Photo By Bev Childress
Photo By Bev Childress
Traditional programs to combat weight gain focus on nutritional (calories in) and exercise (calories out) considerations. Many health-related professionals agree that dietary guidelines likely best serve to promote short-term reductions in body fat, with the impact of exercise being helpful, but perhaps not overwhelmingly. Even so, from the perspective of physical movement, questions can be raised as to which is more meaningful:
- The increased caloric costs associated with daily exercise and activity, or
- An increased ability to oxidize fat.
In fact, the caloric costs of any given format of exercise are typically not great, being at best low to moderate. However, in terms of optimizing the prevention of body fat accumulation, it is suggested that exercise program design should focus on brief, intermittent phases of intense work, followed by more prolonged periods of active recovery.
The Deceiving Apparent Simplicity of Fat Loss
A staggering 2 out of 3 adults are overweight or obese. Equally worrying is the lack of an apparent strategy to ward off such a trend. One basic perspective has been the perhaps overly-simplistic relationship between calories in and calories out. Based on that premise, there are three straightforward approaches to weight loss:
- Curtailing caloric intake (i.e., diet)
- Promoting caloric expenditure (i.e., exercise)
- Some combination thereof
A focus on caloric intake is of course crucial to weight loss, but is negligent regarding the bioavailability of the calories within food, especially processed food. Indeed, the caloric content of what we ingest does not necessarily reflect the amount of energy that is actually obtainable by the human body. Processed food makes calories more readily available as compared to un-processed food, leading to the potential for increased energy availability with subsequent body fat gain.1, 2 In fact, counting calories is often portrayed as a losing endeavor.
The caloric expenditure side of things is similarly suspect. We know that exercise can affect body composition, and a growing body of research suggests that brief, higher intensity intermittent exercise may be even more effective at promoting reductions in body fat than its lower intensity, steady-state counterpart.3,4 Yet exactly how that works remains to be seen. Is the exercise-related loss of body fat more dependent on increases in caloric expenditure (both during and after exercise),5 or is it because of a greater reliance on fat as a fuel?6
The Complications of Calories Out
Exercise invokes an increase in metabolic rate regardless of the design: steady-state or intermittent, low, moderate, or high intensity. From the perspective of caloric cost, steady-state aerobic exercise (e.g., walking, jogging, bicycling) takes time, while brief bouts of intermittent exercise (e.g., resistance training) demand exertion.
Truth be told, dependent on how much time you have or your willingness to apply effort, the calories just don’t add up in terms of making an overwhelming contribution towards the immediate removal of body fat. To be sure, metabolic increases are the result of any kind of physical movement, yet at 4000cal per pound of fat, you need to complete quite a lot of exercise to lose a significant amount of weight.
In the table below, taken from my book, ten bouts of each proposed exercise routine are required to lose one pound of fat. The costs of walking, jogging and cycling come from steady-state measurements; all other examples are estimated in a cost per task format.
|Exercise routines that result in an estimated 400cal cost.|
|Bicycling||~60min at 6mph; ~30min at 15mph|
|Isometric Tabata squat routine||26x4min routines (20sec of isometric hold, 10sec recovery; 104min total)|
|Heavy bag punching||13 all-out 1min rounds (96 punches per minute)|
|Sprint cycling||14 1min sprints at 250w output|
|Squat resistance training||~360 repetitions at 50% 1RM|
Another popular belief is the resulting caloric after-effect that regular workouts provide. Exercise physiologists call this excess post-exercise oxygen consumption (EPOC); fitness professionals refer to this period as the “afterburn.” Regardless of terminology, many of us consider exercise as an unequivocal promoter of an increased metabolic rate, yet recent investigation suggests far worse.
