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Most cycling advice is about doing more — more volume, more intensity, more discipline. This article is about the opposite failure, which is more common than the sport likes to admit and does far more damage: not eating enough to support the training you're doing.
It often starts sensibly. A rider wants to climb better, so they lose a bit of weight. Performance improves, which is encouraging, so they lose a bit more. Training load stays high or rises. And somewhere in there the arithmetic quietly stops working.
Everything below is a description of well-documented risks in endurance sport, written to help you recognise a pattern and go and get it checked. It is not a diagnosis, and nothing here is a recommendation to take any supplement. Deficiencies should be identified with a blood test and treated by a doctor — self-supplementing without testing can be useless at best and genuinely harmful at worst (iron in particular).
Low energy availability, in plain terms
Sports science has a name for the underlying problem: low energy availability. The idea is simple. Take the energy you eat, subtract the energy your training burns, and what's left has to run everything else your body does — hormones, immune function, bone maintenance, repair, thinking, staying warm.
When that remainder is too small for too long, your body starts shutting down the functions it can survive without in the short term. The condition this leads to is often referred to as RED-S (Relative Energy Deficiency in Sport). It affects riders of all genders, and it is emphatically not just an elite problem — amateurs chasing a climbing weight are squarely in the risk group.
Warning signs worth taking seriously
None of these alone proves anything. Several together, over weeks, is a reason to see a doctor.
Performance plateaus or slides despite training
You're getting ill unusually often
Persistent fatigue that rest doesn't fix
Poor sleep, or waking unrefreshed
Loss of libido; menstrual changes or absence
Stress fractures or slow-healing niggles
Feeling cold all the time
Low mood or unusual irritability
The cruel part is that the earliest symptom is usually performance stagnation — and the instinctive response to that is to train harder and eat less, which makes it worse. If you're doing everything right and going backwards, under-fuelling belongs on your list of suspects well before "not enough intervals."
The nutrients most often short in cyclists
Again — test before you treat. These are the ones worth asking your doctor about if the pattern above sounds familiar.
Iron
Central to carrying oxygen in the blood. Deficiency is one of the more common causes of unexplained fatigue and flat performance in endurance athletes — and disproportionately affects menstruating riders.
Food sources: Red meat, liver, lentils, beans, dark leafy greens. Vitamin C alongside plant sources aids absorption.
Vitamin D
Involved in bone health, muscle function and immune response. Low levels are widespread in northern latitudes over winter — exactly when cyclists train indoors and see little sun.
Food sources: Oily fish, eggs, fortified foods. Sunlight is the main natural source, which is the problem in December.
Calcium
Bone density is a genuine concern for cyclists because the sport is non-weight-bearing. Under-fuelling makes it considerably worse.
Food sources: Dairy, fortified plant milks, tinned fish with bones, leafy greens.
Magnesium
Involved in muscle function and energy metabolism. Losses rise with heavy sweating — which indoor training produces plenty of.
Food sources: Nuts, seeds, wholegrains, dark chocolate, leafy greens.
Sodium
The electrolyte lost in the largest quantity in sweat. Heavy or salty sweaters riding indoors can lose a great deal of it.
Food sources: Ordinary salt, and electrolyte drinks on long or hot sessions.
Supplementing a nutrient you aren't short of does nothing, and in the case of iron it can cause real harm — excess iron accumulates and is toxic. A blood test costs very little compared with a season lost to fatigue, and it tells you whether the problem is a deficiency at all. Very often the actual problem is simpler: not enough total food.
Fixing it
- Eat around your training. Carbohydrate before and during hard sessions, and a proper meal afterwards. Fuelling the work is not a failure of discipline.
- Don't run a deficit during hard training blocks. If you want to change body composition, do it in an easier phase, gradually, and not while trying to build fitness.
- Protect protein intake. It supports repair and helps preserve muscle if you are losing weight.
- Eat enough on rest days. A very common error — appetite is often lower, but that's when a lot of the repair actually happens.
- Get bloods done if the pattern fits. Especially if you're persistently exhausted or getting repeatedly ill.
Where training tools help — and where they don't
No app can diagnose this, and any that claims to should be treated with suspicion. What a training log can do is show you the pattern: load climbing while performance flattens, resting heart rate creeping up, sessions increasingly abandoned. Moveee tracks Fitness, Fatigue and Form from your own rides, and adapts your plan when you're clearly not absorbing the work — see Fitness, Fatigue and Form explained.
Indoor sessions are also, for what it's worth, the easiest place to practise fuelling properly — everything you need is in the kitchen ten steps away, and a session in Moveee Indoor makes it obvious how long you'll be riding and therefore how much you need to take on. Our guide to nutrition for indoor cycling has the numbers.
But the underlying message is simpler than any of that. If you are training hard and feeling steadily worse, the answer is very rarely more training or less food. Go and talk to a doctor.
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