Carbohydrate Loading: Effective If Done Right for Even a Day


What better topic is there to discuss after the gluttony most of us experience over the Thanksgiving and other fall/winter holidays than carbohydrate loading? (Actually, it probably should be excess calorie consumption in general, but you get the idea.) The following is excerpted from The Athlete’s Guide to Diabetes (2019) and gives you a better understanding of the topic from an exercise physiology (and diabetes) point of view.

Most athletes can benefit from taking in enough carbohydrate before long-distance events to start exercising with fully restored or even supercompensated glycogen stores. Traditionally, this loading technique consisted of 3 to 7 days of a high-carbohydrate diet combined with 1 or 2 days of rest or a reduction in exercise volume, a method known as tapering. For endurance athletes, loading is recommended to consist of taking in 8 to 10 grams of carbohydrate per kilogram of body weight (e.g., 560 to 700 grams for someone who weighs 70 kg, or 154 pounds)—but that is admittedly a lot of carbohydrate to handle if you have to match it with insulin or are very resistant, and it is not necessary.

Even a single day with enough carbohydrate and food intake and rest or tapering can effectively maximize your carbohydrate stores, so you do not need to spend a week, or even 3 days, overconsuming it. Maximal glycogen storage is dictated by how much muscle mass you have, but it is typically around 300 to 400 grams total in all your skeletal muscle, along with 75 to 100 grams of liver glycogen, for the average person. As long as you consume enough calories and taper or rest for a day, taking in up to 40 percent of your calories as carbohydrates is more than adequate to fully reload your glycogen. For someone consuming 2,000 calories and resting on a pre-event day, that amounts to around 200 grams of carbohydrate—more than enough if you are not starting out fully depleted. It’s also likely that you can fully restore your glycogen on far less carbohydrate, especially if you have been following a low-carbohydrate diet and are fully fat-adapted.

Training Tip: To maximize your glycogen stores, all you really need is 1 day and a combination of rest, enough calories in your diet, and good blood glucose levels for that day. You do not need to do traditional carbohydrate loading to make this happen.

The key for carbohydrate loading to be effective for exercisers with diabetes is to ensure that your muscles can take up any available glucose, which only happens if you have sufficient levels of insulin and enough sensitivity to it to prevent hyperglycemia and promote glucose uptake. Consuming higher-fiber carbohydrate sources and those with a lower glycemic effect will help prevent an excessive rise in your blood glucose and be effective for loading. In fact, a study showed that participants actually end up with higher glycogen stores when they maintain more normal blood glucose levels while loading with less carbohydrate (50 percent of calories from carbohydrate instead of 59 percent in that study). To optimize your liver glycogen replacement, keeping your blood glucose as close to normal as possible is also most effective.

From Colberg, Sheri R., Chapter 4, “Eating Right and Supplementing for Activity” in The Athlete’s Guide to Diabetes: Expert Advice for 165 Sports and Activities. Champaign, IL: Human Kinetics, 2019.

What Is the Best Time of Day to Exercise? The Answer Is…

woman girl silhouette jogger

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I often get asked, “What is the best time of day to exercise?” Like most things related to physical activity—especially with diabetes as an added variable—the answer often is, “It depends.” What is your goal for being active? Are you trying to better balance your blood glucose, or is weight loss your goal? Do you take insulin? What is your normal diet? How much time do you have? Which activities? There are so many questions that likely need answering before you may be able to ascertain the best time for you personally to be active.

A recent article on CNN (1) proclaimed, ”Exercising before breakfast burns more fat, study says.” That article starts out by asking, “Should you eat before or after exercise in the morning?” It then states, “In 30 obese or overweight men, those who exercised before breakfast burned twice the fat as men who ate breakfast before they worked out.”

Before even reading the CNN article or looking up the actual research it quoted, I already had issues with their approach. You have to understand that the fuel your body uses during exercise matters less when it comes to weight loss than the total number of calories you use. Most calories burned during moderate or harder exercise come from carbohydrate sources simply due to the body’s more efficient use of that fuel compared to fat (or protein) (2). Your muscles use fat when forced to or when activity is light—but using more fat during exercise doesn’t mean you lose more body fat, which is how the press always interprets it. What’s more, fat is the primary fuel your body uses during recovery from exercise, which lasts far longer each day than your workouts, no matter what time of day you exercise.

