I know it's a high aspiration of many people to try to spend their whole lives in a perennial state of ketosis, but I think it's ill-advised. 

You know how the body has two states of being: wakefulness and sleep. And the ideal situation is if they are sharply defined, where, when you’re awake, you’re wide awake and sharply focused, and when you’re asleep, you’re out like a light and dead to the world. The more time you spend in that deep, stage 4 sleep, where you’re not even dreaming, the better. That’s what really recharges your battery.

And you want that switch between the two states to work efficiently too, where you fall asleep quickly at night, and when you awake in the morning, you come around quickly and don’t require stimulants like coffee to get going.

Well, likewise, you could say that there are two metabolic states of the body: the state of eating, and the state of fasting, and fasting is marked by ketosis. So, ketosis goes with fasting; not with eating.

Ketones are compounds that result from the breakdown of fat, and they can be burned as an energy source by the body. But, it's mainly the brain that need ketones.  Although glucose is the prime energy source when a person is eating, the body doesn’t store much of it. It stores glycogen in the liver and muscles, but it gets used up pretty fast. After that, the body can keep making glucose from glucogenic amino acids (and most of the amino acids are glucogenic) but it doesn’t want to. And that’s because it would be like burning the furniture. During fasting, the body tries to limit its protein losses to the minimum. It has to break down some protein to derive amino acids for vital functions, but it doesn’t want to burn protein as fuel. Hence, it turns to fat.

The muscles can burn fatty acids directly, and the heart is especially good at it. It's called beta oxidation, and the way it works is that triglycerides are broken down into their constituent fatty acids; then these fatty acids are converted to acyl-Coenzyme A, and then, two carbon atoms at a time, acyl-Coenzyme A is reduced to another substance called acetyl Coennzyme A, and that liberates chemical energy. But, the brain can't do this beta oxidation, hence it cannot burn fatty acids directly. However, it can burn ketones. And that's what makes ketone production so essential during fasting- so that your brain has an alternative fuel to glucose. But, it is a mistake to think that a person has to be in a state of ketosis to burn fat and lose weight. That is a myth.  

Another time besides fasting that the body may enter a state of ketosis is during acute illness. Let’s face it: when people have fever and malaise and the other discomforts, such as when they have the flu, they don’t have much appetite, and they may not have any. Of course, many people will force themselves to eat “to keep up their strength.” But, even if they do, they usually don’t eat much, and their body may reject it anyway, hence, they will often enter a state of ketosis.

So, acute illness and fasting are when ketosis normally occurs. When a person goes beyond fasting into starvation, then vital proteins are lost, and there is a rise in nitrogen excretion.  And that, of course, is pathological, and no one should take fasting that far unless they are trying to end their life.

But, the point is that the body is programmed to oscillate between the two states of eating and fasting. Remember that in the modern world, food is plentiful all the time, but in wild Nature, animals may have to fast frequently. Lions go days at a time without food. Then finally, they make a kill and gorge themselves. The same is true for polar bears. And polar bears build dens out of snow to secure shelter during the harshest part of the winter where they don’t eat at all. Females will give birth in those dens and then nurse their offspring there, all without eating. There are many examples of protracted fasting among wild animals. Even primordial humans had an inconsistent, unreliable food supply that made fasting a common occurrence.

Fasting requires adaptations, and going back to eating does too. There is a metabolic switching involved, and it hinges on blood sugar and ketones. So, as blood sugar drops, that triggers the production of ketones and the other adaptations to fasting, such as, a physiological dehydration. And then when you come off the fast, the consumption of carbohydrate stems the production of ketones and triggers the adaptations that coinside with eating.

Just as with your sleep/wake cycle, in which you don’t want to dwell for very long in the nether region between wakefulness and sleep, likewise concerning eating and fasting, you don’t want to dwell for very long between the two- let alone dwell there permanently.  

So, the people who think you should stay in a state of ketosis all the time are idiots. It is simply abnormal to be in ketosis when you are eating. You should not aspire to do it.