According to Deuteronomy 9:9, Moses did not eat or drink anything for 40 days.
When I was gone up into the mount to receive the tables of stone, even the tables of the covenant which the LORD made with you, then I abode in the mount forty days and forty nights, I neither did eat bread nor drink water.
Is this possible from the scientific point of view? My guess is that no water for that long time must be impossible. How about no meals?
"Comment in passing"—the biblical term "40 days and 40 nights" or similar can be used to mean "a long time". Whether this is the case in any given passage would need to be understood, but is outside the scope of this question.
In any normal circumstance you'd be dead long before 40 days without water.
But hunger strikers regularly live beyond 40 days and purposeful fasting for that period is not unknown.
As an example of what this may do to the body, assume that a person fasting is largely immobile and can survive on a 500 kcal daily energy budget. Available calories from food are about 4 kcal/gram for carbohydrate, 5 kcal/g for protein and 9 kcal/g for fat. Start by assuming that body resources can return say 6 kcal/gram if "living off your own body.
Obviously there are many other factors to consider, and the above mass loss seems 'rather low' compared to expectations, but is an indication that energy availability is not the major factor involved.
This document—Adult Malnutrition in Emergencies—advises that hunger strikers experience "severe metabolic disturbance after 3 to 4 weeks" (ie 21 to 28 days) and death in 6 to 10 weeks (42 to 70 days). ir 40 days would be "pushing your luck".
Wikipedia has different articles in English and German about hunger strikes. The English version talks about people dying after 52 to 74 days of strike and it looks as if it is the lower bound. One Person, Swami Nigamanand, is reported to have died after being on a hunger strike for 115 days. There are several other numbers in between, and not every incident ended in the death of the hunger strike performer.
The German Wikipedia talks about 50 to 70 days strikes, which can be survived, but this sounds like an upper bound. The article is much shorter and concentrates on well known cases in Germany.
But there is a link to the medical aspects of hunger strike and a english version too. To summarize them: In the beginning, a typical weight loss is about 1kg/day, later 0.5kg/day.
Normally, a person will die without water after 3-4 days, but depending on weather circumstances, this can be shorter (1d, hot, dry) or longer (10d, cold, wet). Without food, but with water, healthy persons should survive 30 to 200 days. A critical question is, whether you get minerals and vitamins, or not even that.
One kg of body fat can be used like 30 MJoule or 7.000 kcal energie.
Afaik, we don't know much about the kind of diet which Moses or Jesus would have performed, and in which physical condition they started. Did they drink, and if they did, just water, milk or juice? If I remember correctly, the Bible claims that John Baptist ate locusts, which have a lot of proteins; did Jesus too?
My conclusion from this information is, that - apart from being just a written story - a 40 day fastening is hard, but possible - not a miracle, but a stunt of discipline. But 40 days without water is impossible; that's a lie.
There was a single study published in 1973 in which an obese man, under close medical supervision, fasted for over a year. He took water and some vitamin or mineral supplements but no food.
Yes, it is possible to fast for 40 days. (for food, not water)
Here is a list of about 2 dozen IRA prisoners who participated in a hunger strike in 1981. https://en.wikipedia.org/wiki/1981_Irish_hunger_strike#Participants_who_died_on_hunger_strike
The shortest it took one of them to die was 46 days, the longest someone lasted who didn't die was 70 days.
