Is Fasted Cardio Better For Body Composition?

You'll hear fitness professionals bash 'dieting'. 

restrict oneself to small amounts or special kinds of food in order to lose weight.
— Definition of DIET in the Dictionary

I don't believe in diets of any kind. I don't believe in calorie restriction (it will come back and bite you in the bootay), I do not believe in being a victim to macro-tracking, calorie counting or carb counting and I'm willing to take it as far as I don't think anyone should cut out an entire food group. 

I do believe it is about balance. Choosing this vs. that. Spending more time in the kitchen and less time at restaurants. Making good choices 90% of the time and being forgiving of the other 10%. I believe it's about LIVING your LIFE and not allowing food and weight to control YOU. While, I strive to be as healthy as possible and to take care of my body physiologically and physically, I also think you should strive for acceptance and self love and not perfection. 

With that being said, in my quest to fight hard for my pre-baby body yet refusing to become a victim of ANY "diet", I wanted to experiment with fasted morning cardio (running for me) and see the impact it had on weightless/body fat loss. 

Below, I share with you bits and pieces of peer reviewed articles that I spent hours sifting through as well as some conclusions I came to myself. 

As it turns out, in all my digging through peer reviewed articles, I found that there hasn't been much research conducted on the topic. The research that has been done, came to a little bit of a surprise to me. 

It has been hypothesized that performing aerobic exercise after an overnight fast accelerates the loss of body fat. The purpose of this study was to investigate changes in fat mass and fat-free mass following four weeks of volume-equated fasted versus fed aerobic exercise in young women adhering to a hypocaloric diet. Twenty healthy young female volunteers were randomly assigned to 1 of 2 experimental groups: a fasted training (FASTED) group that performed exercise after an overnight fast (n = 10) or a post-prandial training (FED) group that consumed a meal prior to exercise (n = 10). Training consisted of 1 hour of steady-state aerobic exercise performed 3 days per week. Subjects were provided with customized dietary plans designed to induce a caloric deficit. Nutritional counseling was provided throughout the study period to help ensure dietary adherence and self-reported food intake was monitored on a regular basis. A meal replacement shake was provided either immediately prior to exercise for the FED group or immediately following exercise for the FASTED group, with this nutritional provision carried out under the supervision of a research assistant. Both groups showed a significant loss of weight (P = 0.0005) and fat mass (P = 0.02) from baseline, but no significant between-group differences were noted in any outcome measure. These 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.

Hypothesis: Performing aerobic exercise (cardio) after an over night fast, accelerates loss of body fat. 

Conclusion: IF under a hypocaloric (calorie restricted) diet, body composition changes associated with aerobic exercise are similar despite being in a FED or FASTED state. 


Exercise and nutrition are often used in combination to lose body fat and reduce weight. In this respect, exercise programs are as important as correct nutrition. Several issues are still controversial in this field, and among them there are contrasting reports on whether training in a fasting condition can enhance weight loss by stimulating lipolytic activity. The authors’ purpose was to verify differences in fat metabolism during training in fasting or feeding conditions. They compared the effect on oxygen consumption (VO2) and substrate utilization, estimated by the respiratory-exchange ratio (RER), in 8 healthy young men who performed the same moderate-intensity training session (36 min of cardiovascular training on treadmill at 65% maximum heart rate) in the morning in 2 tests in random sequence: FST test (fasting condition) without any food intake or FED test (feeding condi- tion) after breakfast. In both cases, the same total amount and quality of food was assumed in the 24 hr after the training session. The breakfast, per se, increased both VO2 and RER significantly (4.21 vs. 3.74 and 0.96 vs. 0.84, respectively). Twelve hours after the training session, VO2 was still higher in the FED test, whereas RER was significantly lower in the FED test, indicating greater lipid utilization. The difference was still significant 24 hr after exercise. The authors conclude that when moderate endurance exercise is done to lose body fat, fasting before exercise does not enhance lipid utilization; rather, physical activity after a light meal is advisable. 

