Why Am I Always Tired? 7 Nutrition Mistakes That Can Cause Fatigue in Endurance Athletes

Feeling tired after a hard training session is completely normal. But if you are constantly exhausted, struggling to complete workouts, recovering poorly, or seeing your performance decline, it may be time to look beyond your training plan.

Many athletes assume the solution is to eat more. But good sports nutrition is much more complicated than that. It's not simply "calories in versus calories out." Two foods can contain a similar number of calories but have very different effects on your body, depending on their carbohydrate, protein, fat, fiber, and micronutrient content.

For example, a slice of bread with Nutella and a slice of bread with avocado might provide a similar amount of energy, but they deliver a very different nutritional composition and therefore can have very different effects on satiety, blood glucose, digestion, nutrient intake, and recovery.

This doesn't mean that Nutella or other energy-dense foods are "bad" or that every meal needs to be perfectly balanced. Context matters. A high-sugar food can be extremely useful before or during exercise when you need rapidly available carbohydrate. Avocado may be more appropriate at another meal when you want healthy fats, fiber, and greater satiety.

This is the difference between eating more and eating smarter.

For an endurance athlete, good nutrition is about finding the right balance of carbohydrates, protein, fats, and micronutrients in the right quantities and at the right time to support training, recovery, and health.

Whether you are a runner, cyclist, triathlete, swimmer, or another endurance athlete, your nutrition should reflect what your body is asking it to do.

1. You're not eating enough overall

One of the first things to consider when an athlete feels persistently tired is energy availability.

Your body needs energy not only for training but also for basic physiological functions, recovery, hormone production, immune function, and maintaining healthy tissues. If your training volume increases but your food intake doesn't, you may gradually develop an energy deficit.

Consistently under-fueling can contribute to:

• Persistent fatigue

• Poor recovery

• Reduced training quality

• Increased hunger and cravings

• Poor sleep

• Mood changes

• Increased illness or injury risk

• Declining performance

  
  

However, this doesn't necessarily mean you should simply add more calories.

The quality, composition, and timing of those calories matter too.

An athlete eating enough calories but consistently choosing foods that don't provide adequate carbohydrates around training, sufficient protein, essential fats, or key micronutrients can still have a poor nutritional strategy.

2. You're not eating enough carbohydrates

Carbohydrate is one of the most important fuels for endurance exercise, particularly as exercise intensity increases. Your muscles store carbohydrate as glycogen, which provides readily available energy during training and competition.

When carbohydrate intake is too low relative to your training demands, you may experience:

• Heavy legs

• Difficulty maintaining pace or power

• Poor high-intensity performance

• Increased perceived effort

• Feeling unusually tired during training

• "Bonking" or suddenly running out of energy

• 
  

Your carbohydrate requirements aren't the same every day. A long run, hard interval session, or competition will generally require more carbohydrate than a rest day or easy recovery session.

This is another example of why timing matters.

3. You're starting training under-fuelled

What you eat before exercise can significantly affect how you feel during the session.

Starting a demanding workout with low carbohydrate availability can make the session feel much harder and may limit your ability to hit the intended intensity.

For longer or higher-intensity sessions, a carbohydrate-rich meal or snack beforehand can help provide readily available energy.

Examples include:

• Oats with banana and honey

• Toast with jam

• Pancakes with maple syrup

• Rice pudding with fruit

• Cereal with milk

• Banana and a carbohydrate drink

  
  

The amount and timing should be adjusted according to the athlete, session, and individual tolerance.

Vivify Sport's healthy banana bread. Find more recipes here

4. You're not fueling enough during long sessions

One of the most common causes of fatigue during endurance exercise is simply not consuming enough carbohydrate during the session. During prolonged exercise, carbohydrate intake helps maintain blood glucose and provides an additional fuel source for working muscles.

A useful starting point is:

30–60 g carbohydrate/hour for shorter endurance sessions.

For longer sessions:

60–90 g/hour is commonly recommended.

Well-trained athletes may progressively work towards 90–120 g/hour during prolonged, high-intensity events, provided they have trained their gastrointestinal system to tolerate these amounts. This applies across endurance sports—whether you're running a marathon, cycling a long-distance event or completing a triathlon.

The key is to start fueling before you feel exhausted, rather than trying to catch up once fatigue has already developed.

5. You're not eating enough protein

Carbohydrate gets most of the attention during endurance training, but protein is essential for recovery and adaptation. Protein provides the amino acids required to repair and remodel muscle tissue following exercise. A general target for many endurance athletes is approximately 1.6–2 g of protein per kg of body weight per day, although individual requirements depend on training load, energy intake and goals. Rather than consuming most of your protein at dinner, distribute it across your meals throughout the day.

Useful sources include:

• Eggs

• White Fish

• Fatty Fish

• Lean meats

• Greek Yogurt

• Skyr Yogurt

• Cottage cheese

• Milk

• Lentils

• Beans

• Quinoa

• Protein-rich grains

• Protein powders

  
  

For vegetarian and vegan athletes, particular attention should be paid to total protein intake and protein quality.

6. You're overlooking micronutrients

Calories are only one part of nutrition. Your body also needs vitamins and minerals to produce energy, transport oxygen, support immunity, repair tissues, and maintain normal physiological function.

