It's not uncommon to see claims of “burn up to 20% more body fat by exercising on an empty stomach” .
Bold statements catch readers attention, but often when we look to the research, the facts and findings fall short.
The exercise these subjects performed consisted of treadmill running adjusted to 60% VO2 max for each individual. After exercise (or rest) participants were given a chocolate recovery shake and then allowed to eat ad libitum from a pasta lunch buffet.
The researchers measured caloric expenditure in all four groups, caloric intake from the pasta lunch buffet, participants’ feelings of hunger, as well as metabolic responses including glucose and insulin response in each scenario.
What they found was no surprise. Participants who exercised in a fasted state were in a more negative energy balance after lunch than were those who had eaten breakfast.
What else did they find?
They found that performing fasted cardio utilized a greater percentage of fat for fuel, but those that ate breakfast actually burned more calories during exercise. Those that ate breakfast also had a more favorable glucose and insulin response to lunch.
While the researchers note that fasted cardio may result in a short-term negative calorie balance (they only followed participants through lunch) they are quick to warn that this may not hold true in the long-term. In this case ‘long-term’ is anything that happens after lunch!
There were no conclusions stating that performing cardio in a fasted state equates to a greater reduction in body fat or body weight. None. Period.
So what have we learned here?
1. Don't assume all claims about fat loss & exercise are correct.
2. Fasting through breakfast will result in a greater caloric deficit after eating lunch regardless of exercise.
3. Fasting cardio may result in burning a greater percentage of calories from fat but it may also burn fewer TOTAL calories vs fueled cardio.
4. Fasting, regardless of exercise, will result in impaired glucose and insulin response to lunch.
5. Conclusions 2-4 may apply to healthy, physically active males and may not hold true for females, sedentary, overweight or obese individuals, or elite athletes. Use caution when making broad sweeping statements from the results of one narrow study.
When it comes to fat loss, and building a healthy, lean physique look to the big picture - what's going on for 24 hours of each day, (7 days a week) not just a possibility from 30 minutes or so of your day. Learn more about the science of cardio for fat loss here.
1. Northumbria University. Lose fat faster before breakfast. ScienceDaily.
2. Gonzales J. Breakfast and exercise contingently affect postprandial metabolism and energy balance in physically active males. British Journal of Nutrition.