Why Enhancing Insulin Action with Physical Activity is Critical to Preventing, Reversing, and Controlling Type 2 Diabetes and Pre-Diabetes
Sheri R. Colberg, PhD, FACSM
The number of cases of diabetes is expanding like the American waistline, and the epidemic is a global one, not one that affects Americans alone. According to recent predictions, the worldwide incidence of diabetes will likely rise from 171 million people to 366 million by the year 2030, but that may be an underestimate. This disease has the potential to greatly shorten your life or to reduce its quality with disabling health complications like leg, foot, and toe amputations, kidney failure, heart disease, nerve damage, and blindness.
Some of these problems are found in people without diagnosed diabetes or with pre-diabetes or metabolic syndrome. More than 54 million Americans likely suffer from pre-diabetes, which is an open gateway to the development of type 2 diabetes and other metabolic disorders. Therefore, it is vital that you learn to improve your insulin action, lower your body's levels of inflammation, and improve your blood sugar control to prevent or lower the risk of these problems - which is, hopefully, what you'll learn by reading this article. As you can guess from the title, a key component is physical activity.
By way of background, most cases of diabetes are either type 1 or type 2, the latter being much more common and accounting for 90-95 percent. Type 2 primarily results from a combination of your insulin working less effectively (an insulin resistant state) and your pancreas making insufficient amounts (a relative deficit).
Insulin is a hormone that is normally made in and released from the beta cells of your pancreas. When your beta cells are pushed to their limit and can no longer produce enough to overcome your insulin resistant state, your blood sugars will rise, leading to diabetes (see Table 1 for possible ways to diagnose it).
People with pre-diabetes have fasting blood glucose levels that are higher than normal, but not high enough to fall into the "diabetes" category. Incidentally, the cutoff for the diagnosis of diabetes has dropped over time, from 140 mg/dl down to 126 mg/dl, and the pre-diabetic range has progressed downward to now include all readings between 100 mg/dl and 125 mg/dl (formerly 110-139).
If you have blood sugars in the pre-diabetic range, you have a high risk for developing type 2 diabetes or gestational diabetes during pregnancy, and you can even end up with some additional health complications. Having PCOS (Polycystic Ovarian Syndrome), metabolic syndrome, or excess body weight (without being active) also puts you at risk for diabetes at some point.
Table 1: Diagnosis of Diabetes and Pre-Diabetes
Diabetes of any type is currently diagnosed when:
Pre-Diabetes, or an insulin resistant state, is present when:
People with type 1 diabetes, whose pancreases have lost the ability to produce insulin due to an autoimmune attack of their beta cells, have to inject or pump insulin daily to survive, but they can also become insulin resistant. As a result, the recommendations that follow do not just apply to people with pre-diabetes or type 2.
The key to living long and well with a healthy body is the enhancement of your insulin action, be it your own or injected, which can largely be accomplished through physical activity, among other means. The best way to prevent and control all types of diabetes, pre-diabetes, and other metabolic health problems is through lifestyle improvements: engaging in regular physical activity, improving your diet and nutrition, practicing effective stress management, using selected nutraceuticals, and taking prescribed medications as indicated by your physician.
Why Your Insulin Action Matters
Being able to keep your blood sugars under control relies on the relative amount of insulin you have compared to other hormones, such as blood glucose-raising ones like adrenaline and glucagon, both of which are released by your body during exercise, physical or mental stress, and hypoglycemia (blood glucose levels below 65 mg/dl, or 3.6 mmol). Insulin causes your blood glucose levels to decrease by stimulating the uptake (absorption) of glucose into all of your insulin-sensitive cells, where it is used for energy or stored for later use.
While both your brain and your nerves need a constant supply of glucose, which they luckily can receive without the need for any insulin, your insulin-sensitive tissues (primarily muscle, fat, and liver cells) require an adequate supply of insulin to enable them to store excess glucose after you eat or whenever it goes high for any reason. If your body's cells are sensitive to insulin, you will need only relatively small amounts of it in your bloodstream to keep your glucose levels in the normal range (70-99 mg/dl), but when you're insulin resistant, you will need substantially more of it to accomplish the same blood glucose-lowering effect.
