The Fat-Burning Zone

One day, while I was running on a treadmill at the gym, a personal trainer approached the person next to me to share some advice. “Muriel,” she said, “if you want to burn fat, you should keep your heart rate within a specific zone.” I was so shocked at what I’d heard that I nearly fell off the treadmill!

Perhaps the most popular myth about exercise is that there is a specific range of heart rates in which your clients must exercise to burn fat. All the time I hear fitness professionals tell their clients not to exercise above a certain heart rate, as if it were bad for people to run or bike fast. Target heart rate has become a buzz phrase. Even many cardio machines display a “fat-burning zone” on their panels, encouraging people to exercise in a specific heart rate range. Have you ever wondered if your clients really have to exercise in a specific heart rate zone to lose fat? And what happens if they venture out of that zone?

Fuel Use During Exercise

Clients use both fat and carbohydrates for energy during exercise, with these two fuels providing that energy on a sliding scale. During exercise at a very low intensity (e.g., walking), fat accounts for most of the energy expenditure. As exercise
intensity increases up to the lactate threshold (the exercise intensity that
demarcates the transition between exercise that is almost purely aerobic and
exercise that includes a significant anaerobic contribution; also considered the highest sustainable aerobic intensity), the contribution from fat decreases while the contribution from carbohydrates increases. This happens partly because the body now relies more on glycogenolysis and glycolysis to meet the greater demand for energy (ATP) regeneration and because fatty acid delivery to the exercising muscles decreases at higher intensity levels. When exercising just below the lactate threshold, clients are using mostly carbohydrates. Once the intensity of exercise has risen above the lactate threshold, carbohydrates become the only fuel source.

If clients exercise long enough (1.5–2 hours), their muscle carbohydrate (glycogen) content and blood glucose concentration become low. This metabolic state presents a threat to the muscles’ survival, since carbohydrates are muscles’ preferred fuel. When carbohydrates are not available, the muscles are forced to rely on fat as fuel.

Since more fat is used at low exercise intensities, people often assume that low-
intensity exercise is best for burning fat, an idea that has given birth to the “fat-burning zone.” However, while only a small amount of fat is used when exercising just below the lactate threshold, the rate of caloric expenditure and the total number of calories expended are much greater than they are when exercising at a lower intensity, so the total amount of fat used is also greater. What matters is the rate of energy expenditure, rather than simply the percentage of energy expenditure derived from fat.

Using an incremental exercise intensity test, Achten, Gleeson and Jeukendrup (2002) found that the highest rate of fat oxidation in moderately trained cyclists occurred at 64% VO₂max (74% of maximum heart rate, or max HR) when they were cycling. Comparing fat oxidation in moderately trained women during 15 minutes of running at six intensities (25%, 40%, 55%, 65%, 75% and 85% VO₂max), Astorino (2000) found that the highest rate of fat oxidation occurred at 75% VO₂max, which corresponded to the subjects’ ventilatory threshold. Comparing fat oxidation in endurance-trained men and women during 30 minutes of running and cycling at three intensities (55%, 65% and 75% VO₂max), Knechtle et al. (2004) found that the highest rate of fat oxidation occurred at 75% VO₂max for both running and cycling, which corresponded to the subjects’ lactate threshold for cycling. What this means is that the highest rate of fat use during exercise
occurs when clients are working at a hard aerobic intensity that typically corresponds to the lactate threshold.

You Don’t Need to
Use Fat to Lose Fat

Since clients use only carbohydrates when exercising at a high intensity, does that mean that if they run fast or take a high-intensity indoor cycling class, they won’t get rid of that flabby belly? Of course not. Despite what most people think, your clients don’t even have to use fat during exercise to lose fat from their waistlines. Have you ever seen a fat sprinter? Sprinters primarily train anaerobically, never using fat during their workouts. Yet they’re still very lean. Carbohydrates are actually the muscles’ preferred fuel during exercise. At intensities below the lactate threshold, most of the fat that is used in combination with carbohydrates is in the form of intramuscular triglycerides (tiny droplets of fat within your muscles). This is because during exercise, when you need to regenerate ATP quickly for muscle contraction, it is more efficient to use fat that is physically closer to the mitochondria. To use adipose fat (the fat on clients’ waistlines and thighs), the free fatty acids would first have to be transported to the mitochondria, where they could be oxidized. The adipose fat is burned during the postworkout hours, while clients are sitting at their desks or on their couches.

