How long does it take to get into ketosis?

How long does it take to get into ketosis?

Published: 05 November 2022

Dr Paul B Chell and Dr M Hope-Ross


Introduction

Evolution has gifted humans with two fuelling systems. One uses fats, and the other uses carbohydrates. In truth, we usually use a bit of both, and the balance depends on circumstances. For example, initiating movement, and short power bursts, like jumping or sprinting, are when our muscles use carbs to fuel. Conversely, when we are walking or jogging slowly, we predominantly burn fats. This complex subject is outside the scope of this article but will be the subject of another blog. So, if you’re that exercise fiend, watch this space.

Most of our body’s 37 trillion cells, including skeletal muscle, can fuel on fats or carbs, depending on what we’re doing, and the availability from our diet. But certain cells can only fuel on glucose from carbohydrates. So, in the absence of dietary carbs, glucose must be produced by the liver, from stores of carbs, fats and proteins.

The brain uses 20% (approximately 400 Cal) of our body’s daily energy. And as the liver has a total of 400 Cal of stored glucose —in the form of 100g of glycogen—the liver has sufficient for 12-hours. After the liver glycogen is depleted, and in the absence of dietary carbs —like fasting— the liver can shift into manufacturing mode, known as gluconeogenesis. It does this from fats and proteins, which can trickle out another 100g per day. This produces plenty of glucose for our red blood cells and kidney cells, which can only fuel on glucose.

The brain is a very special case — in times of plenty, its primary fuel is glucose, and it cannot use fats, which cannot cross from the blood into the brain. After 12 hours of fasting, the brain switches to 70% fuelling on ketone bodies, its alternative fuel. Ketone bodies are manufactured in our liver from amino acids and fats. It may well be that this ‘switching’ of fuels, with a lower utilisation of glucose, has beneficial effects in ‘cleansing’ the brain. This is partly why periods of ketogenic diet are favoured by doctors treating neurological disorders like epilepsy and have been doing so for over 100 years. It is also responsible for the clarity of thought described thousands of years ago, by Greek philosophers. Research continues in both animals and humans as to the potential benefits of fasting on neurodegenerative diseases from multiple sclerosis to dementia.

So, when we enter ketosis after 12-hours of fasting, we produce enough glucose and ketone bodies for our organs to continue unabashed. This new metabolic state can continue for many days or even weeks, surviving simply with water.

When you eat, any excess energy from your food goes into storage. You have two separate energy stores for carbohydrates and fats. Your carbohydrate stores are relatively small, amounting to 150 grams in the liver and 450 grams in skeletal muscle. It should be noted that muscle is not an exporter of glucose, keeping its glycogen for use in locomotion. Your carbohydrate stores will last for just of few hours of exercise maximally. Your fat stores are much larger, and in some corpulent individuals, is actually enormous! Normally we can store 15kg of fat— 135,000 calories— and this can last for over 200 hours of exercise. It is possible to live off your fat stores for many weeks, when adequately hydrated.

Ketosis means fat loss, fat loss means weight loss
Ketosis means fat loss, fat loss means weight loss

How long does it take to get into Ketosis?

Well, if only there were a simple answer to that one! Despite big variability in space and time, we can at least point out some basic truths and principles. But first, let’s look at the different stages of body fueling, during a typical night-time meal and sleeping fast.

1. Post-Absorptive stage

0-4 hours: finish balanced meal, around 6.00 pm The body is now in the post-absorptive stage, and the macronutrients from the last meal feed our metabolism, with any excess calories stored as glycogen or fat. Some amino acids will enter muscles for repair and regenerative purposes. Main hormone—insulin, the fat storage hormone. Insulin also lays down liver and muscle glycogen and stimulates protein to enter muscle for repair and regeneration.

