Photo: Portrait. Caption: Sheri Colberg, PhD
Even if you are only an occasional “recreational sports” participant,
any exercise you do can improve your body's ability to use insulin. Normally,
exercise causes a decrease in the release of insulin by your pancreas, at
the same time that muscle contractions are increasing blood glucose uptake.
If you take insulin, however, exercise adds to the effects of your injected
insulin, and your blood sugars can drop rapidly, to dangerously low levels.
As an insulin-user, you frequently need to make adjustments in your diabetic
regimen to maintain blood sugars, especially for higher-intensity or longer-duration
exercise.
One insulin regimen stands out from the rest when it comes to managing exercise
effectively, and that is continuous, subcutaneous insulin infusion therapy,
or more simply, insulin pump therapy. Currently, more than 10% of individuals
with type 1 diabetes wear a portable insulin pump, and some insulin-requiring
type 2 diabetic individuals are choosing to pump insulin as well.
Insulin pumps today are pager-sized and sophisticated enough to give both
basal insulin doses (small amounts given every few minutes to cover your body's
general insulin needs) and boluses (larger doses given to cover carbohydrates
in meals and snacks, or to lower blood sugars when needed). Pumps contain
a reservoir or cartridge filled with fast-acting insulin analogues (Humalog
or NovoLog) that, compared to regular insulin, have a more rapid onset (5
to 15 minutes versus 20 to 30 for regular) and an earlier peak in activity
(90 minutes versus 150 minutes). Use of a pump allows for a more normal glycemic
response following carbohydrate intake, and more rapid correction of elevated
blood sugars.
All insulin pumps currently on the market deliver insulin under the skin in
areas such as the abdomen, buttocks, legs, or upper arms, usually through
an inserted Teflon infusion catheter (using a metal insertion needle, which
is removed after insertion). Pump users replace the infusion set every two
to three days with a new set, at an alternate body site.
As an insulin pump user, you can be afforded many general benefits such as
improved overall blood sugar control, reduced risk of nighttime low blood
sugars, and improved awareness of low blood sugars. Furthermore, as a pumper
with an active lifestyle, the most valuable benefit you will experience is
a metabolic response to exercise that, with experience and frequent blood
sugar testing, is similar to a non-diabetic one. Wearing an insulin pump allows
you unparalleled speed and precision in making insulin adjustments for exercise.
Diabetic or not, you need to have some insulin circulating in your blood to
keep your blood sugars from going too high during exercise, but your actual
levels of insulin have an important effect on your metabolic response. Insulin
pump use is a more effective means to control circulating levels of insulin
during activities, and good metabolic control attained with insulin pump therapy
can result in a nearly normal hormonal and metabolic response to exercise.
With an insulin pump, you have several options to choose from to accomplish
this: you can lower either your basal insulin rates, your insulin boluses
for food, or both. You can make these changes before, during, and after exercise
to optimize your blood sugar control. For example, many people remove their
pumps completely during exercise lasting an hour or less, and they may also
keep their basal rates lowered for one to two hours after the activity and
give smaller boluses for post-exercise snacks.
Unfortunately, even with insulin pump use, your metabolic control with exercise
can potentially worsen instead of improving under certain conditions. If you
have elevated blood sugar levels at the start of exercise (250 mg/dl or higher)
with elevated ketones in urine or blood (resulting from the ineffective metabolism
of fats), you are experiencing a state of insulin deficiency, and exercise
is likely to raise your blood sugars more (and is, therefore, not advised
until blood sugars are lowered). Also, the type of exercise that you do can
cause blood sugars to rise (albeit temporarily); a short period of intense
(near maximal) exercise, especially when done in the morning before eating,
can cause your blood sugars to rise during the activity and stay elevated
for up to two hours afterwards. These effects are found in both non-diabetic
and diabetic individuals and are attributed to an exaggerated hormonal response
to intense activity. For such activities, pumpers can easily elevate basal
insulin rates during and for a period of time afterwards, or give an additional
small bolus of insulin to normalize blood sugars.
Current insulin pumps provide a variety of features such as different basal
increments and duration, basal profiles, frequency of basal insulin delivery,
temporary basal rate settings, bolus increments, bolus delivery, size, cost,
use in water, and other unique features. With regard to exercise, the most
important features to consider are basal rate and bolus increments, basal
profiles, and temporary basal settings. Certain pump models are clearly superior
in their ability to adjust for extended periods of exercise, either planned
or spontaneous, through refinements in basal rate reduction.
Which Pump?
Animas Corp. The Animas R-1000 has the best combination of features of any
of the pumps when it comes to participating in a variety of physical activities.
It has four different basal profiles that can accommodate varying workouts
and non-exercise days. It also has the smallest basal increments (0.05 units/hr.)
compared with other pumps, which allow insulin-sensitive individuals, such
as those on total daily insulin doses of 30 units or less, the flexibility
of making very minute changes in their basal insulin delivery. It also has
temporary basal settings slightly more fine-tuned (+10% for 0.5-12 hours)
than other pumps. It has the capability of delivering a bolus over an extended
period (30 minutes to 4 hours); this feature could be used to extend as well
as reduce meal boluses for exercise occurring after meals. In addition, the
Animas pump is the only one that is inherently waterproof - that is, it does
not require additional plug-ins or casings, so it can be used during surface
water activities (such as swimming, snorkeling, and water skiing).
