Diabetes mellitus is a condition characterized by severely fluctuating blood glucose levels. Hypoglycemia is a condition where blood levels of glucose drop to an abnormally low level.
Brain and blood glucose
Most organs and tissues depend on blood glucose providing energy for various functions. In most organs, a shortage of blood glucose triggers the breakdown of glycogen stores in the liver to produce glucose. Unlike other organs however, the brain depends on a continuous supply of glucose while having a very limited glycogen store (around 0.1% of total brain weight). Despite the fact that the brain forms only around 2% of a human's body weight, it consumes nearly 25% of the blood glucose in the body due to its high metabolic rate.
As the blood levels of glucose falls, the brain quickly utilizes any glycogen supplies and exhausts them. The brain can use some amount of ketones for energy during starvation. However, these ketones are break-down products of fat and cannot meet all of the metabolic demands of the brain.
Diabetic hypoglycaemic coma
The brain is more vulnerable to hypoglycaemia and the consequences of hypoglycemia than any other organ. As the brain levels of glucose fall, the levels of amino acids such as glutamate and aspartate that act as excitatory amino acids are raised. This leads to sodium and water influx causing cellular edema in the brain. Calcium then flows into the cells and causes further dysfunction leading to apoptosis or death of neurons.