Delving into the research itself (3), they actually found that active muscles did use more intramuscular fat in these subjects during prebreakfast training—but after breakfast blood glucose spikes were unchanged after six weeks of moderate cycle training regardless of which time they trained. They released less insulin for breakfast eaten after training, but that is hardly a surprising finding given than eating a meal makes most people (who can) release insulin. Burning more fat during prebreakfast exercise also did not cause those men to lose more weight than those exercising afterward (again, this is unsurprising).

For anyone with diabetes, fasted exercise tends to prevent the typical drop in blood glucose during moderate aerobic exercise—due to higher levels of cortisol and other glucose-raising hormones released when fasting—but many people with diabetes find that exercising in the morning causes their blood glucose to rise, which can also be an issue. I know many people with type 1 diabetes who love to exercise before breakfast, but I personally hate exercising then because I am more insulin resistant for hours afterward and battle to lower my blood glucose. Research agrees with me and has shown this to be especially true for resistance or high-intensity exercise done in the morning in adults with type 1 or type 2 diabetes (4,5).

In all honesty, the best answer when asked what is the best time of day to exercise is: “When you have the time!” In other words, exercise whenever it fits best into your daily life—because no matter when you are active, it’s always better to do something than nothing at all when it comes to your overall health and long-term blood glucose management. Just make sure to adjust your diabetes regimen as needed to keep your blood glucose in check.



  • LaMotte, S. Exercising before breakfast burns more fat, study says, October 18, 2019,
  • Kerksick CM, Arent S, Schoenfeld BJ, Stout JR, et al. International society of sports nutrition position stand: nutrient timing. J Int Soc Sports Nutr. 2017;14:33. doi: 10.1186/s12970-017-0189-4
  • Edinburgh RM, Bradley HE, Abdullah N-F, Robinson SL, et al. Lipid metabolism links nutrient-exercise timing to insulin sensitivity in men classified as overweight or obese. J Clin Endocrinol Metab, 2019 Oct 19. doi: 10.1210/clinem/dgz104 [Epub ahead of print]
  • Toghi-Eshghi SR, Yardley JE. Morning (Fasting) vs Afternoon Resistance Exercise in Individuals With Type 1 Diabetes: A Randomized Crossover Study. J Clin Endocrinol Metab. 2019;104(11):5217-5224. doi: 10.1210/jc.2018-02384.
  • Savikj M, Gabriel BM, Alm PS, Smith J, et al. Afternoon exercise is more efficacious than morning exercise at improving blood glucose levels in individuals with type 2 diabetes: a randomised crossover trial. Diabetologia. 2019;62(2):233-237. doi: 10.1007/s00125-018-4767-z.

What Causes Blood Glucose to Go Down or Up During Exercise

Figure 1 Diabetes Motion Color (rev)

For all the time that I spend praising the “miracle” of being physically active to help better manage diabetes and health, there are times when exercising does lead to better manage blood glucose and times when it does not. It is not always possible to predict the glycemic outcomes in all cases either, although individual patterns and responses can be determined over time. It is helpful to know the main factors that are predictive of outcomes, however, as detailed below:

Exercise Generally Lowers Blood Glucose When:

  • Circulating levels of insulin are higher (such as after eating in those who make their own insulin and within 2-3 hours of the last bolus of mealtime or correction insulin in those who take insulin)
  • Prolonged and aerobic in nature (30 minutes or more when moderate in intensity, an hour or longer when easier)
  • Blood glucose levels are normal (or near normal) at start of activity
  • Muscle glycogen stores are insufficient (either to start or later during activity)
  • On a low-carbohydrate diet and not fully adapted to eating that way
  • Still recovering from recent prior physical activity
  • Done after a recent hypoglycemic episode (particularly if a more severe low)
  • Doing a new or unaccustomed physical activity (greater reliance on blood glucose)

Exercise Tends to Raise Blood Glucose When:

  • Active first thing in the morning when circulating insulin levels are low and cortisol levels are higher (before taking or releasing any insulin)
  • Short and intense (such as heavy weightlifting, sprinting, or high-intensity interval training)
  • Hyperglycemic, especially when ketones levels are also elevated (i.e., relative insulin deficiency)
  • Eating too much during physical activity (or a large amount right before starting)
  • Dehydrated to start or if get dehydrated while active
  • Exercising in environmental extremes (too hot or cold, high humidity, high altitude)
  • A cold, virus, or other type of infection lowers insulin action and raises physical stress
  • An exaggerated release of glucoregulatory hormones (epinephrine, norepinephrine, glucagon, cortisol, and/or growth hormone) occurs for any reason

Despite all the potential influences, the biggest overall impacts on glycemic responses arise from the timing of being active and the activity itself. Those two factors likely explain most of the variance, while the rest comes from people not being able to anticipate what insulin levels are likely to be during an activity and other unexpected environmental or bodily concerns.

