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Experiment and observations after a 48 hour fast

After recently reading a blog by Jouchim Bartoll regarding his thoughts on fasting, I decided to try a 48 hour fast for myself and see what happened. I have done many 18 hour fasts before so I am used to managing when hungry, however, I wanted to explore the practical application, experience and implications, if any, of a longer fast first-hand.

The science and theory behind fasting and its subsequent ‘pros and cons’ are well documented. Here are some of the benefits to highlight:

  1. Cell and protein turnover is elevated after 24-36 hours. This is known as autophagy, damaged cells and amino acids (from non food sources) get cleared out or recycled. This has a certain global healing effect on the body and the mind. I find this fascinating and it is something I want to learn more about ASP.
  2. It allows the digestive system to have a rest and reduces inflammation. This can be very useful and should be considered very important for gut and general health.
  3. A long fast resets the hormones Ghrelin (“I am hungry”) and Leptin (“I am full”).
  4. Increases brain function and mental clarity.
  5. Glycogen gets depleted which encourages the use of ketones and fatty acids as the primary fuel sources. It also improves our ability to become ‘fat adapted’ which is our most efficient biological state. As the name suggests, this means we become efficient in using fat as fuel which in turn general leads to leaner body composition and easier fat loss.
  6. Growth Hormone is elevated which has a host of benefits including increased fat oxidation (fat burning), increased strength, as well as enhanced sensory perception (clearer eye sight, good news for a hungry hunter!)

There are many more claimed benefits such as increased telomere length (the end caps of our gene strands) which increases longevity and so forth, but perhaps, best saved for another blog.

It seems from the experiences of others that results are best if you are already adapted to burn fat instead of glycogen as your primary fuel source. This is achieved by following a low carb diet, keeping protein at around 1-1.5 + grams plus / kg body weight, ideally from animal sources. If the animal produce is from a healthy beast then the fat content generally takes care of itself. Remember that red meat / eggs / offal / chicken / fish etc. all contain fats as well as protein. Add some olives / olive oil / butter / avocado / fish oil / cheese (if you are tolerant) / nuts to bump up fats and you are covered. Always keep veggies high.

I wanted to see if training performance was compromised when hungry. Here is the sequence of events during my 48 hour fast.

Saturday Evening:

8:00pm: Last meal, which was beef and veggies stew

Day 1 – Sunday:
9:00am: Black coffee followed by 60 minute glycogen depleting full body workout with weights. My logic was that if I depleted my glycogen quickly I would enter ketosis faster.
11:30am: Deadlifted a PB so, still strong!
– Attended a Lawrence Shahlaei seminar at my gym until 3pm and had a beef stock drink in a litre of water (cheat 1)
– Walked my dog, went into town for a mince around, did house admin and general life stuff.
10:30pm: Bed, I slept well which I was surprised about, I thought stress hormones would have kept me up. More about this later.

Day 2 – Monday:
6:30am: Coffee.
7:55am: Normal PT working day, 4 clients till 12:30.
12:30pm: Beef stock drink in 1 litre of water (cheat 2).
4:30–6:00pm: Black coffee and a hard training session to test mental and physical fortitude. I thought this would be more difficult than it usually is, but to my surprise I was strong and able to crack on as usual.
6:30pm: 1st meal in 46 hours which was a large chicken salad.

In brief:

Duration: The experiment was a 48 hour fast (which became a 46 hour one).

Fluids: I drank black coffee a few times each day and had 1 beef stock drink each day.I drank water throughout the fast.

Training:  x2 metabolic type training sessions with weight and x1 strength session. The 2nd day was a normal PT working day.

My personal findings from this 46 hour experiment were:

– Mental clarity was good, the brain works just fine on ketones (I talked a lot).
– Strength was up (PB deadlift).
– Training in the function hypertrophy / strength bracket (4-7 reps) was about the same as usual. Tested by x2 arduous full body sessions.
– Appetite was not unduly taxing, I felt I could have fasted longer, although I had a dip for an hour about 18 hours in, which is when I usually ended previous fasts.
– I slept well both nights but after I ended the fast I slept badly (the 3rd night). I think this may have been due to the high amount of circulation stress hormones, perhaps caused by the fast and the harsh training session prior to the end of the fast.
– My urine was fairly pungent after 24 hours, probably due to the increased ketones being excreted. I was well hydrated.


If you start a fast when you generally use carbs to fuel life and therefore are not able to tap into fat as fuel quickly, it will be quite an ordeal I expect. If you have programmed yourself to eat high sugar / carb foods and suddenly stop, it means that other mechanisms (liver glucose and gluconeogenesis) have to kick in to help maintain blood sugar levels. When you are not used to this it can sap mental and physical resolve. Therefore allow 2-3 weeks of the above diet and then proceed.

Also, if your life is a mass of stress then a fast may do more harm than good. Adding more ‘hunger stress’ will probably be counter-productive. In this situation, eating little and often is a good plan to keep blood sugar levels fairly stable and the cortisol / insulin fluctuation less of a dramatic see saw.

There appears to be a profound difference between a fast and a continuous low calorie diet. Counter intuitively, the former will ramp up your metabolic rate which is clearly a good thing from a fat burning point of view. However, the latter will often slow down your metabolic rate over time. This is not a good thing for long term fat loss / muscle mass / brain function / health and so on. On a fast the body will try to hold on to muscle, whereas on a chronic low calorie diet it will try to hold on to fat!

We are designed to hunt and be strong when hungry, we need muscle to overcome our prey which obviously translates to modern times physical activity / training.  After fat adaptation, when we are using fatty acids and ketones instead of glycogen (sugar) our senses are sharper which makes evolutionary sense!

In summary:

I feel the benefits of a fast are worth the hardship, I will continue to do a basic, short 18 hour once a week to give my digestive system a rest and reduce inflammation. I will probably do a 48 hour fast once a month because of the better health benefits of a longer fast. Repairing mechanisms and autophagy appear to work better when we are fully glycogen depleted, post 24 hours.

Please note that there is a difference between being in ketosis and being fat adapted.  Also striving to be in ketosis continually is not a sustainable state to be in. Glycogen (sugar) derived from veggies and fruit provides fast acting fuel for the body and brain. More on these topics for another time.