Running on an empty stomach, also known as fasted running or fasted training, is a practice where individuals run after a period of fasting, typically in the morning after at least 6 to 8 hours of overnight fasting. While some runners swear by it, citing potential benefits like increased fat burn and improved endurance, others are wary of the potential risks, like reduced energy levels and muscle loss. This article explores the science behind fasted running, weighing the benefits and risks to help you determine if it's right for you.
Potential Benefits of Fasted Running
Advocates of fasted running suggest that it offers several advantages, primarily related to how the body utilizes energy.
Increased Fat Burn
The most touted benefit of fasted running is its potential to increase fat oxidation, or fat burn. The theory is that when you run on an empty stomach, your glycogen levels (stored carbohydrates) are low. As a result, your body is forced to tap into its fat stores for energy.
Several small studies have supported this idea. A 2015 study on male participants and a 2017 study on female participants both found that exercise before breakfast increased fat oxidation over 24 hours, attributing this to the body's response to low carbohydrate levels, which stimulates genes that control fat oxidation.
However, the research is not conclusive. A 2018 study found that fasting led to less fat burn after exercise compared to a pre-workout meal of protein or carbs. Additionally, a 2020 review of 71 studies did not find a strong link between fasted endurance training and increased fat oxidation. More research is needed to confirm the effect of fasted running on fat burn.
Read also: Weight loss with running
Reduced Energy Intake
For individuals aiming to lose weight, fasted exercise may help regulate energy intake. Studies have shown that individuals who ran on an empty stomach consumed less energy over 24 hours, potentially due to the liver-brain neural network being affected by depleted liver glycogen stores.
Might Improve Aerobic Endurance
Some evidence suggests that training on an empty stomach may improve aerobic endurance. One study associated fasted exercise with higher VO₂ max, a measure of maximum oxygen uptake during intense physical activity and an indicator of aerobic endurance and overall fitness. However, this study was small and requires further investigation. A well-designed training plan will naturally raise VO2 max and your ability to burn fat as fuel.
Less Digestion Problems
During prolonged exercise, many athletes experience digestive issues, such as stomach cramps, nausea, vomiting, and diarrhea. Running on an empty stomach might be a solution for individuals prone to these problems, as it eliminates the need for the body to digest food while simultaneously fueling the muscles.
Potential Risks and Drawbacks of Fasted Running
Despite the potential benefits, fasted running also carries certain risks and drawbacks that should be carefully considered.
Reduced Training Intensity
While your body can burn fat for fuel, it's not as efficient as using carbohydrates. When your fat stores can't meet the demands of your run, fatigue will set in, making it difficult to maintain a high intensity or pace. Studies have shown that running after fasting is associated with lower endurance.
Read also: Unlocking Athletic Potential with Carnivore Diet
Injury Risk
As your energy stores decline, you're more likely to feel fatigued, which can increase your risk of injury during physical activity. Additionally, your brain needs glucose to function properly, especially during exercise. If you run fasted, your brain might not get enough energy, making it difficult to maintain proper form and pay attention to your surroundings.
Muscle Loss
Cortisol, a hormone produced by the adrenal glands, controls functions like blood glucose and the stress response. High cortisol levels promote protein breakdown in muscle cells, leading to muscle loss and weakness. Cortisol levels are typically highest in the morning. Running on an empty stomach in the morning could negatively affect your muscles. A greater percentage of energy will come from protein, which comes from catabolizing in skeletal muscle if you run without eating first.
Risks for Certain Conditions
Fasted exercise isn't suitable for everyone. Individuals with type 1 or type 2 diabetes may experience low blood sugar (hypoglycemia) when running on an empty stomach, especially if they take diabetes medications like insulin. Similarly, individuals with Addison's disease may experience dangerously low blood sugar levels. It is always recommended to check your blood sugar and eat a snack before running. If you have either condition, consult your doctor about safe exercise practices.
Ineffective for Long-Term Weight Loss
While fasted exercise may increase fat burn, it may not be ideal for long-term weight loss. Studies have shown similar rates of weight loss between exercising before and after eating, suggesting that working out fasted doesn't cause significant changes in body composition. This is because the body regulates its fuel source, compensating for high fat burn during a fasted run by reducing fat burn later on and utilizing more glucose instead.
Potential for Overeating
If you decide not to eat before a run, you may feel hungrier later in the day as your body craves calories, potentially leading to overeating.
Read also: Guide to Running for Weight Loss
Important Considerations
Timing Your Meals, Insulin Needs, and Exercise
If you take mealtime insulin or other medications that can cause low blood glucose levels, you’ll need to plan the timing of your meals and your exercise more carefully. If you’ve just eaten a meal and taken your full dose of insulin, exercising in the three hours afterward can lead to low blood glucose (hypoglycemia). Timing Your Meals, Insulin Needs, and Exercise:
- Reduce the amount of insulin you take for that meal.
- Exercise before eating and taking mealtime insulin.
Most importantly: Be sure to check your blood glucose before you exercise and carry fast-acting carbohydrates with you (like glucose tablets, gummy candy, or juice) whenever exercising in case you experience low blood glucose.
How to Run Safely on an Empty Stomach
If you choose to run on an empty stomach, here are some precautions to take:
- Stick to light to moderate running: Avoid long-distance running or high-intensity training.
- Listen to your body: Take a break if you feel lightheaded, fatigued, or weak.
- Eat a pre-workout snack for longer or more intense runs: Choose a light snack of healthy carbs 1 to 3 hours before running. Examples include whole grain cereal, whole wheat toast, a banana, an apple, yogurt, or a granola bar without added sugars.
- Focus on recovery nutrition: After running, eat lean protein and healthy carbs within 2 hours to support muscle recovery and replenish glycogen stores. Examples include a turkey sandwich with whole wheat bread and vegetables, oatmeal and peanut butter, yogurt and fruit, salmon with avocado and quinoa, or a fruit smoothie with Greek yogurt and nut butter.
- Stay hydrated: Drink water before, during, and after your run to avoid dehydration.
- Consult with a professional: If you're new to exercise, have a chronic condition, have a former or current injury, experience pain while running, are training for a competition for the first time, or want to change your diet, work with a doctor, personal trainer, or dietitian to determine if running on an empty stomach is safe for you.
Some people like to run on an empty stomach. They often do this in the morning, after at least 6 to 8 hours of overnight fasting. It’s sometimes called “fasted running” or “fasted training.” When you haven’t eaten, your glycogen levels are low. Glycogen is the storage form of carbohydrates. It’s what your body primarily uses for energy.
Alternatives to Fasted Running
Instead of running completely fasted, you could eat a protein and fat-based meal to train your body to burn more fat. Additionally, a well-designed training plan will naturally raise VO2 max and your ability to burn fat as fuel.
If exercising on an empty stomach isn’t a great fit for you, you can avoid consuming extra calories by simply timing your workout right after a small snack or a light meal. What should you eat? Stick with real food. It doesn’t need to be complicated. Often, combining a carbohydrate source with a protein source is recommended. Some examples include:
- Fruit + protein (examples: apple with a tablespoon of peanut butter, berries, and a few slices of turkey or nuts)
- Starch + protein (examples: whole grain crackers and cheese or 1/2 sandwich)
- Light yogurt
- Or a lighter snack of veggies and hummus, dip, or string cheese may be desired.