Will Exercise Make You Fat?

Dr. John Blundell

Dr. John Blundell

Yesterday, I had the pleasure of hosting John Blundell, at the Research in Progress seminar series at the Alberta Diabetes Institute.

Dr. Blundell is Professor of bio-psychology at the University of Leeds, UK, and is certainly one of the preeminent authorities on the bio-psychology of ingestive behaviour.

His presentation with the rather provocative title, “Will exercise make you fat?”, started with a broadside at the media, which lately has been quite active in promoting this notion.

However, as Blundell pointed out, this simplistic message is far from accurate in that the relationship between physical activity and its impact on ingestive behaviour and body weight is anything but straightforward.

For one, although short-term studies (days) do often show an increase in appetite, this is by no means regularly observed in longer-term studies (weeks).

In a paper he recently published in the Journal of Clinical Endocrinology and Metabolism, Blundell recently examined the effects of medium-term exercise on fasting and post-prandial levels of appetite-related hormones and subjective appetite sensations in overweight and obese individuals.

The study included 22 sedentary individuals who took part in a 12-wk supervised exercise programme (five times per week, 75% maximal heart rate) and were requested not to change their food intake during the study.

Not only did exercise result in a significant, albeit modest (~3 Kg), reduction in body weight and fasting insulin and an increase in ghrelin plasma levels but also in a reduction in fasting hunger sensations.

A significant reduction in postprandial insulin plasma levels and a tendency toward an increase in the delayed release of glucagon-like peptide-1 (90-180 min) and a greater suppression of postprandial ghrelin.

Thus, although exercise-induced weight loss was associated with physiological and biopsychological changes towards an increased drive to eat in the fasting state, this compensatory effect seems to be balanced by an improved satiety response to a meal and improved sensitivity of the appetite control system.

However, as Blundell pointed out, these mean changes hide the immense diversity between individuals.

Based on these studies it appears impossible to predict in advance how individuals will respond: Some people, in response to exercise, will be hungry and may overeat – others may find that they are much better in controlling their food intake.

Importantly, all subjects, irrespective of their body weight, showed a reduction in their amount of body fat and improvements in risk markers like physical fitness and blood pressure.

Thus, Blundell concludes, exercise does reduce body fat (even in people who do not lose weight) and has beneficial effects on important health parameters.

The answer therefore clearly is: no, exercise does not make you fat, but don’t expect to lose a lot of weight.

The many important benefits of exercise can, unfortunately, not be measured on a scale.

Edmonton, Alberta

Martins C, Kulseng B, King NA, Holst JJ, & Blundell JE (2010). The effects of exercise-induced weight loss on appetite-related peptides and motivation to eat. The Journal of clinical endocrinology and metabolism, 95 (4), 1609-16 PMID: 20150577