How to Fast for Metabolic Fitness and Hormone Balance

STORY AT-A-GLANCE

• The vast majority of people eat across 12 hours or more, which is a recipe for metabolic disaster. Health statistics bear this out, showing that 93.2% of Americans are metabolically unfit

• Time-restricted eating (TRE) is one of the most foundational strategies to stay healthy, and when done appropriately can also help balance your sex hormones

• Your body has two primary energy systems. One of them is activated when you eat. When your blood sugar goes up, your body uses that glucose for energy. When you don’t eat for a period of time, your blood sugar goes down, which switches when your body is metabolically flexible to a different energy system that uses ketones derived from fat instead of glucose

• These energy systems are intimately tied to your circadian rhythm. Food is the most important regulator for the clocks inside your cells. If you don’t get your feeding times right, your circadian rhythms, which are responsible for turning cellular protein production on and off, will become seriously impaired.

• Three key rules of TRE are: Your eating window should be shorter than 12 hours; avoid eating first thing in the morning — wait at least two or three hours; avoid eating right before bed. Have your last meal at least three hours or more before bedtime. With those rules in mind, your eating window could be anywhere from two to 10 hours, tailored to your unique circumstances

In this interview, Dr. Mindy Pelz, author of "Fast Like a Girl," reviews how time-restricted eating (TRE) can improve your health and balance your sex hormones. In general, I believe TRE is one of the most foundational strategies to stay healthy, but the devil’s in the details, and Pelz will tease out some of those here.

Metabolic Health Is at an All-Time Low

The vast majority of people eat across 12 hours or more, which is a recipe for metabolic disaster. Health statistics bear this out, showing that 93.2% of Americans are metabolically unfit.1

In July 2022, the Journal of the American College of Cardiology2 posted an update on the metabolic fitness or flexibility of the American population. In 2016, 12.2% of Americans were considered metabolically fit.3 Two years later, in 2018, only 6.8% of U.S. adults had optimal cardiometabolic health.4 That was four years ago so, today, that ratio is probably even lower.

Metabolic fitness includes things like blood glucose and blood sugar, blood pressure and weight, and metabolic flexibility refers to your body’s ability to seamlessly transition between burning fat and carbohydrates as your primary fuel.

The take-home message is that now 19 out of 20 Americans would benefit from improving their metabolic health, and TRE is one of the easiest yet most powerful interventions for reducing insulin resistance, restoring metabolic flexibility and losing excess body fat.

Pelz has spent the last eight years of her clinical career using TRE, fine-tuning it in the trenches and gaining an understanding of the hormonal components that need to be taken into consideration, especially in women.

Your Two Energy Systems

As explained by Pelz, your body has two primary energy systems. One of them is activated when you eat. When your blood sugar goes up, your body uses that glucose for energy. When you don’t eat for a period of time, your blood sugar goes down, and if you are metabolically flexible your body switches over to a different energy system that uses ketones derived from fat, instead of glucose.

Another interesting component is that these energy systems are intimately tied to your circadian rhythm. Many know that the circadian rhythm is regulated by a master clock in the suprachiasmatic nucleus of your brain. But what most people don't realize is that most all cells in your body also have an internal clock that is unrelated to this master clock.

While light and darkness regulate the master clock, food is the most important regulator or trigger for this other clock inside your cells. So, if you don’t get your feeding times right, your circadian rhythms, which are responsible for turning cellular protein production on and off, will get seriously impaired. Pelz explains:

How Long Is Your Ideal Eating Window?

The basic premise of TRE is that you eat all your meals within a specified window of time each day, and fast for the remainder. Where people differ is the length of that window. Here are three key rules to keep in mind:

1. Your eating window should definitely be shorter than 12 hours

2. Avoid eating first thing in the morning. Wait at least two or three hours

3. Avoid eating right before bed. Have your last meal at least three hours or more before bedtime

With those "absolutes" in mind, your eating window could be anywhere from two to 10 hours. Eventually, you’ll need to find the ideal window for yourself, but to start, here are some other basic considerations.

How to Customize Your Fasting Window

So, how do you go about finding your unique and ideal feasting-fasting schedule? Pelz suggests starting by getting your fat burning system working again. If you’ve been an all-day grazer, start by compressing the window of time within which you eat.

Note the time you normally eat your first meal or snack, and when you eat your last. Count the hours, and then cut that down by one or two hours. Pelz recommends pushing back breakfast by one hour to start.

Do that for one month, or until you’re completely comfortable with it, and then move into a more varied schedule, such as the 5-1-1 schedule mentioned earlier. The key at that point is variation. Feasting one day, and fasting for longer and shorter times on others.

Time Window Once You Are Metabolically Healthy

After I did this interview with Dr. Mindy, I began studying Ray Peat's work and realized an important point. If you are one of the 19 out of 20 people who are metabolically inflexible, insulin resistant, and unable to easily switch between burning sugar and fat as your primary fuel, then the program that Dr. Mindy describes may be beneficial for you.

However, once you regain your metabolic flexibility, which can take anywhere from a few weeks to a few months, you will need to increase your eating window. This is because your body needs glucose and if you deprive it for too long, it will produce cortisol to stimulate your liver to make it. This increased cortisol can contribute to chronic inflammation and cellular damage.