Disetronic Medical Systems.
The latest model from this pump manufacturer (D-TRON) has a second basal rate
profile that can be set for exercise days. The D-TRON can also deliver meal
boluses over an extended period, with meal bolus delivery extended over periods
up to four hours. This pump can also provide a combination of extended and
normal boluses; however, the other main model currently available from Disetronic,
the H-TRON Plus, does not offer these capabilities. The D-TRON offers more
flexibility in setting a temporary basal rate in 10% increments or decrements
over 4 to 24 hours than the H-TRON Plus, which offers an increase in 10% increments
for 12 hours or a decrease for only four hours. Both models can be converted
to be waterproof during surface water activities.
Medtronic MiniMed.
Their current 508 model has the flexibility of three separate basal profiles
(making it also programmable for varying days of activity) and small basal
(0.1 units/hr.) and bolus increments (0.1 units), similar to the Disetronic
D-TRON and H-TRON Plus. The 508 model also offers extended boluses (square
wave) or a combination of extended and normal boluses (dual wave) similar
to the D-TRON. One mild drawback of the MiniMed 508 is that it is only water
resistant, not waterproof, and cannot be worn during water activities. For
shorter water workouts, this pump can simply be disconnected near the infusion
site when using most infusion sets.
Although few and far between, the potential drawbacks of insulin pump use
during exercise are important to know. Excessive sweating can cause your subcutaneous
infusion set to dislodge, which can result in elevated blood sugars or diabetic
ketoacidosis (DKA, a life-threatening condition that may require emergency
treatment in a hospital) if you fail to notice the displacement. To prevent
infusion set displacement due to sweating, you can use liquid skin preparations
like Skin-Tac and stronger adhesives to anchor the set more firmly to your
skin. You can also apply anti-perspirant to your skin at the infusion site
to minimize sweating beneath it. Also, to be on the safe side, it is best
to follow the recommendation to replace your insulin infusion sets every two
to three days, and always check the integrity of your infusion site following
vigorous exercise, sweating, or water contact.
Another potential problem is that insulin is temperature-sensitive. Exercise
in hot or cold environments can potentially cause insulin to degrade and lose
effectiveness. If an insulin pump is placed close to your body during exercise
in the heat, the insulin may become overheated as well. If unexpected high
blood sugars arise after such exercise, replace both the infusion set and
the insulin in the reservoir as a precaution. Especially with the use of rapid-acting
insulin analogues in your pump, diabetic ketoacidosis can begin as few as
five hours following the displacement of your infusion set, and exercise can
hasten its onset.
To experience the most normal metabolic response to exercise possible, learn
your body's response to each and every activity you do by testing your blood
sugars frequently (before, during, and after exercise). After a while, you
can usually predict the effect of the exercise on your blood sugars and make
insulin regimen changes to prevent hypoglycemia from occurring during and
following the activity. Plan out your insulin changes in accordance with your
body's previous responses. Prevent low blood sugars from occuring both during
the exercise and for up to 24 hours afterward. Finally, expect that once you
have been doing an activity consistently over a period of two to three weeks,
you will experience a training response affecting your blood sugar usage.
Training increases fat utilization, which has the potential effect of sparing
blood glucose, resulting in the need for lesser regimen changes. In addition,
with an increase in your muscle mass, overall insulin sensitivity may increase,
causing the need for lower basal and bolus insulin doses.
In order to effectively predict your glycemic responses, you also must understand
the metabolic nature of your exercise. The insulin reductions and/or the carbohydrate
intake you require for aerobic activities will depend on the intensity and
duration of your activity. You should be able to compensate for shorter, less
intense activities with a change in either your insulin (basal and/or bolus
doses) or carbohydrate intake. For short, intense activities such as weight
training, you may not require any immediate regimen changes, but you will
need to anticipate and prevent delayed-onset hypoglycemia from occurring later
on. For longer, more intense exercise, you will generally require a combination
of carbohydrate intake (15-60 grams extra per hour) and insulin reductions
(25-100%) to maintain normal blood sugar levels. For sport- and activity-specific
recommendations and athlete examples for insulin pump users (as well as other
insulin regimens), please refer to my book, The Diabetic Athlete (Human Kinetics,
Champaign, IL).
Thus, even with diabetes, it is possible for you to experience a normal physiological
response to exercise, with more stable blood glucose levels during and following
your exercise. If you are not currently using an insulin pump, try exercising
when circulating insulin levels are lowest (either first thing in the morning,
or three to four hours after the last injection of short-acting insulin) for
the most normal response. If you have decided to get one, keep in mind that
choosing an insulin pump is mainly a matter of your preference: all of the
current models can be used effectively for most types of physical activity.
The benefits of exercise far outweigh the risks, so use your diabetes as an
excuse to exercise and stay fit!