Alternative Day Fasting: The Experiment

With the new year here and the desire to lose over half of my weight, I figured I would write this post and update it monthly for accountability and those interested in the Alternative Day Fasting diet. For those that don’t know, the diet involves eating every other day and fasting on the others. I will be doing a slightly different version for this version.

Generally, for what I will be eating, I will not be changing much and will mostly keep eating junk food, but the rules are for my eating days, I will eat whatever I want while still maintaining “reasonability”, keeping around my TDEE. For the fasting days, I will not eat anything at all. Generally, I will be above my TDEE at the weekend as I will have a takeaway for one of these days. I will keep alternating between days but should the fast fall on the Sabbath, I will eat on those days and keep going as if I didn’t. I will still fast for any Jewish festivals required for those days. For example, Ta’anit Esther is a Thursday this year; should it be an eat day, I will fast instead. I will document how I felt each month and how well I stuck to it – sometimes things will happen.

You will find that in many communities, they say you can’t drink diet drinks, but I will be. I am not doing this for health benefits beyond weight loss. Lastly, it is worth saying I do cycle and this may affect results. The weight loss/gain will be recorded on FitBit and I will then make a graph at the end of each month. To record keeping, the weight you will see will be listed for the first day of the next month (so for January it will be the weight as of the first day of February).

With this in mind, I will start the graph with my starting weight!

Maths

The orange points are my weight at the time of recording. The other data is my expected weight. Everything you read below has been added in February after I noticed my weight wasn’t going down and I decided to keep to 1500 calories. Below, I’ve worked out how much weight I can expect to lose, based on my current weight, my Total Daily Energy Expenditure (TDEE), and the calorie intake on fasting and eating days.

Step 1: Calculating the Daily Deficit

My current TDEE is estimated to be 2300 kcal per day. On fasting days, I consume 0 kcal, and on eating days, I aim for 1500 kcal. This gives me an average intake of 750 kcal per day across the fasting and eating days.

So, my daily calorie deficit can be calculated as:

• Daily Deficit = TDEE – Average Intake
• Daily Deficit = 2300 kcal – 750 kcal = 1550 kcal/day

Step 2: Calculating the Weekly Deficit

Since I’m “generally” alternating fasting and eating days, I can calculate my weekly deficit by multiplying my daily deficit by 7 (the number of days in a week):

• Weekly Deficit = Daily Deficit × 7
• Weekly Deficit = 1550 kcal/day × 7 days = 10,850 kcal/week

Step 3: Estimating Weekly Weight Loss

One kilogram of body fat is roughly equivalent to a 7700 kcal deficit. To estimate how much weight I’ll lose in a week, I divide my weekly deficit by 7700:

• Weekly Weight Loss = Weekly Deficit ÷ 7700
• Weekly Weight Loss = 10,850 ÷ 7700 ≈ 1.41 kg/week

So, I can expect to lose approximately 1.41 kg per week.

Step 4: Estimating Monthly Weight Loss

Each month, I expect to lose approximately 1.41 kg/week. Since there are about 4.33 weeks in a month, my monthly weight loss can be calculated as:

• Monthly Weight Loss = 1.41 kg/week × 4.33 weeks ≈ 6.1 kg/month

Step 5: Tracking Progress Over Time

Now, let’s see how this works out over time. If I continue at this rate, here’s the estimated weight loss for each month:

• Starting weight: 150 kg
• Expected weight after Month 1: 144.4 kg
• Expected weight after Month 2: 138.3 kg
• Expected weight after Month 3: 132.2 kg
• Expected weight after Month 4: 126.1 kg
• Expected weight after Month 5: 120.0 kg
• Expected weight after Month 6: 113.9 kg
• Expected weight after Month 7: 107.8 kg
• Expected weight after Month 8: 101.7 kg
• Expected weight after Month 9: 95.6 kg
• Expected weight after Month 10: 89.5 kg
• Expected weight after Month 11: 83.4 kg
• Expected weight after Month 12: 77.3 kg

Target Weight

My target weight is around 90 kg, so, based on this plan, I should hit that target somewhere between Month 8 and 9, which means by September 2025.

January 2025

This is the first update, and before I post the results, it is worth discussing first what it felt like. I am not sure about others, and I say this with some irony, but I don’t mind feeling hungry and not eating; it is more the food and the amount I eat when I decide to do so. With this in mind, I found it easy to simply not eat on certain days, but did find some days annoying, mostly when I was working on those days when it came to lunches, etc.

At the end of this whole experiment, I will post some data regarding how much I lost each week, and how many calories I ate each week compared to what my TDEE is. I found the results … disappointing. I am not sure if I simply expected too much or maybe I secretly ate a lot more than I thought and while I have measured and recorded what I ate I haven’t seen how many calories I ate in total as the idea of why people promote the ADF diet is to eat what you want one day, and nothing the next and I am sticking closely to that mindset.

February 2025

Honestly, I am not sure what to think about with ADF so far. Carrying out research, many people explained how they had suddenly lost weight, while I seem to have stopped losing weight and maintaining it. This would suggest to me that either the diet doesn’t work, there is a medical condition at play or more likely my TDEE isn’t as high as I think it is and that on the feasting days I am eating enough that it works out enough for two days and thus I do not lose any weight.

I am going to now change this a bit. I am going to try and stick to 1500 calories on the feasting days. If I am correct, I would start losing weight and working out some fancy maths I “should” be at around 100kg within 10 months, but we will see. I have updated the maths above. Just know it is from this time onwards. For now, this is the current weight.