Despite any aggravations associated with balancing blood glucose during physical activity, it is still worthwhile to be regularly active to gain all the physical and mental health benefits associated with it. To help establish patterns and trends, check blood glucose levels before, during, and after various activities and circumstances until it is as predictable as possible.

Stay Hydrated Without Overhydrating for Exercise

glass cup with raspberry inside and outside

Adequate fluid intake is essential to living well at any age, and being dehydrated can impact your health and your athletic endeavors. While it is harder to stay hydrated when exercising in the heat, you can dehydrate under other conditions—even during exercise in cold temperatures if you wear lots of clothing and sweat underneath it. As people grow older, they also begin to lose some of their normal thirst sensations, thereby increasing the risk for dehydration unless they make a conscious effort to drink more.

Diabetes adds its own dehydration concerns. Elevated blood glucose levels (typically when above ~200 mg/dL) lead to glucose loss through urine, which takes extra water with it and can cause dehydration. In addition, taking some of the newer medications like SGLT2-inhibitors that increase urine output whenever blood glucose rises above that level may also lead to excess water losses. If exercising when your blood glucose is higher or after it has been elevated, take care to drink enough fluids to rehydrate. If you are older or have been diagnosed with autonomic neuropathy (central nerve damage), take extra care as your body’s ability to regulate your body temperature through sweating may be impaired as well.

Hydration Tips for Exercise

  • Drink cool water or other fluids before, during, and after you are physically active, especially during warmer or more humid conditions.
  • If you prefer fluids with some flavor, try flavored waters, sports drinks that have no added carbohydrates or calories, and add a pinch of salt if you want it to taste and be more like a sports drink.
  • Only drink regular sports drinks (containing glucose) when you need some carbohydrate to prevent or treat hypoglycemia during activities.
  • Drink only when you feel thirsty and only enough to satisfy your thirst to avoid water intoxication.

Whether you should drink water, sports drinks, or other fluids during exercise when you have diabetes depends on your blood glucose levels. For shorter activities (lasting an hour or less), plain water should suffice for hydration. If you need some carbohydrate, you can supplement and hydrate by using a sports drink like Gatorade or PowerAde or diluted fruit juice. Normally you do not need to replace electrolytes, like sodium, potassium, and chloride, unless you are exercising outdoors in hot weather for more than two hours at a time. Even then, in most cases you can wait to replace electrolytes naturally with your food the next time you eat.

You can also harm yourself by drinking too much fluid at any time, but especially during exercise. If you drink too much (leading to clear urine), you increase your risk of diluting the sodium content of your blood, potentially causing hyponatremia, or water intoxication, and raising the risk of seizures, coma, and even death. To avoid overhydrating, only start drinking when you feel thirsty during exercise. If you have hyperglycemia or autonomic neuropathy, start drinking small amounts of water as soon as you start sweating.

During physical activity, you will be sweating and losing water in other ways (like through breathing), so your body weight should decrease until you have a chance to rehydrate. After exercise, you can rehydrate with water or other non-caloric fluids but replace only the weight you lost. If you already took in a lot of fluid during an activity, wait until you start to urinate before drinking any more.

To hydrate effectively after exercise, consider taking in fluids containing protein and fat that may rehydrate you more effectively than plain water. Eating a piece of fruit (or likely anything) when drinking plain water may also promote more effective rehydration. What’s more, drinking small amounts more frequently instead of large volumes at one time (that is, a slow and steady approach to hydrating) helps you better retain the fluid that your body needs.

Luckily for all the coffee drinkers, caffeinated drinks usually hydrate you as well as caffeine-free ones if they contain enough fluids (so avoid expresso). Taking in too much caffeine can cause your bones to lose calcium, so the decaf options may be better ones. Dehydration also contributes to constipation and taking in enough fluids can help you stay regular.

