You can be fit and still be running on empty

What your performance doesn’t tell you

You finish your sessions. You hold your paces. You lift the same loads as before, sometimes more.

And yet, between workouts, you feel worse. Waking up is hard. The soreness lingers. Your energy dips during the day. You get sick more often, or your menstrual cycle has changed.

Because your results are still good, you keep going. You put the fatigue down to work, to age, or simply to training seriously.

But getting through a session doesn’t guarantee that you have the resources to recover well and repeat that effort over time.

In our previous articles, we explored the factors that can limit performance, then the conditions that turn training into adaptation. In this article, we look at what can go unnoticed while the results are still holding up.

Performance shows what your body can produce today. It doesn’t always tell you what that costs, or whether your needs are covered well enough to keep going over time.

A good session only tells part of the story

Your performance depends in part on your past training, your technique and the physical capacities you have already built. Those capacities don’t disappear as soon as your sleep worsens or your intake falls short.

So you can get through your sessions while your iron stores are low, your energy availability is insufficient or your recovery is deteriorating.

There is no single timeline that applies to everyone. Performance can drop quickly in some people, while others first notice changes in their sleep, mood, cycle or day-to-day energy.

To spot this gap, pay as much attention to what happens between sessions as to the sessions themselves:

With comparable training, do you have to put in more effort to get the same result? And do you get your energy and your motivation to train back between sessions?

Feeling tired after a hard effort is normal. Fatigue that builds up, persists despite rest or reduces your ability to function day to day is worth looking into.

1. Iron: stores that fall before anemia appears

Iron plays a role at several levels. It allows hemoglobin to carry oxygen, supports how myoglobin works in muscle, and is part of enzymes involved in energy production.

Ferritin reflects your iron stores. A hemoglobin test measures the concentration in the blood of the protein that carries oxygen.

This distinction matters: your stores can fall while your hemoglobin stays normal. So you can be short of iron without being anemic.

In some people, this shortfall already contributes to unusual fatigue, heavy legs or poorer adaptation to training.

Heavy periods, blood donation, insufficient dietary intake or absorption problems can make this more likely. Endurance training can add losses and change how available iron is. After some sessions, hepcidin rises: this hormone temporarily slows the release of iron into the circulation.

What a blood test can clarify

Ferritin is read alongside a complete blood count, including hemoglobin and the characteristics of red blood cells, and alongside transferrin saturation when relevant.

Inflammation matters too: ferritin can rise during inflammation and give a misleading impression that your stores are sufficient.

What you can adjust

Include a source of iron in your meals regularly: meat, seafood, lentils, beans or tofu. For plant-based iron, add a source of vitamin C: lentils and bell pepper, chickpeas and lemon, tofu and broccoli.

If your stores are low, keep tea and coffee away from your most iron-rich meals.

A documented deficiency also means understanding its cause. Heavy periods or persistent digestive symptoms need appropriate care. Any supplementation is decided on the basis of your results, with follow-up on how well it works and how well you tolerate it.

2. Energy availability: eating well doesn’t mean eating enough

You can eat good-quality food, get enough protein and still lack the energy to cover your needs.

Energy availability is the energy left for the body’s functions once the energy spent on exercise has been subtracted.

This energy is used, among other things, to repair tissue, renew proteins, maintain bone health and support hormonal and immune function.

The problem can appear without any deliberate restriction. You add two sessions to your week, but your portions stay the same. You train early and push back your first meal. You have little appetite after exercise and skip the snack you planned. You cut your intake sharply on rest days, while repair is still going on.

When this shortfall becomes large enough or lasts long enough, it can contribute to REDs, relative energy deficiency in sport, which affects women and men alike.

Possible signs include:

• persistent fatigue and feeling cold;

• harder recovery;

• menstrual cycles that become irregular or stop;

• lower libido;

• repeated injuries, especially bone stress injuries.

A stable weight doesn’t guarantee that your needs are covered. The body can adapt its energy expenditure. Low energy availability can be accompanied by a drop in T3, a hormone involved in regulating metabolism.

Protein alone doesn’t solve this. In a study of 30 trained women, ten days of low energy availability reduced muscle protein synthesis despite a high protein intake.

What you can adjust

Look at your eating across the whole day, then around your sessions:

1. increase your intake when the length or frequency of your training increases;

2. plan a snack if a meal is too far from a demanding session;

3. keep carbohydrates in the meals before and after efforts that draw heavily on glycogen;

4. keep your meals sufficient on rest days.

For example, after a demanding session, a meal with rice or potatoes, a source of protein and vegetables covers more of your needs than a protein shake on its own.

