Exercise Myths That Won't Go Away

*A myth can be defined as an untrue explanation for a natural phenomenon. Unfortunately, numerous myths remain pervasive and deeply ingrained in the health and fitness world. In the next few posts, we will take a close look at five longstanding myths to determine if they can withstand the weight of scientific scrutiny. We will begin today with the first myth:

Myth #1: Lower-intensity exercise puts you in the fat-burning zone, so it’s preferable to higher-intensity exercise.

Our first myth involves the performance of low-intensity (<40% VO2max) exercise to burn greater amounts of fat. This myth has been perpetuated by many sources of health and fitness information, especially fitness-equipment guidelines. Look at the displays on the latest exercise equipment and you will likely see different “training zones” recommended for fitness and calorie burning. It’s not surprising this myth has lasted so long—after all, most people like the idea that lower-intensity exercise could actually be better for you than sweat-inducing higher intensities.

The metabolism of fat during exercise is a complicated process regulated by multiple hormones. To simplify, first understand that fat used for energy is drawn from two different sources: subcutaneous adipose tissue and intramuscular stores of triglycerides. Stored intramuscular triglycerides are the main source during moderate to higher intensities of exercise. In general, as exercise intensity increases, our usage of fat for energy peaks and then decreases. When does that peak occur—during lower or higher intensities of exercise? The myth and truth lie somewhere in between.

Much of the confusion regarding the optimal intensity to burn maximal fat comes from understanding and applying the non-protein caloric equivalents for RQ (respiratory quotient) chart (Figure 1). This chart details the amount of calories expended per liter of oxygen consumed at a given RQ. The RQ is calculated by dividing the amount of CO2produced and O2 consumed at rest or during exercise. This value can then tell us the relative contributions of fat and carbohydrate to total energy expenditure.

Figure 1. Non-protein RQ Chart


For instance, at a RQ of 0.83, the percentage of total calories coming from fat and carbohydrate are 56.2% and 43.8%, respectively. As RQ increases or decreases from this value, the percentages change. Individuals who perpetuate this myth claim that this chart proves that low-intensity exercise (below RQ of 0.83) will burn more fat because the percent contribution from fat is higher. The problem is that total calories are lower at this intensity. Therefore, the total amount of fat burned is actually lower compared to higher intensities (greater total calories burned and thus more fat) for the same duration of exercise.

So, at what intensity does the “fat burning zone” occur? Researchers have identified where peak fat oxidation in the muscle occurs as a percentage of VO2max. Romijn and others (1993) showed this point as being equivalent to 65% VO2max with decreased oxidation at 85% VO2max. Achten and colleagues (2002) found a similar intensity to Romijn and discovered that maximum fat oxidation occurred within a range of 55 to 72% VO2max. Their data also showed that fat oxidation falls off quickly beyond intensities equivalent to 75% VO2max or greater.

According to past research, if we had to identify a “fat-burning zone,” it would occur at 65% or within the range of 55 to 72% VO2max. This is equivalent to moderate-vigorous exercise intensities. Additionally, to improve the muscles’ ability to oxidize fat at rest and during exercise, one must positively enhance mitochondrial density and function. This is best accomplished through interval training. Therefore, aerobic-program recommendations to augment caloric expenditure (and fat oxidation) during exercise would be to include a minimum of three days of moderate-intensity, longer-duration (45 to 60 minutes) workouts interspersed with one to two days of interval training. This combination program will provide opportunities for maximizing caloric expenditure (moderate intensity) and improving the metabolic function of the muscle (interval training) and overall fitness.

The Bottom Line: The “fat burning zone” at low intensities of exercise DOES NOT exist! The best approach is to think of energy expenditure as a calorie is a calorie is a calorie, rather than partitioning into carbohydrate and fat calories. To burn maximum calories in support of ongoing weight loss, progress to a moderate-intensity/higher-volume exercise program and include interval training.

In our next segment, we will discover whether morning workouts increase metabolism better than workouts performed later in the day.

By Lance C. Dalleck, Ph.D., and Jeffrey M. Janot, Ph.D.