The Best Time to Exercise for Weight Loss

Did you know that you can burn off significantly more body fat exercising before meals, rather than after them. Here’s our first story.

What is the optimal exercise timing for weight loss? Is it better to exercise in the morning or the evening? Before breakfast or after breakfast? There was a Nobel Prize-winning exercise physiologist who said he always ran a mile every morning before breakfast. Was he right? Let’s find out.

More than a dozen experiments have been published comparing the amount of fat burned in a fasted versus fed state, and every single one found more fat was burned on an empty stomach. On average, a single bout of low-to-moderate intensity activity before a meal burned off three grams more fat than the same amount of exercise after a meal. That’s about three-quarters of a pat of butter’s worth of fat––enough to improve insulin sensitivity. The same amount of exercise, but more fat loss, all because of timing.

Now, just because you burn more fat while you’re exercising doesn’t necessarily mean you end up with less fat at the end of the day. Maybe your body offsets the extra fat loss that occurs during exercise with a little extra fat storage when you finally do eat, balancing it out. Researchers in Japan set out to investigate the possibility that your body makes up for it later by measuring 24-hour fat balance after 100 minutes of running, either before breakfast or after lunch. On the exercise-after-lunch day, they burned a total of 608 calories of fat over the course of that day. In contrast, on the exercise-before-breakfast days, in the same 24-hour period, they burned through nearly 90 percent more, 1,142 calories of straight fat. So, the next day, they woke up with about a quarter-cup of fat less after the same amount of exercise. That’s remarkable!

What about just something like walking? Sixty minutes before breakfast, after lunch, or after dinner. Over the 24 hours they exercised in the evening, 432 calories were burned off. On the afternoon exercise day, they burned off 446 calories of fat. They also had a control day with no exercise at all, and on that day, they burned through 456 fat calories. That’s disappointing—it’s like they never walked at all. But the same amount of exercise before breakfast resulted in 717 calories of fat loss. Over the course of a day, timing matters, so much so that when it comes to an hour of walking, exercise increases 24-hour fat burning only when it is performed before breakfast.

All such similar studies on both men and women show we burn through more fat on the days we exercise before, rather than after eating. After reading the chronobiology chapter in How Not to Diet, though, or watching my chronobiology videos, an alternative explanation may spring to mind. Maybe it’s just a morning thing. Maybe it has nothing to do with meals, and your circadian rhythm is just dictating the difference? No. Exercising in the morning after breakfast appears no better than exercising in the evening after dinner, and exercising before breakfast works better than immediately after breakfast––both still in the morning. It really does seem to be a pre- versus post-meal effect. But why?

Carbohydrate is the preferred fuel of the body. When you eat sugars or starches, they get broken down and converted into blood sugar. After a meal, blood sugars rise and your muscles are quick to snatch it up for fuel without having to rely much on your energy stores. If you instead take a siesta, and your muscles have no immediate need for energy, the excess blood sugar from a meal can be stored in our muscles in the form of glycogen for later use. Glycogen is just a bunch of blood sugar molecules strung together into a mass of branches that can be broken off and used for quick bursts of energy any time we need them.

If you exercise after a meal, your muscles can siphon off some of the extra blood sugar floating around for energy. Before a meal, your muscles have to instead resort to dipping into your energy stores and end up burning mostly a combination of glycogen and fat. That explains why you burn more fat during fasted exercise, but what about all the extra fat burned throughout the rest of the day?

Glycogen is more than a store. Glycogen isn’t just an energy reserve, but acts as a sensor capable of activating metabolic pathways. Exercising before breakfast can exhaust as much as 18 percent of your glycogen stores, and that depletion can act as a powerful rallying cry to your fatty tissues to start pulling more of their own weight by breaking down more fat. The lower glycogen stores fall, the greater the sustained 24-hour fat loss.

How long do you have to go without food in order to trigger this effect? Six hours may be sufficient; so, it doesn’t have to be before breakfast. If you timed it right, you could exercise before a late lunch, or if you have an early enough lunch, maybe before dinner when you get home from work. If exercise in a fasted state isn’t possible, does it matter what you eat? Insulin release after a meal appears to play a critical role in suppressing fat breakdown, explaining why lower glycemic foods can have less of an effect. Lentils were identified as a promising option for maintaining athletic endurance (which can take a hit on an empty stomach) while maintaining more of the fat dissolution. Lentils are said to be “unlikely to be consumed by the general population…due to low palatability.” They obviously haven’t tried my mom’s lentil soup.

