They tested the men’s fitness and resting metabolic rates and took samples of their blood and fat tissue.
Then, on two separate morning visits to the scientists’ lab, each man walked for an hour on a treadmill at a moderate pace that, in theory, should allow his body to rely principally on fat for fuel.
Before one of these workouts, the men skipped breakfast, meaning that they exercised on a completely empty stomach, after a prolonged overnight fast.
On the other occasion, they ate a substantial, 600-calorie morning meal, supplied by the scientists, of toast, jam, cereal, milk and orange juice about two hours before they started walking.
Just before and an hour after each workout, the scientists took additional samples of the men’s blood and fat tissue.
Then they compared the samples.
There were considerable differences. Most obviously, the men displayed lower blood sugar levels at the start of their workouts when they had skipped breakfast than when they had eaten. As a result, they burned more fat during walks on an empty stomach than when they had eaten first. On the other hand, they burned slightly more calories, on average, during the workout after breakfast than after fasting.
But it was the impacts deep within the fat cells that may have been the most consequential, the researchers found. Multiple genes behaved differently, depending on whether someone had eaten or not before walking. Many of these genes produce proteins that can improve blood sugar regulation and insulin levels throughout the body and so are associated with improved metabolic health. These genes were much more active when the men had fasted before exercise than when they had breakfasted.