Exercise - Does It Increase Appetite?

by Matt O’Neill (Nutritionist & Director, SmartShape.com.au)

Can exercise produce food cravings that result in calorie compensation and cancel out the calories you've burnt? Nutritionist and fat loss guru Matt O’Neill unravels the mysterious interplay between exercise and appetite management.

Exercise is meant 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 a pound of fat a week. That’s what the math tells us.

But weight management is more than a simple numbers of 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'.

That's why for some people, exercise may stimulate appetite and to a level which results in increased food consumption and complete compensation of calories burnt up in workouts. But how do you know you're at risk of the exercise-appetite drive?

Who's at risk?

Most research on the appetite-stimulating properties of physical activity shows that at least in the short-term (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.

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, we don't really now who may be more susceptible to the exercise-appetite drive. Research studies are conducted on groups of people, with each individual having a different biological response to exercise that is hard to predict.

For example, a September 2007 study published in the International Journal of Obesity examined the affects of 5 x 500 calorie exercise sessions per week for twelve weeks on thirty-five overweight and obese sedentary men. This program accumulates 3500 calories a week which is equivalent to that magic one pound of body fat.

Weight change was highly variable and ranged from a loss of 14.7 to a gain 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 Compensator, 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 in the Journal of Endocrinology, 12 normal weight volunteers exercised for 60 minutes at 65% 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’s 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’s thought that leptin is the master regulator of chronic food intake and falling concentrations are a subtle trigger to munch more 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 men with one study showing that the energy deficit induced by exercise was completely "wiped out" when followed by a high-fat lunch.

It's also worthwhile checking for a decline in daily incidental activity. This can occur if exercisers decide to conserve their energy for workouts or simple 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 for 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 the research has clearly showed that people tend to underestimate food intake, it also shows some people will significantly overestimate their level of physical activity.

Sensible advice

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.

If you start exercising to lose body fat also start monitoring your hunger-fullness levels just as you would your heart rate or rate of perceived exertion. This will help you better deal with any cravings that can undermine your success.

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