For years, pundits in the exercise physiology community — and even those of us at bicycling — have been telling you to train at less than 70% of maximum heart rate if you want to burn fat and lose weight. At high intensities of cycling, the reasoning goes, your body burns more glycogen (stored carbohydrate). This leaves your fat stores intact. Whoa. Sorry.
Your body does use a higher percentage of fat for energy during slow rides but, as we’ll show here, there’s more math to consider.
If you ride at 65% of your maximum heart rate, your body’s fat stores provide about half of your energy needs. When you increase intensity to 85% of max, fat provides only a third of the calories you need to pedal. But there’s more to it than that: The total number of calories burned is more important than the ratio.
Given an hour’s worth of exercise, you’ll always burn more calories at a higher intensity. That’s common sense. The percentage of energy supplied by fat may be lower, but the number of fat calories can be equal or greater. Data collected in the laboratory helps support this.
The chart on page 56 illustrates a study conducted at the University of Texas by Jack Wilmore and Dave Costill (and reported in their recently published book, Physiology of Sport and Exercise). They concluded that moderately fit cyclists exercising at 65% of their max heart rate burn about 220 calories in 30 minutes. But when the intensity increased to 85% of max, the caloric cost soared to 330 calories.
As the chart shows, 50% of 220 and 33% of 330 both equal 110 fat calories. In other words, 30 minutes of cycling, regardless of intensity, burns the same amount of fat. But at the higher intensity you burn an additional 100 calories in the same amount of time.
Keep in mind that you lose weight–and body fat–when you burn more calories than you consume. The mix of macronutrients (carbohydrate, fat, or protein) makes no difference. The more calories burnt, the more pounds you shed.
Another reason to ride harder: Owen Anderson, Ph.D., notes in Running Research News that not only does the higher intensity burn more calories, it has a more beneficial effect on your cardiovascular system.
But that’s not all. Jackie Berning, Ph.D. and sports nutritionist at the University of Colorado, says that because low-intensity exercise burns very little glycogen, when you eat a high-carbo meal after riding your muscles will have no place to store the new supply of carbo. Your body will turn this excess into fat and store it.
WHO SHOULD GO SLOW
Even considering this evidence, low-intensity cycling still has its place.
If your goal is simply to improve your overall health (as opposed to becoming a better rider), cycling at any intensity is beneficial. Recent recommendations published jointly by the American College of Sports Medicine and the Centers for Disease Control suggest at least 30 minutes of moderate-intensity activity at least 4 days a week. At a speed of 15 mph this would equate to riding about 7-8 miles a day.
Of course, that seems slow and short to an experienced rider–which points out the other reason for slow spins. Many novice cyclists might not be able to ride for at least 30 minutes at 85% of max heart rate (or above). But nearly anyone can ride for hours at 60-70%.
Fit cyclists should still include easy days in their training–but for recovery, not because it burns more fat. Another advantage is there’s evidence that relatively slow, extended rides train the body to metabolize fat, sparing muscle glycogen for intense efforts and crucial moments near the end of a race.
But for most of us–especially fit riders whose training time is limited–long, low-intensity rides are not the most practical way to burn fat. To get lean, ride at the highest intensity you can maintain for 30 minutes or longer.