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Contrary to the recent past, when high carb - low fat diets were recommended, it appears that now the opposite type diet is often being recommended. Many doctors who have written books on the subject recommend diets high in fat and extremely low in carbohydrates, limiting them at 10% of caloric intake.

It is unclear how pragmatically one could live on just 10% carbohydrate. The latter represents the most efficient source of energy for the body. Athletic performance, endurance would suffer with only 10% carbohydrate.

I gather the idea of the high fat diet is to change the metabolic state of the body in a state of ketosis whereby given the absence of readily available carbohydrate fuel, the body learns how to use and burn fat for energy. Is this process all that healthy and sustainable over the long term? And, would it enhance cognitive capabilities over the long term as one ages.

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    Genrally speaking, asking a broad "what is best" type question will be closed on all SE sites. – JohnP Sep 15 '17 at 5:08
  • JohnP, I have completely reworded the question to render it extremely specific. I hope it will meet your standards so you can remove the "hold" on this question. Thank you in advance for your consideration. – Sympa Sep 16 '17 at 3:08
  • Narusan and JohnP, can we remove the Hold on this question. JohnP, you put this question on hold because you considered it too vague. So, I completely reworded it to make it very specific. That is only for Narusan to edit it and make it vaguer again. – Sympa Sep 17 '17 at 0:22
  • As with any website across SE network, it takes 5 votes to reopen a question and since we don't have that many high-rep users, patience is key for anything that requires voting here. On the other hand, if you don't like an edit you can always use the roll-back option. For other information on how the site woks, please check out our help center. Thanks! – Lucky Sep 17 '17 at 2:32
  • Voted to reopen, it's a good question. But unfortunately I am not aware of there being significant evidence to support either of those claims. There are doctors who ascribe to different theories on nutrition, but the strength of their evidence varies. One of my go-to's for nutrition/exercise evidence is the American College of Lifestyle Medicine. I've done some of their professional curriculum and find their evidence evaluation to be very strong. Check out the website; they also have education tools here: lifestylemedicine.org/widget/Web-Based-Resources – DoctorWhom Sep 17 '17 at 6:19
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The question is a quite broad since you don't state whether you mean a ketogenic diet, levels of protein and fats, or what you mean by "athletic and mental performance" so I will answer quite generally.

There is some anecdotal limited evidence that shows unhindered athletic performance on a Low Carb diet, such as observations made on Innuit people prior to dramatic changes in their diets. However the bulk of the literature shows impaired athletic performance, particularly in anaerobic events (eg sprints, lifting, field athletics), and no change in endurance events. Most studies also reported a 1-2 week diet adaptation time, during which most performance indicators suffered:

Burke, Louise M. "Re-examining high-fat diets for sports performance: did we call the ‘nail in the coffin’too soon?." Sports Medicine 45.1 (2015): 33-49.

Hawley, John A., and Jill J. Leckey. "Carbohydrate dependence during prolonged, intense endurance exercise." Sports Medicine 45.1 (2015): 5-12.

As for brain function, a recent review article by Koppel (2017) looked at over 50 studies and found that people suffering from neurological conditions such as epilepsy, and more recently, Alzheimer’s benefited from Low Carb, high fat diets (in particular improved cognition). Care should be taken if extrapolating these results to otherwise healthy individuals and/or athletes.

Koppel, Scott J., and Russell H. Swerdlow. "Neuroketotherapeutics: A modern review of a century-old therapy." Neurochemistry international (2017).

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