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For female athletes: Why an energy deficit is so dangerous (including low carb).

What underestimated consequences and risks an energy deficit brings with it and how you can prevent it.

Faster, higher, further. The centuries-old Olympic motto has not lost its meaning to this day and is ingrained in many minds. Those who push body and mind to the limits get recognition.

This is also reflected in the extent with our body weight – whereby especially in endurance sports often mistakenly applies: As slim as possible = the better.

Many athletes and coaches are unfortunately (still) unaware of the dire consequences that can result if nutrition/food is not appropriately coordinated with training.

What exactly happens when the whole thing becomes unbalanced? When the energy supply can no longer meet the required energy demand in the long term?

What exactly does an energy deficit in sports mean?

In Relative Energy Deficit in Sports (RED-S) , repeated energy deficiencies cause hormonal changes, which in turn can lead to decreased performance in sports, changes in bone density, and other health limitations.

RED-S occurs when long-term energy intake is too low to meet total energy requirements for elementary physiological functions.

According to experts, energy deficiency can occur due to the following possibilities:

  • Intentionally, such as by deliberately restricting food intake.
  • Due to disturbed eating behavior or a clinical eating disorder.
  • Unintentionally, such as through ignorance of how many calories the body needs with an increased workout load

The guideline for adequate energy availability is 45 kcal/kg per fat-free mass (FFM) per day. Energy availability describes the amount of energy available to the body for vital bodily functions after deducting the training metabolic rate.

If the intake falls below a value of 30 kcal/kg per FFM, numerous physiological limitations and disorders of various body systems can be observed. Therefore, 30kcal/kg per FFM is considered a critical threshold. However, this marker, fat-free measures (FFM), is difficult to diagnose. More about that later.

Symptoms and consequences of an energy deficit

Persistent low energy availability can significantly affect your performance and, more crucially, your health. Hormone balance in particular plays a role in this. Due to a lack of energy, hormones change, which can lead to various diseases.

In women, a lack of energy is usually more noticeable. In women, menstrual irregularities up to the absence of periods are often the first indication that something is wrong. In the worst case, if the lack of energy lasts for a long time, it can even lead to infertility.

These hormonal changes in both women and men result in decreased bone mineral density. As a result, the risk of fatigue fractures and osteoporosis increases sharply.

But not only physiological functions are impaired, also effects on the psyche, such as depression, could be detected.

In this chart you can see what happens when energy demand and energy intake are NOT in balance: On the left you can see the effects on athletic performance. On the right, the physical or health effects.

Other alarm signs that may indicate RED-S include:

  • Fatigue fractures
  • Quick fatigue
  • Common injuries
  • Increased susceptibility to infections
  • Gastrointestinal problems
  • Increased irritability
  • Common muscular problems

If you experience two or more of the symptoms, it is advisable to reconsider your energy intake and seek professional advice if necessary.

RED-S can therefore occur in both women and men. “Men, however, seem to be more resistant to the consequences of energy deficiency and also regenerate faster,” knows Pia Jensen, sports scientist, active competitive athlete and managing director of where’s the food? sportsnutrition.

Risk factors include, in particular, physical sports in which a slim figure and/or low weight are advantageous. Thus, the prevalence in physical sports such as gymnastics or running (69%) is significantly higher than in the normal population (2 to 5%)!

However, recent studies suggest that RED-S is not exclusive to low body weight sports. Athletes seem to be at risk especially in the preparation phase and in phases of weight reduction. Additional factors include a low BMI, disordered eating patterns, and very high exercise volumes.

“RED-S is not limited to lean women in endurance sports,” Pia Jensen further explains. “Normal-weight female athletes can also be affected. The metabolism has completely changed due to persistent undersupply, and as a result the basal metabolic rate has dropped. As a result, disturbances occur at the fat metabolism level. With the attempt to supply less and less energy in order to lose weight, one only harms oneself.”

Her plea: “Don’t turn a blind eye to symptoms.”

How is an energy deficit diagnosed?

To start with: “RED-S cannot be clearly diagnosed at the moment, but we are getting closer and closer to it,” says the nutrition expert. “This is important because early detection of an energy deficiency should be the goal!”.

One way to track RED-S is to determine its energy availability. You look at what you’re feeding your body. This is then subtracted from the training metabolic rate and divided by the “lean body mass”.

