The scientific consensus on this subject seems to be that "first thing in the morning" is the best time to work out, especially if you're trying to lose weight. Whether you're performing aerobic exercise or strength training (lifting weights), exercising before breakfast burns fat much more effectively than it does later when you have food in your stomach. Early in the morning your blood sugar levels and carbohydrate stores are low because of your overnight fast. Rigorous walking or weight training at that time prompts your body to draw on stored fat to meet the caloric demands of exercise. If a morning workout doesn't jibe with your daily schedule, wait until two to three hours after a meal.
The idea that eating before a workout is helpful is widespread but erroneous; in fact, it may be one reason that some people just can't seem to shed unwanted pounds despite expending lots of effort in the gym. Instead of drawing on fat for energy, your body will draw on the carbohydrates you've most recently consumed, especially if you have ingested foods that rank high on the glycemic index.
You should know, however, that relying on exercise alone to burn calories and lose pounds is seldom successful. In 2009 a study published in the British Journal of Sports Medicine found that average weight loss was a little more than seven pounds among many of the 58 obese participants who agreed to 12 weeks of supervised aerobic exercise without making any changes in their diets. Many of the participants didn't even lose that much. Another study, from the University of Colorado School of Medicine, focused on whether or not exercise can "rev up" metabolism so that you'll continue to burn fat for the rest of the day. The investigators recruited three groups, including lean endurance athletes, sedentary and lean adults, and sedentary and obese adults. All of the participants agreed to spend two 24-hour periods in a walk-in calorimeter, a room equipped to measure the number of calories an individual burns as well as whether those calories are derived from carbohydrates or fats. Results showed that none of the participants, including the athletes, continued to burn fat after exercise. The researchers noted that if you want to burn fat, your best bet is to work out at an easy intensity, because high intensity exercise draws on carbohydrate stores, not fat. The good news is that if you lose weight by cutting calories, regular exercise can help you keep it off. The Colorado researchers documented this only in lab rats but believe that the same physiologic mechanism operates in humans.
Does this mean exercise is useless? Not at all. Weight loss from exercise may be modest, but it is only one of the health benefits that getting fit offers. Whether or not you lose unwanted pounds, exercise can help you control your blood pressure, tone your muscles, and boost your mood.
Andrew Weil, M.D.