I first became curious about the physiological effects of fasting in 2012 and have spent the last 10 years experimenting with different fasting, eating and training patterns. Here I will share a few of the things that these experiences have taught me. I am not a medical professional so please read this blog in the spirit that it is intended, some personal observations and experiences, not medical advice.
Before my fasting experiments I was reasonably healthy. I cycled quite a lot, 30km each day through London to work (lots of racing off the lights!) with more rides in the weekend for fun. Cooking is a passion for me so I prepared most meals from scratch or ate in good restaurants, enjoying a good, standard diet. I was about the same weight as I’d been since my late teens. I didn’t think that the low energy dips I sometimes felt in the afternoon or on my ride home from the office were anything unusual. I also thought it was normal to regularly feel an extreme hunger, sometimes “hangriness”, yes this is apparently a thing!
Then I moved over to the Netherlands to study an MBA. This was another high pressure environment with long working hours but less control over my eating times and limited kitchen space so food quality declined a little. I still managed quite a bit of cycling, often squeezed in early in the morning. Towards the end of my study my snacking and sugar cravings were, in retrospect, noticeably worse but I hadn’t gained any weight so didn’t think too much about it as family life began to take over my spare time.
My curiosity in fasting was first triggered by a British doctor called Michael Mosley who popularised the idea of eating less on two days of the week, the “Fast Diet” or 5:2 Diet. A TV programme also popularised the diet in the UK. It seemed plausible that it might be healthy and sounded like a bearable challenge so I gave it a go. Those two fasting days were quite challenging, requiring conscious effort to maintain.
The next level of fasting came after I was given an inspiring book about the therapeutic use of diet and lifestyle to manage cancer. At about the same time as I heard about Bulletproof coffee and Ketosis from Peter Attia. It did seem odd that there was so much chronic illness around and logical that there must be aspects of our modern lifestyle that caused this illness. It was an appealing idea that we could help reduce some of these risks of illness simply by changing a few eating and lifestyle habits. After reading a few other resources and listening to some podcasts I began experimenting with cutting out refined sugar, then other sugars, then bread and grain based foods. I was on the road to “keto”.
By 2014 I was following a fully ketogenic diet, eating fewer than 50g a day of carbohydrates with a strong emphasis on eating healthy fats (animal fats plus coconut and olive oil). I found I was able to comfortably go for long periods without eating, for convenience, when travelling or just for fun. I remember my first successful 24 hour fast, I skipped supper and breakfast then rode by bike to Amsterdam for a family zoo visit and back home again without any difficulty. I was definitely becoming “fat adapted”. The fasting and diet changes had helped my body to learn to burn fat more effectively. Where I couldn’t previously go more than 3 or 4 hours between meals without low energy and extreme hunger I could now easily go 24 hours. This still feels like a superpower today, more than 5 years later.
Complimentary to the diet changes was my move to “training low”, maximising some of the training effects of sports (rowing and cycling) by doing them in the morning, without eating. The idea is that your body will tend to use up some of its glycogen reserves during the night so that if you then exercise without eating your body would have to burn fat. The more you do this the more efficient your fat burning metabolism would be and the more fat you would be able to burn during exercise by burning fat at higher activity rates. Combined with the diet this means that I became able to ride reasonably quickly (an average of 30 km/h for at least an hour at an aerobic heart rate (around 130 beats per minute).
Today I am no longer following a ketogenic diet and don’t always train low. I would say that I try to be “metabolically flexible”, able to use fat or sugar depending on the situation. So what made me change? I discovered one of the downsides of being too aggressive with ketosis was that I began to miss a bit of top end power. It was also, crazy as this might sound, too easy to eat too little when eating a truly ketogenic diet. This could then lead me to sometimes experience slightly low thyroid symptoms, cold temperature for example. I now try to maintain fat adaptation by eating within a 8 to 10 hour time period every day, fasting for nearly 24 hours every Monday and doing zone 2 training fasted several times a week. For the rest of the week I eat some carbohydrates which helps me be able to reach for that top gear when I need it. I also eat a lot more protein than before, but that is a story for another blog.I have made plenty of mistakes along the way. For example I struggled through the first phase of ketogenic adaptation by not realising the importance of taking extra electrolytes. When fasting you tend to lose more electrolytes, especially when you drink (lots of) coffee as I do. Even today I do have a salty drink for lunch during my fast on Monday and before I exercise fasted. I also found that after the first phase of adaptation I didn't need to eat so much fat and could lose body fat by reducing this when needed, an approach sometimes called “Protein to Energy" or P:E. My fasting journey continues but so far it has been fun and left me feeling better off.