A combination of high and low intensity may be the best and most effective means of fat loss but conventional wisdom still insists that fat burning stops when your heart rate goes above 75%. Some gurus even say that training above the so called “target zone” causes muscle loss and fat gain. As explained in “Forget The Fat-Burn Zone” (Article #10 on this site), total fat burn is actually higher when exercise intensity is high. Happily, people are getting the message.
A 45-year-old policeman in charge of his department’s physical assessment program read my article and wrote asking whether high-intensity intervals might motivate his officers and help them do better on their semi-annual fitness tests. “The vast majority,” he lamented, “work out for two weeks before the test, then go back to beer and TV.” Admitting that he’s not exactly “the poster boy for fitness” himself, he also wondered if intervals might be the answer for his own fitness regimen.
I reiterated that high-intensity intervals are a more effective and a far more interesting way to train. However, I warned him that people who are out-of-shape should feel their way along and check with their doctor if they have any health concerns. I suggested that he and his officers gradually ease into intervals once or twice a week, along with a walking program. “Take it easy at first,” I told him. “Experiment with varying intensity, perhaps one minute harder followed by a minute or two at a moderate to easy pace, and see how it goes compared to steady-state exercise,” I recommended.

Intervals Popular at Pritikin Center
A few days later I received an e-mail from Jeff Novick, RD, LD/N, Director of Nutrition at the Pritikin Longevity Center in Miami Beach, saying, among other things, that he “really pushes interval training” for his patients. “Even those with limited capacity or those who are very overweight seem to benefit from a modified interval program,” Jeff wrote. “They enjoy them and feel they make their workouts less boring.”
I thanked Jeff for the feedback and told him that I often hesitate to recommend intervals to people who are out of shape. Jeff, of course, deals with many people in this category, so his response was both encouraging and instructive.
I am very cautious with the people here when I have them do intervals, and have them begin slowly and work into them. We have many severely obese people and many with joint injuries and/or problems. I have even adapted the interval concept for them to walking programs so those who can only walk can still benefit from intervals. And those who I am most concerned about, I have them do intervals on an Air-Dyne or stationary bike.
Interesting thing about intervals is that once they start them, they rapidly improve their fitness and so even if I start them slowly, they can usually progress fairly quickly and also find their regular workouts are easier.
Jeff also sent along an excellent research review and commentary by Dr. G. R. Hunter and his colleagues at the University of Alabama at Birmingham titled “A role for high intensity exercise on energy balance and weight control,” which was published in the International Journal of Obesity (1998 June: 22(6); 489-493). Observing that recent high profile efforts have focused on low intensity exercise as a means of countering the increasing prevalence of obesity, the authors discuss the many advantages of high-intensity exercise. Some of the details are obvious and others are quite sophisticated. Interestingly, they conclude: “It might be that the best exercise programs should include a combination of low- and high-intensity exercise.”
Advantages of High Intensity Exercise
Quite logically, energy expenditure increases in an approximately linear fashion when the volume of work increases. For example, you burn roughly twice as many calories walking two miles as you do walking one mile. But an interesting thing happens when you increase the intensity by walking faster: Exercise efficiency decreases and the calories burned per mile increase. Dr. Hunter et al say there is a negative relationship between intensity and efficiency. “For example,” they write, “it has been found that 22% more energy is required to perform the same amount of bicycle work at a high intensity than at a low intensity.” The factors causing the inverse relationship are not fully understood, but several reasons have been hypothesized including an “increased dependence on inefficient fast-twitch muscle fibers, increased recruitment of stabilizing muscles, and increased work of the heart and respiratory muscles as exercise intensity increases.”
High intensity exercise – and the inefficiency engendered – is, of course, very fatiguing. The solution, say the researchers, is intervals. “Intervals work (that is, high intensity work interspersed with low intensity work) is one way to combine relatively high intensity with large volume.” In other words, intervals make it possible to do more high-intensity exercise – and expend more energy in relation to the volume of work performed – than is feasible with steady-state exercise.
High intensity exercise also provides another important benefit: improved fitness. The added fitness “will increase the volume of work that can be accomplished at any relative intensity,” say the authors. It is well established that high-intensity aerobic training increases aerobic fitness more than low intensity aerobics. “Training-induced increases in fitness will thus allow individuals to exercise at greater absolute intensities while experiencing the same relative intensity,” the Hunter group writes. For example, an individual with a higher V02max will burn more calories exercising at 50% of his or her capacity than will a less fit individual exercising at the same low relative intensity. Fifty percent intensity, of course, is faster for the more fit individuals than for the less fit. The researchers say that high-fitness individuals can easily burn 33% more calories exercising at 50% intensity. In short, high-intensity aerobics provides an overload which makes you more fit and better able to burn calories to control your weight.
Post Exercice Effect
Perhaps even more important to weight control is the post exercise effect of high-intensity aerobic exercise. “A number of studies have shown that REE [resting energy expenditure] is increased by 5-15% for 24-48 h [hours] as a consequence of aerobic exercise of at least 70% of VO2max, but not increased following aerobic exercise at lower intensities,” the researchers reported (emphasis ours).
The exact causes of the increase in energy expenditure following high-intensity aerobic exercise are not known, but Hunter et al present several hypotheses. One explanation is that heightened sympathetic muscle tone is partially responsible. “It is important to point out,” the researchers write, “that consistent with this hypotheses, elevation in REE only occurs following high intensity aerobic exercise; sympathetic tone is elevated for less than 2 hours following low-to-moderate intensity exercise.”
Two other factors are increased fat burning and glycogen replacement. “Lipid oxidation rates increase to a greater extent following high-intensity exercise, while post-exercise glycogen synthesis is increased, replacing the glycogen consumed during exercise,” says the commentary. Enzyme activity indicative of fat burning increases to a great degree following high-intensity training than after low-intensity exercise, according to the researchers. “It is a logical hypothesis that the increase in lipid oxidation following high-intensity exercise may be at least partly the result of increased glycogen storage,” they add.
Protein turnover may also contribute to elevated energy expenditure. “High intensity exercise may possibly cause greater increases in protein turnover than low-intensity exercise, resulting in greater post-exercise expenditure of energy,” say the researchers. Interestingly, there may be a connection between protein turnover and fat burn. “Increased EE [energy expenditure] induced by protein turnover may also increase the need for endogenous lipid, leading to elevated lipid oxidation,” the researchers write.
hatever the causes, the elevation in energy expenditure following high-intensity aerobic exercise is clearly a significant factor in weight control. According to the Hunter group, the increase in calories burned can be sizeable, amounting to 100 or 200 calories or more per day. That adds up rapidly and makes big difference in body fat level over time. It’s the equivalent of walking one or two miles a day – without taking a step.
High/Low Combination Best For Many
The downside, of course, is that high-intensity aerobic exercise is hard work. “Some people do not like high-intensity exercise, in some cases because of low fitness,” Hunter et al observed. As noted at the outset, their solution is a combination of high- and low-intensity exercise. “The low-intensity exercise, if the more preferred, may be done more frequently and will be the exercise in which the majority of energy is expended,” they conclude.
My own aerobics program is a combination of high and low intensity. I call this “a barbell aerobics strategy” in my new book Challenge Yourself, published a few days ago as I write this (late May 1999). In Challenge Yourself, I explain that “one hard aerobics session [per week] is enough — if combined with frequent walks or some similar activity.” I completely eliminate the moderate-intensity aerobics that most people do. This high/low or “barbell” aerobics strategy works wonderfully well, especially for someone who also trains hard with weights. See my new book for complete details.