Women Athletes: Relative Energy Deficiency and Dietary Recommendations

Brief Summary: Women and men are surprisingly similar when it comes to the basic principles of sports nutrition, but some differences can have significant consequences over time. The main question here is how low energy availability can affect health and performance in women athletes – particularly through menstruation, bone health, and recovery. The conclusion is that the recommendations are fundamentally similar to men's, but relative energy deficiency is often overlooked and can cause problems before one connects the symptoms with energy intake. With simple tools like diet monitoring, period tracking, and wellness follow-ups, much can be prevented.

Women and men differ in many ways — someone once said women are not small men — but physiologically, we are surprisingly similar regarding nutrition. Based on what we know today, women's energy metabolism and energy planning differ very little compared to men's. Therefore, the recommendations for nutrition, energy, health, and performance do not significantly differ between the genders.

But there are indeed differences, and here I go through them based on a new review article: Recommendations and Nutritional Considerations for Female Athletes: Health and Performance. The article strongly focuses on relative energy deficiency — an area increasingly researched and widely discussed among athletes and coaches. Really good!

Background

Women comprise almost 50 percent of all athletes globally. Despite this, female athletes make up only 39 percent of participants in studies within the field, and studies with only women account for a mere 4 percent. Looking specifically at research on sports performance, 63 percent of studies are conducted solely on men, 33 percent include both men and women, and only 3 percent are exclusively on women.

A major reason for this is menstruation. Female participants in a study can have completely different hormonal levels between the first and second visits (especially in cross-over designs, which are recommended). Standardizing studies so that all women are tested in the same phase of the cycle requires daily measurements before the study starts and extensive logistics — which quickly inflates costs and often makes the project unsustainable.

A study of 40 women endurance athletes with optimal energy availability (over 45 kcal per kilogram of fat-free mass per day) compared intake of around 30–45 kcal/kg/day or 30 kcal/kg/day. The result showed that those who consumed about 45 kcal/kg/day had a normal Resting Metabolic Rate (RMR), while the groups who reduced their intake experienced a lowered RMR. The body is adaptive — when you reduce intake, it conserves energy where it can — and therein lies the core issue with relative energy deficiency. You can maintain weight stability yet still suffer from energy deficiency (hence relative rather than absolute energy deficiency).

The Female Athlete Triad

Energy Availability, Bone Health, and Menstruation

A well-established concept aims to avoid relative energy deficiency: the female athlete triad. It focuses on three areas — energy availability, bone health, and menstruation — and as long as these three factors are measured and managed (meaning the athlete eats enough, doesn't lose bone density, and has normal menstruation) the athlete does not have relative energy deficiency. Maintaining these three functions is crucial for a long-term athletic career.

There are many other functions affected by relative energy deficiency that could be measured to determine if someone is developing Relative Energy Deficiency in Sport (RED-S), illustrated in the colorful image below (image a). However, the triad is a practical and relatively simple tool that coaches and sports doctors can use without advanced equipment. Anyone can track diet with energy intake calculations, as well as monitor menstruation and its regularity. Also, ensure that training yields results, motivation is present, sleep is good, and general health is okay. Bone density is measured if the first two factors show a risk of relative energy deficiency.

Some risk factors when you eat too little are disrupted hormone levels — menstruation can eventually stop, which lowers estrogen levels. Estrogen acts as a "bone protector" by inhibiting osteoclasts that break down bone tissue. When estrogen levels drop, the risk of osteoporosis increases, and energy deficiency itself also raises the risk of compromised bone health. Therefore, bone density is a separate aspect of the triad.

The triad is a simplified tool to measure something complex, but it's practical to use.

Issues with Immune System, Gut, and Iron Levels

Relative Energy Deficiency and Iron Deficiency

One of the most common hematological changes with energy deficiency is iron deficiency — something we discussed in episode 108. Depending on the group, studies show that 15–30 percent of women athletes have iron deficiency, while other studies point to over 50 percent. It's very common. The International Society of Sports Nutrition (ISSN) recommends a daily iron intake of 18 mg for women (compared to 8 mg for males), more than twice as much.

Iron deficiency can be linked to reduced training efficiency and can also affect metabolism via the thyroid. Low iron levels can shift metabolism from efficient aerobic combustion to more anaerobic metabolism in case of local oxygen shortages in the muscles. In short: iron is an area that deserves extra focus if you are a woman athlete.

Relative Energy Deficiency and the Immune System

Data from the Rio Olympics shows a connection between low energy availability and infectious diseases (such as colds). However, the research is very limited. For practical advice on reducing illness duration, we recommend our previous article on colds.

Relative Energy Deficiency and Gut Issues

Yes, we know — the term gut issues often pops up as a topic — but it's also a common problem for endurance athletes. The link between relative energy deficiency and gut issues is poorly researched, but there is some research. A study of 1000 athletes showed an increased risk of incontinence and constipation in those with low energy availability. Another study found that participants with low energy availability had a higher risk of gut issues (bloating, cramps, diarrhea, constipation). A small study found here did not find a link, but it is small. Overall, current data suggests a certain correlation between low energy availability and increased risk of gut trouble.

