What happens in your body during a water-only fast?

I tried my first 5-day water-only fast about three years back because I was interested in some of the benefits, such as reduced inflammation, improved gut health and anti-aging. 

I researched the available science to find out how to fast safely, and efficiently for the best outcome. I did a DEXA (dual x-ray absorptiometry) scan before and after to check my body composition and ensure I was not losing lean muscle while fasting.

You can find my results and all the information, including the benefits of fasting, in the Results From My Five Day "Water Fasting" Experiment blog. 

Since then, I felt so terrific that I’ve been fasting every six months since! Here's my latest data.

 

Oura ring stats

While the first time, I was more interested in the body composition changes, this time, I was intrigued by what fasting does to my physiology. I used the Oura ring to track heart rate (HR), heart rate variability (HRV), stress, resilience, and sleep. The results were astonishing, especially when it comes to my HRV.

 

	Before Fast (usual values)	While Fasting
Resting HR	40-50	38
Max HRV	< 120	210
HRV average	< 60	122
Daytime stress	Normal or stressed	Restorative
Resilience	Strong	Exceptional

I wasn’t expecting such a significant increase in my HRV. It’s been nearly a week since the fast, and my resting HR is still 38; HRV elevated above my usual values, and every day has been a restorative day despite working and training lightly.

 

Heart Rate Variability

HRV measures the variation in time between heartbeats, reflecting the autonomic nervous system's control over the heart. It's a valuable indicator for assessing the body's stress load and readiness to perform and recover.

Higher HRV indicates better cardiovascular fitness, stress resilience, and overall health. High HRV is also associated with better executive function, emotional regulation, and decision-making in healthy individuals.

Lower HRV may suggest stress, fatigue, or underlying health issues. It is associated with a lower capacity for self-regulation of physiological, emotional, and cognitive responses and less effective adaption to environmental stress and demands, including food.

HRV fluctuates throughout the day depending on the state of your nervous system. When you are in the ‘fight-or-flight’ state, your HR increases, and your HRV decreases. The opposite happens in the ‘rest-and-digest’ state, which is the case of prolonged fasting.

During fasting, the body shifts into enhanced autonomic nervous system balance, promoting parasympathetic (rest and digest) activity over sympathetic (fight or flight) activity. This shift can lead to a lower resting heart rate, reflecting reduced metabolic demands, increased cardiovascular efficiency, and higher HRV, indicating greater autonomic nervous system balance and cardiovascular health. This is why fasting is not only effective physiologically but also mentally. 

Average HRV decreases with age and in response to lifestyle. Chronic stress, poor sleep, physical inactivity or overtraining, poor diet, dehydration, and excessive alcohol consumption all reduce HRV, which is why HRV is being used as an indicator of physical and psychological well-being.

Furthermore, some research implies that vitamin D and B-12 deficiencies are associated with impaired HRV and that caloric restriction and increased energy expenditure increase HRV.

 

How do I fast?

Preparation

I prepare about a week before to ensure my transition to water fasting is as smooth as possible. The preparation includes getting off caffeine (you don’t want to have caffeine withdrawals and headaches while fasting) and eating a very low-carbohydrate diet of fresh, easily digestible, organic foods. I avoid packaged or bottled foods to reduce the toxic load in my body, fast for 16 hours every day and have a small early dinner. I also go to the sauna and ice bath a few times weekly.

 

During

For me, the 5-day fast is more than just abstaining from food. It’s about disconnecting, removing myself from my environment and any stressors, spending time in nature, meditating, reading and generally chilling, which I never do for a few days in a row. I usually have my last meal on Thursday at 5 pm, go to work on Friday, and then leave for a remote location with 1-3 friends. We get a house in a quiet area with a nice view and stay there until Monday afternoon. No exercise is allowed except for 1-2 hours of nature walking each day and some stretching. It’s a physical and mental reset, and we are in the rest-and-digest mode the whole time. We bring special spring water with us, and to avoid electrolyte imbalances and the crappy feeling that comes with it, I put either salt or sugar-free electrolytes in my water.

 

After

To avoid the so-called refeeding syndrome, which can be dangerous and result in hospitalisation, I follow a specific refeeding protocol for the next seven days after the fast. It involves fermented foods such as saurekraut and kefir, bonebroth and broth based soups and enzyme stimulating herbs and spices. It’s hard to believe I don’t feel hungry or tired after five days of not eating. From all the ketones in my blood, I feel energetic and mentally sharp, as if I had a few shots of coffee but without the jitters, sweats, and anxiety.

 

The best way to start fasting

Despite my love for fasting, I don’t recommend prolonged fasting without consulting your physician. Prolonged fasting is not suitable for everyone and can be dangerous in some cases. I recommend starting with circadian eating, which is safe for most and can bring similar benefits.

So, the best place to start is with our 30-day Chief Life Challenge which includes circadian eating and lost of valuable advice, tips and free downloads. If you need more individualised advice, book a free nutrition tune-up. 

 

Veronika Larisova
Co-Founder, Registered Nutritionist, Exercise Physiologist 

 

 

 

References:

• The connection between heart rate variability (HRV), neurological health, and cognition: A literature review
• Obesity, Nutrition and Heart Rate Variability
• Heart Rate Variability and Cardiovascular Fitness: What We Know so Far
• Effects of long-term fasting and confinement on the cardiovascular activity