7 day (mostly) water fast

This one has been on the backlog for a while. In September 2017 I did a 7 day (mostly) water fast, this post talks a bit about my experiences and the data I collected during this period.

Please do not try this at home. Do your own research, consult with a healthcare professional and make informed decisions.


For those of you that do not know, a water fast is exactly what it sounds like: you stop eating and drink only water. Why would you do that? For me, out of pure curiosity.

There are a bunch of hand-wavy potential health benefits that some people peddle and some more recent more realistic research interest. Clinical fasting is starting to become a more mainstream method of promoting an environment in which the body can use the survival mechanisms it has evolved to potentially benefit overall health. I look forward to new research over the coming years.

Fasting has also been used as a spiritual tool for millennia. It is a great way to wean yourself off of a lifestyle with core values driven by comfort and complacency.


This is very rough, I did not keep a proper diary on this so it is from memory only. Day 1 was the 31st August 2017.

Day 1

No idea what my last meal was but started towards the end of the last day I had at work before the block of time I had booked off for the fast.

Day 2-3

The first few days are the tricky ones in my experience. I did a 5-day water fast a couple of years ago and a number of slightly shorter ones over the past 5 years or so. The likely reason for the trickiness is probably to do with the switchover as glycogen in the liver is depleted and the body turns to fat as an energy source, turning it into ketone bodies that can be used to fuel normal bodily functions.

This can be a bit of a shock to the system. You can expect to feel quite weak, potentially irritable, with some minor headaches. Sleeping patterns might be interrupted too. Having been through this a few times, it was not as daunting this time and just part of the ride.

I started a rough meditation schedule, sitting for periods of between 1 and 3 hours at a time throughout the day.

Day 4-6

I felt like taking in the liquid portion of some vegetable broth to make sure my body had the resources necessary to keep electrolytes in balance. No idea if this actually did anything, but seemed like a sensible thing to do at the time.

At this point, the tricky part of the fast was over and I felt very clear headed. There is nothing quite like the mental experience of an extended fast. It might be a placebo or might even be something to do with the brain switching the primary fuel source from glucose to ketones. Either way, the experience is akin to walking outside on a chilly day in nature; everything seems very still and spacious. There is nothing to do and that feels just right.

It was around this time I had some really great conversations with my brother that led to some breakthroughs in some concepts I was working with at the time. I was on a real high at this point, feeling great for no reason at all all of the time.

Day 7-8

I broke my fast at this point, I think with some melon juice on the evening of the 7th day. Followed by some carrot juice and a few strawberries. The following day I ate mostly grapefruit, did not have much of an appetite at this point and it seemed sensible to just eat some easy to digest fruits.

Day 9+

At this point, I started resuming my normal eating patterns as my appetite returned. Slightly deflated to have ended the fast. I like the challenge and to see how far I can push myself. Next time I will likely try out a slightly longer duration, perhaps 10 to 14 days. It always feels as if just as I am getting to the good bit I have to wind down again and go back to normal life again!

Graphs and Stuff

Blood Glucose/Ketone Levels


This one played out as expected. There are some missing data points at the beginning where I did not have the correct strips for my glucose meter.

Ketone levels go up as the fast begins and peak around the 7mmol/L mark, hovering between 5.8 and 7.7 from Day 3 to Day 8 and sharply dropping back to less than 0.5 from Day 10 onwards. Glucose levels exhibit the opposite behaviour, hovering around the 3.5mmol/L mark from Day 4 to Day 8 and rising to around 5mmol/L as the fast ended.

This pattern occurs predictably as the body uses up the glucose it has stored as glycogen in the liver. At this point, fat begins to be broken down and ketones become the primary energy source. Interestingly, the brain has a preference for these ketone bodies during the fast, preserving the remaining glucose production for the organs that are not able to use the ketones e.g. red blood cells. Upon refeeding, the body switches back to using glucose again.

