Keto and Karate: A Good Fit?

Ketogenic Diets are very popular right now, for reasons I'm not entirely clear about. I've played with ketogenic diets on and off for the past 10+ years, and read a lot of the literature around them, so I figured it was time to talk a little about how useful they are for karateka.

What is Ketosis?
It's easier to start with a brief overview of 'normal' (I really mean typical) metabolism.
Your body needs energy all the time, in every living cell. Most of that energy comes from glucose or from fat. Your body stores both - there is glucose stored in the form of glycogen in your muscles and liver, and fat stored all over the place. When you need energy somewhere it gets glucose and/or fat from the blood and uses it for energy. When the glucose or fat in the blood get too low, more is moved from storage into the blood.
The glucose mostly comes from food (carbohydrates), but your body can make some glucose from proteins and fats (through a process called gluconeogenesis).
Some types of cells are better at using fat, others are better at using glucose. To some extent you can improve your body's ability to use either fat or glucose by eating and training in a certain way, but there are limits (you can never get your brain to run on fat).
If your cells aren't able to get enough glucose (this can happen either because you've run low on glucose OR because it can't get into the cells, as in with a Type I diabetic), your body will start making ketones out of fat. Ketones are just another molecule that your cells can use for energy. Some cells that are bad at using fat directly (like neurons) can use ketones, so you can stay alive even when glucose is in short supply. 'Normally' you make very few or no ketones.
Nutritional Ketosis is what happens when you restrict your carbohydrate intake (don't eat any) and protein intake for a while (specifics aren't important here, but 'a while' means days or weeks, not minutes or hours). At first, your body will just use stored glucose (glycogen) to fulfill its needs, but if that glycogen gets used faster than it's replenished, eventually it runs out (or at least runs very low). Then you start making ketones really quickly, to fulfill your energy needs.
Diabetics can get into ketosis while their insulin and blood sugars are both high. This is a different state and can be very dangerous. Diabetic Ketoacidosis and Nutritional Ketosis are not the same thing.
To be even more clear: on a 'normal' or typical diet, you will have very small amounts of ketones in your blood. If you eat certain foods (Medium Chain Triglycerides, as found in coconut oil) OR deplete your body's stores of carbohydrates enough, your body will start making lots of them.

What's are the benefits of being in ketosis?
There's a lot to unpack here - as with fasting, a lot of the science is mixed.
Health/Body Composition Benefits:
Being on a ketogenic diet (nutritional ketosis - very low carb, not high protein, high fat diets) might facilitate bodyfat loss. This is contentious - there are lots of reputable people who think that ketogenic diets are great for weight loss and others who disagree. I'm not going to take a stand - do the research yourself. I personally suspect that ketogenic diets are better for fat loss but hard to sustain compared to some other successful eating plans.
Having high levels of ketones in your bloodstream might have other benefits. Because ketones can provide cells with energy but get into the cells in a way that's different from glucose, they might help feed cells that are insulin resistant. This is especially interesting with the brain. There is pretty strong evidence that having ketones in the blood can help the brain resist traumatic injury and help the brain recover from injuries. Some also believe that high levels of ketones can prevent symptoms of Alzheimer's and dementia (I'm not going to say this is definitely true; I will say that I suspect the idea has a lot of merit).
There are a lot of other claims made about ketogenic diets, more than I have space to discuss here. Dig around if you're interested.
Performance Benefits:
This is another controversial area.
Your body can use ketones for energy. Your body can also make an effectively infinite amount of them. Suppose you want to run 100 miles. Your body can't store anywhere near enough glycogen to fuel that sort of effort - you'd need to eat carbohydrates along the way, and lots of them, to make it that far. But almost everyone has more than enough energy stored as fat to run 100 miles (it's not even close). So, in theory, if you could get all your energy for the run from ketones, you wouldn't have to slam down carbohydrate gels or eat along the way.
However, power is limited on ketones. You can generate energy forever (practically) using ketones, but you can't generate it very quickly. Yes, you can run 100 miles on ketones, but you won't run any of those miles at anywhere close to your maximum speed.
Having ketones and glucose in your system at the same time hasn't been well studied to my knowledge. It should provide performance benefits without any disadvantages, but it's not easy to get into that state.

So should karateka be on a ketogenic diet?
Short answer: no.
To actually get into ketosis you have to be on a very low carbohydrate diet until your body's stored glucose is very, very low. In other words, you can't have anywhere close to full glycogen levels. If you have lots of glycogen, you won't be in ketosis.
And there is abundant evidence that your body won't be able to generate high levels of power without lots of glycogen around. This makes sense - if you look at the energy systems, the glycolitic system runs on glucose. If you have no glucose around, and really deplete ATP levels, you'll crash, and the energy systems that can use fat or ketones to regenerate ATP from ADP won't be able to keep up with demand. So your body limits power output to prevent a crash.
So if you're in nutritional ketosis, your punches will lack snap, your speed will be limited - basically, you'll be a slower, less explosive version of yourself.
Some people think you can become 'fat adapted' and avoid this fate. Not true. Yes, if you exercise a lot while glycogen levels are low you can get better at using fat (or ketones) for energy, you'll never magically be able to generate peak power off of ketones.
This isn't even really controversial. Without glycogen your body is like a racecar without its highest gears. You can work hard, and for a long time, but you lose that top end speed. No amount of practice or training will completely change that.

HOWEVER:
I want to provide 2 caveats:

1. For you, it's possible that the health benefits of a ketogenic diet are more important than your karate performance. This is something you'll have to figure out for yourself. As a corollary to this, you might find benefits to being in a ketogenic diet in a periodized way - say, 1 month a year, or a couple of weeks at a time, every so often. That's fine, just be aware that your karate will suffer during those times.
2. The reduction in power output ISN'T CAUSED BY THE KETONES BUT BY THE LACK OF GLYCOGEN. So what, you ask? It is quite possible that you could get the benefits of nutritional ketosis without the downsides - by eating ketones. There are supplements out there that are just pills full of ketones. In other words, instead of getting high levels of ketones in your blood by depleting carbohydrates until your body 'thinks' it's starving, just consume ketones and let them get absorbed. This approach is relatively new - the supplements are expensive - and may or may not work. If it does, you'd potentially have a little extra endurance when you work out. Also, more interestingly, you might have some protection against head trauma. Another approach is to eat a lot of MCT (medium chain triglycerides) either in coconut oil or in a dedicated supplement. Your body turns MCT into ketones almost directly, so if you can consume enough of it you can get elevated ketones in your blood without depleting glycogen.

If I were, say, a professional fighter, I would absolutely load up on ketone supplements before hard sparring sessions or fights. We have very little reason to think it could make anything worse (performance or health) and some reasonably strong reasons that it could protect our brains. The downside is that the supplements are expensive. If you have money to burn, go ahead and try them! But nutritional ketosis is not conducive to martial arts training at any level, so I don't recommend it.