Over the years I’ve seen hundreds of gym members, friends, trainers and athletes stumble through the doors early in the morning resembling extras from the walking dead.
Precede to jump on a cardio machine and plod along like they’re chasing imaginary brains. Through many mumbled conversations, a recurring theme emerged: the belief that doing cardio in a fasted state (empty stomach) results in accelerated fat loss.
The theory goes like this – training when you have low plasma glycogen and insulin levels, such as in a fasted state, will cause your body to rely predominately on fatty acids for fuel, when intensity is kept low-moderate.
Does this work in practice?
A recent study by Schoenfeld and colleagues has put a dent in this long held belief.
The researchers looked at changes in fat mass and fat-free mass following 4-weeks of volume-equated fasted versus fed aerobic exercise in young women on a hypocaloric diet (fat loss diet). Young female volunteers were randomly assigned to one of two experimental groups: a fasted training group that performed exercise after an overnight fast or a training group that consumed a meal prior to exercise.
Training consisted of 1-hour of steady-state aerobic exercise performed 3-days per week.
What did they find?
Both groups lost a significant amount of weight and fat mass. HOWEVER, there was no significant difference between the two groups in these measurements. The study concluded that cardio in either a fasted or fed state MADE NO DIFFERENCE to body composition measurements.
Before people scoff and link me to the various studies showing increased fat oxidation acutely during fasted cardio, let’s consider an often important, but commonly misunderstood factor.
Short-term fat oxidation during exercise does not necessarily equate to long-term fat loss and body composition changes.
WAIT! WHAT?
Firstly, our body is dynamic. Rather than fuel oxidation being an on/off switch between fat and glucose, it is similar to a dimmer dial, that is constantly adjusting during the day.
This means you are never exclusively burning fat (fatty acids) or carbs (glucose). Rather, the ratio changes depending on nutrient intake, physical activity and other factors.
Evidence shows greater fat oxidation during a given time period (ie. fasted training session), tends to be compensated by an increase in carbohydrate utilisation later in the day.
For this reason, body composition changes and manipulations require consideration of fuel usage over longer timeframes (day and week), instead of acute durations (training sessions).
Therefore the underlying notion that fasted cardio leads to superior fat oxidation must rely on one of the following theories:
1. It increases 24-hour energy expenditure, so that the overall energy deficit is increased.
2. It suppresses appetite, resulting in less food consumed, so that the overall energy deficit is increased.
3. It has a protein-sparing affect. Meaning that more fat-free mass is retained during a sustained overall energy deficit. We want this! Why spend months gaining muscle only to use it as fuel when you cut down!
For starters, we know that fasted cardio does not increase total 24-hour energy expenditure over non-fasted. In fact there is data to indicate that consuming food before exercise may allow an individual to work at a higher intensity thereby increasing energy expenditure.
So what about appetite?
Once again, evidence suggests that fasted cardio has no appetite suppressive effects compared to non-fasted cardio.
To address point 3 let’s play hypothetical.
We design a study where people do fasted or non-fasted cardio to produce a 500-calorie overall daily energy deficit.
For short term (24-hour) fat oxidation to be favourable for body composition, then the fasted group would have to lose more fat, while also maintaining more lean mass (so that weight loss remains the same...remember the energy deficit is the same).
This means with a given energy deficit, for fasted cardio to be beneficial, it MUST be protein sparing as well. There is no data to suggest that it would be protein sparing, in fact its more likely to have the opposite effect due to elevated amino acid oxidation, especially as you get leaner.
Based on the current research there is no evidence to support fasted cardio increasing fat loss more so than fed cardio.
Does this mean we can rule it out as an effective training method?
No.
More research is needed over longer time frames and with wider populations. However at this time there is little more then jacked gym bro’s anecdotal advice to support this theory.
To quote the authors of the Schoenfeld study:
“...our 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. Hence, those seeking to lose body fat conceivably can choose to train either before or after eating based on preference.”