Sounds quite complicated and who already knows this value? In addition, supply and consumption cannot always be clearly quantified. Therefore, this method is prone to errors.

Better is a symptom check that you can do on yourself. What applies here?

  • General high fatigue
  • Common injuries
  • Menstrual disorders
  • Decreased libido
  • Deteriorated Mood Condition
  • Training status: scope is no longer possible, adaptations remain absent, etc.
  • Eating behavior: strong focus on weight, eating disorders, disordered eating, binge eating, etc.

Even more accurate is the differential diagnosis, which distinguishes between health problems and performance problems:

Health problems

  • Menstrual disorders
  • Delayed puberty
  • Stress fractures
  • Mental problems
  • Gastrointestinal problems

Performance problems

  • Overtraining symptoms
  • Failure to make training adjustments
  • Viral infections (EBV, CMV…)
  • Sleep problems

However, the weak point here is also subjective perception, which is why physicians often fail to recognize RED-S.

A good tip is to have blood work done by your doctor. This is because the so-called “diagnostic markers” change should RED-S be present.

Altered sex hormones (top right in the diagram) and thyroid hormones (T3, left) can provide information about whether something is wrong. There you can also see if the protein synthesis and the carbohydrate content is (too) low – which is of course absolutely not desirable in the performance range. Low testosterone levels may also be a sign.

The following values can be determined by the doctor to find conclusions about RED-S:

  • Sex hormones: LH, FSH, estrogen, progesterone, testosterone
  • Thyroid hormones: TSH, T4, T3
  • Cortisol (also normal weight athletes: Stress hormone shows significant permanent elevation and inhibits fat burning).
  • Insulin-like growth factor 1 (IGF-1)
  • Growth Hormone (GH)
  • Insulin
  • Cholesterol
  • Leptin, Grehlin

What to do when I have an energy deficit?

“Take care of your nutrition like you take care of your training!” What Pia Jensen says sounds logical. However, this is far from being a lived practice.

The competitive athlete continues, “We prepare mentally for training. But we should also think about how we supply the training in advance.”

So if you’ve been diagnosed with RED-S, adjust your diet to match your training volume and associated energy needs!

Pia Jensen, together with her co-founder Jana Borosch, has written four cookbooks written, with which one can supply oneself suitably to the training.

  1. Fix the energy deficit

“If I suffer from RED-S, then I have to increase my energy intake” Pia Jensen sums it up and affirms, “There’s no way around it!”

Unless, of course, you reduce the amount of training very significantly. However, many athletes logically do not want that.

Pia Jensen generally recommends a combination of increasing energy intake and decreasing training volume. “This doesn’t require a complete break from sports.”

Shakes and special carbohydrate drinks can help increase energy intake. Also essential is the timing of your meals around the workout, in other words: ensure sufficient supply before, during and after the workout.

Pia Jensen expressly does NOT recommend taking hormones such as the pill: because this only masks the symptoms. You may seem to get menstruation again, but it is not “real” and the problem is not fixed.

  1. Adjust the training load

As described, the training volume must be lowered if a higher energy intake cannot be implemented immediately

Do not increase the training load again until optimal energy availability can be ensured and when symptoms have improved. “Otherwise, you need to eat more, simple as that,” the expert said.

  1. Return-to-Performance

Supplementation with vitamin D & calcium can assist in getting back to old performance levels when problems arise at the bone level. Because then the bone structure must be strengthened from the inside.

Actually, athletes have 10-15% higher bone density, but doing sports and not eating right does the opposite. If you don’t take proper care of yourself, you ruin the beautiful positive effects of exercise.

Another point is still to increase energy intake and especially carbohydrate intake. Pia Jensen repeatedly hears that many women tell her that they cannot tolerate carbohydrates. “This is actually mostly an excuse to avoid certain food groups. Because an intolerance really rarely occurs. Carbohydrates are THE energy source and important for health.”

Only when the symptoms have disappeared can the training load be gradually increased again!

Prevent an energy deficit with the right energy intake

Ideally, of course, RED-S should not have to be treated in the first place. You have already taken an important step to prevent RED-S by reading this blog.

“The important thing is to know about it,” Pia Jensen emphasizes.

