Fasting is becoming increasingly popular among people who want to improve their health or lose some weight. One of the better-known forms is the 5:2 diet, where you eat normally for 5 days a week and then limit your intake to about 500 - 600 calories on 2 days. But what actually happens in the body if you don't eat for about 16 hours?

What we start to notice after 16 hours of fasting

When the body doesn't receive food, it means it has to access different energy sources (carbohydrates, fats) differently than usual. In the first roughly 4 hours after a meal, the body is still processing the food you consumed. After about 8 hours, blood sugar levels are already somewhat lower and the body starts drawing from glycogen (these are carbohydrate stores) in the liver and muscles. After about 12 hours, glucose reserves (simple sugar) are running low and the liver starts breaking down fats and producing ketones (these are molecules that form when the body uses fat as an energy source).

And at about 16 hours of fasting, an interesting phase occurs: autophagy (auto- means self, phagy means eating — so the process in which the body recycles damaged cells). This is a natural mechanism of renewal and cleansing for the body.

So, after around 16 hours of fasting, we often notice the body transitions from feeding mode to a mode where more restorative processes are activated. This is also why many proponents of fasting notice that weight loss becomes easier and the body uses fat more efficiently as energy.

Why this can help, and what science says about it

From traditional healing practices, we know that giving the body a break from food allows it to reset, to not be constantly engaged in digestion, but to focus on renewal.

Mark Mattson from the National Institute on Aging, in a study published in the New England Journal of Medicine, found that in people who fasted for 16 hours every day, there was fat loss while muscle mass remained almost unchanged. A report from the Harvard T.H. Chan School of Public Health in September 2025 also found that daily fasting (restricting eating to at least 8 hours, but preferably 16 hours) helps people consume about 250 calories less per day, which means a loss of about half a kilogram per week. A summary meta-analysis from 2025 showed that various forms of fasting (called intermittent fasting) have comparable benefits for weight loss as continuous calorie restriction.

To sum up: after about 16 hours of fasting, the body starts functioning somewhat differently, it uses fat and supports renewal processes, which is in line with many traditional practices.

What happens to the brain and why it's important

It's not just weight loss that is interesting. We also need to keep the brain in mind. Many traditional practices say that true fasting also helps the mind, not just the body. And science also demonstrates something similar.

During fasting, a compound called beta-hydroxybutyrate (BHB) is produced, which is an energy source the body creates when it doesn't get many carbohydrates and starts using fat. BHB is also linked to increased production of BDNF (brain-derived neurotrophic factor). This is a substance that helps protect nerve cells from damage and supports brain function.

In other words: when the body switches to using fat and starts producing BHB, it can also trigger a mechanism that supports brain cells, which is very interesting for those thinking about a second youth for body and mind.

However: a larger number of studies on humans is still lacking, and there is a wide range of responses ...

We propose the following home plan:

• Choose a time that suits you: for example, after dinner at 7:00 pm stop eating and have your next meal at 11:00 am the next day. That’s 16 hours of fasting.
• During fasting, drink enough water, you can also have unsweetened tea or black coffee. Everything should be calorie-free.
• When you eat in the chosen time window (for example 8 hours), don’t think you can eat everything you see. It’s important to keep your food quality high: eat plenty of vegetables, nuts, whole grains, less processed food… Hopkins Medicine emphasizes in several articles that what you eat during the eating window is also important. Everything counts!
• Observe your body: if you feel dizzy, too tired, if you’re pregnant or breastfeeding, or have chronic diseases, consult your doctor.
• Very important – start gradually. If 16 hours is too much for you, you can start with 12 or 14 hours and work your way up to 16 hours.

Where is the limit and what are the restrictions

Fasting is not a magic pill, it’s not suitable for everyone and results are not universal. In fact, a study published by EatingWell showed that restricting the eating window to less than 8 hours a day may increase the risk of cardiovascular diseases. In people with eating disorders, pregnant women, small children, and older adults with many comorbidities (the simultaneous presence of several illnesses or health issues in the same person), caution is needed. If you eat mostly high-calorie, processed food during the eating window, the benefits of fasting will be much less.

So, if you follow fasting with common sense (with smart food choices, regular exercise, and good well-being) it can be a great tool. However, without understanding and adjustments, you might fast in vain.

A good home and affordable solution

For all who work, create, and reflect on life, a 16-hour fast is a simple strategy that doesn’t require special supplements or expensive programs. If you do it with the understanding that it’s not just about not eating, but about thoughtful eating and allowing the body to rest, it can yield encouraging results. Simple math shows that just 250 fewer calories a day equals about 0.5 kg of weight loss per week. If you multiply this over a year, that's about 25 kg per year, which is quite a decent result!

Therefore: listen to your body. If you feel good, have energy and see progress gradually, that's great. If you notice that fasting is too hard for you or is affecting how you feel, adjust or stop.