The glucose ketone index or GKI is a simple ratio of blood glucose levels to blood ketone levels, used by Dr Thomas Seyfriend in his research on the managing brain cancer using a dietary controlled metabolic approach. The takeaway is that maintaining a low GKI can reduce tumour metabolism, essentially starving them of the glucose they need. Low means something along the lines of 0.7 to 1.1. During the fasting period, my GKI stayed between 0.4 and 0.6, sharply returning to the more typical huge ratio as I switched back to using glucose as my primary energy source.

Heart Measurements


My resting heart rate stayed between 58 and 73 beats per minute. It seems mostly uncorrelated with the fasting period. The minimum was reached on Day 6 of the fast. This kind of range is pretty decent for a 27-year-old male with a mostly sedentary lifestyle.

My blood pressure also seems mostly uncorrelated with the fast. It stayed between around 94 to 117 mmHg systolic and 57 to 75 mmHg diastolic. This is in the normal range, perhaps slightly low on the diastolic side.

I also recorded some heart rate variability (HRV) measures. These are a bit more difficult to interpret. Using the data from the study of 260 healthy subjects by Umetani et. al, 1998 here is my summary:

  • Inter-beat RR interval was around the 900ms during the bulk of the fast. It was closer to 800ms before and after the fast. This is within the normal 939±129ms for my age group.
  • The RMSSD (Root Mean Square of the Successive Differences) varied a lot from day to day, between 21.5 and 48.0. Again, pretty close to the normal 39.7±19.9 for my age group.
  • SDNN (Standard Deviation of the NN[RR] intervals) was also had a pretty huge variance, ranging between 42.6 and 86.25. This is slightly above the normal 50.0±20.9 for my age group. Higher numbers for this measurement are typically found in younger individuals.
  • PNN50 (The proportion of the number of successive NN[RR] intervals that differ by more than 50ms divided by the total number of NN[RR] intervals) again was all over the place, between 5.4% and 32.4%. This compares with a typical 20±17% for my age group, so nothing too interesting there.

Here is the raw heart rate variability data I collected:

Heart Rate Variability Mean RR (ms) RMSSD LnRMSSD SDNN NN50 PNN50 (%)
01/09/2017 AM 60 803 47.88 3.87 86.25 52 27.957
02/09/2017 AM 50 807 25.82 3.25 42.6 10 5.405
04/09/2017 AM 60 913 47.97 3.87 83.68 49 31.613
05/09/2017 AM 59 995 45.71 3.82 59.37 49 32.45
06/09/2017 AM 54 952 32.73 3.49 53.69 12 7.692
07/09/2017 AM 56 933 39.29 3.67 55.6 30 18.634
08/09/2017 AM 53 960 30.36 3.41 46.3 14 9.032
09/09/2017 PM 47 797 21.47 3.07 37.03 5 2.674

Weight Measurements

As you might expect, weight loss is pretty much inevitable with an extended fast. Over a very short period of just over a week, my weight went from around 134lbs down to around 127lbs towards the end of the fast. Soon after, it went back to around 131lbs mark. My BMI stayed within a healthy range, between just under 20 and 21 throughout.



Interestingly, my body fat percentage dropped from around 19.6% to 16.4% and remained there despite putting most of the weight back on. This is a good indicator that I was indeed literally burning fat for fuel during the fast.


Water Measurements

I took these measurements as a precaution, just to make sure nothing unusual was going on during the fast. Nothing particularly interesting here. You can see the ketone measurements do correlate with the blood ketone measurements. The accuracy of all of these measures is pretty bad, which results in a noisy graph.


There was a bit of a trend in the specific gravity, which did seem to increase a little during the fast but stayed within the typical 1.0 to 1.03 range. The slight elevation could indicate slight dehydration during the water fast, which is quite amusing.



Here are the gadgets I used to collect the data. I messed up a bit ordering incompatible blood glucose strips which explains some of the gaps in the data. Unfortunately, the product links are all Amazon. You can find similar elsewhere too.

2 thoughts on “7 day (mostly) water fast

  1. Thanks for one marvelous posting! I enjoyed reading it; you are a great author.I will make sure to bookmark your blog and may come back someday. I want to encourage that you continue your great posts, have a nice weekend!

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