Monday, July 28, 2008
When we think about blood glucose, we usually think about hyperglycemia, or excess blood glucose. But sometimes we can have too little glucose in our blood. The symptoms are lightheadedness, anxiety, and hunger. Low glucose can even produce shakiness and heart palpitations. This post discusses one of the causes of low blood glucose, reactive hypoglycemia.
Reactive hypoglycemia is important because it can be one of the steps on the path to type 2 diabetes. It is one of the possible responses to a six-hour glucose tolerance test, illustrated in the graph above. The normal person, represented by the red line, has ingested a large bolus of glucose at zero hours. The beta cells of his pancreas have released the appropriate amount of insulin, and while the amount of glucose in his blood has spiked a little, within 2.5 to 3 hours it has come back into the normal range of 70-110 mg/dl, which corresponds to a value between 4 and 6 mmol/L on the graph above. Once the extra glucose has been safely stored, insulin levels decline and the liver takes over.
As discussed in the previous post, the liver can release its stored glycogen in the the form of glucose, and it can also make glucose by gluconeogenesis. Between meals, the liver uses these two processes to keep blood glucose in the range between 70-100 mg/dl or 4-6 mmol/L.
After a person eats carbohydrates, his liver shuts down its production of glucose via glycogenolysis and gluconeogenesis. That makes sense. If the person is ingesting glucose, why would he want to add more glucose to that amount?
When a person eats protein, the situation is a little different. Insulin must be released to store the amino acids building blocks of the protein. But insulin is nonspecific. As it promotes the storage of amino acids, it will also drive glucose from the blood. Without some compensatory mechanism, the process of storing the amino acids would also produce severe hypoglycemia. In steps the pancreas. This time the alpha cells of the pancreas release the hormone glucagon. Glucagon tells the liver to release some of its glycogen in the form of glucose. The liver also begins to do gluconeogenesis to make more glucose. Thanks to the liver, glucose levels can be maintained while insulin is busy telling the body to store its new supply of amino acids. Once the nutrients are stored, the liver goes back to its baseline functions and enables the blood sugar to continue in its normal range.
So far, so good. But as diabetes develops, the liver is one of the organs that becomes insulin resistant. When the liver becomes insulin resistant, its production of glucose becomes dysregulated. The liver can no longer turn off its glucose output in response to carbs, or regulate its glucose output properly in response to protein.
Think about that. The person with insulin resistance may not be eating carbs, but his liver is making carbs (that is, glucose) all the time. In order to control the resultant high blood sugar, the pancreas must produce more insulin. That will get the blood sugar down in the short term, but in the long term it will make the liver more insulin resistant. Eventually, still more unwanted glucose will be produced by the liver, and even more insulin will need to be released by the pancreas.
In the process, the pancreas itself starts to suffer insulin resistance. It releases insulin erratically. Sometimes it allows the blood sugar to go too high. At other times the pancreas overshoots the required amount of insulin and the blood sugar drops too low. This leads to the phenomenon called reactive hypoglycemia, which is shown in the black line in the graph above. The person represented by the black line has ingested a large amount of glucose, but his pancreas has responded by releasing too much insulin, and his blood glucose has fallen below the normal range. Over time, reactive hypoglycemia can eventually progress and intensify to the condition of the person represented by the brown line, which is prediabetes.
We tend to think of type 2 diabetes as a condition characterized by high blood sugar. It is, but for many people, one of the steps on the road to type 2 diabetes is actually low blood sugar. If a person is not a diabetic but is experiencing episodes of low blood sugar, it might be time for him to consult a physician. It could be a important warning sign and an indication that he might need to make some changes in his lifestyle.
The figure is from the Hypoglycemic Health Association of Australia.