This came to my attention during an e-mail discussion I was having about whether the human brain required any glucose – at all. Those taking part were the usual suspects, Richard Feinman, Gary Fettke, Nina Teicholz, Jimmy Moore, Jason Fung, Tim Noakes etc. [Yes, good bit of name dropping there].
The consensus was that the human brain could use Ketone bodies for much of its energy requirement. However, there was an absolute need for about forty grams of glucose per day. The final statement on this matter, the one everyone seemed to agree on anyway, was as follows:
1) The brain requires no dietary glucose. It has a requisite use of 40 grams/day, but these grams can easily be provided from glycerol, and normal ingestion of not particularly high amounts of protein in a high fat, zero carbohydrate diet.
2) But this is a time dependent situation. Short term fasting will not be a problem for most otherwise healthy people. However, more prolonged starvation will eventually kill you as the brain will pirate 40 grams of glucose/day from protein and lipid, until you have neither fat stores, nor adequate diaphragm or heart muscle left to survive.
Don’t worry, there were about a thousand papers quoted in creating these statements, so the science seems robust. This discussion started because I had an interest in how hunter gatherers, who ate no carbohydrates, kept their brains going. What was the mechanism by which the Massai, Inuit and Sami, power their brains with glucose, if they don’t eat any carbohydrates?
Well, it seems that you can get a certain amount of glucose from fat. Fat is made up of triglycerides, and each triglyceride contains three fatty acids and one glycerol molecule. Two glycerol molecules stuck together (by the liver) makes one glucose molecule.
In short, pure fat does contain some glucose, which can be used to power the brain. However – assuming you are eating no carbs – the brain requires more glucose than can be provided by the glycerol held in triglycerides. Thus, you still need to convert some dietary protein into glucose. If you are not eating any food at all, the body will need to break down muscle to get at the protein required to synthesize glucose.
To cut a very long story short, the end point of the discussion was an agreement that you do not actually need to eat any carbohydrates to remain heathy. The body, and the brain, can get all the glucose it requires from glycerol and dietary protein.
The reason why I was interested in this issue was that ‘the absolute need for carbohydrates’ is a ‘fact’ that is thrown at me from time to time by ‘experts.’ I have always known they were wrong, because there are people e.g. the Massai, who never eat any carbohydrates, and remain far healthier than any expert I have ever cast my eyes upon. However, I wanted to be sure of the facts.
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