A friend told me that she has tried to fast, but can’t do it more than 24 hours. And then she gets ravenously hungry afterwards. As you may know I have done 7 day water fasts, and still had enough energy to ride 100km on my bike. She asked me why it seemed so easy for me … so we did the math.
We know the maximum rate that body fat can transfer energy thanks to A limit on the energy transfer rate from the human fat store in hypophagia – Alpert (2005) This paper used Ancel Keyes’ Minnesota Starvation experiment data and a little calculus to derive the value of (290±25) kJ/kg day or (31.5053±2.72) kCal/lb day as the maximum rate that energy can be drawn from body fat.
I’m 230 lbs, my lean body mass is 170 – so I have around 60 lbs of body fat. This means just from body fat alone, I can draw 1890 kCal/day of energy.
She’s 135 lbs, her lean body mass is 110 – so she has around 25 lbs of body fat. Which means from body fat she can only draw 878.5 kCal/day of energy.
My daily metabolic rate is around 2000 kCal, so if I fast I only have to find 110 kCal of energy savings – around 5.5%. If I don’t want my metabolism to slow by 5.5% then I can eat an additional 12.2g of fat per day to remain fasted. But if I don’t eat a little fat I probably won’t even notice a 5.5% slowdown.
Her daily metabolic rate is around 1500 kCal, so if she fasts she has to find 713 kCal of energy savings – around 47.5%. If she wants to maintain her metabolic rate then she would have to eat an additional 79.2g of fat per day to remain fasted. It’s no wonder she can’t fast for long.
And that is why fasting is easier for me, and harder for her.