Why Blood Glucose Rises After Fasting

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What Causes Blood Glucose Rises After Fasting?

In diabetics, blood glucose levels are sometimes high first thing in the morning even after they’ve been fasting overnight. This is known as the “dawn phenomenon”. The dawn phenomenon refers to the body’s natural response to an overnight fast. In the absence of carbohydrates and fuel, the body responds by producing “counter-regulatory hormones”. These hormones, like epinephrine, glucagon, cortisol and growth hormone, have the opposite reaction in your body as the hormone insulin would.

Insulin lowers blood sugar levels by helping glucose enter cells when there’s an excess of glucose, thus lowering blood sugar levels. These counter-regulatory hormones do the opposite. They trigger the breakdown of glucose stores in the liver called glycogen, which release glucose into the blood stream and cause blood sugar levels to rise. This happens because the body senses a lack of glucose due to prolonged fasting and pumps out the appropriate hormones to supply the missing fuel. With the dawn phenomenon, blood glucose levels usually rise between 4:00 A.M. and 9:00 A.M.

There are other reasons why blood glucose rises after fasting in diabetics. It can happen when a diabetic isn’t getting enough insulin to keep their blood sugar under control - or a diabetic can experience another phenomenon called the Somogyi effect.

The Somogyi Effect

The Somogyi effect is similar to the dawn phenomenon. Like the dawn phenomenon, it involves the release of counter-regulatory hormones, but it’s triggered by a drop in blood sugar, usually when a diabetic gets too much insulin. In an overzealous attempt to correct a low blood sugar level, the body produces too much counter-regulatory hormone, and this overshoot raises blood sugar levels. In some cases, vigorous exercise or stress can trigger the Somogyi effect and cause steep blood glucose rises after fasting.

Can You Prevent Blood Glucose Rises After Fasting?

If you have blood glucose rises after fasting, your doctor can help you sort out the cause and prevent it from happening in the future. It may require checking blood sugars during the middle of the night to make sure they’re not dropping and leading to the Somogyi effect. If they’re steadily rising throughout the night, insulin levels in the evening may need to be increased. If it goes up sharply in the early morning hours, it’s most likely the dawn phenomenon. If you are taking a long-acting insulin once per day, your physician may advise you to adjust your schedule to accommodate an injection before bedtime and after getting up in the morning.

The Bottom Line?

Your doctor can help you sort out why you’re having blood glucose rises after fasting, but be prepared to check your blood sugars more often – even during the middle of the night – to find out why.


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Kliegman: Nelson Textbook of Pediatrics, 18th ed.; Chapter 590.

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