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Hypoglycemia (Low blood sugar) is said to be among the potential causes of human-faintings. I was wondering by which mechanism this happens. Google search results are abundant; there are lists of various causes and solutions, but I couldn't find a direct answer to my question.

Is it a controlled reduction of heart function to save energy? Is it an uncontrolled shock reaction (malfunction)? I'm looking for an answer along those lines.


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The following article may be of relevance: – jarlemag Apr 16 '14 at 11:38
up vote 5 down vote accepted

There are regulatory mechanisms that may prevent moderate hypoglycemia to progress to a more severe state, however these mechanisms act before lack of conciousness sets in. The progression of hypoglycemia may depend on overall health and medical status and vary between healthy and diabetic persons, for example.

Rosenthal et al. (1) write that

The brain is normally dependent on glucose for oxidative metabolism and function. Acute iatrogenic hypoglycemia, occurring as a result of insulin excess during the treatment of type 1 diabetes, can cause clinically significant cognitive impairment. In health, such hypoglycemic cognitive dysfunction does not occur. Endogenous counterregulatory mechanisms are activated in response to a small reduction in circulating glucose, preventing more profound hypoglycemia. In diabetes, the inability to reduce circulating insulin levels (1), the failure of glucagon responses (2), sometimes accompanied by defective autonomic and adrenergic responses (3), can allow plasma glucose levels to fall low enough to result in detectable disturbance of cognitive function, ranging from a subtle slowing of reaction times (3) to coma (4).

In short, the brain malfunctions due to lack of glucose (the sole energy source for the brain). Note that cognitive impairment and unconciousness can also occur in otherwise healthy persons, for example in the case of prolonged exercise combined with lack of nutrition. The exact cause of cortical dysfunction is not known:

Impairment of brain function is a recognized consequence of acute hypoglycemia. It is known that different brain functions have different susceptibilities to acute hypoglycemia; for example, the triggering of autonomic activation, a hypothalamic function, occurs in response to quite modest decrements in circulating glucose concentrations (5,6,7), whereas significant cortical dysfunction requires more profound glucose deprivation (3). Even within different cortical functions, there is variation in hypoglycemia susceptibility (26). The mechanisms of cortical dysfunction during hypoglycemia are not known.

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