This is the eighth piece in a series on how to develop a fasting lifestyle. Read the entire series here.

The first week of our plan allows you to slowly shift from a sugar-burning to a fat-burning metabolism (called ketosis), which is the gateway to successful fasting.

Our bodies have two distinct ways of converting food to energy. But most of us haven’t really used the fat/ketone one much since we were infants. We subsist on the carbohydrate/sugar cycle, which makes fasting hard. That’s because it has to be recharged every few hours. And when it’s running, the fat/ketone pathway is mostly offline. Our body is like a hybrid car with a backup gas supply that never gets used because the power charger is always nearby.

For the most part, these two systems don’t run at the same time. As long as you’re eating lots of carbs — especially carbs with a high glycemic load — your body will just stick with sugar burning and store any extra as fat. Think about that. Your body will hoard extra energy from your food as fat on your liver, in-between your organs, on your belly, hips, and thighs. But it won’t be able to use it. No one would do this on purpose. Because of the flood of bad dietary advice, though, many of us do this while trying to do the opposite.

As mentioned in the previous piece, this process is controlled by insulin, a hormone released by your pancreas. Insulin’s job is to keep your blood sugar levels from getting too high. It tells your body whether to store extra sugars and fats as body fat, or to burn fat from your diet and fat stores for energy. If you eat a lot of simple sugars and processed carbs over years or decades, your insulin levels will never get very low. As a result, you could very well become insulin and carb resistant. That means your pancreas has to keep pumping out more and more insulin to clear sugar from your blood.

Think of that insulin as a signal telling every cell in your body: burn sugar, store fat.

Why Two Systems?

Why do we have two systems then? Because we’re designed to survive in different places. For most of human history, people have had to survive lean times and fat times. That’s why people can live on every continent on earth and get by with diverse eating habits. (The 3 meals a day routine is not universal.) Polynesians can do just fine eating fish, coconuts and breadfruit. The Maasai do fine on little more than milk, meat and blood from their cattle. Inuit tribes, and Scandinavian scientists who study them, can prosper year round on little more than blubber and meat from whale and seal.

For thousands of years — outside the tropics at least — people had to endure gaps in their food supply. That means that they had to spend at least some of the time running on fat, which makes sense. Fat is more energy-dense than the other two macronutrients: carbohydrates and proteins.

Every so often, if they were lucky, our ancestors gorged on sweet berries and an unexpected store of honey. This allowed them to build up some body fat, which they then used when the berries, honey bees, and other foods became scarce.

Agriculture brought a steadier supply of grains. Except for some honey, however, no one ate concentrated forms of sugar or highly refined carbohydrates every day. That’s a modern mistake.

These days, we live on sugars and simple carbs (which our body quickly converts to sugar). So, unless you go out of your way to tap the fat-burning system, you probably won’t. The fat on your body will be like guests in Hotel California. They can check out any time they like, but they can never leave.

Fasting and Feasting

Fasting, with a feast every so often, mimics this older, natural pattern. It allows us to use both systems. It harnesses our natural design plan far better than the common modern diet, which has created what some call “diseases of civilization”: obesity, high blood pressure, insulin resistance, inflammation, Type 2 diabetes. Insulin injections treat the symptoms of Type 2 diabetes. But as the body becomes more insulin-resistant, it needs more insulin, and the problems get worse and worse.

Most of this diet damage has happened in the last sixty years. It’s gotten even worse in the last thirty years, since government started to push high carb, low fat diets. Type 2 diabetes used to be called “adult onset diabetes,” but more and more kids are getting it. Just from 2000 to 2007, diagnoses grew by over thirty percent!  And some one in three adults is pre-diabetic.

The plan I’m describing should allow you to get this fat-burning system up and running. And that, in turn, will make regular fasts — from 24 to 72 hours and more – easy.

Depleting the Sugar

Over the first few days of week one, you will deplete the stored sugar (glycogen) in your muscles and liver. Your body will use hormones to keep your blood sugar stable. To do this, it will start using extra proteins and amino acids to produce sugar (glucose) in a process called gluconeogenesis. (Literally: creation of new glucose.)

Soon, though, your liver will start converting your dietary fat to a different type of fuel, ketones. There will be bumps and starts as the new system comes online and your body gets used to the other system. Don’t forget the extra water and salt.

If you work out hard, you might even deplete your glycogen stores and start producing ketones in as few as forty-eight hours.

Why Do I Need to Eat Fat?

But if severe carb restriction (below 50 grams a day) forces your body to start turning fats into ketones, why eat so much fat? Why not just eat protein and wait for your body to start burning body fat? If you’re already insulin-sensitive and metabolically flexible (more on that later), that strategy might be okay. But, remember, protein raises insulin. So, eating nothing by protein is not a good way to turn on the fat-burning system.

Besides, most folks’ bodies aren’t used to using fat stores for energy. In that state, a very-low carb, low-fat diet is too much like starvation. If you cut protein too, then it is short-term starvation. Remember, our goal to make real fasting part of our everyday life, not suffer starvation. So, it’s much better to give your body extra dietary fat while it’s adjusting to its alternate fuel source.

Again, take a cue from Lent, which lasts forty days.

In the next installment, I’ll describe week two.

Jay Richards, PhD, is the Executive Editor of The Stream and an Assistant Research Professor in the Busch School of Business and Economics at the Catholic University of America. Follow him on Twitter.