There are a number of theories circulating about when you should exercise to optimize the efficiency of your metabolism. Many theories suggest a pre-breakfast workout boosts the metabolism, while other theories suggest a little food in your system gives you top-notch results. Another study suggests an afternoon workout to pick up your metabolic rate. It may really all come down to your goals and lifestyle.
Pre-breakfast Workouts
In 2010, "The Journal of Physiology" published a study that concluded a pre-breakfast workout improved the effects of nutrients and hormones related to metabolism. This study compared groups of men who consumed high-fat, high-carbohydrate meals throughout the day and exercised either before breakfast, after breakfast or not at all. The group who exercised before breakfast saw improvements in their metabolism and lost weight, unlike the other two groups.
Time of Day Does Not Matter
In February 2011, a review was published in the "Strength and Conditioning Journal" that reached a different conclusion: Exercising after an overnight fast — before breakfast — results in increased utilization of protein, meaning you are burning through some of your valuable muscle tissue. The same article also argued that you actually burn the same amount of fat as if you had eaten because you are unable to train as hard, thanks to depleted energy. This study tested a group of cyclists, who either ate or did not eat prior to exercise, and then measured the amount of fat and protein burned during a workout.
When Afternoon Is Best
Yet another study, conducted by the Department of Exercise Physiology at IA University in Iran, also published in 2011, discovered that lean and obese women who exercised in the morning saw a significant increase in cortisol, a hormone notorious for slowing metabolism, and therefore, faired better with afternoon workouts. It is important to note that this study also concluded that cortisol secretions are very individualized and highly correlated with one's personal circadian rhythm and body weight. Lean subjects saw greater increases in cortisol in the morning than obese subjects.
Maximizing Your Metabolism
All of these conflicting studies can make it difficult to know when you should exercise to boost your metabolism. It may be helpful to examine your lifestyle and dietary intake to determine which concept works best for you. The first study, published in 2010, fed the subjects very high fat diets. Therefore, in terms of someone who consumes a very high-fat diet on a consistent basis, a pre-breakfast workout may improve glucose-tolerance and insulin sensitivity. On the other hand, the February 2011 study in "Strength and Conditioning Journal" suggests that if you are a more physically fit individual, you could likely benefit from consuming a moderate meal prior to your workout, regardless of the time of day. Finally, the last study implies that your weight and familiarity with your circadian rhythym — determining if you are an early bird or night owl — can help you gauge the best times for your workout according to cortisol secretions, which are elevated when stressed. If you are a night owl, it may be more stressful to workout in the morning, thus increasing cortisol secretion.
References
- "The New York Times"; Really? The Claim: Exercising on an Empty Stomach Burns More Fat; Anahad O'Connor; June 2011
- "The New York Times"; Phys Ed: The Benefits of Exercising before Breakfast; Gretchen Reynolds; December 2010
- "Bratislavske Lekarske Listy"; Daily Timing of Salivary Cortisol Responses and Aerobic Performance in Lean and Obese Active Females; M.A. Azarbayjani, et al.; 2011
- "The Journal of Physiology"; Training in the Fasted State Improves Glucose Tolerance during Fat-rich Diet; K. Van Proeyen, et al.; November 2010
- "Strength and Conditioning Journal"; Does Cardio After an Overnight Fast Maximize Fat Loss?; Brad Schoenfeld, M.S., CSCS; February 2011
