Fasted cardio

You may have heard, or maybe you already tried, fasted cardio exercises for weight loss? So many people, especially in the fitness industry, follow fasted cardio, and some of them are really seeing benefits. But is it really a good idea?

The idea of fasted cardio is that when you wake up, the body’s blood sugar levels are low. Doing cardiovascular activities at that time causes the body to prefer fat as a main fuel source, because high insulin levels suppress fat metabolism. However, the absence of glycogen could also cause muscle loss.

Some studies show benefits of fasted cardio, but others don’t. This can be confusing, but fat loss really comes down to how many calories you consume to fuel your body. However, the most important part is exercise intensity and duration. If you are going to walk on the treadmill for half an hour, low intensity, at a steady state (LISS), then that can probably not make you lose muscle mass. If, however, you are practicing high intensity interval training (HIIT), this can lead to muscle mass loss and/or affect your performance, and increase your recovery time too.

The two main energy fuel sources during aerobic exercise are carbohydrates and fats. At rest and during exercise, most of the fat used for fuel comes from the adipose tissue triglycerides. Fatty acids are released from the adipose tissue (lipolysis, i.e. the breakdown of triglycerides) and delivered to the skeletal muscles for further oxidation (i.e. energy production). The activity of lipolysis is mediated by several hormones, including glucagon, epinephrine, norepinephrine, growth hormone, cortisol, and two main enzymes, hormone-sensitive lipase (HSL) &lipoprotein lipase (LPL). Other factors that may affect lipolysis include gender, fitness level, exercise intensity, and other factors too.

For most athletes, it’s not a good idea to train fasted, or remain fasted in the immediate post exercise period. A recommendation of at least 6g of essential amino acids, with or without 20-30g of carbohydrate, would be a good pre-workout strategy.

In fact, it has been suggested that a high-carbohydrate meal 3-4 hours before exercise may be as beneficial as supplementing with carbohydrate during exercise. Sports nutrition starts with meeting energy needs, fluid requirements, and addressing any micronutrient deficiencies. LISS exercise uses fat as the primary fuel source, while exercise at higher intensities will metabolize more carbohydrates and require more oxygen.

Some examples of studies that have found that carbohydrate feedings just before exercise have no effect, or a positive effect on performance, include a study in the Journal of the International Society of Sports Nutrition, which concluded that the “findings indicate that body composition changes associated with aerobic exercise in conjunction with a hypocaloric diet are similar regardless whether or not an individual is fasted prior to training.”

To conclude, when I did my own research, I didn’t find a study that really shows the individual’s intensity of exercise and their fitness levels. I could ask what would the results be if the studies were over a longer period of time? How much can that affect the results? And how much effect would exercising at high intensity vs. low intensity have on the results? What about the individual’s level of glycogen in the body, their diet, and their lifestyle history? For example, if you follow the Ketogenic diet, which is a high-fat and very-low carbohydrate diet, this can change fat metabolism completely. These are just some of the considerations that must be made when evaluating whether fasted cardio really works.

More information is needed before making an informed decision. Fasted low intensity, moderate duration cardio can be effective for reducing energy intake and increasing fat oxidation, thus can lead to weight loss and improved body composition. Individuals with relatively high body fat may see more benefits compared to a person with low body fat. Someone seeking to lose body fat could conceivably choose to do cardio either before or after eating, based on their preference. But the relatively small size and short duration of the studies means that we can not come to a definite conclusion whether there is either a small benefit to fat loss, or not.

Exercise in a fasted state could provide some short-term benefits, but the downside is that it could also limit the amount of protein available for muscle building. The best suggestions for those who are simply looking to get lean is to focus on total energy and macronutrient balance; whether they perform cardio fasted or fed should depend entirely on personal preference.


Body composition changes associated with fasted versus non- fasted aerobic exercise in the Journal of the International Society of Sports Nutrition; available at


Ayda Ersoy is a nutrition and fitness director at The Diet Doc Hawaii. She can be reached at, or (808) 276-6892


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