Hypothesis: Training in a fasting condition can enhance weight loss by stimulating lipolytic (fat burning) activity.

Conclusion: When moderate endurance activity is done to lose body fat, fasting before exercise does not enhance lipid utilization; rather, physical activity after a meal is advised. 


Recently, my lab sought to determine whether fasted cardio actually promoted differences in fat loss over time. Twenty recreationally trained college-aged women were randomly assigned to either a fasted training (FASTED) group that performed exercise after an overnight fast or a non-fasted training (FED) group that consumed a meal prior to exercise. On training days, the morning meals were consumed in the form of a shake that provided 40 grams of carbs and 20 grams of protein. These meals were ingested under the supervision of the research team to ensure that subjects adhered to either the FASTED or FED conditions.

The training protocol consisted of one-hour sessions of supervised steady-state treadmill exercise performed at 70 percent of maximal heart rate— an intensity that equated to a fast walk/jog. The low-intensity nature of the protocol ensured that any fat-burning advantages associated with fasted cardio would translate into greater improvements in body composition. Training was carried out three days per week for four weeks.

To help control food intake and thus optimize fat loss, subjects were given customized meal plans designed to create a 500-calorie deficit. The meal plans provided a balance of carbs (~45%), protein (~30%) and fats (25%), and allowed ample ability for the subjects to choose their preferred foods. Subjects recorded their daily food consumption in an online journal for continual monitoring of nutritional intake. Nutritional counseling was provided throughout the study period to enhance dietary adherence.

The results were somewhat surprising: Although both groups lost a significant amount of weight and body fat, no differences were noted between conditions in any body composition measure. Adherence to the program was excellent, with average attendance equating to more than 95 percent of total sessions.

Hypothesis: Fasted cardio promotes a difference in fat loss over time.

Conclusion: No differences were noted between conditions in any body composition measure.

**Disclaimer: The credibility of this article is unknown.

My conclusions and two cents...

In all of the research conducted, study groups were small and time frame was relatively short. Also, each differed in their volume and intensity of training (time and percentage of Vo2 max). 

According to my knowledge (courtesy of UNLV Bachelor of Science and Life Time), steady state cardio promote more fat utilization than high intensity interval training. Reason being, you're body's easiest source of fuel is glycogen, which breaks down to glucose (sugar). Fat break down for energy is a longer physiological process and not typically one's main energy source. Therefore, when performing cardio, you're body uses what is available depending on how fast it requires energy. In a high intensity cardio situation, you're body needs immediate energy in order to keep going, thus glycogen is broken down to fulfill that requirement. When you're performing steady state cardio, you're body has time to break down fat for energy utilization. It is natural for our body to take the past of least resistance, and if we feed it easy digesting carbohydrates (perfect to be converted to usable energy), it will use that over breaking down fat for fuel. This is where the "fasted cardio" hypothesis originated from. 

However, from the minimal research done on this particular topic, it seems to be undetermined whether or not the fuel source used actually relates to rate of body composition changed. 

Over the course of the past 3 weeks, I've opted to try fasted cardio. Previously, pre baby, pre pregnancy, I NEVER left for a run without a bowl of oats. Also, back then I was more concerned with performance vs body changes. Anyways, in one week I lost EVERY POUND of pregnancy weight gain without 1 workout (benefits of eating healthy and staying active during pregnancy!). However, I lost 2 additional pounds after starting fasted cardio, not to mention, I'm happy with my progress so far. However, I'm not going to give all that credit to fasted cardio. Taking out a pre-work out breakfast also takes out about 400 extra calories from my day. Now, instead of 2 breakfasts (pre work out and post work out), I only have one large post work out breakfast, which saves me calories without necessarily making me feel as though I'm eating less throughout the day. 

So, while I still don't have an answer on the effects of fasted cardio on body composition changes, I'm a fan and plan to continue to implement this into my training! 

By Tess Chupinsky- BS Biology, PN1

With the help of these articles: