In order to lose weight, is it better to exercise before or after breakfast?

A new study from the University of Bath in the U.K. has demonstrated the different roles fat plays in powering and responding to exercise shows the effects of eating versus fasting on gene expression in fat cells in response to exercise.  The study was carried out with overweight males walking for 60 minutes at 60% of their VO2max (their maximum oxygen consumption) on an empty stomach.  The same volunteers were then asked to complete the same walk on a different occasion, two-hours after consuming a high-calorie, carbohydrate-rich breakfast.  Both blood and adipose tissue (fat) samples were taken after eating or fasting and after exercising.  Fat tissue samples were also taken immediately before and one hour after completion of the walking task.

Gene expression in the adipose tissue differed significantly under fasting vs. after-breakfast trials.  The expression of two genes, PDK4 and HSL, increased when the men fasted and exercised and decreased when they ate before exercising. The rise in PDK4 likely indicates that stored fat was used to fuel metabolism during exercise instead of carbohydrates from the recent meal. HSL typically increases when adipose tissue uses stored energy to support increased activity, such as during exercise.

The results reinforce the view that “adipose tissue often faces competing challenges”.  After eating, adipose tissue is busy responding to the meal and a bout of exercise will not stimulate the same (beneficial) changes in fat cells.  This means that exercise in a fasted state might create a more favourable environment for fat loss.

These findings are of course still dependent on a range of different variables, including gender (study was in males only) and degree of overweight and level of the volunteers. It is also important to consider that a breakfast with a different macronutrient composition (e.g. different level of carbohydrate and the glycaemic index of the carbohyrates in the breakfast) could all influence these results.