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One of the most consequential biases in exercise science has been the historical use of male athletes for studies and the application of those research findings to the body of female athletes. Fortunately, during the past decade, researchers have sought to rectify that scientific research bias, and there is now a substantial amount of current research being done specifically on female athletes, emphasizing some of the physiological and adaptational differences that female athletes experience leading up to competition and in the recovery period.
As it turns out, men’s and women’s bodies respond differently to exercise and nutrition — and thus have different fueling needs.
“Physiologically, women’s bodies tend to have more fat than men,” states Julie Mancuso, Registered Dietitian. “This can result in different energy needs, even without running thrown in the mix.”
However, when running is considered, there are additional elements of physiology unique to females which may impact energy levels, performance and recovery. Here are some ways in which female runners should approach fueling differently from males.
Pay Attention to Iron, Calcium, and Vitamin D Levels
Menstruating women who compete in endurance exercise need to keep tabs on certain micronutrients, such as iron. Iron is a functional component of oxygen transport throughout the body as well as energy production, and is lost as a result of menstruation, sweating, GI bleeding, insufficient intake, and increased iron losses associated with hemolysis.
Low iron can impact fatigue levels, performance, recovery from exercise, immunity and more. Female athletes are considered to be at a greater risk for being low in iron — estimates of iron deficiency among female athletes range from an alarming 15 to 35 percent of the population.
Women also need to take in sufficient amounts of calcium, a vitamin critical to optimal bone health. Those who follow a vegan or plant-based diet can rely on non-dairy calcium-rich foods, such as almonds, broccoli, soy and dark leafy greens, while avoiding calcium inhibitors, such as high amounts of coffee, sodium and oxalates. Athletes at risk for low calcium intake should consume 1500 mg/day to optimize bone health.
Lastly, women should take care to get enough vitamin D. A study of 102 NCAA female athletes at a single institution found 21.5% to have abnormal vitamin D measures. Vitamin D aids in the absorption of calcium and is another important contributing factor to bone health. Since vitamin D is not widely available in the diet, supplementation may be necessary and recommended for many female athletes.
Athletes who are at a risk for low vitamin D include those living at northern or southern latitudes (>35th parallel), athletes training indoors, dark-skinned athletes, and those who often cover their skin with sunscreen and clothing when outside. Female athletes should aim for Vitamin D levels of >50 nM.
Consider Adjusting Nutrition Around Menstrual Cycle
Female athletes may experience physiological changes based on the phase of the menstrual cycle. The proposed changes in physical performance through different phases are likely a result of factors like altered muscle activation, energy use, thermoregulation and body composition. Women can make some changes to their nutrition plans during certain menstrual phases to achieve optimal performance, though there is still much research to be done.
The menstrual cycle is divided into the following phases: 1. the follicular phase (the first day of your menstrual period until ovulation — this phase is on average 16 days), 2. the ovulatory phase (when the ovary releases that mature egg — this phase lasts just 24 hours), and 3. the luteal phase (ovulation until menstruation — this phase lasts 12 to 14 days). These phases differ based on hormonal levels. As discussed in a 2021 review article, Recommendations and Nutritional Considerations for Female Athletes: Health and Performance, estrogen has anabolic effects, such as improved muscle strength and bone mineral density. Peak estrogen levels are reached around days 12–14 of a normal menstrual cycle.
In the follicular phase, when estrogen is rising, women exercising over 1.5 hours a day should focus their diets on protein and aim for at least 1.6 grams of protein per kg of body weight each day.
“When estrogen is high, women may feel better, be better able to tolerate higher intensity and aerobic exercises, and exhibit greater strength and power outcomes,” adds Omega Zumpano, exercise physiologist and menstrual cycle educator.
As progesterone increases, estrogen will start to decline leading into the first half of the luteal phase, and energy levels may start to wane as well.
“As progesterone increases, such as in the early luteal phase, it has a subduing effect on the brain and body,” Zumpano says. “This is a great time to do longer duration, low-intensity training, in contrast to the high strength and power training from the follicular phase.” Protein needs may also be higher during this time to account for the higher rates of protein catabolism.
If women aren’t meeting their basic energy needs, however, working to optimize nutrient composition based on the menstrual cycle phase is ineffective. During exercise, we know that women have higher rates of fat oxidation and lower rates of carbohydrate and protein metabolism compared to men since estrogen has a protein-sparing effect. It is important that women eat enough to perform optimally and avoid signs and symptoms of relative energy deficiency in sport. Female athletes should aim for about 45 calories per kg of fat-free body mass (that is, the body mass when fat is subtracted) for optimal health and performance.
While it is likely that micronutrient and macronutrient requirements may be altered during various phases of the menstrual cycle as a result of hormonal fluctuations, we need more high-quality research before we can prescribe foods and dietary suggestions based on the menstrual cycle phase. Because of the lack of research in this area, it’s best to use subjective judgments and make tweaks. For instance, if a female athlete is feeling more fatigued than normal during different phases of the menstrual cycle, the intensity, duration of training or diet may need to be modified.
Simply put, eating a diverse diet full of various colors and macro- and micronutrients will help women achieve the variety of essential nutrients needed for performance. In some cases, supplementation may be necessary, and women should work one-on-one with a health practitioner or sports dietitian for an individualized plan.