What is best for boosting your metabolism?
Nearly every time you pick up a magazine or newspaper
these days, you see advertisements for products that supposedly raise
your metabolism. Whether they're trying to sell you exercise equipment
or food supplements, these ads claim that the products turn up the heat
in your body so you burn more calories even when you're not exercising.
Sounds logical enough, but is it true?
No question, in today's fitness world people are mad for everything
metabolic. But what exactly is this thing called "metabolism"
and how much do we really know about raising it? For example, what's
better for the boost- weight training or aerobics? And is it true that
eating hot chili peppers fires up your rate of fat combustion? Here
are some burning questions about burning calories, plus some answers
that are sure to surprise you.
What does "metabolism" mean?
Your metabolism is the sum of processes involving energy production-
specifically, the chemical changes in your living cells by which energy
is provided for your vital processes and activities and new material
is assimilated. You can also think about your metabolism as the number
of calories you're burning at any given moment, whether you're watching
"Seinfeld" or pumping away on the StairMaster. Naturally,
your metabolic rate- the rate of energy production and expenditure-
will be higher when you're exercising than when you're crashed on the
couch.
Will aerobic exercise raise your resting metabolic rate?
Several studies have failed to show any significant increase in resting
metabolic rate due to aerobic workouts. One, for instance, found that
men who jogged three times a week for 30 minutes at about 70 to 80 percent
of their maximum heart rate did not increase their RMR. But they were
certainly burning lots of calories on the run.
What is "afterburn"?
This term refers to the fact that your metabolic rate stays elevated
after you do any kind of exercise. But the effect varies greatly depending
on the type of workout, how hard and, to some extent, how long you just
exercised.
In a recent study, Christopher Melby, Dr.P.H., associate professor of
nutritional science at Colorado State University in Fort Collins, and
his colleagues monitored seven men who did a killer, 90-minute weight
workout- six sets of 10 exercises with very little rest between sets.
Two hours after the workout, their metabolic rate was 11 percent higher
than normal. When the subjects were measured the next morning, 15 hours
after the workout, their metabolism was still elevated by a hefty 9
percent, which could translate into as much as 150 extra calories burned.
In another study, Melby cut the workout down to five sets of each exercise
and let the subjects rest longer between sets. The subjects' post-exercise
metabolic rate also was 11 percent elevated after two hours; but by
the next morning, the increase had dropped to less than 5 percent. "What
that indicates is, the post-exercise metabolic rate [of a weight-training
workout] is a function of exercise intensity primarily, and to some
extent duration," Melby says. The numbers in both studies were
impressive, but the workouts were far longer and more strenuous than
most people have the time, strength, stamina and motivation to maintain.
Melby believes that the concept of afterburn gets too much hype. After
all, the calories you burn post-exercise are negligible compared to
what you burn during the workout.
Does aerobic exercise also give you an afterburn?
Yes, but it probably doesn't last as long as the weight-training afterburn-
unless you work out intensely or do interval training. Most studies
have found that mild to moderate exercise elevates metabolic rate for
only a few minutes to a few hours. If, for instance, you jogged at 70
percent of your maximum heart rate for a half-hour, you probably wouldn't
experience an afterburn for more than an hour. "For most people
it'll be less than that," Melby says, burning no more than 15 to
30 calories. On the other hand, highly trained athletes who can run
for, say, 90 minutes at 75 percent of their maximum heart rate, might
have significant afterburn for several hours. The bottom line: Focus
on burning calories during your workout, not after.
As for why strength training has more of an afterburn than moderate
aerobic exercise, scientists aren't sure. One possible explanation,
Melby says, is that anaerobic exercise, such as weight training, might
elevate levels of the hormone epinephrine, which stimulates metabolic
rate. Another theory: The repair of microscopic tissue damage caused
by weight lifting might expend more energy.
What about aerobic interval training?
Go for it. Afterburn seems to depend mainly on workout intensity (duration
doesn't have nearly as much effect). So if you do aerobics, interval
training with alternate bursts of activity will get the best post-workout
rise out of your metabolism. "If you did a number of short bouts,
say on the treadmill or the bike, at 95 percent of your max, the afterburn
would be significantly greater than if you used the same number of calories,
or even more, doing one longer, lower-intensity bout at 50 or 70 percent
of your max," says Melby. "Your metabolic rate would be elevated
for a longer period at a higher magnitude."
So there is truth in advertising. Some kinds of equipment can rev up
your body's calorie-burning rate; just be sure to do high-intensity
interval training for the best metabolic results.
Is there any way to lower metabolism?
Yes. Go on a very low-calorie diet. Your body will sense that it is
being starved and will fight back by slowing down. "If you don't
get enough calories, your metabolic rate can go down profoundly, says
Callaway. Severely anorexic women, he says, burn 30 to 40 percent fewer
calories a day- sometimes 50 percent- than women who eat normally. (In
other words, an anorexic who takes in only 200-300 calories a day uses
it up much more slowly that her normal counterpart.) It's not clear
how much of a calorie deficit triggers a drop in RMR- there's probably
a continuum. But, says Callaway, you can get into trouble by consuming
fewer calories than your RMR requires.
Can't you prevent your resting metabolism from dropping
by exercising when you diet?
Not necessarily, especially if you're on a very low-calorie diet. Two
late-breaking studies divided subjects into four groups: diet only,
diet plus aerobic exercise, diet plus weight training, and diet plus
aerobics and weight training. After many weeks, After many weeks, both
studies found no significant weight loss among any of the groups. "When
you add a lot of exercise on top of a marked caloric restriction, you're
not going to prevent resting metabolic rate from falling," says
Thomas A. Wadden, Ph.D., a psychology professor and director of the
weight and eating disorders Program at the University of Pennsylvania
School of Medicine in Philadelphia who conducted one of the studies.
That doesn't mean you should skip the exercise. To the contrary, you
should diet and work out with moderation. In Wadden's study where the
diet was severe- 128 obese people ate only 925 calories a day for the
first 16 weeks and 1,500 daily for the next 32- there were no significant
differences in weight or body composition changes among the groups.
(Both calorie amounts were severe for this population.)
