Can very-low-carb diets impair your mental faculties?

I wanna tell you a story...

In 2006, I had a test done on my pituitary gland*, called an Insulin Shock Test. It was pretty much what it said on the tin. I laid on a hospital bed, I was injected with an overdose of insulin and I was monitored for blood glucose and growth hormone levels every 30 minutes.

My blood glucose fell and fell and fell and fell until it reached ~1.5mmol/L (~27mg/dL). What happened was interesting. I came out in a cold sweat and developed a tremor. Apart from that, I felt fine. The consultant in charge was chatting to me and I was chatting to him. Eventually, I was given a sandwich, a yoghurt and a banana to eat.

Some time later, when I had stopped sweating and shaking, the consultant returned and told me that I had become confused. My brain had ceased to function correctly due to a lack of blood glucose (the insulin had totally suppressed serum FFAs and ketones, so there were no other brain fuels available), but I was too mentally-impaired to know that I was mentally-impaired. As far as I was concerned, everything was fine & dandy. This is the Dunning-Kruger effect.

Referring to Blood Glucose, Insulin & Diabetes, as blood glucose falls due to either starvation or a lack of dietary carbohydrate, insulin falls and glucagon rises, stimulating the liver to convert liver glycogen into glucose for export to the blood. Eventually, liver glycogen becomes depleted and blood glucose falls again. The pituitary gland notices this and secretes AdrenoCorticoTropic Hormone (ACTH) which stimulates the adrenal cortex to secrete cortisol. The adrenal glands are also stimulated to secrete adrenaline (a.k.a. epinephrine). Cortisol increases the conversion of amino acids and other substrates into glucose by the liver and kidneys. If blood glucose continues to fall, the pituitary gland secretes growth hormone (GH), which has an anti-insulin effect.

From http://anthonycolpo.com/boosting-growth-hormone-with-diet-training-fact-or-fiction-part-2/:-
"after the zero-carb phase, subjects reported symptoms of hypoglycemia that included weakness, irritability, mental confusion, nausea, hunger, cold sweating and disturbed co-ordination. GH levels were higher during exercise after the low-carb phase, but so too were levels of other fuel mobilizing hormones such as epinephrine, glucagon, and cortisol." Epinephrine, glucagon, and cortisol are stress hormones. Chronically-high blood levels of stress hormones disrupt sleep and cause irritable, aggressive behaviour.

Ethanol inhibits gluconeogenesis in the liver (possibly also in the kidneys) resulting in worse hypoglycaemia. Worse hypoglycaemia results in more ACTH & adrenaline secretion and worse hypercortisolaemia, which can adversely affect the hippocampus, impairing memory.

In conclusion, if you want to eat a very-low-carb diet, long-term:-1) Don't do much high-intensity exercise. See "Funny turns": What they aren't and what they might be.
2) Don't drink much (if any) booze.

See also Can Low-Carb Diets Make You Crazy?

*My pituitary gland failed the test by secreting only 40% the amount of GH that it was supposed to. For six months, I was given GH to inject using a special pen with a 8mm x 0.3mm needle. By the end of the six month trial, I had perfected the art of painless injection. The trial was discontinued due to lack of any noticeable benefit (17 days supply of GH @0.3mg/day cost £120!).