As she described things, you either run on glucose or on ketones. In my opinion, she left out an important alternative. She didn't broach running on FFAs.
A properly-functioning "young" body on a mixed diet (we'll leave alcohol out for now) gets a reasonable glucose peak after a meal, and burns the sugar off preferentially until the increase in insulin stores the excess and both glucose AND insulin drop. Then it burns fats. That's what the human body is designed to do.
Everyone observes that it takes a couple of days for ketones to start churning out in bulk, when starting a LC diet. What are you running on before this happens? First you burn through your stored glycogen, and the body has to perceive a glucose shortage ... then you get the magic -- physiological insulin-resistance and gluconeogenesis for those tissues which insist on using glucose. Hell, even THE BRAIN is now known not really to run exclusively on sugar -- the sugars seem to be converted to lactate. And the real magic: the brain is now recognized to turn FFAs to ketones, IN SITU. Essentially, part of the brain DOES run on fat, just not in the same way muscle cells do.
I believe that Amber is missing part of the equation. I believe the traditional fuel use went something like this in a healthy person:
- meal eaten of some mixture of protein, fat and carb;
- glucose preferentially burnt for a couple of hours;
- excess energy stored;
- until next meal (hours after -- no snacking for most of history), body seamlessly goes into lipolysis upon diminution of postprandial insulin;
- ONLY extended undernutrition leads to heavy ketone-body production. Otherwise, rinse-repeat.
It's only in the "damaged" body that you find difficulty in switching between fuels -- that's why restablishing metabolic flexibility is crucial in a person who has been overweight and is rectifying their dependence on glucose as the primary fuel.
That's also why fat-fasting is a tool for forcing a recalcitrant body into burning fat for fuel, and shouldn't be a lifestyle in and of itself -- it's too low in protein. That's why chasing high ketone-body concentrations is an academic sport and shouldn't be our GOAL for weight-loss or -maintenance. Ketone bodies in your blood imply undernutrition, which we consider "good" for weight-loss --
UNLESS you're "cheating" by taking ketone-salts or MCTs! Then it means "nothing." You can produce ketones on a very-high-carbohydrate (percent) diet, if you get the calories low enough. Weight-loss requires burning the fat off your butt for fuel -- not creating lots of ketones from your dietary fat.
When we use ketones for brain-health, that's a different story. Wooo's neurological issues are ameliorated by a diet which is protein-limited and high in the right kinds of fats -- burning too much glucose/lactate gives her a "short circuit." Galina prevents migraines with the diet she has refined. I find that running my brain on fat/ketone makes it function much better -- i sometimes wonder if i wouldn't be an Alzheimers risk, if i had continued to be a glucose-burner.
[aside: from a link someone put on twitter this week, I was inspired to wonder if we "20th-centurions" are predisposed to neurological sensitivity by low fetal DHA....]
Our hearts and other muscles seem to love to run on fats. They are "cheap and easy" fuels, which put no strain on the body to release and utilize ... so long as one has the metabolic flexibility to tap into them.