Fasted Cardio

Cardiovascular exercise is another tool at our disposal to assist in weight loss. Just like we pull calories throughout a fat loss phase, we can add in cardio to create a larger energy deficit. In conjunction with a calorie deficit and a solid training program this is a prime starting point for anyone looking to change and improve their physique, although the timing of one’s cardio and its effectiveness has been a large topic of debate. By timing, this refers to performing cardio in a fasted state (no food/calories first thing in the morning) versus performing it in a fed state.

The rationale

One of the most frequent questions asked is whether or not fasted cardio is superior to cardio performed in a post-prandial state (a fed state). The get an understanding of the rationale supporting fasted cardio, we need to take a step back to some key points: under what circumstances our bodies will metabolize certain nutrients.

Upon waking up first thing in the morning, we have lowered glycogen and insulin levels. The rationale behind using fasted cardio to promote and accelerate fat loss is that the state our bodies are in while fasted essentially forces our bodies to mobilize stored fat as a primary fuel source, as opposed to pulling it from stored carbohydrates. The problem here, however, is that regardless of where this fuel is coming from, we’re burning calories here to achieve weight loss and this process isn’t really something that can/will show instant results from a single training bout. The study to be discussed will truly put fasted cardio to the test to determine if it proves to be the more effective method!

The Study

20 healthy, untrained females participated in this study. They are considered untrained since they were not involved in any kind of resistance training program, though they all reported routinely doing some kind of cardiovascular exercise several days per week. Once baseline measurements were obtained, the women were initially pair-matched according to those baseline body mass measurements and then randomly split into 2 groups. These two interventions consisted of either a fasted training group that performed exercise after an overnight fast, or a post-prandial (fed) training group that consumed a meal before exercising.

Training and nutrition

Each session consisted of 1 hour of steady state cardio at a low-moderate intensity. Frequency for the training sessions was set at 3 times per week for each intervention over a 4-week time period. Now, it is worth pointing out that 4 weeks is a relatively short amount of time for a study to take place. However, one can make the argument that this is still ample time to attain fat loss (mini cuts, for example), which suits this study’s purpose. Regardless, this timeline will be a limitation for the study, as we won’t know if small changes have the possibility to develop over time between the two groups after that 4-week mark.

As for the diet, subjects were provided customized diet plans. This is done in order to place the women in caloric deficits, a necessary factor for any weight loss regimen. Protein was monitored and held at about 1.8 grams/kg/body mass in order to best preserve lean muscle mass throughout the diet, as well as simply aid the women from a satiety and adherence standpoint.

Lastly, a meal replacement protein shake was also provided to the subjects (40g carbohydrate, 20g protein, 0.5g fat) to consume. The fed group took the shake immediately before their cardio session, while the fasted group had it immediately afterwards.

Key findings

This study was the first to investigate the possible differences between fed and fasted cardiovascular exercise while under a calorie deficit. After the 4-week training period, total energy and macronutrient intakes were not shown to differ between the training groups. The reported nutritional consumption for both groups was below that of the individually given meal plans. This does raise some flags, as self-reported intakes can vary substantially between individuals and the underreporting of calories actually being consumed each day is very common. Not only does that relate to research, but in coaching as well!

The key finding of this study counters the theory of fasted cardio promoting fat loss. Both groups had experienced significant drops in body weight; no differences were found between the two feeding protocols in any of the post-diet markers. This signifies that regardless of whether you choose to eat before cardio or not, weight loss will occur granted that you are in a caloric deficit. Fasted cardio does not show to be a superior method to cardio performed in a fed state when following a calorie-restricted diet.

To touch on the rationale behind this theory, the human body will use different fuel sources depending on the intensity and duration of a training bout (high intensity demanding carbs, while low-moderate intensity using fats), and the same goes for fasted and fed states as we discussed earlier. In scenarios such as these, however, the body has a way of compensating for the usage of certain types of energy; essentially, we always end up balancing things out over the duration of the day. For example, when the body uses more fats at a certain time point, it will compensate by using more carbohydrates in the latter part of the day. This idea ultimately ties back to the process and time course for weight loss/gain – we cannot expect the small details of our day to day to immediately impact fat loss overnight. Accumulating consistent days/weeks while employing key principles (high protein, calorie balance, etc.) is ultimately more important.

So, what should I do?

This present study helps to solidify even more so the importance of one’s energy balance when working towards physique-related goals. Granted that your daily protein and total calories are in check/place you in a hypocaloric state, you will be able to facilitate weight loss regardless of the time and/or the conditions under which you choose to perform your cardio. 100 calories burned in a fasted state and 100 calories burned in a fed state still equates to 100 calories expended at the end of the day.

There are plenty of pros and cons to consider when deciding to do fasted cardio (or even fasted training). The take-home response to what is best here is to simply do what you prefer and what you find works best with your unique schedule and circumstances. Every client is different, and a single approach can’t be expected to suit everyone. Not everyone will feel their best hitting the gym on an empty stomach, and the same goes for training while fed (or soon after a meal). In addition to preference, some individuals just cannot make early fasted training work due to unique and challenging schedules, shift work, and the like.

What matters most for the individual, though, is nailing down some key habits (these include nutrition/tracking your intake, training schedule, adherence, etc.) and then modifying those variables in a way that helps you follow through consistently!

Schoenfeld, B.J., Aragon, A.A., Wilborn, C.D., Krieger, J.W., Sonmez, G.T. Body composition changes associated with fasted versus non-fasted aerobic exercise. Journal of the International Society of Sports Nutrition. 2014, 11:54

Be the first to know

Get exclusive, no bullshit content from our coaches that is scientifically based and experience driven.