Relative Energy Deficiency and Its Impact on Performance

Perhaps not entirely shocking: low energy availability (under about 45 kcal/kg/day) over time leads to several negative effects. The RED-S model points to ten physiological areas that can be negatively affected — see image b next to it. Unfortunately, there are few high-quality studies in the area of athletic performance and relative energy deficiency.

One study followed 10 competitive swimmers during a 12-week competitive season and compared 400 m freestyle times. Half of the participants had normal menstruation and the other half did not (due to relative energy deficiency). Those who had relative energy deficiency performed on average 9.8 percent worse (slower times on 400 m) while the group with normal menstruation improved by an average of 8.2 percent. Another study followed elite rowers during a four-week intensive training block and found similar results with decreased 5 km rowing times. A limitation in that study was that the energy deficiency largely was due to increased training volume and insufficient rest (training camp effect). Participants might have overtrained during the block, which in itself can lead to supercompensation later — but acutely, directly after an intense block with too low energy intake and too little rest, performance dropped.

Nutritional Recommendations Specifically for Women

The recommendations do not fundamentally differ between genders. There are metabolic differences and menstrual fluctuations that affect whether you as a woman oxidize more or less fat/carbohydrates and how you store glycogen during different phases of the cycle — we discussed this in a previous article.

Here’s a quick recap of recommended intake of various macro and micronutrients. These recommendations are largely the same as for males.

Carbohydrates

• During activity lasting 1–2.5 hours, 30–60 g of carbohydrates per hour is recommended.
• For activities over 2.5 hours, 90 g or more per hour is recommended.
• These recommendations are mainly based on studies with males, so future research might adjust the levels for women. Individual variations are significant — some can tolerate much more without gut issues, others handle less.
• After glycogen-depleting activity, 1.2 g/kg per hour for 4–6 hours is recommended for maximal replenishment of glycogen.
• Carbohydrate loading: 7–12 g/kg/day for two days prior to competition.

Fat

• About 20 percent of energy intake should come from fat from varied sources.
• A minimum of 1.2 g of omega-3 per day is recommended.
• These recommendations do not differ between genders.

Protein

• At least 1.6 g protein/kg/day (that equals 104 g for a 65 kg person).
• American College of Sports Medicine recommends 1.2–2.0 g/kg/day depending on training status and goals.

Iron, Calcium and Vitamin D

These three micronutrients are particularly important for women athletes to maintain bone health, performance, and overall well-being.

• Iron: Ferritin/S-ferritin can be measured via laboratory or self-tests. If you're low — adjust your diet towards more iron-rich foods or supplement with iron supplements (common iron sulfate is often recommended). If you can't raise levels, you should contact healthcare to rule out other causes.

  “Slow-release ferrous sulfate formulations are recommended as the most effective and tolerable supplement”

• Calcium: Usually obtained via diet, but with a strict diet or energy deficit, you risk depleting reserves and increasing the risk of osteoporosis. Women at risk are recommended 1500 mg of calcium per day. Since the gut doesn't absorb more than about 500 mg at a time, intake should be spread over the day, ideally with vitamin D for better absorption.
• Vitamin D: Studies show that 20–80 percent of women athletes have low levels. A daily supplement of 1000–2000 IU (depending on season and sun exposure) is recommended year-round for athletes.

Conclusion

The summary of the article is fairly straightforward.

• Women athletes should aim for an intake of approximately 45 kcal/kg of fat-free mass per day.
• Keep track of your menstruation so it behaves normally — a missed cycle is not normal.
• Eat a varied diet.
• Supplement with 1000–2000 IU vitamin D every day.
• Drink according to thirst during activity.
• Be cautious of “experts” on social media who readily give advice.

“In brief, female athletes should aim for EA of approximately 45 kcal kg−1 FFM day−1 for optimal health and performance and maintenance of physique. Low EA can manifest as RED-S, a constellation of many symptoms of various physiologic systems that can be negatively affected by inadequate nutrition. Female athletes should pay close attention to their menstrual cycles to monitor for changes or irregularity that may signify nutritional deficiency and are reminded that missing cycles is not “normal for an athlete”. 

Female athletes are encouraged to eat a diverse range of foods to ensure adequate micronutrient intake. Micronutrients that are particularly important for female athletes include iron, calcium, and vitamin D. It is not unreasonable for female athletes to supplement their diets with 1000–2000 IU vitamin D pills daily. 

For most athletes, drinking when thirsty will provide adequate water intake and fluid balance. 

Female athletes are encouraged to consult sports dietitians to develop individualized nutrition plans and to be wary of advice pedaled by so-called “experts” on social media.”