It’s worth mentioning that although there’s increased 24-hour fat burning exercising in a fasted state, the only way to see if this facilitates weight loss over time is to put it to the test, which we’ll explore next.

A systematic review and meta-analysis on exercise timing for fat metabolism found that exercising in a completely fasted state may work best. The Japanese team that published some of the seminal work in this area went as far as to assert: “If exercise were a pill to burn body fat, it would be effective only when taken before breakfast.” Surveys show few people exercise before breakfast, though. Before asking people to make the switch, we need to make sure that these tantalizing 24-hour results translate into weight loss over the long term. There’s a solid theoretical basis, but you don’t know until you put it to the test.

In a study of experimental weight gain, volunteers were fed up to 4,500 calories a day for six weeks while vigorously exercising a total of 300 minutes a week, either always after an overnight fast or after a meal. A control group who didn’t exercise at all gained about 6.5 pounds, compared to 3 pounds in the exercise-after-a-meal group. The pre-meal exercise group worked out the same amount, but only gained about half as much—1 3/4 pounds. What about weight loss, though?

Twenty young women were randomized into three hours a week of before or after a meal exercise. Same diets, same amount of exercise, and, disappointingly, about same amount of weight loss. The pre-meal exercise group did lose about an extra pound of body fat (total weight loss 3.5 lbs. vs. 2.2 lbs.), but this did not reach statistical significance, meaning such a small difference could very well have been due to chance. A study of six weeks of low volume, high intensity interval training before or after meals similarly failed to show a difference.

One explanation that’s been offered for this failure is that the increased fat loss during pre-meal exercise might be “neutralized” by the lesser diet-induced thermogenesis. In other words, because it costs our body fewer calories to process food if we eat after, compared to before, physical activity. When we exercise after a meal, our body gets mixed signals. Exercise is all about mobilizing energy stores for fuel, whereas eating is more about assimilation and storage, and the metabolic challenge presented by the ensuing “hormonal tug-of-war” might be responsible for the 15 to 40 percent greater calorie cost. This has led some to recommend exercising after meals to facilitate weight loss. If you do the math, though, our body is so efficient at digesting that the 15 percent to 40 percent increase might only come out to be 3 to 12 calories. Such a slight difference would be easily overwhelmed by the huge disparity in fat loss, as confirmed by the 24-hour fat-balance studies, showing up to 500 calories of fat-burning difference.

I would suggest a more reasonable explanation might be that the clear body fat deficit on pre-meal exercise days is made up for by extra fat storage on non-exercise days. Your body likes to hold on to extra body fat if it can; and so, on days you’re not driving it down, it may try to even things out. Both of the failed weight loss studies had people exercising only three days a week, and so, their bodies had most of the week to compensate. The study I’d like to see is pre-meal vs. post-meal exercise on all or at least most days of the week, to see if we can continue to drive down fat stores.

For those with diabetes, though, you’d want to do the opposite. You can imagine how that siphoning effect muscles have on excess blood sugar during exercise might be great for those suffering from elevated blood sugars. And indeed, exercising after a meal can bring down blood sugars as well as some blood sugar-lowering drugs. Randomize type 2 diabetics to a 20-minute leisurely stroll (about 2 mph) before dinner versus after dinner, and the after-dinner group blunted their blood sugar spike 30 percent. Same meal, same amount of exercise, but a significant effect on blood sugar control thanks to a little tactical timing. Even just a 10-minute walk after a meal may make a difference. So, for those with blood sugar problems, it’s better to exercise after meals than before them.

Blood sugar from a meal starts appearing in the bloodstream 15 to 20 minutes after the first bite and is ramping up by 30 minutes to peak at around the one-hour mark before declining to pre-meal levels within a few hours. So, for optimal blood sugar control, prediabetics and diabetics should start exercising 30 minutes after the start of a meal, and ideally go for an hour to completely straddle the blood sugar peak. If you had to choose a single meal to exercise after, it would be dinner, due to the circadian rhythm of blood sugar control that wanes throughout the day. Ideally, then, breakfast would be the largest meal of the day, and you’d exercise after that, or exercise after every meal.