Whether cardio on an empty stomach can speed up fat loss is a commonly debated fitness topic. Some believe fasted cardio is more effective at burning fat while others believe it can decrease performance and limit fat loss.
Who is right? The following will inform you of what research has to say about the topic and offer compelling reasons why fueled exercise leads to greater overall fat burn.
Two fuel sources—carbs & fats—are used to generate energy for muscle contraction during exercise. For endurance exercise performed at a moderate intensity, you obtain 50–60% of energy needed from glycogen (which is stored energy from carbs) and the rest from fats.
When you deplete glycogen stores by fasting overnight, or going several hours without refueling, fatty acids break down in the mitochondria to be used as a secondary energy source. As workout intensity increases, your reliance on carbohydrates increases as well.
In one study that tested the fat burning effect cardio on an empty stomach, six healthy men cycled for 60 minutes at a low to moderate intensity:1
Group 1– Fasted overnight before the bike ride.
Group 2– Performed the bike ride after ingesting 0.8g/kg of glucose or fructose to replenish glycogen levels 1 hour prior to the workout.
Results: After 20-30 minutes of exercise, the rate of fat burn was higher in the fasted group than in the glucose or fructose group. This trend continued throughout 50-60 minutes of exercise. There was also a higher quantity of FFAs (Free flowing fatty acids) available in the blood in a fasted state throughout the exercise.
The Take Away: This particular study suggests that more fat was burned by the group that performed MODERATE activity on an empty stomach… DURING THE EXERCISE ITSELF.
Not so fast. Notice how “moderate” exercise is emphasized in the example above? Research shows that people who burn fat during their workouts actually burn less fat the rest of the day. Overtime, fat burning is not an immediate process, rather, it occurs over the course of, not a few hours, but a few days.
As you burn more carbohydrates during your workout, the body will burn more fat post exercise. This “afterburn effect” where your metabolism is elevated for several hours or days following your workout is critical when debating the benefits of fasted cardio.
While you may burn more fat during your workout on an empty stomach, your overall workout intensity may decline.2 Your body’s ability to burn fat post-exercise is compromised. Consider the whole 24 hour period and cardio on an empty stomach is less effective.3
Researchers from Italy investigated the contrasting reports on whether training in a fasting condition enhances weight loss. There were 8 healthy young men who performed early morning slow cardio under 2 conditions:Adaptations to skeletal muscle with endurance exercise training in the acutely fed versus overnight-fasted state.
1. Empty stomach
2. After eating
Eating increased both oxygen consumption (VO2) and respiratory exchange ratio (RER) significantly, 12 hours after the cardio, VO2 was still higher for the group who had eaten, although RER was significantly lower in the FED test, indicating greater fat burn.
The group that ate before the cardio session continued to burn significantly more calories up to 24 hours after the exercise bout. The authors concluded that “when moderate endurance exercise is done to lose body fat, fasting before exercise does not enhance lipid utilization (fat loss); rather, physical activity after a light meal is advisable.” 4 Check out this article for more on pre-workout meal ideas.
During intense exercise that approaches your maximum effort, most of your energy comes from glycogen. If you deplete your glycogen stores, you compromise your energy output. As glycogen stores in the muscles and liver are depleted, and the blood glucose level begins to fall, fatigue, lack of coordination, light-headedness and lack of concentration can occur. Commonly known as “hitting the wall” or “bonking,” fat simply can’t be metabolized fast enough to support the higher pace, so you slow down or even stop.
While research and studies are still ongoing, there are a few certainties. Steady state fasted cardio might burn more fat during your workout, but your post-workout fat burn is compromised. When performing high intensity cardio and exercises, glycogen levels need to be restored to achieve optimal performance and results. Doing cardio on an empty stomach if you aren’t able to sustain adequate energy levels, your workout will suffer.
Always choose energy and sustainability over anything else! Even early in the morning, grab a protein shake, piece of fruit, or handful of trail mix, to help your body to use efficient energy sources to power your workout.