The brain burns ketones and prefers ketones over sugar. You can make glucose from fat. I hope this is useful.

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I read some of your nutrition advice on fad diets. I would like to point out that it has been known for decades that the brain burns ketones (Nelson et al, JBC (140):p.361, 1941; Owen et al. J Clinical Invest.46:1589-95, 1967; Hawkins and Krebs (there’s a famous name) Biochemistry J. 122:13-18, 1971. In fact, the brain prefers ketones over sugar. If my memory serves me this was found out by a group studying fasting conscientious objectors from WW II. In a long-term fast, such as Gandhi would endure, the blood glucose levels fall below that which would sustain consciousness if induced to fall by insulin, for example. Clearly, the brain is burning something else.
Also, you can not make an appreciable amount of glucose from fat, only from the glycerol as fat breaks down into two carbon units, but you are correct that amino acids will do the trick.
In addition, there is research indicating that in hypoxic (low oxygen) situations the brain is protected if it is metabolizing ketones relative to sugar. I also recommend you glance at the article in Scientific American on the effects of caloric restriction and aging, Jan 1996. I hope this is useful, I apologize for the tone but I am in a rush.

Thanks for your input.

Yes, the brain can burn ketones though mental processing seems to become a bit sluggish on ketones as the brain prefers glucose from the blood as fuel. The current accepted practice in nutrition is to not recommend ketogenic or fasting diets as they are short-term weight loss diets that result in yo-yo weight gain and are not long-term plans for sustained weight loss. There are other factors at work with ketogenic diets i.e. the loss of lean tissue mass, which decreases metabolic rate, but because break down of lean tissue (muscles and organs) yields glucose, the blood sugar is maintained in a safe range.

The researcher who had conscientious objectors fasting during W.W.II was Ancel Keys did his work under the bleachers at the University of Minnesota and died in 2004.

Yes, research has proved that people who are slightly underweight and ear fewer calories than their body needs may live longer. People need to examine how this would impact their individual quality of life to have more quantity of life.