Fasting before a workout is something that often gets debated. Will working out without replenishing our body really result in more fat loss, or should you actually be eating something before you get active. We will be looking into the reasons why people think that fasted workouts are better and why they actually might not be.
What is a Fasted Workout
A fasted workout is when someone works out without having eaten anything beforehand. Many people workout right away in the morning and may not be hungry before going to the gym. Others say that it will hurt their stomach too much to eat before morning cardio. One big theory though, is that a fasted workout burns more fat compared to when you eat something beforehand… but why do people think that?
Why Might Fasted Exercise Burn More Fat
There are two main reasons that people think fasted exercise burns more fat. One theory relates to a person’s insulin levels. Insulin levels are lower in the morning before you have eaten. If you were to eat something, these levels of insulin would jump in order to use the sugars you have just consumed. The theory with this is that because these insulin levels are rising, you are using carbohydrates as fuel instead of fat. When insulin levels are lower, essentially more fat will be used to fuel the body.
While there is truth to decreased insulin levels supporting fat loss, it really won’t speed up the process of losing weight. It also won’t necessarily hinder your weight loss either. In fact, it seems that you will break even either way. In a 2011 study by Brad Shoenfeld, he found that those who did eat a good source of carbohydrates before a workout actually ended up burning more fat later in the day than those who didn’t eat anything before their workout.
Another reason for fasted workouts being favored is because, supposedly, there is less muscle glycogen in the morning. This is thought to happen because while you sleep you aren’t eating. This leaves you in a fasted state when you wake up in the morning. Therefore you “should” utilize fat stores for energy in your fasted workout.
This reasoning actually is not true. Your glycogen stores really are not changing much overnight. The body really holds on to the energy you consumed the previous day. That is then used to help fuel the fasted workout instead.
Possible Negatives to a Fasted Workout
While there don’t seem to be any huge negatives from fasting, there are some little things that it could affect. Something interesting that could happen when you don’t eat before a workout is that you may end up eating more the night before than you previously would have. In a 2021 study by Asya Barutca and other researchers, it was found that because people anticipated having to workout in the morning fasted, they increased their energy expenditure the night before. This ended up actually in a 10 percent increase in calorie intake the 24 hours before the workout.
It is also shown that a decrease in performance is possible while fasting. Without any energy intake in the morning before the workout, exercise may feel harder than it usually would. This could lead to a decrease in time spent exercising, especially with cardio. If you are going to do fasted exercise, it is suggested that you stick to low intensity workouts, like walking. High intensity workouts require more glucose to support quick ATP expenditure. The glucose for this comes from the food you eat. Low intensity workouts are more doable and usually don’t lead to a decrease in performance in a fasted state.
Overall, it seems that there just isn’t enough evidence to support a real benefit from working out fasted. It is not shown to increase overall fat loss and it may even make it harder for you to lose weight, as you will be lacking in energy and may even increase your calorie intake the day prior.
It’s really up to you to do what you think is best. As long as you are fueling your body properly throughout the day and previous days if working out fasted, then you should be good to go! The most important thing is to enjoy the workouts you are doing and listen to your body. A registered dietitian can help you to create an individualized plan that fits your needs and lifestyle.
If you’re looking for some guidance on where to get started, our healthy snacking guide is filled with pre-workout snack ideas!
This snack guide can help give you tips on how to prepare a nutritional snack to fuel your morning workout so you can improve performance and reduce recovery time Make snacking easy and build your confidence in your choices through the healthy snacking guide today.
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This article was written by Mackenzie Flug, nutrition intern. Fact checked by Allison Tallman, RD.