When it comes to gender differences in nutrition, the focus is often on specific nutrients such as iron and calcium that women may need more of. As important as optimal levels of these nutrients are, there are also gender differences in how women use energy that need to be accounted for.
New research shows that trained female runners use less muscle glycogen than men during exercise (1). Glycogen is the storage form of carbohydrates that fuels muscle contractions. This study tested glycogen use in the calf and quadriceps muscles during a 10 mile run and following a track workout. Results showed that men used significantly more glycogen during the 10 mile run than women.
Men also had higher muscle glycogen levels at rest. From previous studies we know that women burn more carbohydrates at rest but rely more on fat during exercise than men, which has a glycogen-sparing effect. Because women burn more fat during exercise, they don’t require as much total glycogen overall so storage levels decrease. Greater fat burning and lower glycogen use during long-duration exercise is one reason that women tend to have greater endurance capacity than men. Greater use of fat during exercise may also spare protein, which means women would experience less loss of muscle during long-duration, catabolic exercise.
The typical recommendations to load up on carbs before and after training may not apply to women if they have been calculated based on men’s needs. Further, women can benefit from including adequate healthy fat in their diets—something many women have shied away from in the past due to the incorrect fear of dietary fat is “fattening.”
Why do women and men experience differences in how they use energy?
Scientists have identified differences in the nervous and hormonal systems that are responsible for women’s lower reliance on glycogen. For instance, estrogen stimulates growth hormone release, which increases the use fat for fuel. And women’s bodies are more responsive to the fight-or-flight hormone epinephrine that stimulates fat burning. One study found that when epinephrine is released, such as during exercise, beta adrenergic receptors that burn fat are more responsive in women than men (2).
Such information is important because it helps our understanding of what “normal” versus “abnormal” metabolism looks like in women since fat metabolism forms the cornerstone of metabolic disorders, including obesity, diabetes, and heart disease.
Further lesser need for carbohydrates means the women can consume more fat in their diet. Dietary fat has several positive effects including increased levels of steroid hormones that are beneficial for health and protective of lean muscle mass. Dietary fat is filling, helping to keep calorie intake in check without the struggle of feeling hungry all the time. In fact, a study that compared appetite in response to a Mediterranean diet that provides a moderate amount of fat found that women had less of a desire to eat and lower hunger ratings than men (3).
Seek out research and advice based on female physiology and metabolism if you are a woman.
Don’t rely on research performed on men to identify how women should eat. Women need to plan their diets according to their unique physiology, modifying fat, carbs, and proteins to meet their needs.
Low-fat diets are less likely to be beneficial for athletic women due to their higher rates of fat burning during exercise. A higher fat intake may improve energy metabolism and support hormone release for improved performance (4).
Diets higher in dietary fat may promote appetite regulation, which could allow for a leaner body composition (3).