Written by: Kevin Cann
We all know men and women are made just a tad differently. One of the biggest differences being that females go through a 28 day menstrual cycle. Throughout this 28 day cycle the balance of hormones shifts. Can this balance shift affect what macronutrients women rely on more heavily? The answer to that question is yes.
The main hormone I would like to discuss here is estradiol. Estradiol is responsible for the development of secondary female sex characteristics. A female’s level of estradiol typically peaks at the time of ovulation. During this time, when estradiol levels are high, adipocyte lipoprotein lipase (LPL) decreases (http://www.ncbi.nlm.nih.gov/pubmed/7783659 ). This may result in higher triglyceride use by skeletal muscle during this time. One study showed a difference in fuel metabolism during long duration exercise between men and women (http://www.ncbi.nlm.nih.gov/pubmed/9804587 ). Women relied more heavily on lipids for energy than men. Under the same race conditions this study suggests that macronutrient intake should differ for men and women. Women may benefit from a higher fat intake then men running the same race.
Numerous studies have shown that men and women utilize fuel substrates much differently. This may come as a surprise because there is no difference in muscle glycogen storage or GLUT 4 transporters between the sexes. Muscle glycogen is how we store carbohydrates within the body, and the GLUT 4 transporters are what carry glucose into our muscle cells. However, women tend to utilize lipids more and carbohydrates less for energy in relation to men (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2129154/ ).
The differences in substrate utilization may also explain why the birth control pill comes with a side effect of weight changes. Increasing estradiol levels can shift or further enhance certain substrate utilization. Without accounting for this with dietary intervention, weight gain or weight loss may occur. Yet another reason to discuss the pros and the cons of using birth control pills.
In one study done on male rats, 17B-estradiol supplementation increased glycogen sparing and increased lipid utilization during physical activity (http://jcem.endojournals.org/content/90/11/6218.abstract ). This means that when male rats were given the female hormone estradiol, they shifted their substrate utilization to a higher emphasis on lipids. Another study on male rats had similar effects as the previous one (http://jap.physiology.org/content/71/5/1694.short ).
The only time men and women utilize substrates similarly is during exercise intensities of 85% or greater. Both men and women will shift to utilizing carbohydrates as the main energy source. This is due to carbohydrates being the only macronutrient that can be utilized during high intensity, anaerobic training. In any sport or activity requiring less intense and longer duration activity, women will utilize fats as energy more than men, as the previous studies have shown.
During rest women will use more fats from skeletal muscle for oxidation. However, during activity the shift to utilizing fats from adipose tissue occurs. Even when women increased their carbohydrate intake to 75% of daily calories, depending on the phase of the menstrual cycle they were in, there was no increase in carbohydrate utilization or performance (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2129154/ ). Women who are partaking in carbohydrate loading for performance may want to think twice. There are numerous health concerns that can occur from ingesting too much sugar. Women ingesting 75% of their daily calories as carbohydrate to fuel activity may be doing harm to their bodies without the added reward of increased performance.
In terms of performance, as a female, you may need to tinker with various carbohydrate intakes. Understanding the intensities associated with your activity can help. Remember, as long as the activity is 85% of VO2 max or greater, a higher carbohydrate intake may be effective. However, most of us do not continually work at that intensity, and without expensive equipment it may not be easy to determine what your intensity actually is.
There is an increased need for calories if we are physically active. Most supplements and dietary plans focus on increasing carbohydrate and protein intake for this increased need of calories from physical activity. This may not be appropriate for active females, especially depending on where they are in their menstrual cycle. An increase in healthy dietary fats may be more appropriate when estradiol levels are higher.
Carbohydrates are still important though; we need to make sure that muscle glycogen does not become depleted. This will lead to fatigue and decreased performance no matter what. Women may need fewer carbohydrates to avoid glycogen depletion than men. Utilizing the post-exercise window for carbohydrate intake may be the best spot, as insulin sensitivity is increased. This is where the tinkering comes into play. Try different amounts of carbohydrate intake and see what feels best.
Supplementing with MCT oil and/or using coconut oil to cook with may yield an increase in performance. This increase in performance may be more beneficial for women than men. The reason for this is the ketones that are produced from the breakdown of medium chain triglycerides (MCT). They do not require bile salts to be digested. Instead they are carried to the liver and quickly converted to ketones. If women have a higher reliance on lipids for energy then this may be an added boost because ketones are readily available fats. Some studies have suggested that MCT supplementation can increase glycogen sparing and lipid utilization. This may be an even greater benefit for females during ovulation.
Females and males differ in many ways. One way is in their use of substrates for fuel. Women have a higher reliance on lipid utilization then men and men rely more heavily on carbohydrates then women. As a female, tinker with different macronutrient ratios and see how your performance responds. A higher fat and lower carbohydrate diet might be more appropriate. Also, tinker with various ratios throughout your 28 day cycle as preference for substrate utilization may change. I have heard women tell me before that they have it tougher than men, in this case I might have to agree with them!