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We’ve uncovered the truth about how your workout affects your metabolism, appetite, weight, and overall health
While you were toiling away on the treadmill, a slow-burning debate among fitness experts was catching fire: is exercise an effective strategy for losing weight? Or is the effort ultimately a wash, triggering hunger pangs that make you replace everything you burned and then some? A decade ago no one thought to ask this. Too much food makes you fat. Exercise burns kilojoules. Ergo, exercise makes you less fat. Then research began to find that not everyone who exercises loses weight. Impossible as it may seem, some people actually gain a kilogram or two. The problem: the range of individual responses to exercise is huge, says veteran weight-loss researcher Dr Timothy Church, a professor of preventive medicine at Pennington Biomedical Research Center in Baton Rouge, Louisiana. “We don’t understand all of it,” he says. Fortunately, however, we understand enough to throw cold water on four of todays most insidious weight loss myths.
MYTH 1: Exercise won’t help you lose weight
That would be news to all the people who’ve lost double-digit kilogrammage by pounding the pavement. But it would seem to validate the experience of your mate who trained for a marathon and finished a kilo heavier. Dr Church explains: “The degree to which you respond is probably dependent on genetics. Researchers have found 20 specific genes related to this, and how you score across those genes impacts your responsiveness.” Your diet and the kind of exercise you engage in may play a role too. For most of us, the response is in the middle. Exercise, in fact, helps out in three specific belly-off zones.
Limiting weight gain
“A ton of data shows that leading a physically active life is critical for not putting on weight,” says Dr Church. Beyond the obvious kilojoule-burning rewards, regular exercisers become more attuned to their body’s needs, reap mental benefits, and have a better quality of life, research shows. Regular workouts also help you maintain better body composition (more muscle, less fat), which means a lower risk of chronic diseases in your future.
A recent Cochrane Collaboration review of 43 exercise and weight loss studies determined that exercise helped people lose some weight – about a kilogramme. Crank up the intensity to “high” and you can lose half a kilo more – without dietary intervention.
Preventing the pounds from coming back
Losing weight isn’t easy, but keeping it off is even harder, Dr Church says. Your metabolism downshifts, and hormonal processes kick in to encourage your body to regain that weight. The latest data from the National Weight Control Registry shows that people who successfully keep kilos off exercise for 45 to 60 minutes a day. And as long as you’re not taking in more kilojoules than you burn, daily exercise may remodel your metabolism, so your body burns more fat.
Intensity trumps all. You not only burn more calories while you’re working out but also help your metabolism stay in a higher gear for hours afterward, thanks to a mechanism called EPOC, or excess post-exercise oxygen consumption. In a recent study in Medicine & Science in Sports & Exercise, men who cycled hard for 45 minutes burned an average of 2 170kj during the workout and another 806kj in the next 14 hours. The tipping point intensity level seems to be about 75% to 80% of your maximum heart rate (which is roughly 220 minus your age).
Myth 2 Exercise just makes you hungrier
Doesn’t happen, at least in the short term, says Dr David Stensel, who studies exercise metabolism at Loughborough University in England. In Stensel’s 2010 study, people who exercised for 90 minutes ate just as many kilojoules on the days they worked out as on the days they didn’t. Numerous other studies have shown that vigorous exercise briefly down regulates the appetite-stimulating hormone ghrelin. And while the blood levels of ghrelin rebound quickly after exercise, Stensel says they don’t rise beyond where they were before the activity. Over the long term, however, your body reacts to a serious fitness program as it would to any sustained reduction of its available fuel stores. No matter how much you want that 80cm waist, your body wants homeostasis more. The degree to which appetite amps up varies among individuals and depends on a combination of genetic, behavioural, and contextual factors, says Dr Barry Braun, an associate professor of kinesiology at the
University of Massachusetts at Amherst. “It’s complex, because in most studies we see a poor correspondence between appetite hormones and changes in perceived hunger,” he says. And there’s no clear link between appetite and what people actually eat.
Increasing your incidental activity – kilojoules you burn when you’re not working out – always pays dividends, says nutritionist Alan Aragon. For instance, a study from the University of Missouri found that active non-exercisers burned more kilojoules than people who ran 56km a week but were otherwise sedentary. And one of Braun’s recent studies shows that standing instead of sitting can burn an extra 3 135 a day without triggering an appetite increase. But don’t leave it up to chance: get an activity recorder, such as pedometer, and try to boost your numbers in whatever way you can.