Insulin resistance, or ineffective insulin action, is associated with numerous health risks. For one thing, it results in hyperinsulinemia, or high levels of insulin in your blood, which may be directly damaging to blood vessels and contribute to arterial plaque formation and heart disease; a decrease in your insulin action is also directly related to higher levels of systemic inflammation, which means that your body is releasing more of certain substances called cytokines that lead to this state and other health problems.
High insulin levels (and an elevated state of inflammation) are also associated with high blood pressure, obesity (particularly intra-abdominal), osteoporosis (thinning bones), and common cancers like colon, breast, and prostate. In contrast, most athletic individuals have lower levels and require less insulin release for carbohydrate and other food intake, meaning that their bodies are very sensitive to insulin and their risk for these health problems is much less. Having low fasting insulin levels (assuming it is not due to a deficiency) is associated with greater longevity, as demonstrated by the fact that most 100-year-olds without diabetes exhibit this trait.
Although a hallmark of type 2 diabetes, pre-diabetes, PCOS and other metabolic disorders, a state of insulin resistance can occur in type 1 diabetes as well. In fact, many sedentary individuals with type 1 diabetes who make little or no insulin are now developing a form of "double diabetes," meaning that they are also insulin resistant. In their case, improvements in insulin action can allow them to get by with taking lower daily doses of insulin, which usually leads to better control of their blood glucose levels. Regardless of the type of diabetes that you have (or even pre-diabetes), the good news is that you can lower your insulin resistance and improve the action of insulin simply by modifying your lifestyle, particularly by improving your exercise habits and being more regularly active, as discussed in the following sections.
How Physical Activity Benefits Your Insulin Action
Exercise has a significant and very positive effect on your body's sensitivity to insulin. In fact, being physically active on a regular basis likely has the greatest ability to improve your insulin action and lower levels of insulin in your bloodstream. Any type of physical activity has the potential to make your insulin work better; as mentioned, physically trained individuals exhibit a heightened sensitivity to insulin. Not all types of exercise have the same effect, though, and it's likely that your insulin action may be impacted for different lengths of time after doing various types of physical activity.
For instance, the impact of doing aerobic workouts like walking or running, cycling or spinning classes, swimming or aquatic classes, and other continuous exercise comes primarily from how much muscle glycogen (stored glucose in your muscles) you use up during the activity. In general, exercising longer and/or harder uses more, and while your body works to restore the glycogen afterwards (which can take up to 48 hours), your insulin action is generally heightened.
However, aerobic effects on insulin sensitivity usually start to decline within 1-2 days, and regular exercise is essential to keeping it higher. Studies have shown that six months of thrice weekly aerobic or resistance training improves glucose use, but the benefits of resistance work may have more to do with increasing your overall muscle mass, allowing for greater glucose uptake and storage in muscle (an insulin-sensitive tissue). Having more muscle is always good because it increases the total amount of glucose you can dispose of through glycogen storage: muscles have a limit to how much they can store, and having more muscle mass results in your having a greater potential storage capacity.
Moderate aerobic training still can improve your insulin sensitivity, though. In one study, sedentary, insulin-resistant, middle-aged adults who engaged in 30 minutes of moderate walking 3-7 days a week for six months succeeded in reversing their insulin resistance, without changing their diets or losing any body weight (although their body proportions of fat and muscle likely changed for the better).
In older adults, low- to moderate-intensity "walking" on a mini-trampoline for 20-40 minutes four days per week over a four-month period also enhanced their blood glucose uptake without any additional insulin release or loss of abdominal fat.
If you're obese and currently sedentary, walking on a trampoline can be a good low-impact exercise to start with. Walking more daily steps can also be beneficial, but if you have any lower limb problems (e.g., joint pain with walking), doing non-weight-bearing exercise like stationary cycling or aquatic-based workouts can be a great alternative.
