Recently, a claim has been circulating online that "women should never exercise on an empty stomach, as it can be very harmful to their health".
Upon closer inspection, the explanation is filled with a sense of scientific rigor: the female endocrine system is sensitive, and exercising on an empty stomach will cause cortisol to rise and sex hormones to drop, leading to abdominal fat accumulation, poor metabolism, and irregular menstrual cycles... accompanied by several references, curve charts, and some English abbreviations, making it very convincing.
Image: Screenshot of Xiaohongshu
However, the fact that something appears to be scientific and has literature to support it does not necessarily mean it is true.
In July this year, researchers from New Zealand and the UK published a review in the Proceedings of the Nutrition Society, examining several popular female exercise and diet tips on social media, including the notion that women cannot exercise on an empty stomach. After systematic comparison and evaluation, the authors concluded that these tips all lacked strong scientific evidence.
Women's sports already face more challenges and should not be further hindered by rumors.
Before reading on, take a look at this section
The views expressed in this article are based on the results of exercise nutrition research and are mainly applicable to healthy adult women who engage in regular exercise.
If you are a minor, pregnant, breastfeeding, or have physical conditions such as endocrine disorders, or mental and psychological conditions such as eating disorders, or if you have specific exercise goals, needs, or preferences, please arrange your exercise and diet according to your individual situation and consult a doctor if necessary.
Rumor 1
All women should not exercise on an empty stomach
This rumor claims that women's neuroendocrine and reproductive systems are sensitive, and exercising on an empty stomach can increase cortisol levels and decrease sex hormone levels, leading to menstrual disorders, abdominal fat accumulation, poor metabolism, and muscle loss.
Let's follow this review and take a look at whether these impacts really exist.
Cortisol levels will rise, but it may not be a bad thing
Research has indeed confirmed that exercise can temporarily raise cortisol levels, but this is usually a good thing, as cortisol can increase energy supply during exercise and promote muscle recovery afterwards. It is only when cortisol levels remain elevated over the long term that negative effects occur.
Regarding exercise on an empty stomach, a 2009 study compared the reactions of 8 men and 8 women to 2 hours of cycling, and found that salivary cortisol levels increased significantly after exercise, but the increase was not significantly different between the fasting and post-meal states, and there was no difference between the sexes. This means that, relative to men, women's cortisol levels did not increase more after exercise; and relative to exercising after a meal, exercising on an empty stomach did not cause cortisol to increase more.
Therefore, the conclusion that "exercising on an empty stomach will cause women's cortisol levels to be too high and harm their physical health" is not supported by current research results.
However, for individuals with long-term inadequate energy intake, Cushing's syndrome, or other diseases that affect cortisol levels, which are already elevated in the long term, fasting exercise may have different effects on these populations.
In a normal person, cortisol levels are typically higher in the morning and lower at night, and may increase during exercise, but generally do not exceed the normal physiological range, and then naturally decrease afterwards｜Allara Health
Menstrual disorders are not caused by skipping one meal, but by not eating enough throughout the day.
Proponents of the claim that women should not exercise on an empty stomach often cite research that kisspeptin affects reproductive function and is sensitive to energy availability. In rodent experiments, fasting for 3 days has been shown to decrease kisspeptin levels in females, weaken luteinizing hormone secretion, and disrupt the estrous cycle.
Human trials have also shown that women's reproductive and neuroendocrine systems are more susceptible to energy deficiency than men's.
However, note that the key phrase is "prolonged energy deficiency," referring to the imbalance caused by eating too little and exercising too much over multiple consecutive days, rather than fasting exercise itself. Some women who claim that fasting exercise has stopped their menstrual cycle often simultaneously follow a diet and do not just skip the pre-workout meal, but also eat insufficiently throughout the day.
Currently, there is no research measuring the kisspeptin levels in healthy women with sufficient energy after fasting exercise, nor is there any research confirming that fasting exercise damages the reproductive and neuroendocrine functions of women. Therefore, the conclusion that "fasting exercise will harm healthy women with reasonable diets" is also unfounded.
Menstrual disorders may be caused by insufficient diet throughout the day and excessive exercise｜Xiaohongshu @Bobo Nai Lv/Yi Yima Mi
Abdominal fat, rather than accumulating, has actually decreased.
Proponents of the claim that "exercising on an empty stomach increases abdominal fat accumulation" often cite a mouse study that found increased liver fat in female mice during fasting. However, this study focused on 9 months of alternate-day fasting and did not compare the effects of exercising on an empty stomach versus after eating on fat metabolism, and the results of mouse studies cannot be directly applied to humans.
In human trials, a 2026 study comparing the metabolism of 24 men and 25 women after overnight fasting found no significant difference in carbohydrate and fat oxidation between men and women when cycling at the same ventilatory threshold, an indicator of exercise intensity.
