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 percentage-wise 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.
Strategies for Fat Loss
To maximize clients’ fat loss, try these workouts:
1. Strategy #1: Go Hard
Interval training burns lots of calories in a short amount of time and keeps clients’ metabolic rates elevated for hours following the workout. Have them do one or two of these workouts each week:
- 5–6 x 3 minutes at 95%–100% max HR with 2-minute active recovery periods
- 4 x 4 minutes at 95%–100% max HR with 3-minute active recovery periods
- 8–12 x 30 seconds fast with 1-minute active recovery periods
2. Strategy #2: Go Very Long
Long runs or bike rides (≥ 1.5–2 hours at 65%–70% max HR) that stimulate mitochondrial synthesis and promote the depletion of glycogen threaten the muscles’ survival, since carbohydrates are muscles’ preferred fuel. In response to this threat, muscles “learn” how to use fat more effectively and over time become better fat-burning machines. For additional information on the exercise science behind fat loss, please see the full article, “The Fat-Burning Zone,” online in the IDEA Library or in the October 2009 issue of IDEA Fitness Journal.