For fat and weight loss, what matters most is the difference between the number of calories your clients expend and the number of calories they consume. Fat and weight loss is really all about burning lots of calories and cutting back on the number of calories consumed. For the purpose of losing weight, it matters little whether the calories burned during exercise come from fat or carbohydrates. How people become better fat-burning machines is by enhancing the metabolic profile of their muscles.

Endurance training enhances fat oxidation by (1) increasing fatty-acid binding proteins, which regulate fatty-acid transport; (2) increasing levels of the
enzyme carnitine transferase, which facilitates fatty-acid transport across the mitochondrial membrane; (3) proliferating capillaries within skeletal muscles, which enhances fatty-acid delivery to muscles; and (4) increasing the density of mitochondria in the skeletal muscles, which increases the capacity for fat oxidation (Horowitz & Klein 2000). One of the hallmark adaptations of endurance training is that metabolism is steered away from carbohydrates and toward a greater reliance on fat at the same exercise intensity. Thus, the more fit your clients are, the better they will become at burning fat at the same exercise intensity.

The switch in fuel use from more fat and fewer carbohydrates to more carbohydrates and less fat is referred to as the crossover concept, with the exact split of fuel use depending on the crossover
between exercise intensity (with higher intensity favoring carbohydrate use) and endurance training (which favors fat use) (Brooks & Mercier 1994).

Fuel Use After Exercise

Not only does high-intensity exercise burn more calories in the same amount of time as low-intensity exercise (because the rate of caloric expenditure is greater), but high-intensity exercise also causes a greater elevation in the postworkout metabolic rate. For example, Sedlock, Fissinger and Melby (1989) observed that triathletes who cycled at 75% VO₂max for 20 minutes burned more calories after their workout than they did after cycling at 50% VO₂max for 30 or 60 minutes. Phelain et al. (1997), who compared metabolic rates following two equal calorie-burning workouts—a short-duration, high-intensity workout (51 minutes at 75% VO₂max) and a long-duration, low-intensity workout (78 minutes at 50% VO₂max)—found that the high-intensity workout resulted in a higher postworkout metabolic rate than the low-intensity workout.

A great way for clients to perform high-intensity exercise and decrease their body fat percentage is through interval training, which breaks up the work with periods of rest. Not only does interval training allow clients to improve their fitness quickly; it is also more effective than continuous exercise for burning lots of calories during exercise and increasing clients’ postworkout metabolic rate.

Treuth, Hunter and Williams (1996) found that not only did subjects burn more calories during interval cycling (15 x 2 minutes at 100% VO₂max with
2-minute rest periods) compared with continuous cycling (60 minutes at 50% VO₂max); they also burned more calories during the 24 hours following the workout. Likewise, Laforgia et al. (1997)
reported that an interval running workout (20 x 1 minute at 105% VO₂max with 2-minute rest periods) resulted in a higher postworkout metabolic rate than a continuous run (30 minutes at 70% VO₂max). To obtain a better fat-burning effect, the intensity may need to be very high, since Malatesta et al. (2009) found that the amount of fat oxidized following exercise was similar for a low-intensity continuous workout (60 minutes at 45% VO₂max) and a higher-intensity interval workout (1-minute repetitions at 80% VO₂max with 1-minute recovery periods at 40% VO₂max), even though subjects used more carbohydrates and less fat during the interval workout. Thus, clients need to exercise at higher than 80% VO₂max to have a significant impact on their postworkout fat loss.

Interval training also seems to increase the ability to burn fat during submaximal exercise (below 100% VO₂max). Talanian et al. (2007) found that high-intensity interval training (10 x 4 minutes at 90% VO₂max with 2-minute rest periods)
induced marked increases in whole-body and skeletal muscle capacity for fat
use during exercise in moderately active women. Women who did the high-
intensity interval training seven times over 2 weeks increased whole-body fat use during submaximal exercise by 36%, significantly increased fat-burning enzyme activity and increased VO₂max by 13%. Certainly, during all these types of interval workouts, clients’ heart rates will exceed the traditional fat-burning zone.

For most healthy people without heart disease, exercising at near-max HR is not only safe; it is also necessary to improve the performance of the cardiovascular system and to become really fit. While not all clients are elite athletes, those who are not can still benefit from more intense exercise. Research has shown that interval training at an intensity greater than 90% VO₂max (95%–100% max HR) is the most potent stimulus for improving VO₂max and cardiovascular fitness (Billat 2001; Midgley et al. 2007).

So, for your clients who want to burn fat and lose weight, high-intensity exercise will burn more calories both during and after their workouts and will also increase the muscles’ ability to use fat. And tell clients not to worry about staying in their fat-burning zone—because there’s no such thing. Just tell them they simply need their caloric output to exceed their caloric intake.