How Your Hormones Impact Physical Activity

Insulin injection

The human body only has insulin to lower blood glucose but has five hormones that raise it (with some overlap). This hormone redundancy tells you is that, at least from a survival standpoint, your body is desperate to make sure you do not run out of blood glucose; it is not as concerned about you having too much. Insulin is an important hormone for regulating your body’s storage of fuels (carbohydrate, fat, and protein) after you eat. It tells your insulin-sensitive cells (mainly your muscle and fat cells but also your liver) to take up glucose and fat to store them for later as muscle and liver glycogen (the storage form of glucose) as well as stored fat. During exercise, any insulin in your bloodstream can make your muscles take up extra blood glucose. In people who have a pancreas that functions normally, insulin levels typically decrease during exercise, and levels of a hormone called glucagon (released from the alpha cells of the pancreas) rise to stimulate glucose release (1).

Your blood glucose levels are managed by your liver, which would normally respond to the relative amounts of insulin and glucagon (see table that follows). Insulin and glucagon released from the pancreas go directly to the liver via the portal circulation: after a meal, high insulin and glucose levels tell the liver to store glucose for later use; fasting overnight or doing extended exercise leads to glucagon signaling the liver to release glucose. How people with type 1 diabetes respond hormonally to exercise is a major issue: the insulin and glucagon at the level of the liver are seldom perfectly normal because their insulin is injected or pumped under the skin rather than released directly from the pancreas. People with type 1 diabetes have an altered hormonal response to exercise when their peripheral insulin is relatively high; lowering the circulating level of insulin helps normalize their hormone response.

All exercise causes the release of hormones that increase the production of glucose by your liver and lower your muscular use, based on how long and hard you exercise. Easy and moderate activities only release a small amount of glucose-raising hormones (unless you do them for a very long duration), but intense exercise such as heavy resistance training, sprinting, or high-intensity intervals causes an immediate rise in your blood glucose and leads to an exaggerated release of hormones. These hormones include adrenaline (formally known as epinephrine) and norepinephrine, which are released by the sympathetic nervous system (allowing your body to respond to physical or mental stressors with an increased heart rate), as well as glucagon, growth hormone, and cortisol (also shown in table). The effects of these glucose-raising hormones can easily exceed your body’s immediate need for glucose, especially because high-intensity exercise may not last long. As a result, your blood glucose often rises during and after short bouts of intense activity. In fact, you should expect intense exercise potentially to cause a large increase in blood glucose because of your body’s exaggerated release of glucose-raising hormones such as adrenaline and glucagon. 

Hormones That Affect Blood Glucose During Exercise
Hormone Source Main Actions
Insulin Pancreas (beta cells) Promotes blood glucose uptake into muscle cells and adipose (fat) cells (the latter mainly during rest); stimulates liver uptake and storage of glucose; inhibits fat release from adipose
Amylin Pancreas (beta cells) Supplements action of insulin by slowing digestion and absorption of glucose from food; blocks glucagon release; promotes early satiety (fullness after eating); cosecreted with insulin from functional beta cells but absent in type 1 diabetes and in individuals with type 2 who produce a little of their own insulin
Glucagon Pancreas (alpha cells) Stimulates liver glycogen breakdown and new glucose production from precursors to increase blood glucose; is affected by changes in the insulin-to-glucagon ratio at the liver
Epinephrine (Adrenaline) Adrenal medulla Stimulates muscle and, to a lesser extent, liver, glycogen breakdown, and mobilizes free fatty acids from adipose cells
Norepinephrine Adrenal medulla, sympathetic nerve endings Stimulates liver to produce new glucose from available precursors; acts as “feed-forward” control of glucose during intense exercise along with epinephrine
Growth hormone Anterior pituitary Directly stimulates fat metabolism (release of free fatty acids from adipose) and indirectly suppresses glucose use; stimulates amino acid storage
Cortisol Adrenal cortex Mobilizes amino acids and glycerol as precursors for glucose production by the liver and releases free fatty acids for muscle use in the place of glucose (during fasting, starvation, and long-duration exercise)

You may be more insulin resistant immediately after intense exercise and for a few hours due to these hormones. In one study, after near-maximal cycling to exhaustion, one group of people with type 1 diabetes on insulin pumps experienced elevated blood glucose levels for nearly 2 hours. Similarly, in exercisers with type 2 diabetes, blood glucose also rose for 1 hour in response to maximal cycling, as did their insulin levels (because their bodies still produced their own insulin). You may need some supplemental insulin to bring your blood glucose back to normal (albeit less than normal), or it may drop slowly over time on its own. After these hormones wane, your blood glucose may easily drop later when your body is working hard to restore the muscle glycogen you used during the activity. Be on the lookout for later-onset lows in these cases.