MYTH 3 You can “reward” yourself for working out
Okay, you can, but you shouldn’t – at least not with food. The reason is simple: a few seconds of indulgence can undo an hour’s worth of exercise. Let’s say you run for 40 minutes at a six-minute-kilometre pace in your morning gym session. That’s a good workout, burning about 2 299 kilojoules. Now suppose you grab a Kauai Peanut Butter Bomb for breakfast on the way to work. That’s over 2 451 kilojoules – more than what you just sweated off. Dr Church is now studying dieters who exercise and either gain weight or don’t lose as many pounds as they’d expected to. Why? Because they tend to treat themselves with high-kilojoule foods after workouts. “The problem is, the reward is disproportionate to the activity,” he says. In a 2010 University of Ottawa study, 16 normal-weight people walked on treadmills until they had burned 1 250kj. Then they had to estimate how much energy they’d expended. Some guessed 3 750 – almost triple the actual burn. An hour later, they were told to eat a meal that, in their estimation, would replace the kilojoules they’d burned. This group selected 2 537kj – double the energy they’d used. But there’s some good news: emerging evidence suggests that exercise can help rewire your brain in a way that makes you less likely to seek out indulgent foods.
In a study published this year, researchers at California Polytechnic State University in San
Luis Obispo, asked two groups to either sit quietly or pedal hard on a stationary bike for an hour. When the time was up, the scientists wired them up to measure brain activity and showed them pictures of either high-kilojoule foods or healthy fare. The people who had exercised showed no preference for any particular category of food. But when the non-exercisers saw images of biscuits and ice cream, the reward regions of their brains lit up. “Being fit can have psychological effects,” says Dr Todd Hagobian, the study’s author and an assistant professor of kinesiology in California. “Regular exercise may increase your desire to consume a better diet – and shed kilos.”
First, find an exercise you enjoy; that way doing it becomes the reward. Second, keep a closer eye on the kilojoules you burn exercising. (For most activities, it’s under 1 700 for every 45 minutes). Finally, track your portion sizes. You could be in for a shock. Another key strategy is learning how to fill up on less. Weight loss requires an energy deficit (burning more than you take in), and anytime you’re managing a deficit, your body will notice. You can’t count on self-discipline alone. To win the hunger game, you need to eat foods that fill you up with the fewest kilojoules, says Aragon. Those tend to be foods packed with protein and/or fibre. He advises consuming 30 to 40g of protein at every meal (and snack) and hitting your daily fibre quota of 35g by eating plenty of beans, oats, fruits, and cruciferous vegetables. A recent study published in the New England Journal of Medicine associated three foods – yoghurt, nuts, and fruit – with successful weight loss. Yoghurt and nuts offer protein; fruit satisfies your sweet tooth while providing fibre and water. Most fruits have a lot of water, which lowers their energy density – the number of kilojoules per unit of mass. Low-kilojoule soups have the same effect; the extra fluid helps you feel full with less total food. You can also create the effect by drinking a glass of water before every meal and snack and sipping another one while you’re nibbling.
MYTH 4 As a society, we’re exercising more but still growing fatter
This is a phony correlation on two levels. First, it suggests that people who exercise consistently are also gaining weight. There’s no evidence of that happening. Then it assumes that more people are actually working out. Research flat-out refutes this idea. The peak years of the obesity epidemic – when the average weight of Americans rose by double digits – were roughly 1980 to 2000. In 1980, about 8% of men exercised five times a week, according to University of Minnesota research. In 2000, that increased to 9%. But there was one important change: the number of people who said they sat for more than half the day increased by 14%. To explore this, Dr. Church and his colleagues at Pennington examined how much physical activity people engaged in at their jobs in the 1960s and in 2008. They found a 58% decline in work-related physical activity, with men burning 600 fewer kilojoules a day. “Background physical activity has plummeted,” Dr Church says. “You have to make up for that somewhere. If you don’t, you’re going to gain weight.”
All movement matters. Even a simple evening walk can make up for the kilojoules we no longer burn at work. When we focus on what exercise doesn’t do, we miss a bigger story that goes beyond appetite and even weight control. Exercise pays off in ways that can’t be measured on a scale or seen in a mirror. A 2011 study published in the journal Circulation found that the fitter you are, the lower your chance of having a heart attack or of premature death from any cause, regardless of your girth. Other research reveals that even when dieters regain lost weight, they still come out ahead by maintaining their workout routines. A 2010 study from the University of Missouri shows that the still-exercising dieters were healthier than a matched group of people who’d regained weight but stopped working out. The exercising group had higher levels of HDL (good) cholesterol, greater insulin sensitivity, and lower blood pressure. It’s also worth noting that your own workout routine isn’t part of a controlled scientific experiment. What you take out of it has a high correlation with what you’re willing to put into it and how consistently you adhere to it.
And that’s no myth.