When you are on a ketogenic diet, the mitochondria in your cells — the parts of the cells that produce energy — actually switch from primarily using sugar for fuel to primarily using fat for fuel. They use fat mostly in a form called ketone bodies (or, commonly, ketones), thus a ketogenic diet.
(See Keto-adaptation: what it is and how to adjust for more on this process of switching fuels.)
Sugar-based living (from a diet with more than about 5% calories from carbohydrate)
When you are using the sugar-based system, all of the cells in your body constantly take sugar out of your bloodstream. It’s hard for your body to keep up, and you need to frequently refuel by eating carbohydrate-containing food.
Getting sugar out of the carbohydrates that you eat is a blunt tool. Unless you eat in a trickling stream, you will consume more sugar than is safe to hold in the bloodstream at once. That sugar has to be quickly removed, because high blood sugar damages your cells. So a flood of insulin comes in to initiate the process of sugar removal. There is some limited storage space in the liver, but when that is full, the rest basically gets stored as fat.
Soon however, the job is done. Your blood sugar is back in a safe range. Your body cells are still demanding sugar, though, and your blood sugar starts to drop too low. Your liver can release some sugar back into the bloodstream, but not fast enough to keep up with demand, so you get tired and hungry, and the process starts all over.
- People on carbohydrate-based diets typically have to “snack” every couple of hours.
- Endurance athletes have to stop and eat sugar just to get through their events.
On a sugar-based metabolism, you swing between too little blood sugar and too much, and you have to constantly adjust it “manually” by eating.
On the other hand, when you are using the fat/ketone-based system, there is relatively little demand on your blood sugar. There are only a few kinds of cells that don’t have the ability to use ketones (or fat) for fuel, and have to use sugar. That amount can easily be supplied by the liver, which actually manufactures sugar out of protein on demand in response to changes in blood sugar.
The liver can do this at a rate that is more than adequate for normal energy requirements, when the cells that can use fat/ketones are doing so, and thus not putting extra demand on blood sugar.Your body makes significant ketones only when sugar levels are consistently low.
- It is not uncommon for ketogenic adults to comfortably eat one meal a day.Even my children, when they are ketogenic, can go for several hours at a time without eating.
- Endurance athletes on ketogenic diets don’t “hit the wall”. They have the ability to tap into fat stores for fuel; a supply that could last even a lean person for weeks.
On a fat/ketone-based metabolism, demand on sugar from the blood is gentle, and your liver refills it smoothly on demand as it is used, keeping it remarkably steady.
An argument for ketogenic metabolism as the default human state
Think about the role of the liver here. When you are not consuming sugar in food (from carbohydrates), your liver orchestrates fuel management precisely.
The liver makes ketones out of fat, thereby supplying almost all tissues with all the energy they need. At the same time, it makes a moderate amount of sugar out of protein. That sugar is stored right in the liver in the form of glycogen. Not much is stored, but it is plenty for the purposes of keeping blood sugar steady, because the blood sugar is depleted slowly: only a few tissues draw sugar from the blood. The rest are using fat or ketones.
This system is efficient and effective.
By contrast, on a carbohydrate-based diet, the storage capacity of the liver overflows. The excess sugar can be stored as fat, but that fat is not used efficiently as fuel. For fat to be used efficiently as fuel, the liver would have to be turning it into ketones at a high rate . But ketones aren’t produced much in a high sugar condition. It takes several days of low sugar intake to start producing significant ketones.
Considering how well the liver manages energy when you don’t eat carbohydrates, and the disruption caused to this system by eating them, I would suggest:
The conditions under which the liver delivers optimal fuel on demand may be the conditions under which it evolved.
As pointed out by Valerie and Ash in the comments, and by Carol Loffelmann on Twitter, fat remains an important fuel in its own right, increasingly so after keto-adaptation. I oversimplified here, but the point about efficient fat/ketone-based metabolism under low sugar conditions still stands.
Consider a popular alternative explanation:
Noticing that excess sugar can be stored as fat, and yet knowing that excess fat storage is not healthy in humans, some people have argued that excess fat storage in humans represents an adaptation gone awry:
The thrifty gene hypothesis supposes that we used to go through periods of feast and famine, getting fat, and then using it up. People who were well adapted got fat easily, and thus survived the famines better. Here and now, where famines are rare, those people would simply get fatter and fatter.
There are at least two problems with this idea. First, the evidence doesn’t seem to bear it: famines may not have been particularly common in Paleolithic times, they don’t appear to have occured at all in some populations that we know later developed obesity on modern diets, and modern hunter-gathers don’t get fat in times of plenty. Second, even if this were essentially correct, that would mean that humans were adapted to go through regular periods of using up the fat. In other words, it would mean humans were adapted to regular periods of ketogenic metabolism! So, at best, this theory supports ketogenic metabolism being a regular part of life in some kind of alternation with carbohydrate-based metabolism.
There are animals that use this kind of strategy. Those animals hibernate. Humans can’t hibernate. Even very fat humans need some level of protein to survive, to make into sugar for the few tissues that need it. If they don’t get it, they will start tearing down essential muscle tissues such as heart, and they will die.