  1. Excerpted from Colberg, SR, Chapter 2, “Balancing Exercise Blood Glucose” in The Athlete’s Guide to Diabetes: Expert Advice for 165 Sports and Activities. Champaign, IL: Human Kinetics, 2019.

Debunking Some Physical Activity and Training Myths

woman doing exercise inside gym

Photo by The Lazy Artist Gallery on

By Sheri Colberg, PhD

How often have you heard that things about physical activity and exercise training that you thought sounded correct, but found out later they were totally wrong? If you hang out at a gym or even talk with training coaches, you’ll hear about everything, including contradictory statements about how to be active the right way.

Should you work out in a “fat burning” range? Is weight training going to make you bulk up? Will your muscles turn to fat if you stop working out? Do you need to eat a lot more protein to get bigger muscles? Confused? Here is the truth about some of the more common myths you’ll hear about being active.

Myth: Exercising regularly makes you more tired.

Although you may feel somewhat tired during a workout, when you’re done you usually feel more invigorated after you recover, not less. Doing any regular physical activity is guaranteed to raise your overall energy levels and may you better able to handle everything you have to undertake. If you’re having trouble concentrating at work or getting too stressed, it helps to take a short walk or do any type of physical activity to clear your mind, bump up your energy levels, and decrease your mental stress. Doing regular physical activity also helps you sleep better at night, leaving you more refreshed and energetic during the day.

Myth: If you want to lose fat, you have to work out at a “fat burning” range.

Exactly what is “fat-burning” range you see on a lot of aerobic exercise machines? You have to understand what fuels your body uses during rest and exercise. Typically, during rest 60% of your energy needs are supplied by fat (stored or eaten), with the other 40% coming from carbohydrates. As soon as you start to do any type of physical activity, though, carbs go up to a much higher percentage of your total energy supply. In fact, when you’re doing just moderate aerobic exercise like brisk walking, you’ll use very little fat, so you’re burning mostly carbs even when you’re in a so-called “fat-burning” range. During more vigorous exercise, your body can’t use fat effectively, so almost all energy is supplied by carbs when you’re working out hard. You do use slightly more fat at a lower intensity, but most of its use is during your recovery from exercise, so just try to expend as many calories during exercise as possible without worrying about what types of fuels are supplying them.

Myth: When you don’t use your muscles, they turn into fat.

Have you ever found yourself looking at someone who used to be more fit and thinking that his or her muscles had really turned into flab? While there is no discounting how it looks, it is physically impossible for inactive muscles to turn into fat. What is really happening is this: when you work your muscles out regularly, they can increase in size or simply look more toned; if you stop using them, the muscle fibers will atrophy and disappear—similar to what happens with aging if you don’t fight against it. Then, as your muscle mass becomes less, your caloric needs decrease, and if you don’t start eating less, you’ll gain weight—as fat that then can be stored under your skin (among other places). The reverse is true as well. If you drop body fat, your muscles will look more defined simply because there is less fat in your skin covering them. The bottom line is that it is never good to lose muscle mass, but if you don’t gain fat weight at the same time you lose some muscle, you’ll look thinner, but not like fat replaced your muscles.

Myth: Weight training will bulk you up.

This myth probably arose because you can look bigger as your muscles are stimulated to expand out with heavy weight training. Women are especially worried about bulking up and getting bigger arms or legs. Remember how losing muscle can make you look thinner if you’re not gaining fat at the same time? Well, the same applies here, only in reverse. If you’re losing fat all over (including from under your skin) while you’re gaining muscle mass, you’ll stay about the same size. If you gain muscle without losing fat, you may look slightly bigger, or simply more toned. Either way, most people don’t gain enough muscle from weight training to ever look bulked up. More likely, you’ll just look more toned. When you first start exercising, your weight may go up slightly or just not come down as much as you think it should, simply because as you gain muscle while losing fat, the heavier of the two (muscle) will keep your scale weight higher. Focus less on your scale weight and more on your measurements and how well your clothes fit.

