How Fasting Changes Fat Burning and Brain Fuel

Steven Bartlett: So, on this point of fasting, are you a fan of fasting to combat visceral fat? Could you also give me your thoughts on being in a ketogenic state as it relates to visceral fat?

Dr. Rhonda Patrick: People often think about intermittent fasting as one thing: weight loss. But there is a lot going on here, and I like that you mentioned being in a ketogenic state because there is also a metabolic switch. This is the switch from burning carbohydrates and glucose to burning fatty acids and entering ketosis. Those are two different things.

Intermittent fasting is essentially a tool that people can use to reduce their calorie intake without having to count calories. That is why I like it. You can lose weight by counting calories and reducing your calorie intake, but I personally think that is a lot of work. Some people love doing it, and that is great. Whatever works for a person.

The way intermittent fasting helps people lose visceral fat is by reducing calorie intake. It is a tool I like because I can skip one meal—while making sure I get enough nutrients and protein in the meals I do eat—and create a caloric deficit without having to count everything. It is easier for me.

Steven Bartlett: How do you do that? What is your method?

Dr. Rhonda Patrick: I like to fast in the morning because of ketosis, which I call the metabolic switch. You are not eating while you sleep. If you are in bed for nine or ten hours, you are not eating during that time. It takes about 10 to 12 hours for your liver to deplete glycogen. Glucose taken up by the liver is stored as glycogen so that you can use it for energy later when you do not have energy coming in.

Steven Bartlett: So glycogen is like the petrol station. It runs out of petrol and switches to diesel.

Dr. Rhonda Patrick: That is right. After that metabolic switch, when you deplete glycogen while sleeping or not eating for about 12 hours, your body changes fuels. This timing is relative because it depends on the foods you eat and how physically active you are. If you eat a lot of high-carbohydrate, refined-sugar foods, it might take longer to deplete glycogen because you keep filling the fuel tank. If you eat foods that are lower in carbohydrates, you might deplete glycogen sooner.

When you deplete glycogen, your body still needs energy, but glucose is not available. Fatty acids are mobilized from adipose tissue, including visceral fat. You start using those fatty acids for energy, and ketones are produced in the process.

I typically fast for about 16 hours a day and eat my meals within an eight-hour window. I like being in that metabolic-switch state for several reasons. Ketones provide the brain with readily usable energy, but they also act as signaling molecules. It is as though they are telling the brain, “This is a stressful time. There is no food. You need to be cognitively sharp and find food.”

That is an evolutionary adaptation. For thousands of years, humans went through this metabolic switch because food was not always available. When I enter that state, I feel more cognitively sharp and less anxious. Ketones also help increase GABA, an inhibitory neurotransmitter that can help you feel calmer. When I feel calmer, I am more cognitively focused because the background anxiety is lower. I like being in that state in the morning because that is when I get my work done.

I also like fasting because it gives the body more time in a repair state. A fed state is important for anabolic growth, but repair is also important because growth comes with damage. You want to repair that damage because damage can accelerate aging. I do not want to wake up and eat as soon as I have barely depleted liver glycogen. I want to extend the fasted state a little longer. Repair is still active during a fed state, but it is heightened when you are fasted.

Those are the reasons I like intermittent fasting. I feel good when I do it. I also do a lot of cardiovascular and aerobic-endurance training while fasted, including running and biking. I am not going for a 10-mile run; I am going for a three-mile run. If I were going for a 10-mile run, I would not be fasted because I would need fuel.

Studies have compared aerobic-endurance training—such as running, cycling, and swimming—in fasted and fed states. Much of the benefit from aerobic exercise comes from how the body responds to the work. The work causes inflammation and oxidative damage, and the body responds by activating anti-inflammatory and antioxidant pathways.

Your body also needs fuel. If you are fasted, you become better at burning and oxidizing fat, and you can continue to do that better throughout the day. You also develop mitochondrial adaptations and make more mitochondria. Mitochondria are tiny organelles inside most of our cells that make energy. They are important for the brain, heart, lungs, and everything else. Exercise increases the number of new, healthy mitochondria you make whether you are fed or fasted, but the adaptation can be greater when you are fasted.

There has been a big debate about whether this applies to both men and women. My reading of the literature, along with what I have learned from experts who study male and female responses to exercise, starts with this question: How do you feel when you exercise fasted? If you feel terrible, that is a sign. Listening to your body is important. There are times when I need to eat before I exercise, and I listen to my body and eat.

It also depends on what you are doing. Are you doing a 30-minute run or a two-hour run? If you are doing a two-hour run, you need to fuel. That is a large stress. For a 30-minute run, you do not necessarily need to fuel first.

The problem for women can arise when they are in too much of a caloric deficit, do not refuel enough afterward, and perform very long, high-volume exercise. That can disrupt hormones such as follicle-stimulating hormone and luteinizing hormone. Women can become amenorrheic, stop ovulating, and stop having menstrual periods.

Steven Bartlett: What is the evolutionary reason for that?

Dr. Rhonda Patrick: The body senses that there is not enough food and energy available to sustain a growing fetus. It essentially decides not to allow a pregnancy, so ovulation stops.

This is not common. It occurs in situations such as elite female athletes who are not eating enough food. I did this to myself in my early twenties when I was racing marathons. I ran eight to ten miles a day, five days a week, and I was eating carrots and hummus. I was not fueling myself adequately.

If you feel terrible when you train fasted, do not do it. If you want to train somewhat fasted, you could have a protein shake with a little almond milk so that you are getting something without eating a full meal.

I do a lot of my training fasted. I am 47 years old and in perimenopause.

Steven Bartlett: You are in phenomenal shape.

Dr. Rhonda Patrick: Thank you. As I started to reach perimenopause, I noticed that I had to be more aggressive and put in more effort to prevent fat from accumulating around my belly. It started appearing, and I did not want it. It was not an option for me.