If several signs point to REDs, the assessment needs to bring together diet, training, symptoms and medical context. A single blood test is not enough to diagnose it or rule it out.

3. Total load: your training plan can’t be separated from your life

The same week of training can be well tolerated during a calm period and become hard to absorb when you sleep less, work more or travel.

Even if your training volume stays the same, the resources available for recovery have changed.

This can show up as a usual pace that feels harder, soreness that lasts longer, less restorative sleep or falling motivation.

It’s also important to tell a period of fatigue linked to a demanding training block apart from a problem that is settling in. Overtraining syndrome involves, among other things, a prolonged drop in performance, and other causes need to be ruled out. Not all fatigue in an athlete is this syndrome.

What you can adjust

When several signals are getting worse, adapt the following week rather than waiting for a clear drop in results:

1. cut the number or volume of demanding sessions;

2. keep genuinely easy days between hard efforts;

3. plan rest when your fatigue calls for it;

4. avoid turning every recovery session into extra training.

Walking or very light movement can feel good, but active recovery should stay easy.

Protect your sleep as well. Seven to nine hours is a benchmark for adults; some athletes need more. If your schedule regularly forces you to choose between a session and a full night’s sleep, the way your training is organized deserves a second look.

Resting heart rate and HRV can add to your tracking. Their value lies mainly in your own trend, measured under comparable conditions. A single value or a watch score doesn’t replace your symptoms.

4. Inflammation and infections: look at what keeps coming back

Training triggers a transient inflammatory response. It is part of the repair and adaptation process.

So a raised CRP after a hard effort doesn’t prove you have chronic inflammation. White blood cells can also vary after exercise.

On the other hand, repeated infections, persistent fatigue or deteriorating recovery are reasons to review the whole picture: recent load, sleep, diet, travel, exposure to infections and any possible medical problem.

CRP, sometimes measured with a high-sensitivity method, gives information about inflammation. A complete blood count with differential describes, among other things, the different types of white blood cells.

These tests can help investigate a situation, but they don’t directly measure the “strength” of your immune system.

What you can adjust

Adapt your training during an infection and while you recover from it. Check that you’re eating enough and that you have time to sleep.

If infections keep coming back, talk to a healthcare professional. Routinely adding “immunity” supplements won’t identify the cause.

The useful tests depend on the question you’re asking

A blood test helps explore possible causes of fatigue or poor recovery, as well as aspects of metabolic health that your performance doesn’t reveal. To make sense of it, the results need to be read alongside your symptoms, your diet and your training.

Fatigue, heavy legs or breathlessness that is unusual for your level

• What to explore: Whether enough iron is available for oxygen transport and energy production; possible anemia.

• Useful markers, depending on context: Complete blood count, hemoglobin, ferritin, transferrin saturation.

Unstable energy during the day, energy crashes after meals or frequent cravings

• What to explore: Glucose regulation and, where relevant, insulin sensitivity. These symptoms can also have other causes.

• Useful markers, depending on context: Fasting glucose, HbA1c; fasting insulin depending on your profile.

Your metabolic picture beyond athletic performance

• What to explore: Lipid metabolism and risk factors that can persist despite good fitness.

• Useful markers, depending on context: Lipid panel, to be interpreted with your medical history and other risk factors.

Fatigue together with feeling cold, difficult recovery or changes in your cycle or libido

• What to explore: Thyroid function and possible metabolic adaptation to insufficient energy intake.

• Useful markers, depending on context: TSH, free T4; total or free T3 in some assessments of energy deficiency.

Persistent fatigue, repeated infections or recovery that has become unusual

• What to explore: Inflammation or infection, or a blood cell abnormality that could contribute to symptoms.

• Useful markers, depending on context: CRP or hs-CRP, complete blood count with differential.

Kidney filtration, especially when a result looks unusual in a muscular person

• What to explore: Telling a filtration problem apart from a variation linked to muscle mass, diet, creatine or recent exercise.

• Useful markers, depending on context: Creatinine, estimated GFR; cystatin C and urine tests if needed.

For a follow-up test, try to have your blood drawn well away from a competition or a very demanding session, and keep conditions comparable from one test to the next. Also mention any recent infection and the supplements you take: these can change how the results are read.

Practical benchmarks for fueling your sessions

Total intake comes first. How you spread it out can then make effort and recovery easier.

1. Before a demanding session: Plan food that suits the timing and your digestion. A meal a few hours before, or a snack closer to the session, can help you avoid starting underfueled.

2. During an endurance effort of roughly one to two and a half hours: 30 to 60 g of carbohydrates per hour is a common benchmark, to be adapted to intensity and digestive tolerance. Needs are different for a short, easy session.

