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Matt: Three-day Water Fast including CGM, Ketones, and more


This post is for education purposes only and is in no way medical advice. You should consult with your doctor before doing any diet or exercise.


I completed a three-day water fast in the first part of June. Below I give the reasons why I decided to go through with this fast, what I did to make the fast as bearable as possible, review data from the fast, and reflect on the mistakes I made and how I will fix them next time (probably after residency!).


There are three main reasons why I wanted to do a fast before starting residency:

1. Improvements in metabolic markers. The endpoints of fasting are well studied and reported. For an example, refer to my Fasting-Mimicking-Diet (FMD) write up which shows that 3 months of FMD improves many metabolic markers, including blood pressure, fasting glucose, triglycerides, LDL-C, and CRP. The fasting mimicking diet was created by Dr. Valter Longo to mimic water fasting as water fasting is generally not tolerated by the average American.

2. Changes in the biochemical milieu of cells. The three main cell signaling pathways that cause a differential response within cells in the fasted start are mTOR, AKT3 (Protein Kinase B), and AMPK. These pathways are responsible for the beneficial effects seen in reason number 1 and many other effects such as autophagy. A future write-up will be devoted to each of these pathways and their exact effects on the body.

3. Improving relationship with food. I am more easily irritable when I am hungry. This is a trained body response to a stressor (hunger) that is further reinforced by the postprandial dopamine rush (the reward). In my opinion, fasting is one way to break this Pavlovian conditioning. In essence, a fast proves that you do not need to eat every 3 hours to function.

Strategies and Tactics


Magnesium chloride. 2-3 teaspoons a day as needed mixed in with pickle juice (see below). Reason: prevention of presyncope/ lightheadedness. I took this supplement at baseline and as needed for lightheadedness, which I did not experience as much as I thought I would have.


Pickle Juice. Approximately 3 cups of 1 part pickle juice 3 parts water per day and as needed for electrolyte repletion. One main worry I had about water fasting was derangements in electrolytes and fluid status. I wanted to find a source of concentrated sodium without any calories or additives and pickle juice, specifically the brand Bubbies’, fit this role perfectly. Virta health, a company that specializes in ketogenic diets to cure T2DM, recommends 3000-5000mg of sodium, 3000-4000mg of potassium, and 300-500 mg magnesium on a ketogenic diet. I used these numbers as rough intake estimates while on my water fast.

Topo Chico. These mineral water beverages were a great way to get more water throughout the day. I drank all three flavors of original, lime, and grapefruit as needed for rehydration.

Decaffeinated Coffee. Laura and I decided to stop our caffeine intake during our fast. Our reasons were two-fold: 1.) a caffeine holiday would decrease our sleep latency and hopefully improve our sleep in our hungry state and 2.) caffeine on a caloric deficit/ fast would most likely cause a resting tremor and anxiety. Still, we love the taste of coffee so we opted for black Swiss water process decaffeinated coffee.


Push ups and pull ups. Unfortunately, I was severely limited in the types of exercises I could do as I did not have access to a gym. Ideally, I would do stationary cycling and lift weights. Instead I settled for a few sets of push ups and pull ups throughout the day.

Intra-Fast Data and Pictures

Zero App Fast. I originally planned for a five day fast but stopped at three days. See the “Breaking the Fast” section below for more information.

CGM and Ketones Data. The following graphs detail my blood glucose and ketone levels while on the fast. The CGM data was relatively precise and accurate when tested against the Contour next finger stick glucose monitor. The average of the differences between the CGM and the finger stick monitor was 6.9 mg/dL as seen below. Readings were most inaccurate when my blood glucose was not in its steady state (ie, after exercise when epinephrine induces gluconeogenesis and glycogenolysis). This is because the CGM reads the interstitial glucose which lags behind capillary glucose by approximately 20 minutes. My actual blood glucose was almost always lower than what the CGM was reading even when in a steady state.

The green zone on the graphs below is the same as the green zone on our CGM data page and runs from approximately 60 to 116 mg/dL. This range signifies our preferred ranges for glucose variability and will be covered in my next write up. My average glucose for day 1 was 89 mg/dl, day 2 was 63 mg/dL, and day 3 was 54 mg/dL before I broke my fast.

Nutritional ketosis is defined as a ketone level >0.5 mmol/L. This is the threshold at which your body begins to switch from glucose to ketones for its primary energy source. I hit this threshold about half a day into the fast and peaked at 6.2 mmol/L.

Blood pressure, Pulse, and Weight. Interestingly, my blood pressure crept into elevated, prehypertensive levels during the fast. Additionally, my pulse increased by approximately 10-20 bpm in the middle portion of my fast while my body was still adjusting to the ketones. This increase in heart rate was associated with the subjective feeling of my heart pounding (see below). My weight unsurprisingly decreased while on the fast and I subjectively looked thinner post-fast.


Lethargy. This was the main symptom I experienced and it started approximately 18 hours into the fast. I typically would take a 45-minute afternoon nap and feel decent afterwards. Exercise was difficult on the fast and I did not feel like I was at my normal physical performance.

