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What’s The Best Time of Day to Work Out for Fat Burn?

Morning workouts are best, but they may come at a price.


Ask any personal trainer or exercise enthusiast, and she’ll tell you that the best time of day to exercise is the morning. You get your workout over early — before the day’s schedule makes it impossible. You also start the day with a burst of endorphins and a drop in stress, putting you in a good mood. But when it comes to fat burn, is an early session really the best option? Recent research suggests yes — but there are caveats. Learn about the science and how to time your exercise routine below.

Understanding the Study

In an animal study published in the journal PNAS in February, researchers wanted to know how exercise at certain times of day affects fat burn. So, they conducted a series of tests on mice. (Why mice? It’s unethical to perform certain tests on humans, and mice work well for studies because they are genetically and physiologically similar to humans.)

The study authors had two groups of mice perform high-intensity exercise. Group 1 exercised during an “early active” phase” and group 2 exercised during an “early rest” phase. (The early active phase corresponded to late morning in humans, and the early rest phase corresponded to late evening.) After each exercise session, the researchers collected samples of the mice’s fat tissues for testing.

The Results

The researchers learned that exercise in the “late morning” phase activated genes that do three things: 1) help break down fat tissue; 2) increase heat production; 3) “turn on” mitochondria in fat cells. (Mitochondria are structures inside of cells that turn sugar-based molecules into energy.) All three of these factors indicate that exercise early in the circadian rhythm boosts fat metabolism. In contrast, the group 2 mice that exercised in the “late evening” did not experience an activation of those genes or an increase in fat metabolism.

Would food intake alter the results? The study authors ran another test. During this experiment, a third group of mice (group 3) fasted for 10 hours before exercising in the “late evening.” This was to see whether a 10-hour fast causes just as much fat burn as exercising in the “late morning.” Tests on fat tissue showed that the group 3 mice (the 10-hour fasted, “late evening” group) burned a similar amount of fat compared to group 1 (the “late morning” mice). However, the gene activations that occurred in group 1 didn’t happen in group 3. This suggests that certain genes that aid in fat burn are time-specific — and they are more likely to turn on when a mouse is in the “early morning” phase.

What the Research Means for Humans

“Our results suggest that late morning exercise could be more effective than late evening exercise in terms of boosting the metabolism and the burning of fat. And if this is the case, they could prove of value to people who are overweight,” said Professor Juleen R. Zierath, study author, in a press release.

Indeed, researchers outside this study agree that early-morning exercise is best for fat loss. “In the early morning hours, you have a hormonal profile that would predispose you to better metabolism of fat,” Anthony Hackney, a professor in the department of exercise and sport science at the University of North Carolina Chapel Hill, told Time. In fact, the morning is when you have elevated levels of cortisol and growth hormone, which are both involved in metabolism.

Working Out on an Empty Stomach — A Bad Idea?

Time also noted that a morning workout on an empty stomach is the most ideal for fat burn, because it forces your body to use stored fuel. However, experts at the University of Guelph in Ontario, Canada argue that the cons outweigh the pros. It’s true that cortisol levels are higher in the morning, and that cortisol encourages the body to use its fat tissues as fuel. However, this hormone also encourages the body to use its protein stores. In other words, it promotes the burning of muscle tissue to fuel your exercise.

Also, the body releases cortisol to protect carbohydrate stores in certain organs, like the liver, when those stores get too low. This conservation of fuel can actually suppress the metabolism, which means you may have less energy during your workout. Plus, you may burn fewer calories during the day as your body works to restore the energy it spent burning fat and protein stores.

What You Should Do

So, what should your game plan be? Morning exercise is the best option for fat loss, but it’s not easy to eat right before a workout — especially if you have no appetite or your stomach feels unsettled when you wake up. Food sloshing in your belly as you exercise doesn’t sound pleasant, either. Still, the University of Guelph researchers strongly encourage people to eat something small before an early workout. Here are a few early-morning snacks they recommend:

  • Banana with one cup of milk, soy milk, or oat milk
  • Fruit and ½ cup of yogurt
  • Slice of toast with nut butter
  • Small bowl of cereal and yogurt
  • Small smoothie with banana, berries, milk, and yogurt

If you don’t think you can stomach any of those options, try something more plain, like whole grain toast, apple sauce, one banana, or a handful of protein-enriched cereal. Then, take your time doing a gentle warm-up, and give your stomach time to settle before you get cracking. Once you start forming the habit — an early rise, quick snack, and a workout — you’ll find it easier and easier to stay on track.

Don’t know where to start? Try Denise Austin’s five-move workout for toned legs and the four-move kitchen workout.

This content is not a substitute for professional medical advice or diagnosis. Always consult your physician before pursuing any treatment plan.

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