In 2019, researchers evaluated data from 8,721 adults and found that just one in eight people (or around 12% of the population) had optimal metabolic health. (“Optimal metabolic health” was defined by five distinct markers: ideal levels of blood sugar, triglycerides, high-density lipoprotein (HDL) cholesterol, blood pressure, and waist circumference, without medications.) Contrary to popular opinion, the research indicated that being in a normal weight range didn’t guarantee metabolic health. One thing that did make a difference, however, was consistent, vigorous physical activity. But is all exercise created equal? Does it matter — as far as your metabolic health is concerned — what types of exercises you incorporate into your routine and whether you do them in the morning, afternoon, or evening?
How exercise affects metabolic health
Metabolism is the set of chemical reactions in the body that are involved in transforming what you eat and drink into energy. If you think of the carbohydrate glucose — the body’s preferred energy source — as a monthly paycheck, metabolism is akin to the ways in which your body transforms that paycheck into a currency that it can use to “keep the lights on,” so to speak.
Without a healthy metabolism, the body cannot properly digest and absorb nutrients from food in order to create energy. In turn, this can increase your risk of developing health conditions such as heart disease and kidney disease.
Thankfully, you can reduce the risk of contracting these diseases through lifestyle choices, such as proper nutrition, sleep, and exercise.
After measuring approximately 200 metabolites (which are substances produced during metabolism or involved in metabolism) in the blood of 52 soldiers before and after an 80-day aerobic and strength exercise program, researchers found that the effects of exercise on metabolism were even higher than originally reported, with significant changes to metabolites involved in energy usage by muscle, blood flow, blood clotting, and more, with “positive effects on blood pressure, heart rate, fitness, body fat, and body weight.”
Another study found that six weeks of high-intensity interval training increased levels of Dicer (an enzyme that plays a significant role in combatting pathogens and preserving cellular homeostasis, or balance) in both older and younger adults. It also improved glucose utilization, increased insulin sensitivity, and accelerated lipid turnover (where fats are removed and stored from fat cells) at the whole-body level, which was associated with improved metabolic flexibility and increased physical performance.
The best time to exercise for optimal metabolic health
Because exercise is so vital for metabolic health, it makes sense that we would want to know not only how to exercise but when to do it. The idea of a “best” time to exercise is connected to our understanding that the body’s circadian system (or internal clock) controls many biological processes ranging from body temperature to hormone levels. Because changes in sleep or eating habits impact our circadian rhythm (increasing the risk of obesity, blood sugar changes, and more), researchers have started to explore whether exercise timing also has a similar effect.
The question of the “best” time to eat or exercise is a complex, nuanced question without a perfect answer. Here’s what current research says about the best time of day to exercise for metabolic health.
The case for morning workouts
Exercise can be an important part of a healthy morning routine. One study in Nature that examined overweight, physically inactive young adults in a 10-month supervised exercise program found that people who participated in multiple moderate to vigorous morning aerobic exercise sessions lost more weight than those who exercised in the evenings. (This is echoed in another smaller study from 2015, where researchers found that exercising before breakfast resulted in increased 24-hour fat oxidation, compared to exercising after lunch or dinner.) In a separate study published in Frontiers in Physiology, “total cholesterol, systolic and diastolic blood pressure, aortic and brachial augmentation index decreased significantly in both AM and PM exercise groups” among the women participants, but only those in the morning exercise group experienced significantly lower resting metabolic rate.
If your goal is to lose weight or improve focus and memory, working out in the morning might be the best fit for you.
The case for afternoon workouts
A 2018 paper in Current Biology found that our bodies naturally burn around 10% more calories in the late afternoon than in the early morning or late at night. Since obesity is one factor that decreases optimal metabolic health, exercising at a time when you can burn the most calories may help set you up for success. Another study of men at risk for Type 2 Diabetes found that afternoon exercise led to greater improvements in skeletal muscle and adipose tissue insulin sensitivity (which is a marker of metabolic health — adults with obesity and certain kinds of diabetes have adipose tissue that is insulin resistant). Exercising in the afternoon doesn’t have to mean heading to the gym or going on a run — research shows that even a simple 30-minute walk immediately after a meal can help reduce your blood sugar spike.
The case for evening workouts
In a 2019 study on the impact of high-intensity interval exercises on sleep disruption and energy intake, 11 overweight, inactive men with no prior history of metabolic or sleep disorders were asked to exercise for 150 minutes or less each week. Researchers found that nighttime workouts can reduce levels of the hunger-stimulating hormone ghrelin, which may help with weight loss or management. And another randomized study found that evening exercise may be more prone to improving metabolic health than morning exercise; researchers determined that late-day exercise lowered cholesterol levels and improved patterns of molecules related to cardiovascular health.
Find an exercise (and time) that works best for you
Exercising is a little like learning a language: daily consistency — even for just 30 minutes — is far more effective than occasional cramming. And while it's inevitable that you'll take short breaks here and there, the greatest metabolic and overall well-being benefits of exercise happen with consistent, long-term practice.
Remember that there’s no one “right” form of exercise (though resistance training is a great option, and just one hour a week of resistance training is associated with a 17% reduced risk of developing metabolic syndrome).
Ultimately, however, the best exercise is the one that most resonates with you and works with your lifestyle. If you work early morning shifts, then an AM exercise routine might be off the table; if you don’t live close to a gym and aren’t ready to commit to buying weights, then going on brisk neighborhood walk or doing bodyweight exercises in your living room are still fantastic options.
Lastly, it’s important to take the time to figure out what your personal training goals are. Do you want to lose weight? Are you trying to build endurance, strength, or flexibility (or some combination of the three)? Refining your personal exercise routine is a long-term process that involves experimentation with exercise types, time of day, and even related factors such as nutrition, meal timing, sleep, etc. While you can use tools such as a CGM to help you make sense of your body’s responses to lifestyle changes, listening to your body and plenty of trial-and-error are fundamental ways to help you find an exercise routine you’ll stick to.
- If your goal is weight loss or improving focus/attention, current research suggests that morning aerobic exercise may help you lose more weight than similar evening exercise.
- Taking a 30-minute walk after a meal can help minimize your blood sugar response. If you don’t have 30 minutes, 10 or 15 is better than nothing.
- Working out in the evening may reduce levels of ghrelin, the hormone that causes hunger, and improve metabolic health more so than morning exercise. (Plus, as long as you’re not working out an hour before you plan on climbing into bed, research indicates that evening exercise doesn’t actually impact your sleep.)
- Ultimately, the best exercise is the one you’ll stick to.