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Like the ketogenic diet, intermittent fasting is a trending diet. A quick internet search for “intermittent fasting” will yield more than ten million search results. These results link to an enormous number of articles touting the amazing health benefits of this style of eating. And many of these articles are backed by ample, sound, and positive research. So, what’s not to like about intermittent fasting?
While intermittent fasting can be a great option for men, it is often not as great an option for women. Women need to be aware that intermittent fasting does not affect men and women the same way. Let’s explore.
Before we dive into intermittent fasting and gender, it is important to first answer some essential questions.
Intermittent fasting is a style or pattern of eating rather than a traditional diet. While a traditional diet focuses on what you eat, an intermittent fasting diet focuses on when you eat.
In this style of eating, you cycle between periods of severe or complete calorie restriction—in other words, periods of fasting—and periods of healthy eating. The length of time for these periods of calorie restriction and periods of healthy eating vary according to individual choice.
The concept of intermittent fasting stems from a much, much earlier period in history. Before making revolutionary strides in agriculture, humans lived in hunter-gatherer societies. Without a steady supply of cultivated food, these societies experienced times of plenty and times of scarcity. While food is readily available for most in our current society, intermitting fasting mimics a cycle between periods of plenty and periods of scarcity.
There are many different approaches to intermittent fasting. While all rely on periodic fasting, the frequency and length of the fasting periods in each of these approaches are different.
In an alternate-day fasting schedule, you fast every other day. In other words, you eat whatever you want one day and then reduce your calorie intake to zero the next day. Not eating anything for twenty-four hours, however, comes with some health risks. Such extreme fasting can impact your mood and well-being and affect your ability to train. Additionally, alternate-day fasting can be difficult to sustain.
Other less severe fasting schedules are more popular. In the 16/8 approach, you fast for sixteen hours each day and eat during the other eight hours of the day. Many people who follow this approach skip breakfast and eat lunch at one o’clock in the afternoon. Their fast lasts from nine o’clock at night until one o’clock in the afternoon of the next day.
Another popular type of intermittent fasting is known as the 5:2 diet. In this modified approach to fasting, you eat normally five days of the week and restrict your calorie intake to approximately 500 calories on the other two days.
As will be discussed in more depth below, men in particular can benefit from intermittent fasting. The core benefits of intermittent fasting are weight loss and better body composition. But there are also many other health benefits associated with fasting. These include improved testosterone levels and cardiovascular health as well as decreased insulin resistance.
Although gender can be a sensitive and controversial subject, differences between men and women are important to consider in relationship to health. Political, economic, and cultural arguments aside, biology has formulated men and women differently at a genetic (and thus phenotypic) level. The differences between men and women go beyond X and Y chromosomes. We can see these differences in the fact that men are on average taller than women and the fact that women generally have smaller lungs.
Gender roles dictated by biology have played a part in shaping male and female metabolic responses to exercise, carbohydrates, sleep deprivation, and yes—you guessed it—fasting.
While living in hunter-gatherer societies, men and women adapted to periods of plenty and scarcity differently. Men, with their generally larger physical size, responded to fasting with a giant boost in metabolic rate. This metabolic boost gave them the fuel necessary to hunt. Essentially, men’s genetic makeup says, “Go get food for everyone,” when they haven’t had much to eat.
Research reveals that genetic adaptations to periods of scarcity can still be seen in humans today. During short periods of fasting (twelve to twenty-four hours) men’s metabolisms increase up to 14 percent. Other effects of intermittent fasting on the male body include an increase between 10 and 200 percent in testosterone utilization, an increase between 100 and 200 percent in growth hormone, and an improvement in blood lipids to support the increased hormonal production and decreased risk factors for cardiovascular disease
Women, however, do not respond to intermittent fasting like men do. In hunter-gatherer societies, women’s bodies responded to periods of scarcity differently than the bodies of men did. Women’s metabolisms slowed down to conserve energy and store fat in order to survive a potential long-term famine. What this means for women today is that intermitting fasting may not work well for their bodies.
Some hormones, which we can call “hunger hormones,” have an impact on how hungry we feel. And these hunger hormones impact how intermittent fasting works. The two important hunger hormones are leptin and ghrelin. Created by fat cells, leptin decreases your appetite or level of hunger. Ghrelin, in contrast, is an appetite stimulator. The stomach releases ghrelin, which is believed to signal hunger to the brain.
Research suggests that intermittent fasting can lower levels of ghrelin. This, in turn, can lead to lower feelings of hunger and weight loss.