Studies have also shown that even young, inactive individuals of any weight can improve their insulin sensitivity after doing just one week of aerobic exercise training, without experiencing any weight loss or a true training adaptation in their muscles. The key to getting such results is exercising regularly to sustain the improvements in your insulin action as aerobic workouts likely result in a relatively short-lived effect.
Where You Store Your Body Fat Matters and How Exercise Can Help
Recent research has shown that type 2 diabetes may be preventable and pre-diabetes reversible with lifestyle changes that result in modest weight loss. The Diabetes Prevention Program (DPP) showed that making lifestyle changes – including dietary modifications resulting in some weight loss and regular physical activity – can lower your risk of developing diabetes by 58 percent, even if you already are at risk for it. A recent follow-up analysis of that study showed that weight loss best predicted a lower incidence of diabetes among its participants, all of whom had pre-diabetes. For every kilogram (2.2 pounds) of weight loss, DPP participants experienced a 16 percent reduction in their diabetes risk.
While the strong association between weight loss and diabetes risk reduction might make you think that weight loss is more important, the results from the rest of those analyses are equally important and more in favor of physical activity. Among the 495 participants not meeting the study's weight loss goals (that is, a 5-7 percent loss of body weight) after one year, everyone who met the physical activity goal (30 minutes or more of walking five days a week) had a 44 percent lower chance of developing type 2 diabetes even if he or she didn't lose any weight, showing the absolute importance of exercise. Even though eating fewer calories and being physically active were the main factors that predicted the weight loss in those who met the goals, maintaining regular physical activity was most important behavior for keeping the weight off afterwards. In fact, most of the individuals who lost weight but failed to continue to exercise regained all of it over the course of the study.
Keep in mind, too, that exercise can actually increase your body weight (or keep it from dropping as much) when you gain muscle mass, even while you're losing body fat, so just following your weight on a scale is probably not telling you the whole story. It's always better to keep checking your waist and other measurements (or just how your clothes fit you) than to set your goals on changes in your scale weight alone, which is not necessarily reflective of changes in your body fat and muscle mass.
How important is weight loss to improving insulin action? Everyone wants to jump on the weight-loss band wagon, but not everyone who is overweight or obese is insulin resistant, and it is entirely possible to be "fit" and "fat" and avoid the complications of many metabolic disorders like diabetes and metabolic syndrome.
Exercise also lowers your body's levels of systemic inflammation without weight loss. Moreover, you can have almost a complete reversal of your diabetic symptoms without losing much weight at all – but only if you exercise regularly (and possibly improve your diet somewhat). Where you store your fat also does have some bearing, though, as storing excess amounts deep within your abdomen (the so-called visceral or intra-abdominal fat) can negatively impact insulin action.
Fats (i.e., free fatty acids) released from visceral fat directly enter your liver and have a detrimental effect on how responsive the liver is to insulin; therefore, losing visceral fat (as opposed to the below-the-skin variety) may benefit you the most.
Researchers also recently examined the effects of dieting by following a low-calorie diet high in healthy fats, exercising (moderate walking for 50 minutes three times a week), or a combination of both in older women with type 2 diabetes. After 14 weeks, only women who exercised, irrespective of how much weight they lost, experienced a significant decrease in their visceral fat levels. In fact, dieting by itself reduced subcutaneous (below the skin) and total abdominal fat, but not the visceral type without exercise. Moreover, exercisers maintained all of their muscle mass while the diet-only group lost some muscle in the process. So, you should include regular exercise to lose the worst type of fat and to keep the muscle that you have, especially when you go on a diet.
How to Choose the Best Types of Training
How an activity will impact your insulin action in the short run has a lot to do with how physically fit you are. For instance, if you're mostly or completely sedentary now, starting a walking program will initially be effective enough to enhance it. If you're already engaging in regular aerobic exercise, you will benefit more from either doing it more vigorously or adding in some resistance training to your exercise routine.
