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How Long Does It Take to Go into Ketosis

Nora Minno
"Andrea is a copywriter and editor specializing in holistic nutrition, health, and wellness. She holds a Bachelor of Arts in Communication Studies from McMaster University, is a certified personal trainer with the National Academy of Sports Medicine (NASM), and Certified Nutritional Practitioner (CNP) with the Institute of Holistic Nutrition."

The keto diet is all the rage right now and the lengthy “benefits of keto” list are why people of all ages are following it. Whether it's for metabolic reasons, digestive health, or chronic disease prevention, the hype around it is warranted.

But to experience all the benefits keto has to offer, your body must be in what’s called ‘ketosis’ and meaning it is producing ketones.

If you’re struggling to get yourself there or are just trying to understand how long it may take, this article will help break down the journey your body takes on the road to ketosis. We’ll give you what you need to know about getting into ketosis: how long to get into ketosis, factors that affect getting into it, and how you can propel yourself there faster.

How long does it take for your body to go into ketosis?

The exact amount of time it takes for a person to enter ketosis is highly individual and may vary from person to person. We're all biochemically unique and our metabolisms operate differently. However, there are some estimates based on general human physiology that we can use as guidelines for when someone may enter ketosis. We'll discuss more below.

The cardinal rule of the keto diet is to significantly reduce carbohydrate intake. The average adult will need to keep their carb intake at or below 30-50g per day. By doing so, insulin levels in the body will drop and the body can enter a catabolic state whereby glycogen stores are depleted. This causes the body to undergo metabolic changes [1]. From here, two metabolic processes are triggered to produce energy: gluconeogenesis and ketogenesis.

Generally speaking, ketosis can be achieved by fasting for longer than 72 hours (3 days) [2] or by following a very low carb (30-50g/day), keto diet for up to several weeks. Below are some key timeline benchmarks for achieving ketosis while fasting.

 

12 hours of fasting

Flipping the Metabolic Switch

During this phase, your body is no longer using stored carbohydrates for energy. Instead, your body starts to release stored fats to be converted into usable energy. This "switch" typically begins around 12 hours, but your body will continue to use alternate sources of fuel, such as fat, throughout the prolonged fasting period.

12-24 hours

Body Enters Fat Burning Stage and Ketone Levels Rise

Since your body is no longer burning carbohydrates, dietary or stored, for fuel, it needs to find another source of energy. During this phase, your body starts to release stored fatty acids to be converted into ketone bodies, which can be used by the brain and the muscles for energy. As you continue through this phase, your body will continue to burn fat and ketone levels will continue to rise. The start of this phase will vary per individual based on the amount of stored carbohydrates in your body prior to the fast as well as what you had for your last meal.

72 hours - 120 hours

Insulin and Glucose Significantly Decrease

Fasting for 3 or more days has shown to reduce circulating insulin and glucose by about 30%, in some cases even more, which can help reduce risk for metabolic disease, such as diabetes.

96-168 hours

Fat as Fuel

Although ketosis has begun earlier in the fasting phase, your body is now fully dependent on using ketone bodies and free fatty acids to produce fuel for the brain and the muscles. Around day 4, there is a dramatic increase of ketone production, which continues to rise up to (and past) day 7.

How to tell whether you’re in ketosis

In general, there are a few ways to determine if you’re in ketosis:

  1. Blood testing: Blood ketone testing is the preferred method of testing if you’re in ketosis because it’s the most accurate; this can be done through a lab or using an at-home test. Blood tests measure blood levels of the main ketone beta-hydroxybutyrate (BHB). Generally, anything over 0.3 mmol/L is considered elevated [3], but optimal levels may exceed 1 mmol/L.
  2. Breath testing: Ketone breath tests are an easy way to measure ketone levels, specifically acetone—the ketone responsible for the “fruity keto breath.” Breath tests aren’t as reliable as blood tests, but the concentration of breath acetone [4] is associated with the extent of glucose metabolism and lipolysis.
  3. Urine testing: This method is probably the easiest and most common method of testing ketone levels. Simply pee on a test strip, match the color to the corresponding ketone level, and you’re good to go. If you’re choosing this option, you ideally want to test early morning or after dinner [5] for the best accuracy.

However, you want to keep in mind that ketone levels aren’t necessarily a sure-fire confirmation that you’re in ketosis. Oftentimes, the body produces ketones faster than it gets into a state of nutritional ketosis, which means your test could show you’re in ketosis but your body isn't fat-adapted yet.

How do i know i am in ketosis? Some of the most common signs are:

  • Fruity or bad-smelling breath
  • Weight loss
  • Reduced hunger
  • Frequent urination
  • More energy
  • Increased focus and attention
  • Insomnia or fatigue (short-term)

Why do some people take longer to enter ketosis?

If getting into ketosis was as simple as baking a cake at 350F for 40 minutes, we’d all be there. So, how long does it take to enter ketosis? There’s no exact recipe for exactly how long it takes and there are several variables that can interfere along the way.