Research on intermittent fasting demonstrates gender inequality. Out of seventy-one studies found in Harvard’s database for intermittent fasting, only thirteen include women at all. Beyond that, absolutely none of the controlled studies focus on the female population in general. There are no controlled studies that allow us to draw intelligent conclusions about how intermittent fasting affects the female population.
One of the thirteen studies related to intermittent fasting that includes women is on pregnant women fasting during Ramadan. This study found no improvement in insulin sensitivity and an increase in blood lipids for women. This suggests that at least some of the health benefits touted by other studies of intermittent fasting apply only to men. Additionally, as every woman who has been pregnant knows, the female body is completely different when pregnant versus not pregnant.
When you sift through the precious little data on women in a fasted state, you find something fascinating: women don’t respond to fasting like men do.
In fact, instead of much-celebrated metabolic boosts and weight loss, women might find a 50 percent increase in cortisol and a decrease in insulin sensitivity as a result of intermittent fasting. This means that intermittent fasting could contribute to obesity and diabetes instead of health benefits for women.
Overall, men tend to do better with intermittent fasting. They experience the most proven health benefits from this style of eating. Women, in contrast, tend to benefit from other diets and eating styles discussed below.
Although we could benefit from more research on this subject, women should consider what we do know about intermittent fasting before deciding to try this eating style.
As suggested above, an intermittent fasting diet is likely not going to help women lose weight. Men are likely to benefit from skipping breakfast or restricting their calorie intake for an entire day once or twice a week. These actions can boost metabolism and lead to weight loss. Women, however, are not likely to benefit from the same diet.
Women who want to optimize their body composition, have consistently high energy, and continually improve their workout performance would benefit the most from a constant calorie intake each day. An unchanging flow of high-quality calories—rather than intermittent fasting—is likely to lead to the best weight loss results for women.
Intermittent fasting might actually prevent women from getting enough nutrients. By maintaining an unchanging flow of high-quality calories, women can ensure that they are getting enough nutrients to maximize their health. Because of this, women, more than men, might benefit from sticking to the old guideline of eating four to six small meals per day.
For help with nutrition strategies, check out the ISSA article “8 Ways to Eat a Balanced Diet.” As this article notes, listening to your body’s natural hunger cues and not skipping meals can help you achieve your health and nutrition goals. By making purposeful, consistent food choices, you can boost your energy while reducing mood swings and overeating.
Women who are trying to become pregnant should not try intermittent fasting. Diets with consistent food intake rather than severe calorie restrictions are particularly important for women who want to conceive. This is because intermittent fasting can impact a woman’s menstrual cycle, leading to missed periods and infertility.
Intermittent fasting does not only have to be for men. However, we recommend that women ask their doctors about fasting and consider the potential health benefits and risks before trying this or any diet.
Something that is particularly fascinating for trainers is the impact of limited research on health and fitness recommendations and practices.
Why are there so many different recommendations out there? Why do some experts recommend a high-carb diet while others recommend a low-carb diet for maximum health? Or weight training versus cardio training for fat loss? Or volume versus intensity for exercise? The answer can often be found in the way that scientific research is conducted. Focused research on one type of individual is likely to produce different results than focused research on a different type of individual.
Research on the science of intermittent fasting—especially for women—still has a long way to go. That said, individuals who are interested in trying this type of diet can be confident that there is some science behind intermittent fasting.
Intermittent fasting affects many chemicals in the body, including the hunger hormones leptin and ghrelin. As discussed above, intermittent fasting reduces ghrelin and can help you lose weight.
In addition to helping with weight loss, intermittent fasting can lead to lower blood sugar and insulin levels, lower LDL (“bad”) cholesterol and blood triglycerides, and reduced inflammation. It is important to note, however, that most research on how intermittent fasting affects body chemicals comes from studies done on men.
There is certainly science behind the intermittent fasting diet. That said, it is good advice to be skeptical of any newly hyped diet or exercise fad that claims to be effective for everyone. One size does not fit all. This is a core reason why our industry exists.
Inherently, we know men and women need different things from us as trainers, in the same way that people of different ages, training experience levels, and body types need different things from us as trainers. We hope that the science of nutrition and exercise will catch up to our intuition as trainers one day.
The takeaway from all of this is that science suggests that some males can skip breakfast and receive possible benefits. However, it seems that intermittent fasting for most females is not currently supported by research. Because of this, it is better for most women to eat on a consistent and frequent schedule. Coach your clients accordingly.
Inspired to learn more about how food can fuel your clients’ fitness goals? Explore ISSA’s Nutrition course to educate yourself and provide more expertise to your clients!