Hyperglycemia means high (hyper), glucoce (gly) in the blood (emia). As you see in the chapter symptoms diabetes (1), hyperglycemia is defined above 200 mg/dl or 11.1 mmol/L, even though there is no consensus for an exact level here as well. There is no evidence a temporary high glucose is dangerous, and you can actually feel well with it as well. If continuously having high glucose you might use your body that it´s normal, and feel quite ok. You though get thirsty and polyuria, frequent urination, and you can get dehydrated.
Too high values, too often, can in the long-term lead to serious health problems. One short-term consequence is, if having insufficient insulin the glucose cannot enter the cells to be used as energy and the body start burning fat. The liver converts fatty acids to ketones which also can be used as energy. As can be seen in the chapter insulin (2), insulin have many tasks in the body. Everything affect the glucose and insulin need. With insulin treated diabetes, these are the challenges:
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