I've read a book[1] -- by an actual doctor -- saying the exact opposite. If you think about it from an evolutionary standpoint, our ancestors would have had many times when food accessibility was scarce and they would have needed to maintain their strength through a long fast. An animal that burned muscle/protein over fat is an animal that wouldn't have survived long, that would be a terrible evolutionary disadvantage. The design of our body isn't stupid -- we store fat for a reason, so that we can burn it in times when food is scarce. Burning fat for energy is both normal and totally healthy.
From a personal standpoint, I barely eat any carbs and I've lost zero strength despite dropping 25 pounds. Apparently the diet that has made me thinner and increased my energy and mental clarity is the "dangerous" one, and the diet that has made everyone obese is "healthy" and "balanced".
Fats do not become glucose (with the exception of the glycerol from triglycerides...a minute amount of the total kcals from one trig) but they do become energy. The majority of energy is in fatty acids which have no pathway to glucose.
For example, it is believed that when we sleep our energy comes from fats. We are not doing anything exerting, so the process of (fat catabolism -> energy) >= energy needs. The brain needs glucose however, so it gets it from glycogen catabolism or protein catabolism (gluconeogenesis).
Edit: For some reason I cannot reply to the comment below. The recommendation is 45-65% based upon individual need.
I've mentioned elsewhere in this thread that the brain can use ketone bodies.
I disagree that glucose = obesity as you seem to see it. Many, many things (nutrition, PA, stress, hormone issues, etc) are related to obesity.
A lot of what you emphasize is related to the lack of physical activity that the population no longer gets. If we truly want to solve obesity, it takes adjustments to diet and consistent purposeful physical activity. There's a lot that can be done with nutrition, but continuing our highly sedentary ways will always have consequences for health that nutrition alone will not completely solve.
I think it's important to separate health and obesity when considering exercise. A large body of evidence shows us that exercise makes you healthy. A smaller body of evidence shows that exercise helps prevent obesity. But there is virtually no evidence that shows exercise has a large part to play in curing obesity.
Ketone bodies can be used by the brain. It prefers glucose, but you don't absolutely need it.
Lets say for a second you're right and you need glucose, though. 65% is a really high number, and we basically know exactly what it leads to (obesity).
I'm only giving advice that I've personally tried myself, and have seen it work. I actually have a coherent theory for why people get fat, you just have... anger
I'm not comfortable with the word "designed." There is evolutionary adaptation, there is no design.
Would you have lost the same 25lbs. if you had just eaten less? In other words, does it really matter that you cut out carbs only? Are you chowing down on bacon and steaks?
From a personal standpoint, I barely eat any carbs and I've lost zero strength despite dropping 25 pounds. Apparently the diet that has made me thinner and increased my energy and mental clarity is the "dangerous" one, and the diet that has made everyone obese is "healthy" and "balanced".
[1] The Obesity Code by Dr. Jason Fung