FASTED EXERCISE AND AVOIDING THE “BONK”

Now that we have a dog, Gus, hiking is our favorite summer activity. But we used to mountain bike frequently with a group of athletic and fit friends. On those rides, constant consumption of energy bars and other snacks was as critical as water and a spare inner tube. There was a deep fear of “bonking” on the trail. The idea of fasted exercise was unthinkable.

Today, fasted exercise is very important to my goal of cardiovascular health and longevity.

Since there are many ways to fast, there are many ways that fasting and exercise interact. Time-restricted feeding (TRF) with an 8-hour feeding window is by far the most popular way that people fast. Most of the research we have regarding fasting and exercise comes from this experience. Exercise during longer fasts is less researched, but there is a lot of knowledge related to the physiology of exercise and fasting that is useful for helping us know how our bodies will react to longer fasts.

Glycogen, fatty acids, and ketones

Technically, fasting starts about 6 hours after a meal. That is when the energy supplied by that meal is dissipated and we need to dip into stored energy supplies. We store energy in two tanks. The little tank contains about 2,000 calories of glucose molecules linked together in muscle cells; it’s called glycogen. The big tank contains hundreds of thousands of fat calories stored as triglyceride in our fat cells. The idea of fasting, whether for 16 hours or for 5 days, is to use up the little tank and tap into the big tank. 

Anyone who’s felt their blood sugar crash during a workout would assume that glucose is needed for exercising muscles. Actually, there are a variety of fuel sources, and different organs in our bodies use different sources. Muscle also uses a variety of fuel sources at different times, depending on whether we are in the fed state, fasting state, at rest, or exercising. 

In the fed, or absorptive, state, insulin levels are elevated, our muscles use glucose for energy, and our bodies are storing–not burning–fat.

In the fasted, or postabsorptive, state, insulin levels fall and the liver makes glucose for use mostly by the brain. Our brains are largely made up of fat, but can’t use fatty acids as a fuel source. During fasted exercise, our muscles first use the glycogen stored in the muscle for energy. With lower insulin levels, fatty acids can be released from fat storage and enter muscle cells for additional fuel.  

When resting, muscle derives 85% of its energy from fatty acids. When we exercise, our muscle cells need additional energy. Glycogen is the most readily available energy source when we need bursts of energy. 

Fueling our muscles

Healthy muscles can use a variety of fuels during exercise. Very intense exercise, such as heavy weight lifting or high intensity interval (HIIT) training, uses a lot of energy very quickly, so glycogen is the best energy source. Exercise also stimulates the release of adrenalin, which increases the availability of fatty acids to the muscle for energy. This supply of energy is more appropriate for sustained energy needs, like running or biking. 

During longer fasts, our bodies create another fuel source from fatty acids: ketones. Ketones are a very efficient fuel source for both our brains and our muscles. The liver converts fatty acids into ketones, which enter the blood and provide efficient energy to the brain, heart, and muscle. 

This system of energy utilization is metabolically very flexible. Muscle can use whatever fuel, glucose, glycogen, fatty acids, or ketones, that is available and best for the circumstances.

The trouble is that 88% of Americans have a hard time using fatty acids for fuel in our muscle cells. When this happens, insulin is less effective in bringing glucose into the cells and there is less glycogen stored in muscle. This condition is called insulin resistance. It affects muscle by making it hard for the mitochondria to generate energy. Our muscles cannot increase the use of fatty acids and are stuck with utilization of glucose driven by higher and higher insulin levels. Over time this can lead to metabolic syndrome, heart disease, or diabetes.

Should you exercise while fasted or fed? 

1. Muscle glycogen stores: 

The purpose of longer fasts is to use up the glycogen stored in muscle, and exercise can deplete glycogen more quickly. When muscle glycogen is gone there is a strong stimulus for ketone production and stimulation of autophagy. Light to moderate aerobic exercise is useful in this circumstance. On the other hand, heavy weight training uses energy very quickly and only glycogen stored in the muscle can provide very fast energy. When glycogen stores are low, heavy weight training will result in early muscle fatigue.

2. Ketone production: 

Muscles work well when fueled by ketones. Ketone production in the liver increases when insulin levels are low and fatty acids are elevated. People with good ketone production, through a ketogenic diet or a regular fasting practice, have an efficient way to use fat stores for energy and often feel well exercising. There are some endurance athletes who prefer to compete in a ketotic state and do well.

3. Metabolic flexibility:

Poor metabolic flexibility makes it more difficult to exercise while fasting because our bodies cannot efficiently use fatty acids and glucose for fuel. On the other hand, exercise is the best way to improve metabolic flexibility! Exercise in both the fed and fasting state is beneficial. Strength and endurance need to be built gradually but the rewards will be great. 

What happens when you exercise in a fasted state

During protracted exercise, glucose production by the liver may be slow and glycogen stores can run out. This is the situation that leads the dreaded “bonk.” Endurance athletes might want to take advantage of the nearly unlimited production of energy from stored fat in the form of ketones to improve performance. Exercise further helps us reduce visceral fat stores. Weightlifters and bodybuilders, on the other hand, rely on short bursts of intense energy utilization that can only be provided by glycogen stores. Avoid these workouts when fasting or trying to build muscle.

When fasting for a long time (two to five days), exercise with light and moderate aerobic and weight training to give your muscles anabolic signals to reduce muscle wasting. People who are fasting can accelerate the depletion of glycogen by exercising and get into ketosis faster. Ketones provide very efficient energy to the brain and muscles. Ketosis is the state associated with healing and autophagy and one of the goals of longer fasting.

Exercising in a fasted state helps us establish or maintain metabolic flexibility. The cellular mechanism responsible for the entire cascade of insulin resistance and metabolic dysfunction is the accumulation of abnormal fat in the muscle cells, called diacylglycerol (DAG). DAG is similar to triglyceride, but has only 2 fatty acids attached to glycerol instead of 3. Getting the DAG out of muscle cells through exercise, low carb or ketogenic diets, and fasting prevents and reverses insulin resistance. Fasted exercise burns 20-30% more fat than when we are fed, and is a very effective way to get the DAG out of muscle cells.

Time-restricted feeding and exercise 

With time-restricted feeding (such as 16:8 or one-meal-a-day), you have the choice of exercising in the fed period or the fasting period. Exercise, other than intense weight training, can proceed normally. Metabolic flexibility is improved with exercise in the fasted period, and muscle building is maximized by exercising in the fed state. There is an anabolic window 30 minutes to 2 hours post-exercise during which calorie and protein will stimulate more muscle growth. 

Athletic performance may suffer with fasted exercise until you are adapted. With longer fasts, exercise should be increased gradually. Exercise can be difficult if the transition from glucose burning to fat oxidation and ketone production is slow. This is the cause of the “keto flu” which is characterized by dehydration, headache, and weakness.Only walking is appropriate if these symptoms occur. Everyone should walk a lot when fasting. As your body learns to transition into ketosis more easily, more vigorous exercise can be tolerated, including some weight training to preserve muscle mass. 

With refeeding at the end of a fast, anabolic potential for muscle building is high. This is a great time for a nutrient dense high protein diet and vigorous weight training to build muscle.

Gus and I frequently hike in the fasted state, or at least I do (Gus never fasts). I am amazed that hunger and lack of stamina are not an issue and there is no desire to fuel up for several hours. With practice fasting, this can be your experience, too.

Remember:
Exercise when fasted for metabolic flexibility.
Exercise when fed to increase muscle mass.
Exercise during fasting to preserve muscle mass.

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