Additionally, research has made direct comparisons of the impact of fasting and post-meal exercise on body composition. The results found that after 18 sessions of high-intensity interval training over 6 weeks, 16 overweight or obese women saw decreases in fat percentages in their abdominal, leg, and whole-body areas, as well as increases in lean body weight in their legs and buttocks - and these changes were unrelated to whether they exercised on an empty stomach or after a meal.
In another study, 16 women of normal weight saw decreases in weight and fat after 4 weeks of aerobic exercise and diet control, with no difference between the fasting and post-meal exercise groups.
Of course, these studies have limitations such as small sample sizes and limited populations, but so far, no research has confirmed that there are gender differences in indicators such as carbohydrates, insulin, or fat during fasting exercise, nor have any differences been found between fasting and postprandial exercise in women.
As for the concern about losing muscle, the reality is that performing resistance (strength) training on an empty stomach doesn't cause muscle loss, and the rate of muscle gain may be comparable to training after a meal.
A 2025 study had 22 women undergo 12 weeks of resistance training and found no significant difference between fasting and fed states, with muscle volume, strength, and power all improving after training.
Research on metabolism also supports this: whether you do resistance training while fasting or after a meal, the effects on body composition and resting energy expenditure (similar to what people commonly call "basal metabolism") show no difference.
In brief,
There is currently no sufficient evidence to prove that healthy women will be significantly adversely affected by exercising on an empty stomach when they have sufficient energy throughout the day.
For most healthy women, fasting exercise is safe as long as daily diet and exercise intensity and duration are well-controlled.
As for whether exercising on an empty stomach or after a meal is better, there is no standard answer, as it depends on your exercise goals, daily energy needs, and personal feelings.
Additionally, regardless of whether you are a woman or a man, consider the following before exercising on an empty stomach:
It is recommended that individuals with diabetes, thyroid disease, Cushing's syndrome, gastrointestinal disease, cardiovascular and cerebrovascular disease, eating disorders, a history of hypoglycemia, or those who are pregnant or breastfeeding consult a doctor before use.
If the goal is to improve athletic performance, training on an empty stomach is generally not recommended.
For those who have a habit of exercising and want to work out immediately after waking up in the morning, and experience bloating, stomach pain, or nausea after eating and exercising, consider trying exercising on an empty stomach.
When exercising on an empty stomach, it is necessary to ensure sufficient energy intake throughout the day, especially carbohydrate intake the night before.
When starting fasted exercise, strictly limit it to low intensity, short duration, and low frequency—significantly below the volume and intensity of your usual post-meal workouts. Carry emergency food with you, and eat promptly once you finish.
Rumor 2
Women are more prone to dehydration than men
The second rumor claims that women's hormone levels fluctuate with their menstrual cycle, and during certain phases, their core body temperature, weight, and sweating in response to exercise change, making them more prone to dehydration than men.
Body water fluctuates with the menstrual cycle, but the response to exercise remains unchanged.
A 2025 study tested 11 women with regular menstrual cycles during the early follicular phase and mid-luteal phase. The results showed that core temperature, estradiol, progesterone, and aldosterone levels were higher during the mid-luteal phase, and body weight was slightly increased, but there were no significant differences in fatigue, sweating rate, urine output, hematocrit, or dehydration indicators during a 180-minute heat walking test.
Another study compared responses to exercise during three periods of the menstrual cycle and found no differences in fluid intake, urine output, sweat rate, or body weight changes.
Although current research evidence is limited, existing findings suggest that while women's body water levels fluctuate with the menstrual cycle, there is no significant change in their response to exercise.
The percentage of water in women's bodies is lower than that in men's.
While the saying goes that "women are made of water," on average, women have a lower percentage of body water relative to body weight than men, due to lower muscle mass and higher body fat percentage.
Additionally, due to women's generally smaller average body weight and lower total body fluid, when they sweat 500 milliliters, the proportion of sweat to total body fluid is higher than that of men, making women more prone to dehydration from this perspective.
The question is, if men and women exercise with the same intensity and training status, does the gender difference in sweating still exist, and current research has not provided an answer.
At the other end of the dehydration spectrum is overhydration, which can cause exercise-related hyponatremia, also potentially life-threatening. In 2005, a study that conducted blood tests on 488 Boston Marathon finishers found that 13% had hyponatremia, with a higher incidence in women than men, but the difference between the sexes disappeared after adjusting for factors such as body mass index and finishing time.
Regarding the supplementation of water and electrolytes, there is currently no strong evidence to support that women require a different approach than men.
Moreover, there is no standard supplement quantity for either women or men, as the amount of sweat and electrolyte quality in sweat can vary by more than 10 times from person to person.
Replenishment is needed during exercise based on individual sweating, weather conditions, and exercise duration. To replenish more accurately, one must conduct multiple measurements to calculate the required replenishment amount for the next exercise.
