Q.Exactly how long does an aerobic workout have to last in order for fat to start being burned? Some people have told me 10 minutes, and others say it is actually 20 or more. Which is the correct amount of time?
A.Time to straighten out this longstanding confusion about “when does fat-burning kick in.” What do you think you’re burning right now, sitting in that chair, reading this response? Glucose? Protein? Rocket fuel? You’re burning fat — and maybe a little bit of glycogen, with some dipping into the phosphate pool when you squirm in your chair.
During a workout session, there is no set magical time for your body to burn fat versus glycogen (sugar stored in the liver and muscles). Your body burns some of both all the time. It makes a mixed-potion cocktail for itself, based on muscle demand, available fuel sources and various metabolic factors.
It’s true that at lower intensities, this cocktail is proportionately higher in fats than sugars, and that switches as you increase the intensity. Once you’re pretty exhausted, panting like crazy, you’ve reached your anaerobic threshold (this might take 10 minutes in the unconditioned, 60 minutes in conditioned people), and you begin to deplete essential nutrients and glycogen. The more you train, the better your tolerance of exercising at higher levels, and you will be able to sustain a higher oxygen-uptake level of exercise for longer periods.
Here’s what is important for you to know: If you’re interested in losing body fat, burn as many overall calories as possible in a given 24 hours. You can do this through short bursts of anaerobic work, or longer, aerobic sessions — it doesn’t really matter. What does matter is the number of total calories expended. The reason fitness trainers emphasize low and slow activity over high, fast, quick bursts is so you can extend the time you’re burning calories and remain at a steady state longer. This is where your muscles seem to balance between the energy required to do the work and the aerobic energy-releasing aspects.
Moderate activities such as brisk walking, low resistance cycling and other types of moderate-intensity aerobics are usually recommended for weight loss because people can tolerate these lower intensities much easier — especially if you’ve been sedentary.