Therefore, once you are no longer insulin resistant, it is best to vary your eating window between 8 and 12 hours and avoid going lower or higher than that window. It is also best to avoid eating before sunrise or after sunset and at least three hours before bedtime.

Another Starter Tip

Interestingly, research has shown that when you compress your eating window to around 16 hours or more, you become relatively metabolically immune to the damage that otherwise unhealthy foods would cause, such as processed food and refined carbohydrates.

This is not permission to eat junk food forever, but it does allow you to have a less than perfect diet all the time and still lose weight. This also means that when you’re first starting out, don’t change WHAT you’re eating. Only change WHEN you’re eating. So, if you eat a lot of processed foods, continue as normal, and just compress the eating window.

Once you’re used to that, THEN you can start making nutritional changes, start cutting out some of the unhealthiest foods and add in more wholesome choices you may not be used to. This will give you the greatest chance of success.

Dip Into Longer Fasts

In her book, "Fast Like a Girl," Pelz describes six different fasts, ranging from 13 to 72 hours, all of which have been tested on millions of people. She suggests thinking of fasts as "switches," where the longer you stay in a fasted state, the more healing is allowed to take place in your body.

Goal Considerations

Aside from improved mood, those benefits also include autophagy and increased stem cell production. All of that said, though, longer fasts are not ideal for everyone. While in the past I’ve done up to 10-day water fasts, I no longer go beyond 20 hours or so, due to my age and my already optimized metabolic flexibility.

At this point in my life I’m far more concerned about maintaining muscle mass and preventing sarcopenia. I am more than convinced that the daily 18-hour fasts I do provide me with more than enough autophagy.

If I cut calories by 500 or 1,000, I can lose a pound or two, maybe even 5 pounds if I do it over a weekend of travel. So, again, there are no hard and fast rules. You really need to determine what works best for you, and keep your eye on your goal. What is it that you’re trying to achieve? Weight loss? Disease reversal? Antiaging benefits? Muscle mass maintenance?

Each may benefit from a different fasting schedule. "As long as fasting is working for you and you're not getting stuck , then you don't need to take that one-day ," Pelz says. For the record, muscle loss is not a given, even with longer fasting intervals, so if you’re younger and fit, you can still do them. The key then lies in making sure you’re feeding your muscles correctly when you do eat. As explained by Pelz:

The Importance of Protein

For muscle maintenance, you want at least 30 grams of protein, twice a day, to activate mTOR and provide enough raw materials to sustain your muscle mass, if not increase it. The reason muscle mass is so important is because within your muscle you have receptors that drive sugar into the muscle, so they act as a glucose sink. That’s how increasing your muscle mass helps counteract high blood sugar and lower your risk of diabetes.

Guidance for Timing Your Eating Window

As mentioned earlier, you want to eat your last meal at least three to four hours before bed. One reason for this is because digestion continues for five to six hours after our last bite or drink of any food (calories). So, if you eat your last meal three hours before bed, you’ll remain in digestive mode for another three while sleeping.

Assuming you sleep for eight hours, that gives you five hours in repair and regeneration mode while sleeping, plus however many hours you fast into the morning. Getting this timing right is important, as it affects your insulin level and related hormones, such as melatonin. When you’re digesting food, melatonin will be inhibited, and hence the quality of your sleep will go down and your insulin resistance will go up.

Fasting and Female Hormones

Men and women differ in terms of the sex hormones that predominate and drive key health processes. For men, testosterone is a primary one, and research shows intermittent fasting has a beneficial impact on testosterone in men. A 15-hour fast can raise testosterone by as much as 1,300%.

In women, testosterone, estrogen and progesterone are a key triad. While women don’t have as high a testosterone level as men, it’s still a very important hormone. After the age of 40, estrogen starts to fluctuate, often resulting in weight gain and insulin resistance. Timing your fasting to your menstrual cycle can help smooth out these estrogen dips.

Glucose is required to make progesterone, so the week before a woman’s cycle, her body will raise glucose and become more insulin resistant.

A woman who follows a one-meal-a-day lifestyle will often see adverse changes related to her sex hormones, such as hair loss, cycle changes and early menopause, because her progesterone is tanking. The good news is you can normalize your hormones by changing the length of your eating window during the different phases of your menstrual cycle.

A Woman’s Fasting Cycle

One of the prime reasons for the interview was to understand how time-restricted eating is modified based on a woman’s menstrual cycle. In her book, Pelz maps out what she calls the fasting cycle, which is a tool women need when they first start fasting.

The graphics below from Pelz’s book can be helpful in understanding the female fasting cycle.

Fasting Cycle for Perimenopausal and Menopausal Women

If you no longer menstruate because of your age, this next section is for you:

If you’re in menopause, Pelz recommends doing the 30-day reset detailed in her book, but timed to the moon cycle. You may notice that the four cycles in the charts above have the same names as the moon cycles. So, you’d start your reset based on the moon cycle that is present at the time that you begin. For example, the day of the full moon is ovulation at Day 11, so you’d start at Day 11 and follow the schedule for the next 29.5 days from there.

More Information

To learn more, be sure to pick up a copy of "Fast Like a Girl." She’s also releasing an app that will guide you on what you should be doing each day, which can be a very helpful tool.