Assume an otherwise-healthy adult man, say 40 years of age, 80kg, neither obese nor possessing a remarkably low percentage of body fat, is denied food. How long until he’s so weak that he wouldn’t be able to stand? Two weeks? Three?
The survival adage is three minutes without air, three days without water, and three weeks without food.
Agree. A month under controlled conditions.
That’s until he dies. I’m asking about how long until he’s conscious and breathing but too weak to move. (Need this for a story I’m writing.)
There were actually experiments done on this during World War II. See the Minnesota Starvation Experiment.
This (longish) article Death by GPS includes the story of Albert & Rita Chretien, who in 2011 got lost & stuck in their minivan in northern Nevada.
Albert walked off for help, but didn’t find it. Rita stayed with the car, where elk hunters found her alive - 49 days later. She’d had unlimited water from a nearby stream and a very small amount of candy and trail mix. She was 56 years old, and had effectively zero survival or wilderness experience or training.
The linked article doesn’t give this detail, but I believe she was (marginally) able to stand. She needed only a few days in hospital after being rescued.
This study seems not to closely match the OP’s specified conditions:
Read up on the 1981 Irish hunger strike where 10 prisoners starved themselves to death. Here is some preliminary information:
80kg male will probably have a 2000 calorie a day requirement assuming he is sedentary.
Each pound of fat has about 3500 calories in it. Assume the 80kg male has 10kg of fat, that is 77,000 calories. Enough for 38 days assuming all his energy comes from fat.
I think skeletal muscle only has about 400 calories per pound.
Realistically, he is going to burn a lot of skeletal muscle which will drop his metabolism. He may burn through 50+ lbs of muscle, which would drop his metabolism to closer to 1300 a day.
if I had to guess, somewhere in the 1-2 month range. A month is probably enough for him to lose 1/3 of his lean body mass and become very weak.
He can’t last that long, nowhere near 2 months. While fat can provide calories the brain needs glucose, around 130 grams per day to survive. For people consuming a normal diet this glucose is provided by dietary carbohydrates, for people on carbohydrate restricted diets the glucose is provided by gluconeogenesis of dietary protein, and for people eating nothing this glucose is provided by by gluconeogenesis of muscle tissue. This process doesn’t differentiate between skeletal muscle which can be spared and cardiac muscle, which is kind of important.
It would be nice if during starvation the body first burned all of the fat then switched to consuming muscle but that’s not how it works.
Can’t your liver make glucose out of broken down fats? Why does it need to be proteins? I’m looking online at the biochemistry charts though and I don’t know if the body can convert acetyl COA to pyruvate. So maybe not. Either way, I thought the brain could use ketones in a low carb dieting situation.
I did find this, which implies it can happen but it isn’t efficient.
https://www.nasw.org/users/mslong/2011/2011_07/Gluconeogenesis.htm
One argument I’ve heard is that a pound of fat can only mobilize about 31 calories a day. I get the impression that isn’t set in stone, but each pound of fat can only break down and release so many calories per day. Beyond that your body starts breaking down lean tissue.
Either way, as far as OPs situation I assume at the end of the month the guy will still be alive, but he’ll be 50 pounds lighter (mostly muscle loss) and very weak.
You can use ketone bodies for energy in a low carbohydrate dieting situation for sure, but I had always been told that fat could not be metabolized to glucose as there was not a biological pathway for that to occur. That being said, I just found this article on the web that says it is possible. The article is not scholarly, and certainly lacks peer review and all that, but the references given at the end look like they have a good deal more rigor.
So it looks like you can make at least a bit of supplemental glucose starting from fatty acids. I’m certainly guilty of not keeping up with the research and apologize. I’m only a regular chemist, not a biochemist, and went to college when the periodic table was only Earth, Air, Water, Fire, and Aether. It was a simpler time.