TL;DR I eat lots of fat to switch my body into metabolic fat burning.
Ketosis is a state in which your body makes a fundamental metabolic switch from burning glucose (sugar) to burning fat as the primary fuel source. To enter ketosis you must restrict carbohydrate intake to less than 50 grams a day for a minimum of two weeks.
There are many benefits to ketosis that will warrant another blog post. But I'll indulge you briefly:
- Increased consistent energy
- Profoundly anti-inflammatory
- No cravings for sugar
- Ketosis is the best dietary intervention for weight loss (not my ultimate goal, but still).
To get a better understanding, let's do some quick metabolic homework.
Fat, exercise, and your brain
Warning: This is a simplification of a complex process.
During exercise you mainly burn sugar, not fat. This is because of a simple metabolic requirement: your brain. It runs exclusively on glucose, and a lot of it! It's 25% of the body's glucose needs, even though it's only about 3% of your body mass. Because of this need, you must constantly metabolize glucose for energy. The problem is that you arent't accessing your fat stores.
Let's think about this. How much sugar is in your blood right now, assuming you aren't diabetic? And a follow up question, how much sugar can your body store at any given time? Ask the nearest person to you and see what they think.
- In your blood: about 8 grams, or 2 teaspoons
- Total storage: about 400 - 500 grams in your liver and skeletal muscle.
Side note: for every gram of glucose you store in your muscle (called glycogen), you need to store about 3 grams of water, because the body metabolizes the glucose in a process called hydrolysis, which requires water. Hence, people who start working out gain "water weight" because their muscles store more water along with glycogen.
Now for the kicker: how much fat can a human being store? The answer is basically: infinite.
As human beings, why would our brains be reliant on something like sugar, which we can store so little of? At 10% body fat, you have more than 40x more calories to burn than all your sugar stores combined. Why would we constantly be in a state (non fat adapted) where we can't access that boundless energy?
Insulin and why you're fat
Remember how you only have 8 grams of sugar in your blood stream? If you have much more, called hyperglycemia, you go into a diabetic coma. Your body heavily regulates glucose to keep it within normal levels. But look out! The only way for your body to deal with that glucose is to send it through your blood to somewhere else. So what happens?
Eating a sandwich = 2 slices of bread = 50 grams(!) of sugar dumped into your blood as you digest it. Your body has to keep that down, so your pancreas releases the hormone insulin, which tells a whole bunch of mechanisms in your body to clear glucose from the serum.
One of three things will happen:
- You will metabolize (burn) the glucose if exercising heavily. What were you doing the last time you ate a sandwich?
- You will store the glucose in your muscles as glycogen, assuming your stores are empty (they aren't, but get depleted by exercise)
- Glucose will be converted to triglycerides (fat!) in the liver in a process called de novo lipogenesis.
You read that right, what increases your circulating triglycerides is not dietary fat, but dietary carbohydrate! 
Perhaps more importantly, insulin tells your fat cells to do the following:
- Stop releasing fat.
- Start storing fat.
Now you have excess triglycerides (fat) floating around, you are repeatedly spiking your cells with hormones (insulin) and your body is in pure fat storage mode. Congratulations on eating a "balanced" meal!
Finally: What is ketosis?
Ketosis is when your liver metabolizes fat and converts it to ketones (pronounced key-tones). A ketone is an organic molecule that is used by your brain, heart, and muscles as fuel. Ketones themselves are not fat, but they are produced by the liver metabolizing fat. They are the energy molecules that your brain will now use for up to 75%(!) of its fuel.
If you remember the Krebs Cycle from high school biology, ketones replace glucose in the Krebs Cycle to make ATP. In fact, ketones are used by mitochondira more efficiently than glucose .
When you restrict carbohydrates to less than 50 grams a day for at least two weeks, your brain will realize it needs a different fuel, and tell your liver to metabolize fat and produce ketones. Your fat cells are in release mode now without excess insulin to keep them in storage mode, and your body happily and efficiently uses all that fat for energy. That is how you become fat adapted.
Is it safe?
All humans are capable of safely entering ketosis. It's deeply rooted in our evolutionary history. Consider these points:
Babies are born in ketosis
. Their brains run on only 1/3 glucose.
We've only had access to abundant carbohydrates since the dawn of agriculture, which is less than 1% of the 4 million years we have been humans. What about the other 99%?
The only macronutrient you can restrict is carbohydrate. There are three macronutrient groups: protein, carbohydrate, and fat. If you eat very low fat, you die. If you eat very low protein, you die. If you eat very low carbohydrate, you safely enter ketosis, and thrive.
What about diabetic ketoacidosis, bad breath, alcoholic ketoacidosis, and excess artery clogging fat? What about all that cholesterol you're eating? What about scurvy? What about fatty liver? (Ironically, ketosis is a treatment for fatty liver ).
These are interesting questions that require examination. Rest assured the answer is: yes, ketosis is safe.
I'll let you know when I get scurvy and die from acid blood, don't worry.
There are many incredible resources out there, but be warned! This isn't a topic you can fully understand from a blog post. If you are curious about the benefits of ketosis, pick up a book!
The Art and Science of Low Carbohydrate Living Voleck and Phinney 2011. Written by doctors, for doctors, based in decades of research and patient studies.
The Art and Science of Low Carbohydrate Performance Voleck and Phinney 2011. Answers the questions of how to live on low carbohydrate, aimed at the average (non diabetic) person and athelets.
1hr Lecture by Dr. Voleck If you learn well by video, and have some time to kill, this lecture covers many of the points in his book.
How much Glucose Does Your Brain Really Need? Mark Sisson. The paleo blogger to follow, answers the common misconception that your brain runs entirely on glucose.
In Part II I talk about my first month of keto adaptation.