Fasted Cardio for Fat Loss
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Fasted Cardio for Fat Loss: Fasted exercise has been around for ages, even bodybuilders back in the day used to wake up and take a jog on an empty stomach. To this day you still get people who swear by fasted cardio and others who simply think it’s unnecessary.
I personally, am a firm believer in fasted cardio or just any sort of fasted activity for that matter. I think it’s valuable for the body to be familiar with performing activity while relying on its fat stores for energy. However, this doesn’t mean that I think fed state cardio is useless or worse, because it is not. Both can be utilized effectively.
Fasted exercise has been shown to lead to greater mobilization and utilization of fat for fuel as opposed to fed exercise  and has been shown to burn 20% more fat . The one important point which I won’t discuss too much in this article is the potential improvement for metabolic flexibility with fasted activity. My E-Book on fasting will talk in depth about metabolic flexibility, how to improve it and why it’s so important for health and body composition.
The study, “A Mini-Fast with Exercise Protocol for Fat Loss,” offers some really interesting take home points. The authors suggest that, “exercise is of most value if oxidation of stored fat is maximized during and following the exercise session.”  I absolutely love and fully agree with this view point. I never understood why people would say it doesn’t matter if you are burning stored fat, because only calories in vs. calories out matter.
Those who are metabolically inflexible are at risk for developing a host of metabolic disorders mainly because of how inefficient they are at using different fuel sources. So please tell me again why being able to run on alternate fuels is not a good thing? Remember, weight loss is different than body composition changes and while energy in/energy out cannot be understated, human metabolism is very complex and can’t be looked at in a vacuum.
The author goes on to say, “this is best achieved if prolonged exercise of moderate intensity is performed during a 12-14 hour mini-fast that entails skipping a meal…” He goes on to say that, “if subsequent food consumption features low-fat foods, the fat stores expended during and after the exercise will not be fully repleted by dietary fat…thus, prolonged compliance with such a regimen should lead to a steady loss of body fat until a much leaner equilibrium body composition is attained.”  As I have mentioned in previous articles, one goal you should aim to achieve during a diet is maximizing the breakdown of fat for energy as you are teaching the body to become efficient at using this fuel source.
There were 34 participants in the study of which only 27 returned for their follow ups. They were asked to perform moderate aerobic exercise 3-5 times per week during which they were in a 12-14 hour “fasting” period. The researchers did not impose any strict calorie requirements; they only recommended that participants try to eat low-fat, low-glycemic index carbohydrates. After 12 weeks, the fat loss averaged out to be 7.4kg or 16.3lbs and their fasting insulin levels dropped by 25% . Surprisingly, the weight loss during the second block of 6 weeks was “at least as great as in the first week”, which suggests that the participants could have kept losing more weight/fat had the study been conducted longer .
What do the authors propose as the reasons for success in the study?
– Fasted exercise promoting the selective usage of stored fat 
– This happens when insulin is low (at basal fasting levels), which happens during a fast 
– Fasting post-exercise prolongs the oxidation of fat, as the body attempts to spare glucose
– The fast post-exercise increases hepatic fatty acid oxidation, which can potentially lead to ketosis and further, appetite suppression 
– Insulin sensitivity through GLUT4 translocation
The sample size was small and it wasn’t exactly the perfect study by any means, however, it’s hard to ignore the results. Whether it was the fast, diet, exercise or combination of the three; it worked. Lack of compliance is the main culprit of any diet’s failure (assuming it’s a rationally thought out plan), yet these participants were able to follow this plan. I personally believe that the fast and fasted exercise served their purpose in enhancing the effects of insulin, improving metabolic flexibility, and giving the body periods to use its own stored energy. Lowering dietary fat intake and choosing lower glycemic carbohydrates played a role as well. The lower GI carbs help control insulin levels so as not to disrupt the desired level of fat oxidation during the day. For more information on the glycemic index you can read, “Do Carbohydrate Sources Matter.”
I looked at this study more to understand exercise in the fasted state and some of the physiological changes that occur because of that. It’s also important to note that the majority of the people I work with want to have a lot of lean muscle AND lower their body fat or stay lean, not just “lose weight.” For this person, I don’t recommend resistance training every session in the fasted state. Most of the fasted work I prescribe to clients is aerobic work or restorative/metabolic workout type of days. With this style of training you can continue the fast afterwards as there isn’t as much of a need to replete as there is after intense resistance training. My clients also use a specific dietary set up and certain supplements before/during fasting to maximize the benefits of their fasting period.
I’m not saying that you can’t train with weights fasted and make gains; I just think it’s less effective than some alternative means. Depending on how long you have fasted and how depleted your glycogen stores are when entering your lift, your current fasted state could lead to decreased peak power output and have you leaving some reps in the tank. Because of this, fasted training definitely shouldn’t be a staple in a strength athlete’s regiment, but it still has value for those seeking more cosmetic goals. In a previous article of mine (Fasts And Sprints For Fast Fat Loss) I discuss which type of cardio you should use (Steady State vs. HIIT).
Other Articles That Discuss Fasting:
 “Metabolic Responses to Exercise After Fasting,” http://jap.physiology.org/content/61/4/1363.
 “Lose Fat Faster Before Breakfast,” http://www.sciencedaily.com/releases/2013/01/130124091425.htm
 “A Mini-Fast With Exercise Protocol For Fat Loss,” http://catalyticlongevity.org/prepub_archive/minifastwithexcercise.pdf