Hits and myths: exercise may not help you lose those fast-food calories

PUBLISHED : Monday, 09 December, 2013, 10:06am
UPDATED : Monday, 09 December, 2013, 10:06am

If you eat junk food but exercise often, can you still gain weight?

The straight answer: Yes

The facts: Energy balance is the difference in energy intake and energy expenditure. Positive energy balance - that is, when the amount of energy intake is greater than the amount of energy expenditure - will lead to an increase in body weight. Conversely, achieving a negative energy balance can lead to a reduction in body weight.

According to Daphne Wu, a British state-registered dietitian and PhD researcher, energy expenditure can be calculated via metabolic equivalents, also known as MET. One MET is the equivalent of one calorie per kg of body weight per hour. In explaining energy expenditure, she gives three examples using someone who weighs 60kg. In the first example, the MET value of sleeping is 1 MET.

Age, genetics, stress, sleep, and lifestyle all played 

Thus, a 60kg person who sleeps for eight hours would burn 480 calories (1 MET x 60kg x 8 hours). In the second example, brisk-walking at a speed of 6.4 km/h has the MET value of 5.

So if the same 60kg person brisk-walked for 30 minutes he would burn 150 calories (5 MET x 60kg x 0.5 hours).

In the third example, running at a speed of 12.8 km/h has the MET value of 11.8. Running for a half-marathon (21 kilometres) would therefore cost this person 1160 calories (11.8 MET x 60kg x 1.6 hours).

A typical fast food meal, made up of a Big Mac, 350ml of cola, a medium-sized fries and three chicken nuggets, has approximately 1148 calories. In order to burn off these calories, the same 60kg person would have to run 21 kilometres at a speed of 12.8 km/h.

The important question is, how often can a person run 21 kilometres? He might consume a fast-food meal every day, but unless he expends the calories in that meal, he will gain weight.

Of course, one's metabolic rate does stay up for few hours after exercise, depending upon the type and duration of exercise performed. This is one of the reasons why regular exercise is so important - it helps the body burn more energy, even during periods of rest. But people who overestimate the energy expenditures from exercise might take in more energy from foods, thereby failing to get their weight down.

Another reason why some physically active people who eat junk food cannot stay lean, while others can - even if they eat the same foods and do the same kind of exercise - is that everybody burns energy at a different rate.

A person with more muscle mass has a higher basal or resting metabolic rate than someone with less muscle mass, even if the body weight of both people is the same.

But exercise is not the only factor that affects your weight. Age, genetics, stress, sleep duration and quality, some medications and lifestyle habits all play a role in weight gain and weight loss.

Wu advises us to think about more than just weight management. Just because a person is lean does not mean he or she is in good health.

In deciding what to eat, she asks that we consider how much cholesterol, sodium, fibre and saturated fat we are taking in.