2. Glycogenolysis stage

4-16 hours: Sleep for 8 hours —7.00 am wake up, no ketosis. On wakening, you either fast or eat a keto diet. Exercise now will shorten this stage. During this phase, energy is provided from glycogen stores. The liver has 150g of stored glycogen and the muscles 450g. Although the muscles do not export glycogen and use it for their own energy, when depleted by exercise, they exert a “pull” effect on the liver to produce more glucose. Main hormones involved—glucagon and adrenaline

3. Gluconeogenesis stage

16 hours to 10 days: this stage is incredibly variable. For some people this stage lasts for 2 days and for others all the way to 10 days. There are so many variables here, but let’s examine a few. This is the phase where the liver creates glucose from amino acids (predominantly) and fats. In other words, your own protein is broken down to maintain your blood glucose levels, a prerequisite for life. You will also lengthen this stage by too much dietary protein via insulin (opposite effect to glucagon), and stress via cortisol. Again, exercise will increase insulin sensitivity, reducing the effect of dietary protein lengthening this stage. Main hormones involved—glucagon, growth hormone, adrenaline and cortisol.

4. Protein conservation stage

2 days to 10 days: This begins when stage three ends. This can be measured by the nitrogen disappearing from the urine. This tends to occur more quickly in people who are already fat adapted. Also fasting will induce this faster than ketogenic diet, because of the insulin effects of the protein in the ketogenic diet. No matter which route you take, you will need to pass through all these stages sequentially. The speed at which you achieve this, can be highly variable, and influenced by fat adaptation and the method utilised. Main hormones involved—low insulin is the main trigger for ketogenesis, increasing free fatty acids as well as their uptake into the liver mitochondria for ketone production. Ketogenesis is also increased by glucagon, cortisol and thyroxine.

Broadly speaking these are the timings for ketosis, once you start to fast, eat a keto diet, or both.

Becoming the Breath Monster!
Becoming the Breath Monster!

The Fastest Way into Ketosis

  • Get your body fat adapted.
  • Start fasting or eat a keto diet or better still do both.
  • Fasting achieves ketosis more rapidly than a keto diet.
  • The lower the carb content of your keto diet, the quicker that you deplete your glycogen stores and start fat burning and become ketotic.
  • Fasting or a keto diet runs down your liver glycogen stores after 12 hours.
  • Gentle exercise lasting 2-3 hours, or HIIT for 30-60 minutes on the first day, uses up all your muscle glycogen stores.
  • As you have now depleted your glycogen stores, from both your liver and your muscles, your body will now burn fat and your liver mitochondria will rapidly produce ketone bodies.

You can only do this rapid process if you are fat adapted. If your body is not accustomed to fuelling on fat, you will not be in full ketosis by 24 hours. But what is the rush?

“Rivers know this: there is no hurry, We shall get there someday” A.A. Milne

Fasting and ketone bodies are incredibly good for your health, helping with repair, regeneration and preventing long term nasty diseases. Fat adaption is worth the effort. Once you learn to fat adapt, you have acquired a lifelong skill, which keeps you healthy, frees you from the tyranny of snacking, and allows you long-term weight control! Good luck!

Whisperings

  • Ketosis is a metabolic state, characterised by raised ketone bodies.

  • Ketosis happens when your body uses fat to fuel.
  • When you are ketotic; you lose fat, and you lose weight.

  • Carbohydrates are stored in your liver and your muscles, in the form of glycogen.

  • Your carbohydrate or glycogen stores are limited.
  • Your fat stores are virtually unlimited
  • Fat burning and generation of ketones starts when glycogen stores are depleted

  • Ketosis or becoming ketotic starts with fat burning.

  • Fasting puts your body into fat burning mode and ketosis.

  • Restricting carbs in a keto diet puts your body into fat burning mode and ketosis.

  • The greater the carb restriction in a keto diet, the more ketone bodies are produced and the greater the degree of ketosis.

  • Exercise, by depleting muscle glycogen, accelerates the development of ketosis.

  • Once you are fat adapted, becoming ketotic is much easier.

Please read: The information on this website is not intended to constitute medical advice, nor is it intended to replace or conflict with the advice given to you by your doctor or other health professional. Before embarking on the plans set out on this website, you should discuss them with your doctor, especially if you have any medical condition or if you are taking any medication. The author and publisher disclaim any liability directly or indirectly from the use of the material in our books and on our website by any person.

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Please read: The information on this website is not intended to constitute medical advice, nor is it intended to replace or conflict with the advice given to you by your doctor or other health professional. Before embarking on the plans set out on this website, you should discuss them with your doctor, especially if you have any medical condition or if you are taking any medication. The author and publisher disclaim any liability directly or indirectly from the use of the material in our books and on our website by any person.

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