Inadequate fueling. RED-S

Dec 12, 2021Nutrition

Athletes and active individuals wanting to maximize their performance and those pursuing aesthetic goals can restrict their energy intake too much. At first, they will lose weight, which leads to better performance but if practiced chronically as it is common in endurance and weight-based sports, it will lead to health problems.

In this comprehensive article you will discover:

  • what Relative Energy Deficiency is
  • who is at risk
  • the symptoms and consequences of RED-S


When you eat matters

If you not only want to maintain weight but also feel energetic and light, it is important what foods you eat and how you distribute them during the day. Did you know that you can take in enough calories to support your training within a day but still stagnate, feel heavy, or even get into the RED state?

It is necessary to look at within-day energy intake and energy intake at times around your training differently.

The typical signs of inadequate fueling include:

  • Your performance during training is lower despite giving it your all
  • Gradual loss of performance
  • Stagnation in training
  • Delayed recovery between training sessions
  • Skipping or missing menstrual cycle (Female athlete triad)
  • The body holds onto more fat
  • Disturbed sleep
  • Fatigue throughout the day
  • Cravings in the evenings (especially sweets cravings)
  • No motivation for training

If you spot multiple of these signs, there is a good chance you are underfueling.

Underfueling can be intentional such as during weight loss but often it is unintentional.

Underfueling can be intentional as when you are trying to cut fat and lose weight. From a short-term perspective, it does not need to have be a detriment to your exercise performance, and it can even improve your weight to power ratio if done correctly.

But underfueling is often unintentional. Athletes who want to eat better often eat more vegetables, and avoid gluten-containing grains, which can decrease their energy intake.

Having enough energy around the training is especially important for the female athletes!

Clinically, it is called relative energy deficit or RED-S, which manifests itself mainly in athletes with high energy expenditure and insufficient energy intake.

Who is at risk?

Typically, athletes who compete in sports where body weight is a concern (running, cycling, MMA, and other weight-categorized sports) and aesthetically driven arts (bodybuilding, bikini fitness, dancers) are at the highest risk.
But it’s happening across all sports – and even in recreational exercisers.
Due to the psychological factors, females and young female athletes are at higher risk.

Although until recently RED-S was more often observed in the female population, it was partially because men did not receive enough attention.

While a female can notice skipping or missing the menstrual cycle as a quick sign of low energy availability, the male’s body does not react as quickly.

Endurance athletes, female athletes, and people pursuing aesthetic goals are at increased risk.
Among endurance athletes, cyclists seem to be at a higher risk than runners. (source)
Although until recently RED-S was more often observed in the female population, it was partially because men did not receive enough attention.

How low energy intake is too low?

There is not a clearly established threshold for low energy availability, but the most often used formula defines it as less than 30kcal/kg of lean body mass.

What happens to your body during chronically low energy availability?

The body fuels activities of the highest energy demand first. During periods of low energy availability, you can still have energy for training, but your body’s systems start to shut down as a result of low energy availability. An athlete in training with high expenditure and low energy intake can hinder his/her health if energy availability stays low for an extended period of time.

That is when athletes start to notice the negative consequences.

  • Body creates a lower setpoint weight
  • Lower metabolic rate, which makes athletes frustrated and they eat less to lose fat!!!
  • Thyroid functions lowered (sex steroid pathways, IGF-1)
  • Muscle loss due to lower Muscle Protein Synthesis(IGF-1) and higher protein breakdown (Muscle oxidative capacity stays the same)
  • Cortisol up (fat storage, bad body composition)
  • Digestion problems, IBS
  • Lower peak power
  • Low performance and recovery
  • Bone density loss (lowered bone turnover)
  • Amenorrhea


Carbohydrates seem to be important to prevent RED-S

Carbohydrates fuel performance, but they also seem to have an association with RED-S.

Some studies even found that low-carb diets elicited small negative effects on athletes’ iron levels, immune functions, and response to exercise, but no substantial alterations to athlete health were observed when athletes restricted energy availability with sufficient carbohydrates. The authors concluded that “Short-term restriction of CHO, rather than energy, may have greater negative impacts on athlete health.” (source)

Poor chronic CHO availability, beyond or instead of LEA (low Energy Availability), is emerging as a potential mechanism also associated with some RED-S related outcomes (source)


What are some of the best practices to make sure you are fueling enough for your workouts so you feel light and energetic for your training.