3. After exercise: Combine carbohydrates and protein. A snack can bridge the gap if your next meal is a while away.

For hydration, adapt what you drink to the duration, the heat and how much you sweat. During long efforts with large losses, sodium can also help. The aim is to replace enough of what you lose, not to drink as much as possible.

Look at what happens between your sessions

For seven to fourteen days, note a few simple things:

• your energy when you wake up and during the day;

• how long you sleep and how well you feel you slept;

• how hard your usual sessions feel;

• any pain or soreness that lingers;

• skipped meals and long stretches without eating.

If it applies to you, add changes in your cycle or libido.

This tracking should help you answer one concrete question: does your current routine let you get your energy back between workouts?

Then use these observations to choose one or two concrete adjustments:

1. If you skip meals or eat too little around your sessions: Plan something to eat before training and a full meal afterward. If your meal is a while away, prepare a snack combining carbohydrates and protein, such as fruit with yogurt or a soy alternative. Keep your intake sufficient on rest days too.

2. If your usual sessions are getting harder and fatigue is building up: Lighten the following week: reduce the volume or the number of intense sessions, and plan more rest or very light movement. Watch whether your energy, your aches and your motivation to train improve.

3. If your nights are regularly too short: First set aside enough time to sleep, then adapt your training times. Shortening or moving an early-morning session can do more good than continuing to cut your sleep to keep it.

The first sign of improvement may show up in everyday life: waking up more easily, steadier energy, fewer lingering aches or more desire to move. Your time on the clock isn’t the only result worth tracking.

If the fatigue persists, your periods stop or injuries keep recurring, look into it further with a healthcare professional, even if your athletic results are still fine.

With Lucis, understand what deserves your attention

Your performance is one source of information. Your daily energy, your recovery, your symptoms and your blood test results are others.

At Lucis, we help you bring this information together, spot what needs a closer look, prioritize the adjustments that matter and track how things change.

You train regularly, but your energy or your recovery has changed? Take stock with a Lucis health assessment.

To go further, read our articles on performance and recovery, and listen to the episode with Anaïs Gautron and Marion Paret on nutrition, stress and sleep for athletes.

To go further

Listen to the Lucis Podcast episode with Anaïs Gautron and Marion Paret on nutrition, stress and sleep for athletes.

Scientific references

1. Australian Institute of Sport. Iron: How and when do I use it?

2. Burke LM, et al. Carbohydrates for training and competition. Journal of Sports Sciences, 2011.

3. Jäger R, et al. International Society of Sports Nutrition Position Stand: protein and exercise. Journal of the International Society of Sports Nutrition, 2017.

4. KDIGO. Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney International, 2024.

5. Meeusen R, et al. Prevention, diagnosis, and treatment of the overtraining syndrome: joint consensus statement of the ECSS and ACSM. Medicine & Science in Sports & Exercise, 2013.

6. Mountjoy M, et al. IOC consensus statement on Relative Energy Deficiency in Sport (REDs). British Journal of Sports Medicine, 2023.

7. National Institute of Diabetes and Digestive and Kidney Diseases. The A1C Test & Diabetes.

8. National Institutes of Health, Office of Dietary Supplements. Iron: Fact Sheet for Health Professionals.

9. Oxfeldt M, et al. Low energy availability reduces myofibrillar and sarcoplasmic muscle protein synthesis in trained females. The Journal of Physiology, 2023.

10. Peeling P, et al. Iron status and the acute post-exercise hepcidin response in athletes. PLOS ONE, 2014.

11. Saw AE, Main LC, Gastin PB. Monitoring the athlete training response: subjective self-reported measures trump commonly used objective measures. British Journal of Sports Medicine, 2016.

12. Sawka MN, et al. Exercise and fluid replacement. Medicine & Science in Sports & Exercise, 2007.

13. Schwellnus M, et al. IOC consensus statement on load in sport and risk of illness. British Journal of Sports Medicine, 2016.

14. Sim M, et al. Iron considerations for the athlete. European Journal of Applied Physiology, 2019.

15. Stellingwerff T, et al. Development and validation of the IOC REDs Clinical Assessment Tool, version 2. British Journal of Sports Medicine, 2023.

16. Walsh NP, et al. Sleep and the athlete: expert consensus recommendations. British Journal of Sports Medicine, 2021.

This content is provided for educational purposes and does not replace medical advice, a diagnosis or a prescription. Tests and any supplementation should be adapted to your individual situation. Persistent fatigue, cycle disruption, repeated injuries or suspected REDs require assessment by a healthcare professional.