But this applies not only to the athletes, but also to the coaches. Ideally, there is good cooperation between trainers, nutritionists, psychologists and medical doctors.

Certainly, a fundamental rethinking of the body image also helps to prevent RED-S.

“Lighter does not always mean faster and/or better” is a very important statement.

After all, it’s all about finding your individual performance weight. And that is completely individual. The decisive factor here is whether the body is functioning physically and mentally. There is no evidence that less body fat equals better performance.


  • Constant training is the key to success!
  • Performance is only possible with a sufficient and correct energy supply!
  • Only a healthy body is a powerful body!

Nutritional strategies can also help prevent an energy deficiency: the prerequisite is to get a good overview of your own energy needs and energy intake.

“Almost all of us have a watch these days – a gadget that shows energy consumption. It may not be 100% accurate, but it still helps to get an idea of what I am consuming and should be consuming,” explains Pia Jensen.

Nutrient intake during training should receive special attention during high training loads (high volume or intensity). You can find out how best to feed yourself during training, for example, in our Guides for the four sports Cycling, Running, Triathlon and Cross-country skiing read up.

If the goal is really to optimize weight, it is advisable not to do it on your own, but to work with a nutritionist.

Our tips for care around the training

When it comes to diet/food, it pays to take a long-term view. For example, if I have an intense session coming up the next day, I should eat accordingly (higher in carbohydrates) the night before.

Catering around the training should also be planned in advance. “It’s crucial that you eat at all,” explains the active competitive athlete, who still gets funny looks when she eats a bar during an hour-long session.

“Food is often still perceived as a sign of weakness, but that’s just not the case anymore,” she says. “If I’m still going to workout after a day of work, I just need another snack.”

The well-known emergency bar should rather become an everyday bar! (for example super suitable, the 100% natural PORRIDGE BAR from MoN Sports)

Basically, it can be said that a carbohydrate intake is extremely important for a load duration of 1.5 hours. If the last meal was more than four hours ago, the intake should take place soon after the start of the load.

For relaxed units, for example, our SLOW CARB is especially good. The carbohydrates go slowly into the blood and you are well supplied for a long time. For intensive units offers itself e.g. FAST CARB is recommended.

A big advantage of a good carbohydrate supply: You are ready for action again faster afterwards. “You can always get through a training session somehow without supplies. The question is, what happens afterwards? Insufficient food catches up with you the next day at the latest,” says Pia Jensen.

Consequently, a rapid supply of energy after the end of the load is just as crucial. Keyword here: “Open Window”. Ideally, the supply consists of a mixture of carbohydrates and proteins.

Pure protein shakes are therefore not very suitable, especially for endurance athletes. Therefore also our RECOVERY SHAKE contains plenty of carbohydrates – besides proteins (or essential amino acids).

Study: Why low carb doesn’t make sense for athletes

One study looked at how the same energy availability with different nutrients affects the body. In the chart above, you can see a “high carb diet” and below, a “low carb (high fat) diet.”

In both cases, energy availability was 60 kilocalories per kilogram of body weight. The subjects exercised twice a day for five days. Then they looked at how the carbohydrate content in the muscles changed, blood tests and performance tests were performed.

This found that bone resorption markers increased with the low carb dietary strategies. Thus, increased bone resorption took place.

The authors’ conclusion was that carbohydrates have a critical impact on reasonable bone health.

You should therefore – especially as an athlete – include carbohydrates in your daily diet to prevent stress fractures and stay healthy.

Reduced carbohydrate availability before and after exercise also did not result in better muscular and mitochondrial training adaptations. Furthermore, subjects on the low-carb Diet performed worse on the performance tests than those on the high-carb Diet.

Carbohydrate intake, along with general energy intake, seems to be the key performance factor!

About author Pia Jensen /
where’s the food? sportsnutrition

where’s the food? sportsnutrition was founded by Pia Jensen and Jana Borosch. Under the motto “Sports Science Meets Kitchen,” the two have set themselves the goal of creating functional and healthy sports cuisine whose recipes taste really good. The cookbooks help professional and amateur athletes to easily integrate a healthy sports diet into their daily routine. Furthermore offers where’s the food? sportsnutrition Group and individual coaching sessions. To the cookbooks and further information you reached

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