Fasting Process and Hyperinsulinism (HI) Diagnosis

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Congenital hyperinsulinism (HI) is a genetic disorder where the pancreas makes and releases too much insulin. Excess insulin causes low blood sugar (hypoglycemia). Children with hyperinsulinism do not have a normal response to fasting. Fasting studies can diagnose hyperinsulinism or tell your team more about your child’s condition and treatment plan.

What is the normal response to fasting?

Our bodies and brain need a constant source of fuel (energy) for normal functioning.
The 3 main sources of fuel are:
- Food that we eat
- Glucose released from glycogen stores in your liver
- Ketones produced from breaking down fat
In the first 3 hours after eating, food provides the fuel that your body needs. Any extra glucose is stored in your liver as glycogen.
When the food fuel is gone, your body activates a series of hormonal and metabolic responses that changes liver glycogen into glucose that can be used as fuel.
If fasting continues, your body breaks down fat in the liver. It is used to make ketones that can be an alternate source of fuel when glucose is scarce.
For these responses to be activated, the release of insulin needs to be turned off. Insulin is the most important hormone to keep blood glucose steady.
After a long fast, as the blood glucose decreases, one would expect:
- Insulin release to stop (insulin concentration in blood to be undetectable).
- Ketone production to increase (blood ketones to be high).
- Glycogen stores to be depleted (no rise in blood glucose after a glucagon dose).

How is fasting different for children with HI?

In children with HI, the beta cells in the pancreas make too much insulin, and do not turn off insulin release when blood glucose is decreasing. The elevated insulin level interferes with the body’s ability to adapt normally to fasting and to activate the responses to produce fuel. Thus, the liver is unable to release glucose from glycogen or to produce ketones. In a child with HI, as the glucose decreases during fasting:

Insulin release does not stop (insulin concentration in blood is higher than expected).
Ketone production is not activated (blood ketones are low).
Glycogen stores are preserved (after a dose of glucagon blood glucose increases more than 30 points).

Types of fasting and other studies

What is a diagnostic fast?

This test will help diagnose the cause of your child's low blood glucose. Your child will fast until they have a low blood glucose or until their ketones are high. The maximum length of time that your child will fast depends on age:

Less than 1 month: 18 hours
1-12 months: 24 hours
Older than 1 year: 36 hours
Adolescent: 48 hours

What can you expect during the fasting study?

An IV will be placed at the start of the fast. Your child will be allowed to drink water throughout the fast. Infants will be given saline water.
We will monitor your child's blood glucose levels and ketone levels frequently by a finger or heel stick.
When the blood glucose is higher than 70 mg/dL, we will monitor every 3 hours. When the blood sugar is lower than 70 mg/dL, we will start checking every 1 hour and once lower than 60 mg/dL, every 30 minutes.
Once the blood glucose is lower than 50 mg/dL or the ketone levels are higher than 2.5 mmol/L on 2 consecutive checks, we will send a glucose to the lab. Once the lab confirms the blood glucose is lower than 50 mg/dL, a blood sample will be drawn from the IV to measure hormone and metabolic markers.
If the fast ends for a blood glucose lower than 50 mg/dL, your child will receive a medicine called glucagon, this will be given through the IV. If it cannot be given through the IV, it is given as an injection into the muscle.
We will then monitor your child's blood glucose every 10 minutes for 40 minutes to see the glucose level increases. If after 20 minutes the blood glucose remains low, then your child can eat.
Please keep in mind glucagon can cause nausea and vomiting, which is a common side effect.

What happens once the fasting test ends?

Each child is different. Your team will work with you to develop the safest plan. Sometimes the plan may change. If treatment or additional testing is needed, your child's discharge from the hospital can be delayed. We will communicate any possible changes with you.

What is a safety fast?

A safety fast tests your child's current medical routine to learn if it is effective in preventing hypoglycemia. How long your child will fast depends on their age and medical routine. Please keep in mind your healthcare provider may decide to change the time, but this will be discussed with you prior to the fast.

