How a 72-hour fast pushes your body into ketosis - Performance Optimal Health (2023)

A few months ago, physical therapist Shane Foley chose to complete a 72-hour fast, putting his body into a state of ketosis. Here's what he got out of it.

Feb 22, 2023 | Shane Foley, DPT, OCS, CSCS

How a 72-hour fast pushes your body into ketosis - Performance Optimal Health (1)

A 72-hour fast, or fasting for three consecutive days without any food intake, can help your body enter a state of ketosis. Ketosis is a metabolic state in which your body burns stored fat for energy instead of glucose from carbohydrates. Here are some potential benefits of a 72-hour fast that forces your body into ketosis:

Weight loss: When you're in ketosis, your body becomes more efficient at burning fat for energy, which can lead to weight loss. Fasting for 72 hours may also lead to a reduction in overall calorie intake, which can further aid in weight loss.

Improved insulin sensitivity: When you fast, your body's insulin levels decrease, which can help improve insulin sensitivity. This can be particularly beneficial for people with insulin resistance or type 2 diabetes.

Increased mental clarity: Many people report feeling more alert and focused when they're in ketosis. This is thought to be due to the fact that the brain can use ketones (produced during ketosis) as an alternative source of energy.

Reduced inflammation: Some studies suggest that ketosis can help reduce inflammation in the body. This could potentially benefit people with inflammatory conditions such as arthritis or autoimmune diseases.

Improved metabolic health: Ketosis may help improve various markers of metabolic health, such as triglyceride levels, blood pressure, and HDL (good) cholesterol levels. These benefits may also be related to the weight loss that often occurs during ketosis.

However, it is important to note that fasting with zero caloric intake for 72 hours can be challenging and should be done under medical supervision, especially if you have any underlying health conditions. Anecdotally, it was an interesting experience to realize that after the 72 hour mark, I had more energy than before, and that physical activity and exertion helped increase my energy levels, which should help force the synthesis of ketones.

For more on the benefits of fasting, check out our podcast on how fasting can help you lose weight and gain energy:


Will a 72 hour fast put me in ketosis? ›

Here's what he got out of it. A 72-hour fast, or fasting for three consecutive days without any food intake, can help your body enter a state of ketosis.

How to do a 72 hour fast properly? ›

Once the 3 day water fast begins, you will drink only water (and potentially herbal tea or black coffee if you choose) for a full 72 hours. You cannot eat any caloric food or beverages, and most people avoid all artificial sweeteners as well.

How long does it take to enter ketosis while fasting? ›

Your body may go into ketosis after just 12 hours of not eating, which many people do overnight before they "break fast" with a morning meal. (A midnight snack obviously sabotages this process.)

How can I trick my body into ketosis? ›

Strategies for encouraging the body to enter ketosis include:
  1. Significantly reducing the carbohydrate intake. ...
  2. Increasing physical activity. ...
  3. Fasting for short periods. ...
  4. Increasing healthful fat intake. ...
  5. Testing ketone levels. ...
  6. Checking protein intake. ...
  7. Consuming more MCT oil.

How much weight can you lose in a 72 hour fast? ›

You'll lose weight

“As you release glycogen, you lose water and that is usually the reason for the rapid weight loss. Losing fat takes more time.” Fasting proponent Fung, however, disagrees and maintains that you could lose 1.5 pounds of fat over a 72-hour period.

What supplements should I take during a 72 hour fast? ›

Electrolytes: The supplements you'll most likely need to replenish during a fast are not vitamins at all, but electrolytes, substances the body needs to regulate and maintain a number of key bodily functions, and which are depleted through physical activity and fasting.

What foods should you eat after a 72 hour fast? ›

Try some bone broth soup and well-cooked vegetables as the first meal after a long fast. Another idea is to sip on some broth for an hour, then eat a small first meal. You could also include some apple cider vinegar with lemon water as part of your first meal.

How much water should I drink on a 72 hour fast? ›

Water fast (24–72 hours)

Most people drink two to three liters of water per day during a water fast. The water fast lasts for 24–72 hours. You should not water fast for longer than this without medical supervision because of health risks.

How do I know when I'm in ketosis? ›

Testing blood ketone levels with a monitor is the most accurate way to determine whether you're in ketosis.

What mineral puts your body in ketosis? ›

Magnesium balances blood sugar and immune system functions. Sodium BHB is the star ingredient, inducing ketosis and supplying necessary electrolytes. Calcium and potassium supplement the nutrients your body misses out on without carbs.

What are symptoms of ketosis? ›

Other symptoms of ketosis may include:
  • Bad breath (“keto” breath).
  • Constipation.
  • Insomnia.
  • Dehydration.
  • Low bone density (osteopenia) and bone fractures.
  • High cholesterol (hyperlipidemia).
  • Kidney stones.
Aug 15, 2022

What happens if you fast 72 hours every week? ›

Fasting of 72+ hours is also shown to improve the body's response to toxin exposure and stress hormones. 24 to 72 hours: you start producing ketone bodies to fuel your brain, and see an increase in fat loss and autophagy. You also start to see an increase in BDNF which supports brain function.

What not to eat after a 72 hour water fast? ›

Foods and drinks that can be even more shocking to your system after a fast include those like a greasy cheeseburger, slice of cake, or soda. Even high-fiber raw produce, nuts, and seeds may be difficult to digest.

How much fat can you lose in a 72 hour water fast? ›

Water fasting helps you lose weight quickly. According to studies, you can lose 0.9 kg per day by drinking water in 24 to 72 hours. However, this loss of body mass is only from water, carbs and even muscle mass, not fat.

What happens if you stop eating for 72 hours? ›

Fasting for 72 hours is difficult for most people without any medical assistance. The body will also begin breaking down muscle tissue and can start leading to vitamin and mineral deficiencies and digestive issues. And that is what happens if you eat nothing for 3 days.

Why am I not in ketosis after a 3 day fast? ›

A first time faster won't reach optimal ketosis until well past the 24-hour mark. Most people are so poorly adapted to burning fat that it can take several months of regular fasting before the body easily enters into ketosis. You have to first reverse the insulin resistance and metabolic syndrome.

How to get into ketosis in 3 days? ›

Consume 20g or less of net carbs per day on this keto menu plan, not too much protein, lots of fat, drink at least 100 ounces of water per day, get plenty of salt and supplement with potassium and magnesium. Boom, three days later you're in ketosis (which you can measure with your ketostix,) and on your way!

Will a 24 hour fast put me back in ketosis? ›

Fasting will cause your body to burn through it's stored glucose and force it to create a new kind of fuel - ketone bodies. But fasting alone won't bring you to the promised land of nutritional ketosis. To reach ketosis in 24 hours, you need to add another element to your game... exercise!

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