From Mark's Daily Apple:
When describing someone that has successfully made the transition to the Primal way of eating I often refer to them as “fat-adapted” or as “fat-burning beasts”. But what exactly does it mean to be “fat-adapted”? How can you tell if you’re fat-adapted or still a “sugar-burner”? I get these and related questions fairly often, so I thought I’d take the time today to attempt to provide some definitions and bring some clarification to all of this. I’ll try to keep today’s post short and sweet, and not too complicated. Hopefully, med students and well-meaning but inquisitive lay family members alike will be able to take something from it.
As I’ve mentioned before, fat-adaptation is the normal, preferred metabolic state of the human animal. It’s nothing special; it’s just how we’re meant to be. That’s actually why we have all this fat on our bodies – turns out it’s a pretty reliable source of energy! To understand what it means to be normal, it’s useful examine what it means to be abnormal. And by that I mean, to understand what being a sugar-dependent person feels like.
A sugar-burner can’t effectively access stored fat for energy. What that means is an inability for skeletal muscle to oxidize fat. Ha, not so bad, right? I mean, you could always just burn glucose for energy. Yeah, as long as you’re walking around with an IV-glucose drip hooked up to your veins. What happens when a sugar-burner goes two, three, four hours without food, or – dare I say it – skips a whole entire meal (without that mythical IV sugar drip)? They get ravenously hungry. Heck, a sugar-burner’s adipose tissue even releases a bunch of fatty acids 4-6 hours after eating and during fasting, because as far as it’s concerned, your muscles should be able to oxidize them (PDF). After all, we evolved to rely on beta oxidation of fat for the bulk of our energy needs. But they can’t, so they don’t, and once the blood sugar is all used up (which happens really quickly), hunger sets in, and the hand reaches for yet another bag of chips.
A sugar-burner can’t even effectively access dietary fat for energy. As a result, more dietary fat is stored than burned. Unfortunately for them, they’re likely to end up gaining lots of body fat. As we know, a low ratio of fat to carbohydrate oxidation is a strong predictor of future weight gain.
A sugar-burner depends on a perpetually-fleeting source of energy. Glucose is nice to burn when you need it, but you can’t really store very much of it on your person (unless you count snacks in pockets, or chipmunkesque cheek-stuffing). Even a 160 pound person who’s visibly lean at 12% body fat still has 19.2 pounds of animal fat on hand for oxidation, while our ability to store glucose as muscle and liver glycogen are limited to about 500 grams (depending on the size of the liver and amount of muscle you’re sporting). You require an exogenous source, and, if you’re unable to effectively beta oxidize fat (as sugar-burners often are), you’d better have some candy on hand.