Fasted cardio...to do or not to do?

There is a vast amount of information on fasted cardiovascular exercise freely available on the web. Different masters of sports recommending to perform exercises such as running, cycling or x-training on an empty stomach whilst others urging to never even consider it if desiring to build lean muscle.

For however long I can remember if I have done a morning cardio workout it would be in a fasted state. Simply for no other reason than I dislike exercising directly following food consumption. I like the feeling fasted cardio gives too, a different type of workout (generally a sweatier one) than a postprandial session. I presume this is due to my body going into a state of minor ketosis – especially as I was previously following a very low carbohydrate diet.

Ketosis itself is a good state to be in (I believe). Using ketones as the main source of fuel occurs when insulin levels fall leading to glucagon being produced to release the glucose from the liver and muscles for the body to use as energy. When these stores have run out lipolysis occurs, utilising carnitine and breaking free fatty acids into ketones (known as beta-hydroxybutyric and aceto-acetic acids). Being in a ketotic state increases the body’s ability to utilize fat stores, whilst sparing protein stores. Research suggests that if the body has ketones as fuel there is no need for it catabolise muscle for gluconeogenesis. A downside when attempting to gain lean muscle is that it may reduce appetite and therefore prevent enough calories being consumed to maintain weight. I have definitely suffered appetite loss when trialling “fat burners” pre workout, which obviously utilise ketones as fuel (but we will leave the fat burner discussion for another day).

HOWEVER I see the problem arises if the body is no longer using an aerobic pathway as ketones can only be used by mitochondria which require oxygen. If exercise intensity increases above a moderate pace and anaerobic pathways come into play then not only will excess ketones build up leading to acetone breath and ketotic urine, but also protein catabolism may occur as the body requires glucose to use as its energy source.

So my initial opinion by considering the body energy systems is that fasted cardio is beneficial to reduce body fat whilst maintaining lean muscle mass IF it is at a moderate intensity only utilising the aerobic system.

BCAAs are something I haven’t started yet however taking these prior/during/post cardio will further prevent any catabolism of muscle if the body does go into anaerobic respiration.

Studies:

A study at Loughborough University found that there was little difference in appetite, daily energy intake and resting metabolic response whether performing a 60 minute treadmill run fasted or postprandial state. Deighton K, Zahra JC, Stensel DJ.

A 1985 study introduced that fasted state had a positive effect on endurance due to its “glycogen sparing” effect and fatty acid utilization. This study was however on rats, despite this endurance exercise mainly utilises the aerobic system meaning fatty acids would indeed be the main fuel source. Dohm GL, Tapscott EB, Barakat HA, Kasperek GJ.

Another study looked at the metabolic responses to exercise after fasting, it concluded that plasma glucose remained equal in both fasted and fed state, however lactate production was increased in fasted state. It is thought that the blood glucose remained at homeostasis during a fasted state due to decreased utilization of muscle glucose due to reduced pyruvate dehydrogenase activity (transforms to acetyl CoA, which links glycolysis pathway to citric acid cycle – since no glucose, glycolysis does not occur and pyruvate is reduced). In practical terms this study means that blood sugar levels won’t decrease if fasted cardio is chosen, despite liver glycogen stores being used, so there should be no concern over feeling faint.