Cardiovascular and lightheadedness. The second most striking symptom was the sensation that my heart was pounding. This started on day 2 and lasted for a good portion of that day. I am not sure of the physiological mechanism but it might be related to the spike in ketones affecting the cardiovascular system. Another striking aspect of the water fast was that I did NOT experience lightheadedness/ presyncope while fasting. This seemed to be one of the most reported side effects of water fasting when I did my research. It is possible that my magnesium and pickle juice prophylaxis with Topo Chico kept my electrolytes and fluids in the correct proportions throughout the fast, thus relieving me of orthostatic hypotension.

Hunger. Unlike others who do a water fast my hunger never truly went away. The hunger pangs were worst on the first day and in the mornings.

Bad breath. By the end of the fast I was brushing my teeth three times a day and it did not touch my bad breath. My breath was NOT the fruity odor of ketoacidosis (only 30% of people in ketoacidosis have the fruity breath anyways). I am not sure of the physiological mechanism of bad breath on a fast but overgrowth of certain oral bacteria due to the differential energy intake is a plausible explanation.

Sleep. My sleep did not seem to be positively or negatively affected by the fast.

Headache. I developed a 3/10 right posterior headache on day 2 that eventually resolved before I ended my fast on day 3. It is difficult to know if this was a caffeine withdrawal headache or if it was due to the water fast.

Breaking the Fast

Decision. I was aiming for a five-day water fast. It was an ambitious goal as I’ve never fasted for an extended period of time but I did have some hard stops for conditions that would make me break my fast early. These included symptomatic effects such as excessive and refractory presyncope/ lightheadedness and blood glucose below the semi-arbitrary cutoff of 50mg/dL. I hit 47 mg/dL and dropping blood glucose during the evening of the third day and, thus, stopped my fast.

Breaking Fast Meal. I broke my fast with a high fat, high protein, low carb meal that consisted of 8 oz bone broth, 30g macadamia nuts and 10 manzanilla olives. A high protein meal is essential after a water fast to induce IGF-1 levels to stimulate new cell growth after the fasted state. A low carbohydrate meal is also essential as muscle is insulin resistant after a fast due to the ketosis state.

The day after breaking my fast I ate a more substantive but still low carbohydrate meal that consisted of 4 scrambled eggs, ½ onion, and spinach with Cholula and 2 strips of bacon. I ate lunch and dinner as well but I did not record them precisely. Both lunch and dinner consisted of small portions of salmon, squash, zucchini, salad, and bone broth.

Post Fast Data

Exercise. One of the most striking differences after the fast was my athletic performance. I am not a jogger by any stretch; my preferred cardio is studio cycling. However, I do jog occasionally and it was the only cardio afforded to me during and after the fast. The 1.7 mile run I did the morning after breaking the fast was miserable. My legs felt heavy and it was a mental chore to pick them up and put them back down. There was no groove to my jog and my cardio frequently gave out. I’d rate it as easily the worst feeling jog I’ve ever been on.

I did a T25 workout the following day with Laura and felt OK, then the day after that (three days total since breaking fast) I went on another jog. This jog felt fantastic. I was able to not only obtain a new personal record, but sustain it for three miles. My cardio was fantastic and my heart rate held a record maximum of 187 for about 7 minutes, which is impossible for me to sustain on a stationary bike. Below is a chart of the Polar Beat heart rate monitoring data for an easy comparison. Due to inclusion of my cool down walk, time spent jogging, distance, and pace average are reported first with totals in parentheses.

Alertness. Although I felt sluggish during exercising, my fatigue and lethargy were gone by the morning after breaking my fast. In fact, my mood was elevated for the week after the fast and I woke up more refreshed than baseline.

Weight. My current weight is 158.5 lbs about 2 weeks out from the fast. I believe most of the weight I lost was subcutaneous or visceral fat as I did not notice a large change in my muscle mass. Regardless, I am now lifting weights again with post-exercise bone broth to continue to build my muscle mass in this post fast period.

Mistakes and Corrections for the Next Fast

Lack of Exercise. By far my biggest mistake was my lack of exercise. Exercise is incredibly important while fasting for at least two reasons: inhibition of muscle degradation and increasing blood glucose levels. In an ideal environment I would be doing relatively heavy weights for 45 minutes followed by 30 minutes of mild to moderate cycling. My lack of exercise caused my blood glucose to dip to 47 mg/dL which was below my threshold for breaking the fast. For an example of how important exercise is for blood glucose maintenance, look at the day 2 fasting graph: I hit my daily peak of 72 mg/dL after doing sets of pushups and pullups with a relatively higher average level of glycemia post-workout when compared to pre-workout levels.

Collection of More Biomarkers. There are many additional biomarkers that would be fascinating to measure pre- and post-fast to determine how an individual responds to a water fast. These include, but are not limited to, measurements of LDL-p, Lp(a), cortisol, thyroid function tests, LFTs (if previously elevated), hs-CRP, oral glucose tolerance tests, insulin, antibody tests if autoimmune disease is present (anti-TPO, anti-gliadin, anti-SS-A,, etc), etc. The results of these tests would be more for intellectual curiosity rather than clinical use but long-term trends after single fasts or multiple fasts could have implications for individualized treatments. Additionally, a daily point of care chemistry panel would alleviate my concerns about electrolyte derangements.

Schedule More Activities. Fasting is boring. An immediate realization when fasting is how social and enjoyable eating is and you will certainly miss it over the fasting period. Filling in extra time with fun, active activities such as putt-putt or long walks would help pass the time.

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