When is the Best Time to Exercise?

Does working out on an empty stomach burn more body fat?

Monica Reinagel, M.S.,L.D./N,
May 27, 2009
Episode #045

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When is the Best Time to Exercise?

Scott writes:

I've heard that the best time to exercise is in the morning before you eat breakfast because your body is still fasting from not eating overnight which causes you to burn more calories. Is this true?

Well, Scott, how many calories you burn during exercise depends on how hard and long you exercise, and not how long it’s been since you’ve eaten. But there is some research showing that if you exercise after a fast (such as first thing in the morning) you will burn more body fat than if you did the same exercise later in the day.

Why? Your body stores energy (or calories) in a variety of formats and places in your body. You store a little bit in your blood, a little bit in your muscles, some in your liver, and the rest you store as body fat.

Fat Bank Accounts

It’s a little like storing your money in a number of places. You probably have some in your wallet, possibly some more in your dresser drawer, some in a checking account, and maybe the rest is in a money market account. Which account you withdraw funds from will probably depend on how much you need and how fast you need it.

Similarly, what type of fuel you burn during exercise depends in part on the balances available in your various fuel storage accounts. In the morning, your blood, muscle, and liver “accounts” are going to be at their lowest, because your body will have been drawing on them to pay for metabolic activities during the ten or twelve hours since you had your last paycheck, I mean, meal.

So, if you exercise first thing in the morning, before you’ve refilled those energy accounts by eating some breakfast, you’re going to have to tap into those fat stores that much sooner. The question is, does this really change how much body fat you will lose in the long run?

A Simple Matter of Cash Flow

Just like with your money, regardless of which account you withdraw from, you’re still spending the same amount. If you spend more than you deposit, your net worth goes down. And when you burn more calories than you take in, you’re going to lose body fat.

Exercising on an empty stomach may increase the amount of body fat you burn during exercise. But your body alternately makes and burns body fat all day long, transferring fuel in and out of its various accounts. So, you might burn a bit more fat while you’re exercising on an empty stomach but then burn a bit less fat later in the day. Over the long term, the amount of fat you have in your body depends mostly on how many calories you take in versus how many calories you burn.