Says Dr. Jeffrey F. Horowitz, the senior author of the study, "Differences in what you eat after exercise produce different effects on the body's metabolism." Commenting on the transient nature of the benefits of exercise, he added, "many of the improvements in metabolic health associated with exercise stem largely from the most recent session of exercise, rather than from an increase in 'fitness' per se. But exercise doesn't occur in a vacuum, and it is very important to look at both the effects of exercise and what you're eating after exercise."



The study took place at the Michigan Clinical Research Unit of the University of Michigan Hospital and examined nine sedentary but otherwise healthy men between the ages of 28 and 30 years. The men were required to undergo four separate sessions each of which lasted around 29 hours. Prior to attending each session, they were required to fast overnight before coming in the next morning.
The first session consisted of a control trial with the subjects eating meals that matched their daily caloric expenditure but not engaging in any exercise.
The second session had the subjects exercise on stationary bicycles or treadmills at medium intensity for 90 minutes followed by meals that equalled their caloric expenditure. The levels of carbohydrates, fats, and proteins in the meals were matched to the subjects' expenditure.
The third session repeated the level of exercise of the second session but provided low carbohydrate meals. These meals contained around 200 grams of carbohydrate, which is less than half that of a balanced meal.
The last session repeated the level of exercise of the previous two sessions but gave the subjects low-calorie meals that provided about one-third the caloric energy expended. However, these meals were relatively high in carbohydrates in order to replace those used up in exercise.

Although there was a trend towards enhanced insulin sensitivity throughout the exercise trials, when the subjects ate less carbohydrates there was significantly greater insulin sensitivity.
Weight loss is an important aspect to exercise in improving metabolic health especially in overweight and obese individuals but, according to Dr. Horowitz, the results of the study show that important health benefits (in this case, enhanced insulin sensitivity) from exercise can be achieved without under-eating or losing weight. Dr. Horowitz and his team are now conducting studies with obese subjects in order to identify the minimum amount of exercise need to still show improvements in insulin sensitivity.