A study of what can only be considered an above-average active-population—a hunter-gatherer tribe—revealed that daily caloric costs were comparable to those of a “standard,” similar-sized, sedentary apartment dweller.7 Intuitively, it would appear that the physically demanding lifestyle of a society that eats only what it catches or forages, would result in a daily energy expenditure greater than that of someone who, with a smart-phone, can voice-command a pizza delivery at any time of the day. The author of the investigation suggests:
“…the body makes room for the cost of additional activity by reducing the calories spent on the many unseen tasks that take up most of our daily energy budget; the housekeeping work that our cells and organs do to keep us alive. Saving energy on these processes could make room in our daily energy budget, allowing us to spend more on physical activity…”8
What Are We Measuring?
How could the exercise science and fitness communities be so wrong? Many of us have been trained to champion the idea that regular exercise, at the very least, provides some kind of addition to overall daily caloric expenditures, not a subtraction. The answer may not lie solely with absolute caloric costs per se, but also in terms of relative considerations, as the type of substrate or fuel from where those calories come from.
The calorie is a historical unit, created when the direct measurement of heat served as the gold standard in the quantification of life’s energy exchanges. Due to time and expense, heat measurements—calorimetry—have been replaced with oxygen uptake measurements, the latter serving to estimate heat production.
In terms of the measured volume of oxygen (O2) consumed, glucose oxidation results in a greater caloric cost (~5.0cal per liter of O2) as compared to fat oxidation (at 4.7cal per liter of O2), a difference of about 7%. But let’s look at the inverse. When caloric expenditure is estimated by units of oxygen consumed, the following conversions are noted, per calorie:
- Glucose oxidation = 0.20 liters of O2
- Fat oxidation = 0.21 liters of O2
From that perspective, the oxidation of fat compels a greater volume (~5%) of oxygen consumed per calorie; that is, an equivalent need or demand for energy results in a greater amount of oxygen consumed when fat is “burned” as a fuel, compared to glucose.9
The daily energy needs of the hunter-gatherers mentioned earlier utilized a methodology where the estimate of daily caloric costs came from the calculated amount of carbon dioxide produced, not the volume of oxygen consumed. From the perspective of energy demand, the oxidation of fat results in proportionately less carbon dioxide production, and a greater oxygen uptake than does the oxidation of glucose.10 This means that a similar rate of carbon dioxide production between sedentary and active populations may falsely indicate a lower than actual metabolic rate for active people.
Can We Just Burn Fat?
If caloric costs don’t provide a straightforward explanation for exercise-induced weight loss, perhaps the answer lies more with the ability to oxidize fat.
While the association between a decreased ability to oxidize fat with subsequent weight gain is weak, it is also statistically significant.11, 12 Increases in fat oxidation have been found after exercise.6, 13, 14, 15 Yet evidence also is available to suggest that the rate of fat oxidation between sedentary and active populations is not much different.16 Factor in the frustrating variability associated with the measurement of substrate oxidation during non-steady-state exercise and activity, as well as the ever-present influence that diet also provokes,17 and a conclusive take on an increased ability to oxidize fat cannot be made.
One bit of solid evidence regarding substrate utilization and working skeletal muscle is that the higher the intensity of exercise, the greater the reliance on glucose (glycogen) as a fuel. This has led to the conclusion that exercise designed to lose body fat should be of low to moderate intensity and longer duration: steady-state cardiovascular exercise (e.g. walking, jogging, bicycling). Times have changed.
We Need More Accurate Research
The focus of exercise and its associated energy costs within most exercise science labs has been on low to moderate intensity, steady-state exercise. Treadmills and bicycle ergometers serve as traditional equipment, and standard costs are reported in a per-minute format (i.e., liters O2 or calories per minute. As a direct yet baseless result, equivalent descriptions are also used for higher intensity, non-steady-state (intermittent) exercise.
