We all know that human brain depends on glucose for only energy storage. Then, what if a body is completely lack of glucose, can energy supplied from fats or proteins support the functioning of his brain?

Yes. The fatty acids from triacylglycerides (fats) and the amino acids from proteins can be broken down to TCA cycle intermediates (such as oxaloacetate). The enzyme phosphoenolpyruvate carboxylase converts oxaloacetate to phosphoenolpyruvate, which can be made back into glucose through a process known as gluconeogenesis (essentially the reverse of glycolysis).

If the brain couldn't function without glucose, I'd have died a long, long time ago. (I'm a diabetic). The problem with the metabolism of fats and proteins instead of glucose is that it results in the formation of various ketones and aldehydes which aren't all that healthy for your body. (Does a real number on your liver). So a long period of time without sufficient glucose levels in the brain can cause severe damage to other parts of the body.

Actually, that's not quite right, jdurg. Diabetes means that you simply cannot naturally control your blood glucose levels, not that they aren't there. Hypoglycemia is, I've heard, a really big problem for those with diabetes, in part for the very reason that nerves can only take in and use glucose.

 

Mokele

Hypoglycaemia is a serious problem for diabetics and can lead to coma and death. This is why it is important that diabetics recognise the signs of developing hypoglycaemia so they can take prompt action to remedy it.

Oh I am fully aware of that. Insulin is like a 'key' for the glucose molecules to get into the cells and be metabolized into energy. If there's too much insulin and not enough glucose, then all the glucose gets metabolized and the brain suddenly starts to run out of 'fuel'. I believe that in cases of hyperglycemia, simple diffusion allows some of the glucose molecules into the cells so you don't get the sudden 'oh crap, there's no more fuel' effect you get in cases of hypoglycemia. With both hyper and hypoglycemia, ketones can be detected in the urine from the brain's need to get energy and having fats and proteins as the only available source.

True, but it's not actually the neurons (or any other brain cells) making the ketones or utilizing fats or protiens. The fats and protiens are being broken down into ketones in order to make glucose, which is then dumped into the blood for the neurons to use. Iirc, the actual process of gluconeogenesis occurs in the liver.

 

Mokele

The brain MUST have glucose.

 

Fats cannot make glucose but there is a metabolic pathway for proteins to make glucose. As mentioned, it also releases ketones into the blood and therefore is a very short term substitute.

 

During TAG turnover, the body can recycle the glycerol backbone into glucose via the liver, but it's pointless as it will ultimately all go back to reform the fatty acid.