Myth: No pain, no gain.

If you’ve ever hung around a gym, you’re sure to have come across this myth. The “pain” part of exercise results from the build-up of acids in active muscles (like lactic acid), and acids drop the pH of your muscles and sensitize pain receptors. Usually, it’s just a sign that you’re working hard or that your muscle is fatiguing. However, you can certainly have gains in your strength and endurance without pushing yourself to the point of having a lot of pain in the process. The more fit you become, the more easily your body can clear out those excess acids produced by physical activity. Too much pain can also signal that you’re likely to get injured.

Myth: Lifting weights slowly builds larger muscles.

Remember how we just debunked the “no pain, no gain” myth? If you try lifting weights more slowly, you’ll certainly feel the pain, but it absolutely doesn’t mean that your muscle or strength gains will be more. On the contrary, lifting weights slowly when you could lift them faster will build more muscular endurance, while lifting the heaviest weight as quickly as possible will recruit extra muscle fibers and cause you to build bigger muscles. So, the rule of thumb should be that if you are lifting a weight slowly, but could lift it faster, you either need to move it faster or try a heavier weight for optimal results.

Myth: Working on your abdominal muscles will give you a flat belly.

You’ve probably always heard that if you want to get rid of that stomach flab that you have to do a lot of abdominal work, but don’t be fooled into believing that. As much as we’d all like to pick and choose where we lose our fat, it is not possible to spot reduce, and doing hundreds of crunches will not make you lose stomach fat any faster than you lose it from the rest of your body. If you want a flat belly, you can certainly work on toning up your abdominal region, but focus more on simply burning off excess calories. Doing harder workouts will also build more muscle, and having more muscle increases your daily caloric needs. One side benefit of including abdominal exercises, though, is that having toned abs makes it easier for you to pull in your stomach in case anyone is looking at it, even if you can’t spot reduce there.

Myth: The more exercise you do, the better off you’ll be.

There is a limited benefit to anything and that includes exercise that is excessive. When you do more than 60 to 90 minutes of aerobic exercise daily, you’re much more likely to develop overuse injuries—such as stress fractures, tendinitis, bursitis, and other joint issues. You don’t want to get injured because then you’ll have trouble working out. You are better off doing slightly more intense exercise for less long, which you can do with any type of interval training (including some of the latest crazes like HIT and CrossFit). You can push yourself a bit harder from time to time during a workout, or do the whole thing at a higher intensity if you can, while cutting back on the duration—and you will gain the same benefits, or even more, from your workout. Most of us don’t have time to work out all day anyway, so it’s good to know that we really don’t need to.

Myth: If you want to gain muscle mass, eat more protein.

Ah, yes, the protein myth. It is true that you have to eat some protein to gain protein (muscles are made of amino acids, the building blocks of protein). And, yes, physically active people do need more protein than sedentary ones, but not that much more. In fact, no training athlete needs more than 1.6 to 1.7 grams of protein per kilogram of body weight (~0.75 grams per pound), or just twice that of a sedentary person. Does that mean you need to take protein supplements or up the protein in your diet? Not usually. Most Americans already eat well over 15% of their calories as protein: about 75 grams of daily protein in a 2,000 calorie diet (or 112 grams per 3,000 calories), more than enough to cover protein needs. Taking in some protein (especially whey) with carbs right after hard workouts may be beneficial, but make sure your protein is coming from good sources without a lot of extra saturated or trans fats. Instead of spending money on supplements, try eating more egg whites or drinking chocolate milk post-exercise.

Myth: If you’re not sweating, you’re not working hard enough.

Everyone equates sweating with working hard, but that simply isn’t always the case. People vary in their sweating rates. Being physically trained improves your ability to sweat more and to start sweating sooner, but men always tend to sweat more than women. Sweating is related to not only exercise intensity, but also to the environment. If it’s hot and humid, you’re going to sweat more, even if you’re not working hard. You will also sweat less if you’re dehydrated or lose too much fluid while you’re working out as your body has mechanisms to limit fluid losses to keep enough in your blood. So, sweating is often not reflective of your effort level.

