To optimize exercise for fat loss, you must consider three factors: energy expenditure, compensatory behavior, and compliance.
Energy expenditure matters because fat loss depends on a negative energy balance. But more is not always better. Increasing activity to create a large energy deficit can have undesirable consequences, including fatigue, appetite enhancement, a decrease in nonexercise activity, and poor adherence. If endurance workouts result in overeating or reduced activity, the intensity and duration of workouts should be reduced.
Risk factors for negative outcomes due to endurance training include a) limited aerobic fitness, b) current involvement in intense, sport-specific training, and c) energy-restricted dieting. At least one of these risk factors is likely to be present for sedentary individuals, recreational athletes, and strength or power athletes. The presence of multiple risk factors is particularly likely for aesthetic- or weight-class athletes, such as bodybuilders, dancers, wrestlers, and boxers. For these athletes, the potential for compensatory behavior and poor compliance is elevated.
Fortunately, more exercise isn’t the only option for fat loss. In a review of research on activity-based interventions, Thompson et al. (2012) found that, when energy deficits from diet and exercise programs are matched, reductions in fat mass are equivalent. The decision to eat less or exercise more should therefore be based on preference.
So when is it appropriate to increase activity? You may find activity-based strategies for fat loss useful if you
- experience physical and psychological benefits from exercise
- prefer exercise to (additional) caloric restriction
- have extra time and energy to devote to fat loss efforts
- already support muscle mass with resistance training two or more times a week
Low-intensity activity, such as comfortable walking, is easy on the body and predominantly dependent on fat as a fuel source. However, the absolute energy cost of walking is low, so its impact on fat loss will only be evident over extended periods of time. This strategy is most useful for those who are unable to engage in intense exercise.
Moderate- to high-intensity activity, such as running, cycling, or elliptical training, can be performed for an extended duration and the energy cost is substantial. However, elevating the intensity and duration of activity to increase energy expenditure may result in compensatory behavior and poor compliance, particularly for athletes following calorie-restricted diets. This strategy is most useful for endurance athletes or those who wish to prioritize energy expenditure as a strategy for losing body fat.
Interval training, such as sprinting or other supramaximal activities, can improve multiple aspects of fitness in brief periods of time. However, interval training is less energy intensive than continuous exercise, which makes it better for fitness than fat loss.
Although advocates of interval training cite elevations in metabolic rate and fat oxidation following activity as evidence to support the effectiveness of this method, the contribution of post-exercise metabolism (also known as excess post-exercise oxygen consumption, or EPOC) is minimal when compared to the energy demands of activity itself. This was highlighted in a review of EPOC research by LaForgia et al. (2006), who concluded that only 6-15% (depending on intensity and duration) of the total energy cost of exercise could be accounted for by EPOC. To optimize fat loss, it is therefore preferable to concentrate on energy expenditure during, rather than after, activity.
Of the three strategies, moderate- to high-intensity activity (traditional “cardio”) is optimal because intensity and duration are balanced to maximize energy expenditure during exercise.
What’s “Better” Cardio?
As I suggested earlier, when energy expenditure is considered on its own, cardio is the ideal activity-based strategy for fat loss. Unfortunately, the effectiveness of this approach is often compromised by compensatory behavior or poor compliance. Although the precise cause of these consequences is unclear, there is some evidence to suggest that exercise intensity plays a role.
Recently, Hopkins et al. (2013) examined post-exercise energy intake among overweight and obese women. After engaging in a 400 kcal bout of exercise, participants were given ad libitum access to a standardized meal. Individual differences in post-exercise energy intake were dramatic. The data indicated that more than 1/3 of the variability in participants’ post-exercise energy intake was accounted for by carbohydrate oxidation during exercise. Given that the proportion of energy from carbohydrate increases as exercise intensity increases, it may be possible to minimize compensatory responses by reducing the intensity of exercise.
Conventional wisdom is to “work as hard as you can for as long as you can,” but better cardio involves “work at a moderate intensity for a sufficient duration.”
In addition to increasing activity to promote fat loss, it’s important to a) develop a plan for calorie and macronutrient intake and b) engage in resistance training to maintain lean mass. Once these conditions are met, the frequency, intensity, time, and type (F.I.T.T) of cardio can be considered:
Frequency: As an adjunct to a resistance training program, 3-5 days a week of better cardio will go a long way toward reducing body fat.
Intensity: For trained endurance athletes, the range of maximal fat oxidation is 55% to 72% of VO2max (the capacity to take in oxygen), with a peak of 64% VO2max (Achten, Gleeson, & Jeukendrup, 2002). For non-endurance athletes, this is likely to be too intense. Continuous activity at 50% VO2max will still have a meaningful impact on energy expenditure without being too physically demanding. If you decide to increase intensity beyond 50% VO2max, it will be important to monitor fatigue, appetite, and daily low-intensity activity in the days that follow exercise bouts.
An easy way to estimate %VO2max is to use maximum heart rate (MHR) data. 65% to 70% MHR should be equivalent to 45% to 50% VO2max. You can use an age-predicted MHR calculator like this one to identify an approximate workout intensity.
Time: 60 minutes should be sufficient.
Type: Running is a poor choice for modality because comfortable speeds are too intense for non-endurance athletes. Running can also be hard on the body. Treadmill walking on an incline is ideal for this protocol. Ehlen et al. (2011) found that the metabolic cost of slow walking up a moderate incline was equivalent to that of fast and level walking, but with less force production. For most people, a maximum speed of 4.8 km/h (approximately 3 mp/h) is a comfortable pace. For greater intensity, increase the grade rather than the speed. Walking at 3.0 mp/h at 10% incline should be manageable for those with average levels of fitness. If this is too challenging, work up to this grade over time.
Sample Workout and Impact
According to American College of Sports Medicine (ACSM) estimates, the net (after accounting for resting metabolic rate) energy expenditure for a 180 lb. man walking for 60 minutes on a treadmill at 3.0 mp/h with a 10% incline is 553 kcal. For a 120 lb. woman, it’s 368 net kcal. You can estimate your results with this calculator.
This approach will
- have a meaningful impact on energy expenditure
- minimally impact appetite and subsequent sport-specific training or performance
- limit risk for injury or overtraining even if performed most days of the week
- be comfortable enough to perform regularly
If you’re used to intense and long-duration exercise, recognize that aggressive approaches are unlikely to result in long-term success. Combining better cardio with a moderate caloric deficit will result in substantial fat loss over time. Incline treadmill walking at a comfortable pace is low impact and energy intensive without being exhausting. In addition, this protocol decreases the likelihood of compensatory responses and increases the likelihood of compliance.
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Achten, J., Gleeson, M., & Jeukendrup, A. E. (2002). Determination of the exercise intensity that elicits maximal fat oxidation. Medicine and Science in Sports and Exercise, 34, 92-97.
Ehlen, K. A., Reiser, R. F., & Browning, R. C. (2011). Energetics and biomechanics of inclined treadmill walking in obese adults. Medicine and Science in Sports and Exercise, 43, 1251-1259.
Hopkins, M., Blundell, J. E., & King, N. A. (2013). Individual variability in compensatory eating following acute exercise in overweight and obese women. British Journal of Sports Medicine. Epub ahead of print.
Thompson, D., Karpe, F., Lafontan, M., & Frayn (2012). Physical activity and exercise in the regulation of human adipose tissue physiology. Physiological Reviews, 92, 157-191.