For most people, ketosis kicks in after about 72 hours, but even that is still a ballpark. The same reasons that differentiate you and your best friend are many of the same reasons that dictate the speed at which your body will enter ketosis. An athlete may get into ketosis after an overnight fast, whereas someone else may take up to a week to start seeing ketones in the urine. We’re all programmed with different metabolisms that influence how we metabolize glucose, which is an important consideration for how fast you’ll get into ketosis.

So, if you’ve ever wondered “why am I not in ketosis?” here are some factors to consider:

  • Eating schedule

When you eat is an important factor in how long to get into ketosis. If you’re abstaining from food regularly (i.e. fasting), you’re going to burn through glycogen stores and start oxidizing fats quicker. Greater fat oxidation equates to greater ketone production.

  • Activity level

Highly active people tend to get into ketosis faster, as exercise depletes glycogen stores and disposes of excess blood sugar, thereby accelerating the switch from being a sugar-burner to a fat-burner.

Studies show that skeletal muscle fuel selection shifts with increasing exercise intensity [6], which means that carbs are seen as a premium fuel source. High-intensity work relies almost exclusively on glucose, which means that prolonged exercise at a high intensity will deplete carb stores faster. 

  • Current carb intake

All other factors aside, your carb intake is the #1 biggest thing you need to look at if you’re struggling to get into ketosis. Many people think they’re eating “low-carb” but when they break it down, they’re not. Carbs are abundant in the diet, and if you’re not paying close attention, you could easily be surpassing your carb threshold.

The typical keto macronutrient breakdown is 70-80% fat [7], 20-25% protein, and 5-10% carbs, which translates to roughly 30g net carbs or less. However, some experts will advise that up to 50g of carbs daily is still low enough to maintain ketosis—but it may not be for you.

If you’re consuming 50g of carbs daily and struggling to get into ketosis, you may need to reevaluate your carbohydrate intake, consider reducing it.

How to enter ketosis faster?

1. Watch your carbs

Like we said before, carbs are sneaky and they’re everywhere! Keeping track of your carbs is the biggest key to getting into ketosis fast. Reducing carbs:

  • Keeps blood sugar low
  • Reduces insulin levels
  • Low insulin signals your body to burn fat

Athletes or highly active people can probably afford a bit more than 20-30g, but for most people, play it safe and stick to those guidelines. If you struggle to stick to your carb budget, download a food tracker so you have the numbers in front of you.  

2. Fasting and ketosis

Your body still needs to operate even when you’re fasting and with no incoming energy, your body is going to turn to what it has—glycogen stores. Once those are depleted, fat is next in line because it’s preferable to protein. During a fast, your body oxidizes fatty acids to produce ketones, so if you fast long enough, you will enter ketosis regardless of your carb intake before.  

Intermittent fasting combined with a keto diet works for most people to get into ketosis, but you can also try an extended 24 or 48-hour fast to speed up the process.

3. Exercise more

High-intensity activity relies primarily on glucose for energy, which means it’s going to burn up your glycogen stores faster. And assuming you’re not replenishing with carbs, your body is going to switch into fat-burning mode when there’s no glucose left. 

4. Boost your fat intake

Similar to carbs, not eating enough fat can be a culprit for not being able to get into ketosis. If you’re eating 70% fat, 20% protein, and 10% carbs, it could be that either your carbs are still too high or you’re not eating enough fat.

Try knocking back your carbs to 5% and upping your fat to 75% and see if that helps. On keto, fat takes the place of the carbs you just lost, so by cutting back on incoming glucose and increasing the substrate needed to produce ketones, you may find that it takes a bit less effort to reach ketosis.

5. Use exogenous ketones

If you're struggling, there is some research behind exogenous ketones to help you get into ketosis faster. However, the thing is that supplementing with ketones increases blood ketone levels, but it doesn't mean your body is producing ketones on its own. 

Takeaway message: how long to get into ketosis?

The speed at which you get into ketosis is likely not going to be the same as someone else’s. But if you’re sticking to a keto diet (<30g carbs), chances are you’ll get there pretty quickly with a bit of patience.

Just always remember the golden rules to enter ketosis:

  • Eat fat
  • Keep carbohydrates low

Stick to these and you’ll be in ketosis and producing ketones in no time. 

As always, remember to discuss any significant dietary changes with your doctor or dietitian, especially if you are pregnant, have underlying conditions, or a history of eating disorders.

References

[1] Ketogenic Diet

[2] Ketones and Human Performance

[3] Blood Ketones: Measurement, Interpretation, Limitations, and Utility in the Management of Diabetic Ketoacidosis

[4] Breath Ketone Testing: A New Biomarker for Diagnosis and Therapeutic Monitoring of Diabetic Ketosis

[5] Monitoring for compliance with a ketogenic diet: what is the best time of day to test for urinary ketosis?

[6] Nutritional Ketosis Alters Fuel Preference and Thereby Endurance Performance in Athletes

[7] The Effect of Medium Chain Triglycerides on Time to Nutritional Ketosis and Symptoms of Keto-Induction in Healthy Adults: A Randomised Controlled Clinical Trial

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