I don’t think a measily 5 days proves much, especially with that poor attitude…kk
One thing to consider is that wheat used to make most bread is much, much different nowadays…less nutritious, less delicious, less everything. To properly do this experiment, you’d have to have bread made from heirloom Russian wheat.
Also, in those days, with his kind of existence, even 1000 calories would be more than enough food per day. Currently, Buddhist monks in Korea and elsewhere live on such a diet…really only one meal a day, mainly of rice and a few vegetables, totalling by most accounts under 1000 calories. When you live a simple, stress-free existence in a mountain temple your body simply doesn’t need more that that. I suspect that the heightened stress levels of modern city living necessitate the “standard” 2500–3000 calorie diet you usually hear of.