What scientists, serveral studies, and doctors are saying:
If you want to lose more fat while exercising, you should skip snacking before your workout.
Cyclists who train without eating first, burn more fat than those who eat.
Carbohydrates usually give muscles energy, that’s why athletes eat large amounts of food in advance of their event, but scientists say by not eating before you exercise, your body doesn’t have many carbohydrates in reserve, so this forces your body to burn fat instead.
In a group study it did show that the group who didn’t eat before exercising did not perform as well in their workout activity, but even though the group performed worse, they did burn a higher proportion of fat to carbohydrates than the group that ate.
Peter Hespel, a professor of exercise physiology at the University of Leuven in Belgium, recommends people do this kind of training before breakfast, since eating carbohydrates interrupts the process of metabolizing fat for about six hours afterward. But he also acknowledges the method isn’t for everyone and that aside from the pain of struggling through an exercise session while hungry, there are other potential pitfalls. He says, “When you postpone breakfast to exercise, it is possible you might eat more afterwards. People exercising (without eating) need to respect all the normal strategies of weight control like not overeating.”
Other experts don’t agree, they say that even though people may burn more fat this way, it is mostly fat within the muscles that you will lose and it will not make a big difference to those who are trying to lose weight.
Andrew Greenberg, director of the Obesity and Metabolism Laboratory at Tufts University says, “When you exercise (without eating), fat is broken down more quickly in the muscle. You may enhance how you burn the fat in the muscles, but it doesn’t affect your overall body fat,” and he said, “more intense exercise may prompt the body to burn more fatty acids in other regions of the body, but that a lot of training would be required to see a big difference.”
But Ron Maughan, a professor of sport, exercise and health sciences at Loughborough University in Britain says, “Science is finally catching up with what smart runners have always known. If you have a long, hard run without breakfast once a week, that hard run will train you to burn fat. And for the rest of the week, have plenty of carbohydrates so you can train hard.” But he also said, “That might help you get very good at burning fat, but you won’t be very good at whatever exercise it is you’re doing. Without enough fuel, you won’t get the intensity of training you need to get improvements.”
Dr. Alexis Chiang Colvin, who is a sports medicine expert at Mount Sinai Hospital in New York, and she has worked with professional football and hockey teams, says “I think it’s actually a pretty bad idea. If your blood sugar is low, you could wind up getting dizzy and you might not be able to exercise as well as if you were well-nourished.” She recommends eating something small like a banana before training. She also warned the strategy might make people more prone to injury and that eating was important so the body would have enough nutrients to recover from a bout of exercise.
Daniel Kobbina, who is a personal trainer, and he also runs a martial arts school in London, says “If you train on an empty stomach, you’ll see that six-pack a lot sooner.” He says the method requires discipline – but it works.