// Can exercise make you fat?
by Matt O'Neill
EXPLORING THE EXERCISE-APPETITE DRIVE
Can exercise produce food cravings that result in calorie compensation and cancel out the calories you’ve burnt? It’s time to unravel the mysterious interplay between exercise and appetite management.
Exercise is supposed to help you get into shape by burning up calories stored as body fat. Burn around 500 calories more than you eat each day and you’re on target to lose nearly half a kilo of fat a week. That’s what the maths tells us.
But weight management is more than a simple numbers game. It’s a complex physiological process that centres around the human body’s biological drive for homeostasis. Put simply, your body tends to compensate for an attempt at reducing it’s energy stores. You may know this as the ‘set point theory’. This is why, for some people, exercise may stimulate appetite to a level which results in increased food consumption and complete compensation of calories burnt up during a workout. But how do you know if you are at risk of the exercise-appetite drive?
WHO’S AT RISK?
Most research on the appetite-stimulating properties of physical activity shows that, in the short-term at least (up to 14 days), appetite does not increase enough to cause a drive for full compensation of the energy burnt during exercise. This is good news and means you can keep on promoting the same beneficial calorie-burning, fat loss messages about exercise to your clients and participants.
However, for some individuals you may want to tweak your message to include an alert about the potential for exercise to stimulate appetite. The challenge is that we don’t really know who may be more susceptible to the exercise-appetite drive. Research studies looking at this effect have shown different individuals having different, and unpredictable, biological responses to exercise. For example, a September 2007 study published in the International Journal of Obesity examined the effects of five x 500 calorie exercise sessions per week for twelve weeks on thirty-five overweight and obese sedentary men.
This program accumulates 2,500 calories a week which represents around a third of a kilo of body fat.
Weight change was highly variable, ranging from a loss of 14.7kg to a gain of 1.7 kg. To examine why some men did fantastically and others not so well, the men were classified into two groups – ‘Noncompensators’ who lost around 6 kg and ‘Compensators’ who only lost around 1.5 kg on average.
Energy intake increased by almost 300 calories a day in the Compensator group, but decreased by 130 calories in the Noncompensator group. Subjective hunger rating also increased for Compensators, but not for Noncompensators.
Previous studies have also shown huge variations in response to exercise, so what’s going on?
SEARCHING FOR ANSWERS
Scientists have only really just started to untangle the ball of biological wires that may reveal a clear mechanism to link exercise and appetite. However, some potential chemical culprits have been identified.
In a May 2007 study published in the Journal of Endocrinology, 12 normal-weight volunteers exercised for 60 minutes at 65 per cent of their maximum heart rate, then had levels of gut hormones measured a few hours later. Exercise increased levels of peptide YY, glucagon-like peptide-1 and pancreatic peptide – all chemicals known to suppress appetite. It is possible that in some people the levels of these hunger-buster hormones fail to respond positively to exercise.
A longer-term reduction in body fat may also act to stimulate food intake. Levels of the hormone leptin, produced by fat cells, fall in tandem with body fat. It is thought that leptin is the master regulator of chronic food intake and that falling concentrations are a subtle trigger to eat more in order to rebalance body fat levels.
WHAT ElSE IS HAPPENING?
Another explanation is cognitive calorie compensation, which means that exercisers may allow themselves to eat more as a reward for being active. This has been observed more in women than in men, with one study showing that the energy deficit induced by exercise was completely ‘wiped out’ when followed by a high-fat lunch.
It is also worthwhile checking for a decline in daily incidental activity. This can occur if exercisers decide to conserve their energy for workouts or simply feel they can be less active because they ‘go to the gym’.
Perhaps, most importantly, a check on compliance to the exercise regime is vital to ensure enough calories are being burnt to achieve fat loss. Objective measures, such as heart rate or daily pedometer steps, give a much better indication of activity level than subjective reports of ‘doing workouts’. Just as research has clearly shown that people tend to underestimate food intake, it also shows some people will significantly overestimate their level of physical activity.
According to a 2003 review of exercise and appetite in the Proceedings of the Nutrition Society, ‘It is likely that the adoption of a more active lifestyle will have a number of consequences for appetite control’. In fact, appetite management is often the missing link between a diet plan, exercise and actual results.
TIPS FOR FITNESS PROFESSIONALS
• Be alert to the exercise-appetite drive in particular clients.
• Monitor your clients’ hunger-fullness levels just as you would heart rate or rate of perceived exertion.
• Help your clients with advice to better deal with food cravings.
• Integrate a nutrition strategy into any fat loss program you deliver.
• Enrol in specialist training on nutrition and weight management to enhance your knowledge and skills in appetite management.
Matt O’Neill, BSpSc, MSc( Nut&Diet)
Matt is a nutritionist and fat loss guru who provides practical tools and specialist training in nutrition and weight management for fitness professionals. He appears regularly on television providing expert advice and practical tips. For more information, visit www.SmartShape.com.au or call 02 9620 9511.
NETWORK • SPRING 2008 • PP28-29