By Sheri R. Colberg, PhD
Insulin is a hormone that is normally released by the beta cells of the pancreas. When a person’s pancreas cannot produce enough insulin to sustain good health, insulin can be injected into the body with a needle, inhaled with an inhaler, or infused with a pump.
One of the main functions of insulin is to lower blood glucose levels by enabling glucose to enter the cells of the body, where it is used for energy or stored for future use. A person who is insulin-sensitive needs only a relatively small amount of insulin to keep blood glucose levels in the normal range and to keep the body’s cells supplied with the glucose they need. A person who is insulin-resistant, on the other hand, needs a lot more insulin to get the same blood-glucose-lowering effects.
Insulin resistance is associated with numerous health risks. For one thing, it causes hyperinsulinemia, or high circulating insulin levels, which may be directly damaging to blood vessels. Hyperinsulinemia is also associated with high blood pressure, heart disease and heart failure, obesity (particularly abdominal obesity), osteoporosis (thinning bones), and certain types of cancer, such as colon, breast, and prostate cancer. In contrast, having low circulating insulin levels is associated with greater longevity; most centenarians without diabetes have low circulating insulin levels.
Insulin resistance is a hallmark of Type 2 diabetes, but it can occur in Type 1 diabetes as well. In fact, there is a growing number of people who are said to have “double diabetes” because, in addition to having Type 1 diabetes, they also have the insulin resistance characteristic of Type 2.
The good news is that you can lower your level of insulin resistance — and raise your level of insulin sensitivity — by modifying your lifestyle choices, particularly your exercise and eating choices, regardless of the type of diabetes you have.
Physical activity has a significant positive effect on insulin sensitivity. Indeed, it may have the biggest effect of any measure you could take to improve your insulin sensitivity. Any type of physical activity has the potential to make your insulin work better, and combining aerobic activities — such as brisk walking, swimming, and cycling — with resistance training, or weight training, appears to have the greatest effect. Aerobic activities burn more calories (and glucose) per session, but resistance training builds muscle, which is what burns glucose during exercise, so having more is better.
Numerous studies bear out the value of physical activity and exercise in improving insulin sensitivity. In a study of lean, sedentary, young adult women (ages 18–35), both six months of thrice-weekly aerobic training and six months of resistance training improved glucose use in the body.
In another study, sedentary, insulin-resistant, middle-aged adults who engaged in 30 minutes of moderate walking three to seven days per 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 a study of older adults in their 70’s, low- to moderate-intensity “walking” on a mini-trampoline for 20–40 minutes four days per week over a four-month period enhanced their glucose uptake by cells without any additional insulin release by the pancreas or loss of abdominal fat.
No matter what your age, weight, or current level of physical fitness, studies have shown that exercise training can improve your insulin sensitivity within just one week of training without weight loss.
While combined aerobic and resistance training is recommended for everyone, the benefits you will reap from any given activity will depend on how fit you currently are. If you’re completely sedentary now, starting a walking program or moderate resistance training program will initially be effective enough. If you’re already doing regular aerobic exercise, you will likely benefit more from adding some interval training or resistance training to your exercise routine. In interval training, you intersperse periods of intense exercise with periods of slower, easier activity. For example, on a stationary bike, you might pedal faster and harder for 30–60 seconds, then pedal at a moderate pace for a few minutes, then repeat this pattern throughout your exercise session.
Once again, research confirms that this approach works. In a recent study of previously sedentary people with Type 2 diabetes, four to six weeks of moderate-intensity resistance training improved their insulin sensitivity by 48%, even without causing any significant changes in their body fat or muscle mass. Similarly, unfit men who were newly diagnosed with Type 2 diabetes who did 16 weeks of “progressive” resistance training (meaning that the weight they lifted was increased over time) just twice a week gained muscle mass, lost body fat (particularly in the abdominal region), and greatly enhanced their insulin sensitivity — all despite a 15% increase in the amount of calories they consumed.
For older women with Type 2 diabetes, studies have shown that the combination of aerobic and resistance training may afford even greater improvements in insulin sensitivity and a more significant decrease in abdominal fat than aerobic training alone, with increased muscle mass to boot.
