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The process should be straightforward: when we exercise we burn calories to create an energy deficit, prompting our bodies to use up stored fat to fuel bodily processes and other daily activities. If only it were that simple.
“Exercise on its own is not a very useful tool for weight loss,” Javier Gonzalez, professor of nutrition and metabolism at the University of Bath, says. “Dieting to consume fewer calories than you expend is a more effective way to shed pounds, but the healthiest approach is when the two — healthy eating and exercise — are used hand in hand.”
The reality for many people is that they don’t lose anywhere near as much weight as they expect when they start working out, and scientists are beginning to establish why.
Some researchers have suggested that the human body responds in unexpected ways when we increase our activity levels. In one study, Herman Pontzer, an evolutionary anthropologist and metabolism scientist, and his colleagues at Duke University in North Carolina, measured the energy expenditure of members of the Hadza tribe, hunter-gatherers from Tanzania. They found that although the Hadza were active for much of the day, the number of calories they burnt was similar to that of couch potatoes once data was adjusted for body size and other factors.
More recently, for a paper in the journal Science Advances, Pontzer looked at the calorie-burning of ultra-endurance runners who ran about 150 miles a week for 20 weeks in a transcontinental race. Surprisingly, even they burnt relatively the same number of calories as sedentary people. Pontzer’s theory is that this effect is down to what he calls “constrained total energy expenditure” — the more calories we burn doing regular exercise, the more internal adjustments our bodies make to other components of our metabolism, ensuring overall calorie burning is maintained within a narrow range.
“If you exercise today, you will burn more energy today,” Pontzer told the Washington Post, “but if you really change your lifestyle and start exercising frequently and that becomes your new normal, your body adjusts and you wind up not burning more calories overall.”
Gonzalez says there are many reasons why there may not be a linear increase in calorie burning when you start to exercise more — if you run for an hour and burn 500 calories, it’s not a given that your total energy expenditure for the day will be 500 calories higher than usual.
“It’s well documented that a lot of people tend to compensate for a morning run or gym class by being less active later in the day, bringing down their overall calorie burning for the day, despite adding a workout. And if someone substitutes another daily activity for an exercise class, it may impact their calorie output positively if they would otherwise have been lying on the sofa, but maybe negatively if they would have been digging the garden and did a yoga class instead.”
Of course, exercise, and resistance training in particular, is important for creating a leaner look when you lose weight. “It’s incredibly important for preserving muscle mass, which has aesthetic and metabolic benefits when weight is lost,” Gonzalez says. Here’s how exercise will (and won’t) help you to get into shape:
A review of 149 studies, published in the journal Obesity Reviews, that looked at how supervised aerobic, resistance or high-intensity interval training (HIIT) exercise in combination or alone affected weight loss in overweight adults concluded that exercise alone does lead to weight loss, albeit only a modest amount. “The key is to create an energy deficit so that you burn more calories than you consume,” Gonzalez says. “If you start exercising and don’t change what you eat, you will do this to some extent, but the effect is more powerful if you reduce your calorie intake too.”
A common complaint is that exercise makes you hungry, so you end up eating more, compensating for any calories burnt during a workout. A lot depends on the type and amount of exercise you are doing. Counterintuitively, gentle jogging, swimming and yoga are more likely to leave you hungry than a tough HIIT session.
One recent study by researchers from the University of Virginia showed that HIIT sessions reduced levels of ghrelin, an appetite-stimulating hormone, so that people felt less hungry afterwards. Meanwhile, researchers at the University of Kentucky reported overweight people who exercised for five hours in total every week displayed changes in levels of leptin, an appetite-suppressing hormone, allowing them to better regulate their desire to eat. Those who worked out less showed no such hormonal changes.
The evidence suggests that frequent exercise is much better at preventing weight regain than it is at helping people to lose it in the first place,” Gonzalez says. Although Dr Mark Homer, an exercise physiologist at Buckinghamshire New University, adds that you need to be diligent and to gradually increase the intensity or the duration of activity for the weight to stay off. “Vary your exercise and keep at it are the rules,” he says.
Contestants on the American TV show The Biggest Loser were tracked for six years after losing weight and researchers reporting in the journal Obesity concluded that those who exercised at the gym for about 80 minutes most days regained fewer pounds than those who didn’t exercise.
“Frequent exercise reduces inflammation and stress levels and helps to counter some of the undesirable side-effects of excess body fat, such as raised cholesterol and blood-sugar levels, and there is absolutely no argument that it is not good for us.”
People who combine exercise with dieting have been shown to achieve double the metabolic health benefits, such improved blood-sugar control, as people who only dieted. And when it comes to longevity, exercise is the winner with a review from researchers at the University of Virginia showing that obese people reduce their risk of heart disease and early death far more effectively if they improve their fitness levels than if they just lose weight.
Studies have shown that resistance training will help to reduce harmful visceral fat, the type that settles around our organs and raises the risk of metabolic conditions, including type 2 diabetes.
A review paper in the journal Exercise Physiology confirmed that even though it may not induce faster weight loss than plenty of moderate aerobic activity, a short-duration HIIT workout also helps to blast belly fat.
“Exercise of any type will bring a host of metabolic benefits that help to prevent disease,” Homer says. “The important thing is to do it frequently for maximum benefit.”