For children receiving therapy with diazoxide:

Less than 1 year: 12 hours
Older than 1 year: 18 hours

For children receiving therapy with octreotide:

6-8 hours

For children receiving therapy with NG or G tube dextrose:

12 hours or overnight

For children receiving therapy with lanreotide:

12-18 hours

What can you expect during the fasting study?

We will monitor your child's blood glucose levels and ketone levels frequently by a finger or heel stick.
When the blood sugar is higher than 70, we will monitor every 3 hours. When the blood sugar is lower than 70 mg/dL we will end the fast.
Please note that at times we may fast your child until blood sugar is lower than 60 mg/dL depending on current medical routine.
When the safety fast ends, we will send a confirmatory blood glucose and ketone level by finger stick or heel stick.
No blood drawing IV will be placed for this study unless other testing is done during the admission. Also note that labs may need to be drawn by phlebotomy which will be decided by your healthcare provider.

What happens once the fasting test ends?

Your child may eat at the end of the study. We will continue to monitor the blood glucose to make sure your child is safe for discharge. Please keep in mind how your child fasts will determine when your child can be discharged. If adjustments need to be made to the current treatment plan, this may prolong your hospital stay.

What is an OPTT? (Oral Protein Tolerance Test)

Some forms of HI have a component of protein sensitivity associated with it causing hypoglycemia. This test will help us decide if your child has low blood glucoses in response to eating protein.

What can you expect during the OPTT?

Your child will not be allowed to eat or drink sugar containing fluids for 3 hours prior to the testing.
We will place a blood drawing IV.
Your child will need to drink a protein shake (typically 4 ounces) within 5 minutes. This can also be given through a NG or G tube.
Blood samples are taken to measure blood glucose and other labs very frequently over a 3-hour period. Testing ends at 3 hours or sooner if the blood glucose goes lower than 50 mg/dl.

What happens once the OPTT ends?

Your child may eat at the end of the study. We will continue to monitor the blood glucose to make sure your child is stable.

What is a cure fast?

This test will help us determine if your child is cured from the current condition causing low blood glucose. Your child will fast until they have a low blood glucose or develop high ketones. The maximum length of time that your child will fast depends on age:

Less than 1 month: 18 hours
1-12 months: 24 hours
Older than 1 year: 36 hours
Adolescent: 48 hours

What can you expect during the fasting study?

An IV may be placed at the start of the fast. Your child will be allowed to drink water throughout the fast. Infants will be given saline water.
We will monitor your child's blood sugar levels and ketone levels frequently by a finger or heel stick. When the blood sugar is higher than 70 mg/dL, we will monitor every 3 hours. When the blood sugar is lower than 70 mg/dL we will start checking every 1 hour and once lower than 60 mg/dL, every 30 minutes.
Once the blood glucose is lower than 50 mg/dL or the ketone levels are higher than 2.5 mmol/L on 2 consecutive checks, we will send a glucose to the lab. Once the nursing staff gets confirmation from the lab that the blood glucose is lower than 50 mg/dL, a blood sample will be drawn from the IV to measure hormone and metabolic markers.
If the fast ends for a blood glucose lower than 50 mg/dL, your child will receive a medication called glucagon, this will be given through the IV. If unable to be administered through the IV, it is given as an injection into the muscle.
We will then monitor your child's blood glucose every 10 minutes for 40 minutes to see if there is an increase in the glucose level.
If after 20 minutes the blood glucose remains low, then your child can eat.
Please keep in mind that glucagon can cause nausea and vomiting, which is a common side effect.

What happens once the fasting test ends?

Your child may eat at the end of the study. We will continue to monitor the blood glucose to make sure your child is safe for discharge. Please keep in mind that how your child fasts will determine when your child can be discharged. If treatment needs to be given or additional testing is required, this will prolong your hospital stay.