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Q2 2019: 6-Day Fast

Brian Crain
Brian Crain
5 min read

I've been interested in fasting for many years. Around 2010, I read a book called "Eat Stop Eat" by Brad Pilon about intermittent fasting. Afterward, I started fasting for 24 hours once or twice a week. I did that pretty consistently for a few years, although less so the last three years.

Then my brother started doing longer fasts and I joined him in January and did a 5-day fast. It was an amazing experience and really revived my interest in fasting. So I decided to do a longer fast once a quarter. This week, I finished the first of these fast and am writing up some notes on it here.

Start: Wednesday April 3 - 3.50pm
End: Tuesday April 9 - 4.06pm
Duration: 144 hours 17 minutes (exactly 6 days)

I made a big bone broth on day 1 of the fast and started drinking that on day 2. There were a few pieces of meat and fat (probably <100g) in the soup. From day 2 on I started having 2-3 cups of the broth per day and a little bit of the meat of the soup.

I don't know how many calories I consumed through that. The bone broth was certainly quite fatty and also quite gelatinous. Based on this, I guess I might have consumed between 150 and 300 calories per day. Otherwise, I only drank water and tea.

Subjective Experience
In my January fast, I had good energy throughout the entire time. This time it was more mixed. On Friday, I started being quite low on energy and struggled to focus at work. On Sunday, my energy dropped even lower and I took a nap during the day. After taking 50mg of modafinil, I finally start feeling clear on Sunday afternoon.

I ended sleeping poorly on Sunday night and decided to take most of Monday off. I took a microdose, went to an infrared sauna and then took a long walk back. Wandering through LA in the beautiful afternoon sun, I felt more relaxed and at ease than I had in weeks.

On Tuesday, my last fasting day, I was full of energy. I worked well and went to CrossFit in the early evening. This was my first workout on the fast and I expected that I'd be quite weak. Instead, I ended up performing at least on par with a normal workout if not better.

I was a bit sad that the fast already ended then, though I had also been so excited about eating again that I decided not to extend it. Physically, I felt great and going longer would have been easy.

Ketone & Glucose

I measured my glucose and ketone level throughout the fast. One thing that is noteworthy is how long it took me to actually reach a decent level of ketosis. The first time I measured >1.0, I was already over 60 hours into my fast.

Glucose Ketone Index (GKI)

I recently read about the Glucose Ketone Index (GKI). This is the glucose level (in mmol/L) divided by the ketone level. Various studies are using this as a way of assessing whether a therapeutic level of ketosis has been reached.

The logic behind GKI is as follows:
1. Cancer cells can only function on glucose. If there is no glucose, they can't grow and die.
2. GKI is a way of measuring whether you are in a range where this is happening.
3. Dr Seyfried, who pioneered the research of ketosis as a means of treating cancer, has done various studies targeting GKI levels. He argues that the therapeutic range to shoot for is below 1.0. One of his papers is here.

Only at the end when I broke the fast, did I reach a value of around 1. At least, between  hour 90 and 144, I was consistently below 2. I only read about the GKI towards the end of the fast and didn't specifically target to reach a low level this time.


What I remember most vividly about my January fast is how I slept less and less, yet woke up each day wide awake without a trace of fatigue. Back then my resting heart rate (as measured by the Oura ring) dropped lower every day to a low of 35. My normal average is 41.

This time this effect was less consistent and strong. The lowest my resting heart rate went was 37 on Friday and it was at my normal level on other nights. My sleep overall was fine, but not exceptional.

Lipid Panel

I did a pretty comprehensive lab test with WellnessFX a few months ago. I found some pretty concerning lipid values that point to a high risk of heart disease. I've since been learning more about the topic and am starting to get a better understanding of how to read a lipid panel.

I was very curious how fasting would affect this. So I did a NMR LipoProfile at the start of the fast and just before breaking the fast. The results were interesting and included some big surprises:

LDL Particle Number
Decreased from 2013 to 1787 nmol/L. (My understanding is that this is the most important risk factor.)
Increased from 177 to 219 mg/dL. (My understanding is that this is primarily meaningful as an inaccurate proxy for LDL-P. Since I measured that directly, this may not have separate relevance.)
Increased from 55 to 111 mg/dL. This was a big surprise. Generally reducing carbohydrate consumption reduces triglycerides, so I expected a further decrease. Instead, I had my highest triglyceride measurement so far. A cursory search indicates that this might be a common response as triglyceride removal is impaired during a prolonged fast.
Total Cholesterol
Increased from 248 to 292 mg/dL. (My understanding is that if you control for LDL particle number, this doesn't matter.)
Decreased from 32.4 to 24.7 umol/L. I have no idea what this means.
Small LDL-P
Decreased from 800 to 459 nmol/L. This seems to indicate that the decrease in LDL-P was solely due to a decrease in the small LDL-P. I'm unsure whether this is relevant. Petter Attia seems to say, the only thing that matters is the number of LDL particles.
LDL Size
Increased from 21.2 to 21.7 nm. Again, Attia claims it doesn't matter.
Lipoprotein Insulin Resistance (LPIR) Score
Decreased from 36 to 27. This seems to be an improvement, but I'm not familiar with this score.

To summarize, the most relevant point seems to be that my LDL particle number decreased by 11%. This is an improvement, but still leaves me in the high risk category. I plan to do a follow-up test in two weeks to see if this remained low or increased again post-fast.


For me, this was again a great experience. Living without food for almost a week feels like diving in an alternate universe. It's a great tool for learning more about how the body functions and responds to different environments. It's also been the most fruitful way for me to explore ketosis.

I'm looking forward already to my next fast in June!

Future Experiments

Here are a few ideas that I might experiment with next time:

  1. Get into ketosis more quickly. I might make an effort to get into a deep state of ketosis more quickly. Most likely through exercising and potentially taking exogenous ketones early on.
  2. Remove bone broth. I might try doing it without the bone broth. Or consume it in a more limited way. Perhaps, the collagen in the bone broth is preventing me from reaching a deeper level of ketosis.
  3. Go longer. The day I ended the fast, I felt the best. My GKI was also the lowest. So I might try fasting a little bit longer next time. Probably 7 days instead of 6.
  4. Sauna. I'm not sure why, but I suspect sauna might help getting into deeper ketosis. I may try going to the sauna a few times and test that.

Let me know if you have any observations or questions. I'd be particularly curious to hear from anyone more knowledgeable about lipid panels.