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Harvard Commentaries
Harvard Commentaries
Reviewed by the Faculty of Harvard Medical School

Dangers of High and Low Blood Sugar

July 17, 2014

Diabetes Type 1
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Dangers of High and Low Blood Sugar
Dangers of High and Low Blood Sugar
Illness, insulin, food and exercise all can affect blood sugar. When these elements get out of balance, sugar can spike or plunge. A little preparation can help you to prevent some incidents, or handle them if they occur.
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Reviewed by the Faculty of Harvard Medical School

Dangers of High and Low Blood Sugar
Most of the time, good diabetes care can help to keep your blood sugar close to normal levels. This care includes careful monitoring and appropriate adjustments in insulin dosage, diet and exercise. Despite these efforts, episodes of high or low blood sugar may occur.
All people with diabetes will have times when blood sugar levels are abnormally high. This condition is called hyperglycemia. Hyperglycemia can damage the body. It causes many of the long-term health problems associated with diabetes.
Symptoms of severe hyperglycemia include:
  • Frequent urination
  • Thirst
  • Weakness
  • Fatigue
  • Weight loss
  • An urge to eat more frequently
  • Blurred vision
In type 1 diabetes, hyperglycemia occurs when you don't take enough insulin. This can happen if you eat more than usual or if you exercise less than usual.
Other causes of high blood sugar include:
  • Infections
  • Any severe illness or injury
  • Unusual emotional stress
  • Thyroid disease or other hormone imbalances
  • Certain medicines, including:
    • Corticosteroids
    • Antipsychotic medicines (to treat psychosis, depression or some sleep problems)
    • Diuretics
    • Beta agonists
    • Statin drugs
    • Niacin
    • Phenytoin (Dilantin)
    • Thyroid hormone
    • Pentamidine
    • Alpha interferon
If your sugar is over 240 milligrams per deciliter (mg/dl), check your urine for ketones. If the urine shows a "moderate" or "large" number of ketones, you may be developing ketoacidosis. This is a dangerous complication of diabetes. Talk to a doctor right away.
Exercising usually will help to bring down high blood sugar. However, insulin is the most important treatment if the sugar is far out of its usual range. If your sugar is over 240 mg/dl, whether or not you have ketones, you may be quite dehydrated. Exercise can be dangerous at a time that you are dehydrated. You should never exercise at a time when your urine test shows ketones.
Call the doctor, too, if blood sugar is high for several days in a row. You may need to make changes in your diet, insulin dose or the timing of insulin shots.

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When blood sugar levels are too low, this is called hypoglycemia. It's also called an insulin reaction. Hypoglycemia occurs when you inject more insulin than you need. It happens to all people with type 1 diabetes.
It is important to recognize the symptoms of hypoglycemia. Then, you need to boost your blood sugar back to a normal level as soon as possible. Symptoms include:
  • Tingling around the mouth
  • Rapid heartbeat
  • Dizziness or shakiness
  • Feeling disoriented or confused
  • Having a hard time focusing or concentrating
  • Clumsiness
  • Sweating
  • Hunger
  • Headache
  • Pale skin color
  • Mood or behavior changes
  • Seizure or coma (in severe cases)
When you notice any of these symptoms, check your blood sugar at once. If your blood sugar is low, treat yourself right away by eating or drinking something containing sugar. Your doctor (or your child's doctor) can give details about what to use to treat these reactions. After you eat, check your blood sugar again. Make sure it has returned to normal.
Some people do not have noticeable symptoms when they have hypoglycemia. This is most likely to occur in people who have had diabetes for a long time. For these individuals, it is especially important to check blood sugar regularly throughout the day.
If you think a person may have low blood sugar, but a monitor is not available, go ahead and boost the blood sugar. It's best not to wait until you can confirm that the level is low. If hypoglycemia is not treated quickly, symptoms could get more severe. Low blood sugar can cause permanent damage to the brain.
If a person with diabetes is unconscious, don't give any food or drink even if low blood sugar has caused the problem. Call 911 immediately for emergency help. If it is available, you also can give an emergency medicine called glucagon. This medicine is injected (given as a shot). Glucagon raises blood sugar. If a doctor prescribes glucagon, a person with diabetes can carry it for use during an emergency.

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Sick Days
If you are a type 1 diabetic, being sick — for example, with a cold or the flu — can make your blood sugar level higher than usual. The stress of an illness or an injury can make your body less sensitive to your normal doses of insulin. You also may be less active during an illness. This can raise blood sugar as well.
Sometimes, sick days have an opposite effect on your blood sugar. They can cause unexpected low levels. If you are sick, you may not feel like eating your usual diet. A child with nausea and vomiting may eat or drink very little. When you eat fewer calories (especially carbohydrates), you don't need as much insulin. Hypoglycemia may occur if your insulin dose is not correctly adjusted.
It's often possible to prevent low or high blood sugar caused by illness. Talk to your doctor about how to manage "sick days." Together, you can make a plan for handling them.

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