Does It Matter When You Exercise?
How the time of day you exercise affects blood glucose
You’ve probably heard that exercise aids diabetes management. It’s true: Moderate aerobic exercise—the kind recommended 30 minutes a day, five days a week—can help you lower your blood glucose.
And exercise has benefits beyond blood glucose control, says Michael Fowler, MD, assistant professor of medicine and director of diabetes outreach at Vanderbilt University Medical Center. “[Regular exercise is] highly, highly, highly beneficial in folks with type 2 diabetes,” Fowler says. It lowers risks of cardiovascular events in people with type 2 diabetes. The cardiovascular protection is less well proven in people with type 1 diabetes. “But we know that exercise, for everybody, makes our bodies more sensitive to insulin,” he says. That includes any insulin the body makes or that a person injects. Greater sensitivity to insulin helps the body use it effectively to lower blood glucose levels.
You may wonder when to fit workouts in and whether the time of day you exercise makes a difference in blood glucose levels. Let’s take a closer look.
Around the Clock
Whenever you can move your body—a 20-minute walk in the morning or playing catch at night—it’s good for you and your diabetes, says Sheri Colberg-Ochs, PhD, a fellow with the American College of Sports Medicine. “The best time is when it fits in your schedule,” she says.
Of course, during and after exercise with diabetes, some people face the risk of blood glucose going too low. That’s more likely to happen if you use insulin and sulfonylureas, which can put you at greater risk for lows overall.
One thing that may help you manage blood glucose during and after exercise is to set a regular time for it. A consistent exercise routine, says Fowler, can help you learn more about how exercise affects you. “What we typically recommend is that somebody does exercise at the same time—that way things are more predictable. If it’s going to cause low blood sugar, they learn how to predict it or adjust to accommodate [that].”
So let’s rock around the clock to see what you might expect with regard to your blood glucose and exercise, keeping in mind, as always, that your response may vary—from other people’s and from day to day.
Some people like to rise and shine. And this can be a good time to exercise to combat what—for some—are the day’s highest blood glucose levels. During sleep, the body typically maintains blood glucose levels by making hormones that tell your liver to release glucose. The body with diabetes often needs help from insulin or other diabetes medications to make sure the liver doesn’t produce too much glucose, which can result in high blood glucose overnight and on awakening in the morning. Plus, the dawn phenomenon—thought to be the result of a normal surge of hormones that the body produces to kick-start the day—may also raise blood glucose levels.
Non-Insulin Users: Teresa Anderson, 45, of Myersville, Maryland, fits fitness into her schedule by working out first thing in the morning. “It’s what I can stick with. That is the key,” says Anderson, who has type 2 diabetes.
Insulin and Sulfonylurea Users: Talk to your doctor about what to eat before and after morning exercise and how to adjust insulin dosing and food to avoid lows.
Working out before a meal can be a great way to fit in some exercise. And while you may not see as much of a drop in blood glucose as at other times of day, it’ll still do your body good.
Non-Insulin Users: If you don’t take insulin, you probably need no extra preparation when exercising before a meal, says Colberg-Ochs. “The main effect of working out pre-meal—other than breakfast—may be a slightly lower rise in blood glucose after eating,” she says.
Insulin and Sulfonylurea Users: If you frequently go low while exercising, it may help to exercise before meals instead of after meals, when bolus insulin or pills that cause the body to secrete insulin are the most active. Fast-acting insulin injected or pumped for your previous meal takes about four hours to be cleared from the body.
Working out increases your body’s insulin sensitivity, making it easier for insulin to transport glucose to the cells that will use it. So working out after a meal can be a good way to “use up” excess glucose after eating.
Non-Insulin Users: “I test after I exercise. I noticed that my numbers were definitely lower [after working out],” says Jacquelyn Weller, 37, of Reed City, Michigan, whose after-dinner exercise helps control her type 2 diabetes. “When I saw the numbers, I kind of celebrated a little bit. It really pushed me to make sure that I keep exercising so that I keep seeing those numbers.”
Insulin and Sulfonylurea Users: Do exercise safely: Carefully monitor your blood glucose during and after post-meal physical activity to avoid surprise lows. “Generally, if you’re really worried about hypoglycemia, avoid exercising within two hours of a meal,” Colberg-Ochs says. This tends to be the time that injected or pumped mealtime insulin is most plentiful and active.
Any exercise, no matter the time of day, is a good idea. Be aware, however, that working out at night can interfere with your sleep—which can affect your diabetes. The Division of Sleep Medicine at Harvard Medical School reports that the body secretes the stress hormone cortisol during exercise. That activates your brain’s “alerting system.” If you want a good night’s rest, make sure to finish your workout at least three hours before bed.
Non-Insulin Users: Working out at night should not increase hypoglycemia risk for non-insulin users, says Colberg-Ochs.
Insulin and Sulfonylurea Users: Exercising too close to bedtime can be risky. When you’re asleep, you may not feel a low coming on. So, if you’re an insulin user who wants to work out at night, you’ll want to carefully monitor blood glucose before, during, and after exercise and before bedtime. A snack before you go to sleep may be a good idea. You’ll also want to check blood glucose in the middle of your sleep time (say, 3 a.m.), at least a few times while developing a regimen to be sure you’re not going too low. Talk to your doctor about the best hypoglycemia prevention approaches for you and your routine.
The bottom line? There’s no best time of day when it comes to exercise. Pick a time that works for you and just do it. As Cynthia Zuber, 39, of Minneapolis, has found, consistency is key. She prefers to walk after meals, but the timing is less important than doing it nearly every day. “For me, a walk is the perfect balm,” says Zuber, who has type 1 diabetes. “If I walk three miles a day, diabetes is so much easier to manage. I always try to get that movement in.”
Different types of activities will affect your blood glucose in a variety of ways. We’ve already talked about aerobic exercise, but anaerobic exercise, or strength training, can increase your adrenaline, and that can raise your blood glucose. The same goes for interval training, including workouts such as CrossFit, and activities with short bursts of intense activity, such as basketball or hockey, says Jen Hanson, RKin, MEd, a registered kinesiologist and executive director of Connected in Motion, a Toronto nonprofit designed to get people with type 1 diabetes involved in adventurous exercise. Hanson, 31, lives with type 1 diabetes.
“When you’re doing activities that are anaerobic, with short bursts of energy, the body is releasing large amounts of adrenaline,” Hanson says. “That signals to your [liver] that it needs to dump large amounts of sugar into your bloodstream. Often then you’ll see spikes in your blood sugar.” This is because anaerobic exercise prompts your liver to produce glucagon, a hormone that raises blood glucose levels. When your body is being pushed to its limit—not with gentle or moderate exercise, but real heart-pumping, high-intensity stuff—its “fight or flight” reflex kicks in. “Your body doesn’t want you to go low,” Colberg-Ochs says. So instead, it signals your liver to kick out lots of glucose so you have energy.
“Usually, the spikes in blood glucose following high-intensity work come down on their own within a couple hours in non-insulin users,” Colberg-Ochs says. “It can help to follow such activity with a lower-intensity cooldown.” Insulin users, she says, should talk to their health care providers about whether or not to take insulin to correct post-exercise highs; if you take too much, you’re likely to come crashing down later.
Talk to your doctor before making any big change in your exercise plan.