The exercise studies mentioned so far all examined the effects of longer-term training, but each bout of exercise also exerts its own effect. During exercise, your body burns glycogen, a form of glucose that is stored in your muscles. After exercise, your muscles replenish their glycogen stores with glucose from the bloodstream. The more glycogen that is burned during a bout of activity, the longer the body’s insulin sensitivity is improved.
More intense and longer-duration activities can improve insulin sensitivity for up to one to two days, as muscle glycogen that was used during the exercise is being restored. A short, low-intensity activity such as weeding a small garden, however, may not have any lasting impact on insulin sensitivity since very little glycogen is used during it.
Therefore, to improve insulin sensitivity on a continuing basis, you should plan on exercising at least every other day, with near-daily workouts exerting an even more beneficial effect. In addition, you should try to do some more intense workouts (like brisk walking and resistance work) in addition to normal, less strenuous daily activities.
For example, in a recent study of older individuals with diabetes, even when they were already walking at least 10,000 steps a day, their fitness and diabetes control benefited from doing “pick up the pace” training that consisted of walking at a 10% faster pace for 30 minutes three days a week. For a person who usually takes 90 steps per minute, walking 10% faster would mean taking 99 steps per minute instead.
Given that the effects of more intense or longer-duration exercise may last slightly longer, the best plan is to intersperse these types of workouts throughout your week, or to do a more intense activity every other day to maximize its insulin-enhancing effects. Alternating between more challenging activities and easier or more moderate activities is a good way to keep yourself motivated, prevent athletic injuries, and keep your exercise training on track.
The major reasons that people stop (or fail to begin) regular exercise are perceived lack of time, injuries from workouts, inconvenience, loss of motivation, and difficulty of workouts. By varying your activities from day to day, you can fit in whatever you have time for more easily, you’re less likely to injure yourself, you can pick activities that are convenient for you on any given day, your motivation is likely to stay higher, and the exercise won’t feel too hard for you since you’ll have plenty of time to recover between more intense workouts.
To improve your insulin sensitivity, aerobic training should either be more intense for an extended period of time (such as during the “pick up the pace” training mentioned earlier) or just during interspersed intervals (like periodically walking faster between two mailboxes or telephone poles) during the whole activity.
Resistance work (with weights or rubber resistance bands) will benefit you the most if you build up to a weight or resistance that you can lift with good form at least 8 times but not more than 12 times in each set. (Plan to do one to three sets of 8–12 repetitions per exercise.) When you’re first starting out with resistance training, however, it’s OK to work with lighter weights or less resistance while you familiarize yourself with the equipment and exercises.
See the sample weekly plan for an example of an alternating schedule of activities that can help to optimize insulin sensitivity.
Alternate-day fasting, or fasting every other day, lowered fasting insulin and insulin resistance more than calorie restriction in adults with overweight or obesity and insulin resistance, according to a recent study in the journal Obesity. Among alternate-day fasters, the was a 52% reduction in fasting insulin and a 53% reduction in insulin resistance compared to 14% and 17% reductions, respectively, in those on a calorie-restricted diet.
In addition to performing more physical activity, losing excess body fat is well known to improve insulin sensitivity, and other factors can help, too. For instance, avoiding elevated blood glucose levels after meals can improve your insulin sensitivity. You can prevent such elevations by choosing foods with a higher fiber content and lower glycemic index and glycemic load (such as vegetables and legumes) and, if you take insulin, adjusting your premeal doses according to the amount of carbohydrate in the meal. Exercising just before meals can also help to lower after-meal blood glucose levels since insulin sensitivity is heightened right after exercise.
You can also lower your insulin resistance by getting enough sleep, moderating your stress levels (exercise can help with this), and reducing your body’s level of inflammation (which can also be accomplished through physical activity and better food choices). For more ways to increase insulin sensitivity, see “Insulin Sensitizers in Brief.”
You can exert a large amount of control over how effectively your insulin works in your body, and being insulin sensitive has many health benefits. If you take insulin, a heightened sensitivity may allow you to lower your insulin doses. If you take insulin-sensitizing oral medicines, these prescriptions will work more effectively, and you may also end up requiring smaller doses. Regardless of any medicines you take, improving your insulin sensitivity — particularly when you accomplish this goal through increased and varied physical activity — is a winning proposition for your diabetes control and your overall health.
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