By Gretchen Reynolds
A few minutes of brief, intense exercise may be as effective as much lengthier walks or other moderate workouts for incinerating body fat, according to a helpful new review of the effects of exercise on fat loss. The review finds that super-short intervals could even, in some cases, burn more fat than a long walk or jog, but the effort involved needs to be arduous.
I have written many times about the health, fitness and brevity benefits of high-intensity interval training, which typically involves a few minutes — or even seconds — of strenuous exertion followed by a period of rest, with the sequence repeated multiple times. Most H.I.I.T. workouts require less than half an hour, from beginning to end (including a warm-up and cool-down), and the strenuous portions of the workout are even briefer.
But despite this concision, studies show that interval workouts can improve aerobic fitness, blood sugar control, blood pressure and other measures of health and fitness to the same or a greater extent than standard endurance training, such as brisk walking or jogging, even if it lasts two or three times as long.
People being people, though, the most common question I hear about quickie intervals and have asked, on my own behalf, is whether they also will aid in weight control and fat loss.
Only a few past studies have directly compared the fat-burning effects of endurance training to those of short interval workouts, however, and their results have been inconsistent. Some indicate that intervals prompt significant fat loss and others that any losses are negligible when compared to the effects of endurance training.
But those studies have almost all been small and short-term. They also used many different approaches to interval and endurance exercise, making the findings difficult to interpret.
So, for the new review, which was published in January in The British Journal of Sports Medicine, researchers from Brazil and Britain decided to pull together as much data as possible from all of the existing, high-quality studies looking at intervals and body fat.
Scrolling through medical library databases, they eventually found 36 studies that involved randomized experiments — not surveys or other epidemiological data — comparing the effects of endurance training to those from interval workouts. The experiments had to have lasted at least a month and included body-composition measurements at the start and finish, even if changes to body fat had not been the primary focus of the study. (And in most of these experiments, they were not.)
Then the researchers pooled numbers from the studies, giving them a total of more than 1,000 participants, young and old, male and female.
The moderate-exercise routines used in the studies varied considerably, with some involving walking and others jogging, cycling or swimming. In general, the endurance-style workouts lasted for about 40 minutes.
The interval sessions likewise differed from one study to the next, but most involved intervals lasting for a few minutes at a time, at a pace just below all-out effort, which is typical of H.I.I.T. Others required a few seconds of absolutely all-out exertion, an approach the researchers dubbed sprint-interval training, or S.I.T.
Then the researchers simply compared fat loss after the different exercise programs.
The results should be encouraging for anyone who exercises. Both moderate training and intervals, of all types, led to reductions in body fat, the researchers found. These reductions were absolute, meaning that people shed some of their actual fat mass, and also relative, meaning that they lowered the percentage of their body mass that was fat.
The changes also for the most part occurred whether or not people lost a noticeable amount of overall weight, suggesting that they might be losing fat while gaining muscle.
Perhaps most important for people drawn to quickie exercise, interval training, especially S.I.T. workouts, often burned more fat, in absolute terms, than prolonged, moderate exercise, with interval trainers dropping an average of about 3.5 pounds of fat during most studies, versus about 2.5 pounds for moderate exercisers.
It is worth noting that this is a one-pound difference, which in real-world terms is almost negligible.
In fact, the primary takeaway of the review could be that, “due to the similarity of outcomes, there can be flexibility in choice of exercise approach,” for anyone hoping to trim fat, says James Steele, an associate professor of sport and exercise science at Solent University in Southampton, England, who conducted the review with colleagues from the Federal University of Goiás in Brazil and other institutions.
Plan your workouts around your preferences and schedules, he says, and not concerns about which type of exercise might better trim fat.
Of course, even with 1,000 participants, the review remains relatively small. In addition, interpreting the data from individual volunteers can be somewhat baffling. In every study and every exercise program, some people lost more fat than others and some much less, for reasons that remain unclear.
“If you prefer longer yet easier-effort exercise, then go for it,” he says. “If you want to pump it up in a H.I.I.T. session, be my guest.”