In fact, the muscle mass you can potentially gain from doing resistance work with weights or bands may be of greater importance for diabetes control and prevention and pre-diabetes reversal, particularly if you're currently sedentary. In a recent study on people with type 2 diabetes, 4-6 weeks of moderate (40-50 percent of maximal) resistance training improved their insulin sensitivity by 48 percent, even without causing any measurable changes in their body fat. Similarly, men newly diagnosed with type 2 diabetes undertaking 16 weeks of progressive resistance training just twice a week gained muscle mass, lost body fat (particularly visceral), and greatly enhanced their insulin action – all while eating 15 percent more daily calories (now, if that isn't a bonus, I don't know what is!).
A combination of aerobic and resistance training may afford even greater improvements in your insulin sensitivity and a more significant decrease in your visceral fat than aerobic training alone, with increased muscle mass as a side benefit. Even if you don't have diabetes, but are contending with pre-diabetes, obesity, metabolic syndrome, PCOS, or another health problem, doing a combination of both types of training is likely to offer you the greatest overall improvements in your health by optimizing your muscle mass gains and insulin action for the longest duration.
How You Can Maximize the Benefits of Exercise Training
Each exercise session that you do exerts its own effect. Depending on the type of activity – how hard it is, how long it lasts, what type of exercise it involves – the improvements in your insulin action can vary. All activities cause your muscles to use some stored glycogen as fuel, but more intense and longer duration, moderate exercise results in greater overall use, even a total depletion at times. Thus, these types of activities generally have a longer-lasting effect on insulin action, but still usually no longer than 1-2 days while your blood glucose is used to restore your muscle and liver glycogen. As the primary fuels your body uses during exercise can vary, easy exercise being the greatest user of mobilized fat stores, mild activities like weeding or slow walking may not have any lasting impact on your insulin action since they leave your glycogen stores virtually untouched.
Therefore, to really impact your insulin action on a continuing basis, you should exercise at least every other day, and doing near-daily workouts is even more beneficial. In addition, you are advised to engage in some more intense physical activity (like brisk walking and resistance work) in addition to your normal, less-strenuous daily activities.
In a recent study of older individuals with diabetes, even when they were already walking at least 10,000 steps a day (about five miles or 8 km), their fitness and diabetes control benefited from doing "pick up the pace" training that consisted of walking at a 10 percent faster pace for 30 minutes three days a week. If you normally take 90 steps per minute, walking 10 percent faster would mean taking around 100 steps per minute instead.
How to Optimize Your Daily Activities Plan
Since the effects of more intense or longer-duration exercise may last longer, the best plan is to intersperse these types of training sessions throughout the week, or to simply do a harder activity every other day to maximize its insulin-enhancing effects. Doing "hard" and then "easy" or "moderate" days of activity is a good way to keep your motivation high, prevent athletic injuries, and allow you to continue doing regular exercise training.
The major reasons that people stop (or fail to begin) regular exercise programs include a perceived lack of time, athletic or overuse injuries (caused by working out), inconvenience, loss of motivation, and difficulty of workouts. If you choose to vary your activities from day to day instead, you can make time for them more easily; you're less likely to get injured or too sore to participate; you can pick activities that are convenient on any given day; your motivation to exercise is likely to stay higher; and your exercise will feel easier to do as you will be giving your body enough time to recover fully between harder workouts, all while optimizing your insulin action and overall health.
To improve your insulin sensitivity the most, your aerobic training should either be harder for an extended period of time (such as by doing "pick up the pace" training) or just include interspersed, faster intervals (like periodically walking at a faster pace between two mailboxes or telephone poles) throughout the activity. Moreover, varying your exercise from one day to the next allows you to get more physically fit, as well as improving your insulin action to a greater extent and for a more prolonged period of time afterwards.
Likewise, resistance work done with weights or rubber resistance bands is most beneficial when your workout is a moderate or hard intensity, such as using a weight or resistance that is hard enough to only allow you to complete at least 8, but not more than 12, repetitions for each set, and you should complete 1-3 sets of repetitions on each resistance exercise. Doing so will allow you to maximize your muscle mass gains and how much glycogen you use during each workout.
