I've been working out for almost thirty years, and low carb for almost ten. I didn't notice any real difference, in the long run, with or without carbs.
Ben Greenfield is a triathlete, and that's skewed his perception some. If a person is keto-adapted they might be less prone to bonking--but there are limits to what the human body was meant to be put through on a regular basis. He ran into thyroid problems (pun unintended), so now he buys into the theory that a certain minimal amount of carbohydrate is necessary to maintain proper thyroid function. Maybe that's what he's talking about.
During total starvation, a human being will manage to burn a fair amount of fat--but total oxidation of glucose will only be somewhere around twenty to thirty grams. If fat burns in the fire of carbohydrate, it sure doesn't take much.
In that state, almost half the glucose that's oxidized is replaced by synthesis from glycerol that's released during lipolysis.
He's right about more glucose being used during weightlifting type activities, but not necessarily about this having anything to do with needing to eat carbohydrate. Lifting weights doesn't completely oxidize glucose--it breaks it down to pyruvate and lactate, this releases energy faster than you can get it from fat. Pyruvate can feed into the mitochondria as Acetyl-CoA, and that's an irreversible step--glucose carbon is lost to the body. But if you're fat/keto-adapted, your body will have lots of Acetyl-CoA available from breaking down fat, so production of Acetyl-CoA from pyruvate is greatly downregulated--so a more likely fate of the pyruvate or lactate is that it will be used in the process of gluconeogenesis to replace the glucose that was broken down. During starvation, hundreds of grams of glucose are normally broken down to pyruvate--this is how red blood cells get all their energy, they have no mitochondria--but only that twenty to thirty grams or so of glucose is permanently lost to the system.
I've pushed this thing so far as doing my usual workouts while doing a fat fast. Long term, I'm sure the low protein would hurt--but short term, my workouts got better
, not worse. Although I put that under the do-not-try-this-at-home category.
There are mouse studies where a ketogenic diet of 95 percent fat and 5 percent protein calories resulted in mice with a higher percent body fat. When researchers tried putting a running wheel in their cages, suddenly the diet wasn't fattening anymore. The ketogenic diet didn't make them fat unless it was paired with a particularly unstimulating environment that kept the mice from being their naturally active selves. If you've ever seen a rodent in a cage or in a field, the difference in activity is pretty obvious.
Point is--these mice managed to burn fat with no carbs, and with so little protein that they didn't dare make much of it into carbs. More evidence that "fat burns in the flame of carbohydrate" might be true to a certain extent--but we really aren't talking much carbohydrate here.