Suddenly “Keto” is everywhere. I even saw these Slim-Fast Keto bars this week at my local Giant supermarket:
What is this new fad, and what’s the science behind it?
Quick background in case it’s helpful: Foods contain three macronutrients (substances that provide energy/calories): carbohydrates, proteins and fats. (Interestingly, alcohol is technically a fourth macronutrient, at 7 calories per gram, compared to 9 for fat and 4 for both carbs and protein.)
The body breaks down carbohydrates (sugars and starches) into glucose, which provides energy for the body and especially for the brain. Although the brain makes up only about 2% of body weight, it can use up to 20% of daily calories. And yes, it seems that demanding mental work does burn slightly more calories, so keep up the lifetime learning!
This summary is better than mine
For easy access, the body stores about a 1- to 2-day supply of glucose in the muscles and the liver. Note: when this storage is full, if you consume more carbohydrates, the body stores them as fat (insulin is the messenger giving this signal).
The body can also pull energy out of stored fat as needed — BUT the energy from fat isn’t in the form of glucose. The rest of the body can use it just fine, but the brain prefers glucose.
There’s even a backup system (gluconeogenesis) to produce glucose out of protein.
And there’s one more backup system in our incredibly complex and brilliant bodies (here’s where keto comes in, finally!). When we run out of glucose, our bodies can use fat to produce a different fuel for the brain: ketone bodies.
So … if we eat very little carbohydrate, and not too much protein (enough to repair and maintain our body tissues, which is the usual function of protein, and not so much that the excess is just begging to be turned into glucose), after our 1- to 2-day glucose supply runs out, the body will burn fat for energy and produce ketone bodies from fat to fuel our brains. That’s called being in ketosis.
Clearly one way to get there is to fast for more than 1-2 days, since we aren’t replenishing our carbohydrates. You’ve likely done this before, for instance when you had a stomach bug and couldn’t eat for a couple of days. (You were in ketosis! Not so strange or revolutionary … )
Question: in this situation, why doesn’t the body just reach into its protein stores and form glucose via gluconeogenesis? Answer: Protein isn’t really stored in the body — it’s in the form of functional tissues such as muscles. It would be pretty foolish for the body to eat up its own muscles when it has plenty of fat cells hanging around full of juicy 9-calorie-per-gram energy. Of course in the case of starvation, when fat stores are used up, muscle will be broken down to fuel the brain. Some muscle can also be broken down when a lot of energy is needed quickly, because extracting energy from fat is a relatively slow process.
Besides fasting, another way to get into ketosis is to eat mainly fats, a very low amount of carbohydrate (less than 20-30 grams per day), and moderate protein.
This is the ketogenic diet, which has a well-accepted medical use to prevent epileptic seizures in children.
But is it good for the rest of us?
My own January challenge for myself was to get into ketosis for a week, try it out and form my own conclusions. Find out what happened in my next blog post!