Rumor 3
It is necessary to consume protein within 30 minutes after exercise
This is probably the most widespread rumor in the gym. Indeed, adequate protein intake is beneficial for maintaining health and can also promote muscle repair after exercise and reduce muscle loss when energy is insufficient.
However, there are rumors that women's metabolism differs from men's, and the window for consuming protein after exercise is shorter, requiring protein intake within 30 minutes.
In terms of resistance exercise-induced muscle protein synthesis, most existing studies have indeed focused on men, but there are also some studies that include women, and these have not found significant gender differences.
Regarding the anabolic window, a 2019 study divided 43 women into two groups, one of which received carbohydrate and protein supplements before and after training, while the other did not. After 5 weeks of resistance training, there was no significant difference in body composition or lower-limb muscle strength between the two groups of women.
Another study in 2025 analyzed this in more detail, with 36 women undergoing 8 sets of lower-limb resistance training. Researchers found that compared to the group that took a placebo after training, the group that took 20 grams of protein had higher levels of essential amino acids in their blood, but the rate of muscle protein synthesis after exercise did not change significantly. In other words, the raw materials for building muscle were delivered, but the factory did not work overtime as a result.
According to existing research, there is no significant difference between men and women in the "protein window" after resistance exercise. Moreover, for both sexes, the most beneficial dietary approach for muscle growth is to consume sufficient protein throughout the day, evenly distributed, rather than focusing on the 30 minutes after exercise.
For ordinary people, instead of rushing to drink protein powder or find something to eat after exercising, it's better to carefully calculate: how many grams of protein do they need in total every day, and whether they are getting enough and eating it evenly.
Additionally, it's generally not advisable to consume high-protein, high-fat, or high-fiber foods before exercise, as they offer no significant benefit to your workout and may cause gastrointestinal discomfort.
Insufficient research on women.
But rumors should not be used to fill the void.
All of the above rumors do not withstand scrutiny. Compared to the rumors themselves, what is more worth exploring is why there are so many rumors in the field of women's health and fitness.
Insufficient scientific evidence is a major reason. The lack of evidence reflects a broader issue - compared to men, there is a relative scarcity of research on women's health in the field of exercise, and many studies are not rigorous enough, such as not using hormone tests to determine the phase of the menstrual cycle, and not distinguishing between women with natural cycles, those using birth control, and those using intrauterine devices.
A lack of evidence can easily be filled with semi-true and misleading information that is sensational enough to spread wildly on social media. The insufficient evidence also makes it difficult for professionals to refute or provide accurate advice.
In the face of such confusion, it's better to ignore all the advice on social media and focus on the physical activity and dietary guidelines set by the national health and sports departments, as well as the personalized advice given by doctors.
Differences between people can transcend gender.
What suits oneself is the best
"Women are not small men," and this statement holds true, particularly in the realm of sports nutrition and health, where women have numerous differences from men.
However, the differences between people can sometimes transcend gender, and the differences between two women can be greater than those between women and men.
Moreover, many times the differences do not stem from gender itself, but from factors such as low body weight, minimal muscle mass, or insufficient exercise. It just so happens that women are more likely to possess these characteristics, and as a result, are often stereotyped as a group and unfairly judged as being unsuitable for certain sports or diets.
In fact, these methods may help some women maintain good health habits, but may not be suitable for some men.
What suits you is the best. First, clarify your own exercise goals, health conditions, exercise needs, and preferences one by one, then try, observe, and adjust under safe premises, and ultimately you will find the best solution that belongs to you.
References
Researchers Fraile-Martínez, Liviu Boaru, de Castro-Martínez, Ortega, and García-Montero published a narrative review in the journal Nutrients, volume 18, issue 10, with the publication number 1502, in 2026, examining sex-specific responses to intermittent fasting across physiological, clinical, and psychosocial contexts.
Astorino, T.A., MacDonald, A.J., Anderson, A. et al. No sex differences in fat oxidation in response to exercise prescribed according to maximal heart rate or ventilatory threshold. European Journal of Applied Physiology 126, 1645–1656 (2026).
Researchers from various institutions, including Boot N, Hermans WJ, Kuin LM, Malowany JM, Hendriks FK, Warnke I, Senden JM, Overman A, Goessens JP, Zorenc A, Kornips E, Verdijk LB, and van Loon LJ, conducted a study published in the Journal of Nutrition in December 2025. The study found that consuming 20 grams of whey or canola protein does not further increase muscle protein synthesis rates during recovery from resistance exercise in healthy, young females, as reported in the journal's 155th volume, 12th issue, on pages 4123-4135.
Matsuzaki T, Iwasa T, Kinouchi R, Yoshida S, Murakami M, Gereltsetseg G, Yamamoto S, Kuwahara A, Yasui T, Irahara M. Fasting reduces the kiss1 mRNA levels in the caudal hypothalamus of gonadally intact adult female rats. Endocr J. 2011;58(11):1003-12.