Iron is particularly important for endurance athletes because it plays a key role in oxygen transport and energy metabolism. Low iron status can contribute to tiredness and impaired exercise performance. Athletes with high training loads can be particularly vulnerable. However, fatigue does not automatically mean you are iron deficient.

If you suspect low iron, appropriate blood testing and professional assessment are important before supplementing. Other nutrients that may deserve attention depending on the individual include vitamin B12, folate, vitamin D, calcium, and magnesium.

This is another reason why eating a varied diet is important. You can meet your calorie requirements while still having gaps in micronutrient intake.

7. You're not recovering properly

Nutrition is only one part of the recovery equation.

You can have an excellent diet and still feel exhausted if you are consistently:

• Sleeping too little

• Training too frequently

• Taking too few recovery days

• Increasing training load too quickly

• Stacking multiple hard sessions together

• Training while unwell

• 
  

Your body adapts to training during recovery, not simply while you are exercising.

After demanding sessions, prioritize carbohydrates, protein, fluids, and sleep.

If you're training frequently, recovery nutrition should be part of your overall training strategy rather than an afterthought.

It's not just about calories

This is perhaps the most important message.

Nutrition is more complicated than calories in versus calories out. Calories matter. Energy balance matters. But where those calories come from, how they are distributed, and when you consume them also matter.

Think about two foods with a similar calorie content:

Bread + Chocolate Spread

versus

Bread + avocado

They can provide a similar amount of energy, but their nutritional composition is very different.

The chocolate spread option provides considerably more rapidly available carbohydrate and sugar, while avocado provides more fat, fiber, and micronutrients. Neither is inherently "better."

The context determines the right choice.

Before a hard training session, rapidly available carbohydrate may be exactly what you need. At another meal, a source of healthy fats, fiber, and micronutrients may be more appropriate. This is what smart nutrition means: not labelling foods as good or bad, but understanding what your body needs, when it needs it and in what quantity.

At Vivify Sports, we don't believe in obsessing over calories

At Vivify Sports, we don't believe that constantly counting calories is the best way to approach sports nutrition. Calories are important, but nutrition is about much more than a number.

Instead, we focus on macronutrients — carbohydrates, protein and fats — and how they are distributed according to your training, recovery and individual goals.

An athlete completing a hard interval session will have very different carbohydrate requirements from someone having a rest day. Protein requirements need to support recovery, while dietary fats provide essential fatty acids and support overall health.

We also consider food quality, micronutrients, fiber hydration, timing, and individual preferences.

The goal isn't to make you spend your day weighing every ingredient or worrying about whether you've eaten 100 calories too much.

The goal is to understand what your body needs, when it needs it and in what quantities. That's what we mean by eating smarter, not simply eating more or less.

At Vivify Sports, we use nutrition to support your training—not to make food another source of stress.

Is fatigue always caused by nutrition?

No.

Persistent fatigue can have many causes, including inadequate sleep, excessive training load, psychological stress, illness, medication, nutritional deficiencies, and underlying medical conditions.

If fatigue is persistent, worsening, or accompanied by other symptoms, it is important to seek appropriate medical assessment rather than assuming that you simply need more food or supplements.

Nutrition is one part of the bigger picture.

How do you know if you're under-fueling?

Some potential warning signs include:

• Persistent tiredness

• Declining performance

• Poor recovery between sessions

• Constant hunger

• Difficulty concentrating

• Poor sleep

• Frequent illness

• Feeling unusually cold

• Loss of training motivation

• Increasing perceived effort during normally easy sessions

  
  

One symptom alone doesn't necessarily mean you're under-fueling. But if several of these signs occur together, it is worth reviewing your nutrition and training load.

What should you do if you're constantly tired?

Before reaching for another supplement, ask yourself:

Am I eating enough overall?

Am I getting enough carbohydrate for my training load?

Am I eating before demanding sessions?

Am I fueling longer workouts adequately?

Am I consuming enough protein?

Am I getting enough healthy fats and micronutrients?

Am I sleeping and recovering enough?

Have I had relevant blood markers checked if appropriate?

Has my training load increased recently?

Sometimes the answer isn't another supplement. Sometimes it's better food choices, better timing, appropriate quantities, and more recovery.

The bottom line

Fatigue is your body's way of telling you that something needs attention. For endurance athletes, under-fueling is one factor that should never be overlooked. But solving fatigue isn't necessarily about simply eating more.

It's about eating smarter.

Your nutrition should provide the right balance of carbohydrates, protein, fats, fiber, vitamins, and minerals in the right quantities and at the right time to support the demands placed on your body.

Because good sports nutrition isn't about eating as little as possible or as much as possible.

It's about giving your body exactly what it needs to train, recover, adapt, and perform.

Scientific sources

• Thomas DT, Erdman KA, Burke LM. Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance. Journal of the Academy of Nutrition and Dietetics.

• Burke LM et al. Carbohydrates for training and competition. Journal of Sports Sciences.

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

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

• Sim M et al. Research on iron considerations for athletes and iron deficiency in sport.

Marine Lenehan, Sports Nutritionist