An example of an alternating schedule of activities for individuals to optimize insulin sensitivity is shown in Table 2.
Table 2: Sample Weekly Exercise Plan for Optimal Insulin Action
Day 1 (Harder aerobic): 20-60 minutes of aerobic training done at a more challenging pace or with higher-intensity intervals interspersed throughout
Day 2 (Moderate resistance): 10-minute easy aerobic warm-up, followed by 30-60 minutes of resistance training using your major muscle groups (at least 8 to 10 exercises total) doing 1-3 sets of 8-12 repetitions each
Day 3 (Easy aerobic): Set a goal of taking at least 10,000 steps (counted with a pedometer) at any pace throughout the day
Day 4 (Harder aerobic): 20-60 minutes of aerobic training done at a more challenging pace or with higher-intensity intervals
Day 5 (Moderate resistance): 30-60 minutes of resistance training
Day 6 (Easy to moderate aerobic): Take as many steps as you can, but include a shorter, planned activity (such as a 30-minute walk)
Day 7 (Rest): No planned activities, but plenty of activities of daily living
Other Means to Enhance Insulin Action
Along with physical activity and loss of visceral fat, there are other factors that can impact your insulin action that can be found in Table 3 that follows. For instance, simply avoiding elevations in your blood glucose levels after eating can improve insulin's effectiveness, which you can accomplish by choosing foods with a higher fiber and lower glycemic index and glycemic load (such as vegetables and legumes), balancing your food intake more effectively with certain medicines (e.g., insulin, or certain oral diabetic medicines), and exercising before or after meals to enhance your body's ability to process the carbohydrates in meals.
Along those lines, picking your foods more carefully to choose ones high in nutrients – including fiber, vitamins, minerals, natural antioxidants, and phytonutrients – and possibly supplementing with selected vitamins, mineral, and/or other nutraceuticals can enhance how well your insulin works. In addition, you will want to choose foods that are lower in calories, refined sugars, and flour to get the most benefit. Your insulin action is also improved by getting enough sleep, controlling your emotional stress levels (which exercise can also help with), and reducing your body's level of low-grade, systemic inflammation thought to be integrally involved in causing insulin resistance. Not surprisingly, you can help lower this inflammation through regular physical activity and better food choices (i.e., more natural, less refined products) as well.
Table 3: Factors that Potentially Improve Your Insulin Action
In conclusion, after reading this article, you should be thoroughly convinced that you hold your health in your hands and that you can likely reach an optimal level of well being on your own without needing to take lots of expensive prescription drugs (with potential side effects). You can exert a great deal of control over how effectively your insulin works, and being insulin sensitive has numerous health benefits – prevention, control, and reversal of type 2 diabetes and pre-diabetes just being some of them.
If you have type 1 diabetes, you can't reverse your need to take insulin altogether, but having a heightened sensitivity can allow you to take lower insulin doses, which leaves less room for error. If you've been prescribed insulin-sensitizing medicines, these prescriptions will work more effectively, and you may also need to take smaller doses when you start engaging in regular training. There remains no doubt that improving your insulin action – particularly when you accomplish it through increased and varied physical activity and other easily-implemented lifestyle changes – is a winning proposition for diabetes and pre-diabetes prevention, reversal, and control, as well as for your overall health and longevity.
About the Author:
Sheri R. Colberg, Ph.D., FACSM, is an associate professor of exercise science at Old Dominion University in Norfolk, Virginia. Funded by the American Diabetes Association and others, her research continues to focus on exercise and diabetes, including prevention, control, and reversal of diabetes and its complications with physical activity. She is the author of numerous articles and seven books, including The Diabetic Athlete, Diabetes-Free Kids, The 7 Step Diabetes Fitness Plan, 50 Secrets of the Longest Living People with Diabetes, The Science of Staying Young, Matt Hoover's Guide to Life, Love, and Losing Weight (co-author) and The Diabetic Athlete's Handbook. Dr. Colberg is also the author of an upcoming book, Diabetes? No Problema, to be published in Spring 2009.
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