And Intermittent fasting. There are many variations on the practice. Some people eat normally for five days a week and fast for 24-hours, once or twice a week. While others look to a more acute fast:eat schedule of restricting their eating window daily to a super-short 4 hours, with others extending their eating window to 8 or, at the outer limit, a 12 hours. Terminology here matters, too. Intermittent fasting (IF) is the practice of restricting your calorie intake to a shortened window with calorie restriction.
Time Restricted Eating (TRE) is often interchanged terminology with IF, but the difference is that during the eating window, there are no limitations on how much a person eats (e.g. no calorie restriction). Time-restricted eating usually aligns the eating and fasting cycles to the body’s innate 24-hour circadian rhythm. IF has garnered all the buzz in the medical and fitness world as the panacea of weight loss, longevity and health; and if your training falls into the fasted window, well, fasted training supposedly improves the way your body fuels during exercise. Let’s step back and unpack this a bit.
The Concept of Fasting
The basic health premise behind intermittent fasting is that if you deprive your body of food for long periods, it will lower your insulin levels and burn your stored fat for fuel. Proponents believe that by putting your body under this stress, it forces it to adapt in healthy ways, much like it does when you put it under exercise stress.
Much of the original fasting and health research came out of the mid 1930s to understand what soldiers endured in war and how nutritional deficiencies impact people during times of widespread economic hardship. Scientists wanted to see what would happen to people’s health if they restricted their food intake for economic reasons. As was standard practice, they only included men in their human studies. And they found that restricting calories by about 20 percent was actually beneficial for health. Follow up research (again only with male subjects) confirmed that calorie restriction leads to increases in telomere length. (Telomeres are a region of DNA that shortens over time and acts as a biological marker for aging.) IF also appeared to increase autophagy, which is your body’s way of cleaning out damaged cells in order to regenerate newer, healthier cells.
More recently, medical scientists have been investigating intermittent fasting as a way to help people with obesity. These studies have been done on people who are reflective of the general population, so they are overweight or have obesity, and have unhealthy blood lipids and blood glucose levels.
Generally speaking, the men in these studies do really well. Regardless of age, they have improved insulin sensitivity, so they are at lower risk for diabetes; they have lower LDL and triglyceride levels so their cardiovascular health improves; they have better autophagy, so that cellular debris is cleaned up; and they have lower levels of oxidative stress, so their cells are protected from free radical damage. Their brains even work better. They score better on memory tests. So, it’s easy to look at that research and think, “Wow. That’s fantastic.”
But … this data has mostly been collected on men, and sedentary men at that. When we look at the sparse data from women, we find different outcomes; however, most of the outcomes are from sedentary and/or obese women. When you look at sedentary women, there’s no benefit for insulin sensitivity. In fact, women who have high blood sugar and who are on the cusp of becoming diabetic see a worsening in their insulin sensitivity and blood sugar. On the plus side, women see an increase in their good HDL levels, but there’s no shift in their LDL or triglyceride levels. Unlike men, women have minimal, if any, improvement in autophagy. And they have increased oxidative stress. Worse, fasting has adverse effects on their endocrine system. The thyroid function slows down and takes their metabolism with it; women’s bodies will fight to preserve energy with a lack of nutrient density and available calories.
This picture gets worse for women who exercise. If you exercise on top of fasting—which is what many women try to do to lose weight—all these negative effects become amplified. Which is pretty much the opposite of what you’re looking for as an athletic woman working to maintain a healthy metabolism with good metabolic and cardiovascular health.
Why? Gender differences in physiological responses to food availability. When we look at early cellular research on IF, it was found that the gene alterations to improve fertility was five times greater in males than females; and gene alterations to reduce fertility occurred in females. We see this in current studies of low energy availability and Relative Energy Deficiency in Sport (RED-S); the threshold for the onset of endocrine dysfunction is ~30-35 calories per kg of fat-free weight in women; but around 15 calories per kg in men. This difference comes down to the hypothalamic-pituitary-gonadal (HPG) axis, which is responsible for regulating key hormones that are involved in functions like appetite control and reproduction is incredibly sensitive to energy intake. The effect of a negative energy balance is thought to work through kisspeptin. Kisspeptin, is a neuropeptide that is responsible for sex hormones and endocrine and reproductive function, which also plays a significant role in maintaining healthy glucose levels, appetite regulation, and body composition. Women also have two areas of kisspeptin neurons in the hypothalamus, whereas men have one; meaning women have greater sensitivity to changes in energy balance. Intermittent fasting can disrupt kisspeptin regulation. When our brain perceives we have a deficiency in nutrients, especially carbohydrate, we have a marked reduction in kisspeptin stimulation, which not only increases our appetite, but also reduces our sensitivity to insulin. This is why research shows intermittent fasting is more likely to cause impaired glucose intolerance in women, but not men.
How the Body Reacts to Fasting
What happens when we layer exercise stress on top of the stress of denying our bodies an important fuel source? Stress hormones like cortisol rise even higher. As you keep increasing that stress, it keeps your sympathetic drive high and reduces your ability to relax. Your thyroid activity is depressed, the key hormone for regulating menstrual cycles; luteinizing hormone (LH) starts to lose its daily pulse and this messes with your menstrual cycle. Your body also starts storing more belly fat. Now you’re facing disrupted menstrual cycles, higher anxiety and stress, impaired performance and often weight gain—pretty much the opposite of what you’re looking to achieve. Additionally, fasting kicks up the sympathetic (a.k.a. fight or flight) drive in women, producing more anxiety, greater risk of depression, and impaired sleep patterns (a significant reduction to REM and SWS, the two patterns responsible for mental and physical growth and reparation), which again hurts our hormonal health, performance, wellbeing, and body composition.
Due to biological differences, if women train fasted, they are more likely to tap into lean muscle as an energy source as the body is trying to conserve all fat and carbohydrate stores for critical functions. Research with mice following IF indicates livers of male mice stop producing energy storage molecules whereas livers of females make use of all available resources in an effort to stock the energy necessary to maintain the reproductive capacities. Biologically, women are designed to hold onto fat and carbohydrates because, as far as evolutionary fitness is concerned, her primary purpose is to feed, nurture, and cultivate an entire other human being inside her body for nine months. Women have to create and support another life inside their bodies. They have to provide the food, the water, and the shelter at all times and under all circumstances. If something goes terribly wrong in the outside world, that nutrient flow to the fetus could be interrupted, thus putting her evolutionary purpose at risk. So, having adequate fat stores is key to the success of the baby. Hence a female athlete is more likely to tap into lean muscle mass to fuel her fasted training session than fat stores, further compromising strength and muscle function for performance.
Final Thoughts on Fasting
Many women will contend that intermittent fasting works well for them. And it might actually work to some degree for the first three months, because nearly any diet will yield some effects in the short term. The long-term effects for female athletes, however, is endocrine dysfunction, increases in abdominal fat, more depression, and a backlash of subsequent fat gain. The bottom line is that female athletes should focus on fueling themselves adequately so that they are able to cover all metabolic processes and also the energy expended during exercise. To improve training adaptations and performance, fueling before exercise and recovering from exercise are equally critical.
If you’re a female athlete training at any level, it’s best to avoid intermittent fasting.
This article originally appeared in Women’s Running.