The exercise routines were identical, mixing short, intense intervals on stationary bikes one day with lighter, longer workouts the next. The practitioners exercised for five consecutive days while continuing the high-fat diet. The researchers then repeated the original tests.
The results were a little unsettling. After the first five days of eating fat, the men's cholesterol had risen, especially their LDL, the most unhealthy type. Your blood also contained altered levels of certain molecules related to metabolic and cardiovascular problems, the changes indicating a higher risk of heart disease.
Early morning exercise, meanwhile, did little to mitigate these effects. The morning athletes showed the same high cholesterol and worrying molecular patterns in their blood as the control group.
Evening exercise, on the other hand, lessened the worst effects of poor diet. The late trainers showed lower cholesterol levels after the five workouts, as well as improved patterns of molecules related to cardiovascular health in their bloodstream. Surprisingly, even on the nights after exercise, while they slept, they developed better glycemic control than either of the other groups.
The result of these findings is that "evening exercise reversed or reduced some of the changes associated with the high-fat diet," says Trine Moholdt, an exercise scientist at the Norwegian University of Science and Technology who led the study in Australia a visiting researcher. "Don't do morning exercises."
This study doesn't tell us how or why the later workouts were more effective at improving metabolic health, but Dr. Moholdt suspects that they have greater effects on molecular clocks and gene expression than morning exertions. She and her colleagues hope to examine these issues in future studies and also to examine the effects of exercise timing in women and the elderly, and the interplay between exercise timing and sleep.
For now, however, she cautions that this study in no way suggests that morning exercise is not good for us. The men who exercised got aerobically, she says, regardless of when they were exercising. "I know people know," she says, "but any exercise is better than not exercising." However, exercising later in the day can have unique benefits for improving fat metabolism and blood sugar control, especially if you are on a high fat diet.