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Written by Morgan Schaack, RD, LD
on September 09, 2019

Exercise is a very important part of living a healthy lifestyle and is recommended to many of our patients as it promotes weight loss, preserves your muscle mass, and improves your mood. But is it true that working out on an empty stomach can help you burn fat faster? In this blog, I am going to talk about the science behind this thought and provide you with my opinion on whether you should engage in fasted or fed cardio.

So what exactly is fasted and fed cardio? 

Fasted vs. fed cardio is an idea that you will lose more body fat and more weight when you either eat before or after your workout. Fasted being you workout in the morning before eating breakfast and fed being you eat a light snack before working out. 

Bariatric patient walking outside to get exercise in the morningTo clarify, in order to engage in a fasted workout, you have to exercise in the morning after an overnight fast (or period of not eating). 

Some studies predict that during fasted cardio, you are likely to burn 20% more calories than you otherwise would if you had eaten. This theory has to do with the way the body metabolizes fat and carbs. It is important to note that this theory will only be accurate if you maintain consistency in your workout intensity and how many calories you are consuming. 

Let's dive into the science a little. 

The Science

We all know that your weight is, in general, predicted based on the balance between your food intake and your energy expenditure—how much you eat versus how much you move. So, it makes sense that the fewer calories you consume, the more weight you will end of losing. 

A similar process happens in your body when you workout on an empty stomach. As we have explained in a previous blog, carbs are used as the body's source of fuel. When your body's insulin (sugar) levels are low—as a result of not eating carbs for a long period of time—the body will turn towards your stored fat for energy. This is known as fat oxidation, or the burning of fat for energy. 

In summary, insulin inhibits the breakdown of fats for energy. When we eat before working out, our insulin levels will rise and therefore our body will turn towards carbohydrates as our source of energy. 

So this is where the idea that "fasted cardio will result in greater weight/fat loss" comes from

Challenging the Science 

As with every good research conducted, there is evidence out there that challenges this thought. The contradictory theory is that fat oxidation was not much higher for individuals who performed fasted cardio opposed to fed cardio.

Therefore, this would suggest that there really was no difference in fat loss when working out before or after eating. 

Lady pinching her stomach fat

Similarly, it is important to look into fat oxidation over a longer period of time—not just when you are working out—as your body burns energy throughout the day. And there have been studies that point to evidence that fat oxidation is higher post workout when you engage in fed cardio versus fasted cardio. 

This suggests that you are likely to burn more fat longer term when you eat before working out. 

Fasted vs. Fed Cardio: The Verdict

So now that you have heard both sides of this argument, what do I have to say about it? In general, there is no significant evidence that points to fasted or fed cardio being better for fat loss. Actually, there have been many studies that have proven that there really is no difference in fat loss when the circumstances were consistent (workout intensity and calories consumed). 

In one particular study, researchers looked at the effect of high intensity training (HIIT) in overweight/obese women when performed in a fasted or fed state. Their conclusion? Regardless as to whether they ate before or after working out, there was no difference in fat loss.

Yet they did find HIIT training to be a great way to burn calories and improve body composition long term (so download our bariatric HIIT guide!). 

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In general, I do not believe that everyone should engage in fasted cardio—it really depends on you!

As I said above, fasted cardio must be performed in the morning after an overnight fast. And for some people, working out in the morning is not their thing. Similarly, some people get nauseous when they workout on an empty stomach because their blood sugar levels are too low. 

If you do want to eat before working out, I recommend eating a food that is rich in carbs and protein, yet low in fats as fat tends to sit heavy in the stomach. I want to note that this is not your excuse to eat sugary and fatty foods—you want to aim for nutrient dense foods that will fuel your workout.

Some great pre-workout snacks are: 

  • low carb greek yogurt (less than 10g carbs)
  • protein shake 
  • protein bar (more than 10g protein and less than 5g net carbs)
  • cottage cheese

So in conclusion, one is no better than the other. It depends on your schedule and what you prefer. You will likely end up burning the same amount of fat and calories regardless of whether you workout before or after eating. 

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