Interpretation and Management of Hyperglycemia and Exercise

In some circumstances, elevated blood glucose levels can be indicative of medical concerns like insulin deficiency. People with type 1 diabetes are more susceptible to insulin deficiency since they have almost no ability to produce any insulin; therefore, they need to receive instruction on why and when to check for ketones (1). This is especially important if the individual is using an insulin pump. If ketones are present, then the higher blood glucose levels are a result of insulin deficiency, and corrective action should be taken immediately.

People with type 2 diabetes can experience hyperglycemia from a combination of insulin resistance and inadequate insulin secretion; in their case, extremely elevated glucose levels in combination with severe dehydration can result in hyperosmolar hyperglycemia, which may be aggravated by other extenuating health variables such as severe illness and infections (2). These individuals typically do not produce ketones; if ketones do exist, they may be due to dietary restriction, as opposed to insulin deficiency.

Most diabetes specialists teach people with type 1 diabetes to check for ketones when their blood glucose levels are consistently above 300 mg/dL (16.7 mmol/L), but they should check whenever they have unexplained hyperglycemia (≥200 mg/dL, or 11.1 mmol/L) that persists more than a couple of hours. Exercise should be postponed or suspended if blood ketone levels are elevated (≥1.5 mmol/L or 8.7 mg/dL), equivalent to moderate to large urine ketones, since blood glucose and ketones may rise further with even mild activity (3).

Insulin regimens paired with frequent blood glucose checks greatly diminish the chance of insulin deficiency developing, and significant levels of ketones are rarely found when performing blood or urine checks. In most circumstances, slightly elevated blood glucose levels should not interfere with exercise performance; however, some people report headaches, blurry vision, or lack of energy with even mild hyperglycemia, which may be reason enough to avoid physical activity until the glucose level improves. The health care facilitator must consider the ability of the individual to perform blood glucose and ketone testing and understand the complexity of the information.

In other situations, physical activity itself can raise normal blood glucose levels when performed at high intensity (4). The catecholamine response to very intense activity results in an exaggerated hepatic production of glucose for fuel, and after the activity is stopped, the insulin need can double during the post-activity period. If not corrected with insulin dosing in insulin users, this hyperglycemia may last for several hours before drifting down, or it may not decrease without additional insulin (5).

Those using insulin pump therapy may bolus with a small amount of insulin to address this physiological need. If the injecting insulin by syringe, an additional dose of short- or rapid-acting insulin can also be administered. The timing and amount of insulin given require careful consideration and monitoring to accomplish the desired blood glucose result. Individuals must consider any insulin remaining from their last injection or bolus in making subsequent adjustments to doses, as well as factor in the residual effects of the last bout of activity on blood glucose use (ie, postexercise enhanced insulin action). Regardless of the delivery method, this additional insulin dose can result in hypoglycemia and may not be advisable in all cases.



  1. Kamata Y, Takano K, Kishihara E, Watanabe M, Ichikawa R, Shichiri M. Distinct clinical characteristics and therapeutic modalities for diabetic ketoacidosis in type 1 and type 2 diabetes mellitus. J Diabetes Complications 2017;31:468-72. doi: 10.1016/j.jdiacomp.2016.06.023.
  2. Umpierrez G, Korytkowski M. Diabetic emergencies – ketoacidosis, hyperglycaemic hyperosmolar state and hypoglycaemia. Nat Rev Endocrinol 2016;12:222-32. doi: 10.1038/nrendo.2016.15.
  3. Riddell MC, Gallen IW, Smart CE, et al. Exercise management in type 1 diabetes: a consensus statement. Lancet Diabetes Endocrinol 2017;5:377-90. doi: 10.1016/S2213-8587(17)30014-1.
  4. Fahey AJ, Paramalingam N, Davey RJ, Davis EA, Jones TW, Fournier PA. The effect of a short sprint on postexercise whole-body glucose production and utilization rates in individuals with type 1 diabetes mellitus. J Clin Endocrinol Metab 2012;97:4193-200.
  5. Aronson R, Brown RE, Li A, Riddell MC. Optimal Insulin Correction Factor in Post-High-Intensity Exercise Hyperglycemia in Adults With Type 1 Diabetes: The FIT Study. Diabetes Care 2019;10-16. doi: 10.2337/dc18-1475.