Clearly, low to moderate intensity steady-state exercise and higher intensity non-steady-state exercise are not the same, yet many exercise scientists continue to estimate the caloric costs of both using standard per-minute measurements. Likewise, it needs to be kept firmly in mind that the published energy cost estimates of steady-state exercise do not include recovery.18 For intermittent non-steady-state exercise, this practice has been questioned, as it may misrepresent both the absolute and relative aspects of the caloric cost and fat-burning benefits of intermittent exercise, respectively.19, 20
While not yet a mainstream methodology, energy requirements also have been estimated in the context of a cost per task, where an amount of work is completed along with the total energy costs of that task. A total energy cost estimate consists of three specific measures:
- Exercise oxygen uptake
- Anaerobic costs (based on blood lactate levels)
- Recovery oxygen uptake (measured between bouts or sets, as well as after exercise is completed)
How Building Muscle Burns Fat
Resistance training, where work is performed as the number of repetitions completed in a per-set format, serves as a wonderful example of high-load, non-steady-state, intermittent exercise. Under these conditions, it has been reported that as the number of sets increases within a given workout, the amount of recovery oxygen consumed between sets increases in apparent proportion to the decrease in anaerobic costs.21 Because there is little to no intensity associated with recovery, conditions for fat oxidation appear optimized.
Within 3 sets of a specific resistance exercise, aerobic recovery costs grow larger, while the estimated anaerobic exercise-related costs decrease (based on blood lactate levels), setting the stage for moments of increased fat oxidation.
Higher intensity exercise most certainly demands a greater cost to physical movement. A decreasing blood lactate cost-to-work ratio among sets of repeated high-intensity resistance exercise along with an increasing recovery oxygen uptake indicates an increased use of the high-energy phosphate stores of ATP and phosphocreatine (PC) within muscle during the repeated act of weight lifting. Afterwards, in the recovery between sets, a good deal of oxygen is consumed to replenish those high-energy phosphates stores, and fat may be the preferred substrate that fuels that energy exchange.
The take home message is that an intense intermittent exercise workout, or rather, the presence of multiple recovery periods within that workout, may serve to better oxidize fat as compared to a single lengthy bout of steady-state exercise followed by a single recovery period.22 With a focus on recovery, a case can be made that brief bouts of heavy work followed by a somewhat lengthy recovery period might optimize the use of fat as fuel.
When examining resistance training work between separate workouts performed on different days of high and low loads, an analysis of the work to total energy cost ratio revealed that efficiency actually improves as more repetitions are completed.6, 22 That is, lower loads with a high number of repetitions are more efficient, as compared to higher loads with few repetitions—even as more work is completed with the former, at a greater exercise-related cost.
When comparing energy cost efficiency among different resistance training workloads, lifting a heavier load with fewer repetitions is less efficient, as compared to lifting a lighter load for many repetitions. In fact, efficiency rises to a maximum as more work is completed. Data from our lab indicate that as aerobic and anaerobic exercise costs rise proportionately with resistance training work, recovery costs do not—EPOC or afterburn costs are somewhat similar among high-load and low-load workouts involving dissimilar amounts of work.22, 23
However, as work increases with muscular endurance type resistance training, the amount of recovery-related oxygen consumption (i.e., recovery cost) appears similar for high-load, low repetition, strength training where less work is completed.23, 24
To summarize, aerobic and anaerobic exercise energy costs rise linearly with increasing work, but recovery energy costs do not, being somewhat similar for high-loads with less overall work and low-loads where a much greater amount of work is completed.
Go Hard and Heavy, and Stay on Your Feet
Regular physical movement of any kind necessitates an increase in metabolic rate, if only temporarily. Coupled with caloric restriction, this almost certainly helps to reduce body fat. With the knowledge that most forms of exercise offer only a low to moderate overall caloric cost, exercise design should focus on what a person actually enjoys doing.
In terms of a more meaningful impact on longer-term, gradual body fat loss, exercise program design is suggested to consist of intermittent, brief, intense, high-load periods of inefficient work, each married to an extended recovery period. Recovery periods should, in turn, be active, involving lower-intensity, steady-state, large muscle group activities like walking, as opposed to passive (seated) rest, where higher rates of fat oxidation may be best achieved. A small but significant ability to oxidize fat day-to-day may better serve in the prophylactic prevention of long-term body fat accumulation.
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