If you want to get a better overview of your health, you have the option of testing blood biomarkers:

  • Cortisol – increased levels of cortisol
  • Testosterone – usually reduced
    Vitamin D
  • Ferritin (iron stores). A typical problem of endurance athletes,especially women, vegans and vegetarians.
  • Inflammatory markers – in runners, they are usually manifested by painful joints and repetitive injuries.

These markers will give you a better overview of your nutritional and physiological status and where your health is trending e.g. reduced low ferritin levels will show up before you have reduced hemoglobin – at which point athletes tend to be chronically fatigued.

What is the root cause of these conditions?

Too much stress (physical and mental) and reduced availability of energy (little food).
Therefore, if you take your training seriously, you should also take your rest and diet seriously.

Fueling best practices:

  • Eat 3-6 times a day – smaller meals are digested more quickly and make you feel lighter.
  • Include pre-workout snacks if your training is more than 4 hours after your main meal.
  • Fuel during sessions longer than 60 minutes.
  • If you had no pre-workout, and after training, you have more than an hour till your next meal, include postworkout protein and carbs, ideally in a 1:3 ratio. (banana and a scoop of protein)
  • Don’t skip breakfast, even if your workout is in the evening.
  • If you are training hard for 10 or more hours a week, then don’t limit carbs on your rest days. – you have relatively high physical activity, even if it doesn’t feel hard.

Why do athletes have trouble getting enough carbohydrates in their diet?

There are several reasons why athletes try to eat carbs with the problem of eating enough carbs. Some are physiological, some mental.

However, if your appetite is low, it may be due to exercise-induced stress or suboptimal food choices.

Typical cases are vegans, but also people who try to eat healthily. Their diet is dominated by vegetables, whole grains, lean meats, nuts, and legumes. While such a diet is undoubtedly healthy and helps to lose weight and subsequently maintain weight, it prevents people with high energy demands from getting sufficient energy.

Legumes, vegetables, whole grains are full of fiber, which is essential for the satiety and the health of the digestive tract.

In addition, vegetables are low in calories. Therefore, an athlete with a high energy expenditure should not have diet centered around vegetables but rather look at higher calorie sources of healthy nutrients.

Lean meat provides high-quality proteins, but at the same time, it can feel heavy for an athlete. Excessive protein intake is also not a benefit if it prevents the athlete from consuming sufficient total energy and carbohydrates. Quantity is essential! You do not need to eat 400g of chicken breast 3 times a day. 100g of chicken breast contains approx. 30g of protein.

Psychological aspects of underfueling

The psychological aspect is also important and I deal with it mainly in personal work with clients, because each person is psychologically very different.

In general, there are several dysfunctional patterns of thinking that athletes have.

“It’s only 10km, that doesn’t even count as training.”

Whether you consider running 10 km as training or not, the fact remains that you had to spend energy on it. Just because running 10km now seems easier compared to when you started with running, it doesn’t mean you’re not spending energy.

“Today I have a day off and I should eat less.”

If you are active outside of training, e.g. you walk 10,000 steps, or you have a job where you are physically active, you are expending energy. This energy expenditure can easily exceed the energy expended during training.

In addition, the training volume during the week should be taken into account. If you train e.g. 9 hours or more a week, you don’t need to eat enough calories and carbs. Therefore, limiting food during rest days can lead to insufficient energy intake in terms of the week.

“Unless I’m training hard, I don’t need as much energy.”

You may not feel this way, but you can burn more energy during light activity than during intense training. This is mainly because we cannot train at high intensity for long. On the other hand, we can walk for hours and not feel exhausted. Evaluating the activity in terms of intensity is not at all beneficial. (see table below)

In the table, you will find an example of the energy expenditure of 70 kg (154 lbs) runner who runs 10 km (6.2mi).

Pace (min per km) min per mile Energy expenditure
6:00 9:39 720
5:30 8:51 741
5:00 8:03 698
4:30 7:14 678
4:00 6:26 627

The data in the table is based on and your individual expenditure may vary. The table serves to illustrate the principle.

As you can see, the runner has the biggest energy expenditure at a pace of 5:30 per km. Faster pace and more effort does not automatically mean more energy expenditure.

After intense strength training, people usually feel hungry. After intense cardio, quite the opposite. Here you need to look at the facts, not just the feelings.

Learn how to best fuel your body

If you want to learn how to best fuel your body, stop tracking your food and meet with Daniel who specializes in sports nutrition and can help you create a sustainable diet/food plan that fits your unique needs and lifestyle.

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