quality workouts, and eating until you're satisfied allow you to improve your power-to-weight ratio without the risks of an energy deficit. Our running weight loss calculator clearly shows that increasing your expenditure through training is much safer than restricting your diet.
My takeaway: Losing weight can help you run faster — but a chronic deficit will destroy you. Eat enough to fuel your training, and let your body composition optimize itself naturally. A well-fed and well-trained runner will always outperform a starved runner in the long run, believe me.
Frequently Asked Questions
Does losing weight make you run faster?
To a certain extent, yes: it's estimated you can gain 2-3 seconds per km per kilogram lost. However, below a critical threshold, performance declines and health risks increase.
What is RED-S?
RED-S (Relative Energy Deficiency in Sport) is a syndrome caused by chronic energy deficit. It leads to hormonal, bone, and immune system dysfunctions, as well as a decrease in performance.
Can you diet during marathon training?
It's not recommended. Marathon preparation requires sufficient caloric intake to support the training load. It's better to lose weight during the off-season, not during peak training.