Greetings,
Suppose you have two individuals, one with 15% BMI and one with a 39% BMI.
Now, suppose these two individuals were stranded somewhere with unlimited drinking water but no food of any kind.
Would the person with the higher BMI stay alive longer, living off the stored fat deposits so to speak? Would both starve at equal rates?
For the purpose of this discussion lets avoid all cannibalism scenarios and concentrate on stored body fat as it relates to starvation.
Thanks for all replies.
First, BMI is not a percentage - it does have units (kg/m squared).
Someone with a bmi of 15 is already well underweight, and would probably starve before the overweight individual, but there are plenty of factors that can influence that (metabolic rates, essential nutrient stores).
Suffice to say, in general, an overweight individual has more material for the body to self-cannibalise to use for survival than an underweight one, and will probably last longer in starvation conditions.
Well, there are a lot of factors, but the reason humans store fat is for just this contingency, so I’d say the broad answer is ‘yes’…someone with a higher body fat is going to take longer to starve (presumably to death) than someone with lower body fat (lots of caveats and such in this of course).
There’s a really famous case of an obese Scotsman who went on a crash diet and did not eat for over a year.
I doubt many people with a normal BMI could last a year without food.
Yes. Also, the person with the higher BMI would be able to sustain a certain level of physical activity for longer, allowing him more activity to find food. If possible. He’d also suffer less damage from the starvation, as the body would not cannibalize essential bilogical matter as quickly.
This is what fat is for. It is a tissue type with a purpose, not a disease.
Just a few days ago, I saw something online about a person who weighed something like 1,000 pounds, and starved to death on a crash diet. He weighed over 600 pounds at the time of his death, and his cause of death was vitamin deficiency, although which one, IDK.
BMI is a bit weird in that identically proportioned people of different heights will have different BMIs. One of those little round venus figures a few inches tall would have a tiny BMI, and the fantastically tall statue of fairly slender Abraham Lincoln in the Lincoln memorial would have an enormous BMI. Weight divided by the cube of height would be a better measure for predicting how long somebody could survive without food.
Each pound of stored body fat is 3500 calories of energy. So, yes, the fat guy would last longer, though as nearwildhaven points out, there are other vitamins and nutrients besides simple fuel that we need to maintain our bodies. However, that is not “death by starvation”, that is death due to vitamin deficiency. I expect he’d get rickets and all the other bad stuff before he died.
Well it’s made out of marble, so yeah. 
Do we really want another thread on the practical limitations of BMI for individual assessment and why using a cube for height (the Ponderal Index) is not much better?
I thought when the body self-cannibilizes it dips into muscle more and faster than fat. But I learned that a long time ago, maybe someone can enlighten me? I learned this as the reason a fat person does not get a real advantage in a starvation scenario.
The body generally uses for energy, in order: eaten sugars, eaten complex carbohydrates, eaten fat, stored carbohydrates in the form of glycogen, stored fat, eaten protein and finally stored protein. It will use stored protein if it has to, but because of the number of molecular bonds it has to break to do so, it’s metabolically expensive, using up more energy (than other sources) to get energy.
If the body needs amino acids to build new cells or hormones it will break down stored protein to get them. But again, it’s easier for your body to make them from food, so as long as you’re eating, those proteins will be used preferentially.
You might find this helpful: Carbs, Fats, and Proteins: How the Body Burns Fuel - VirtualMedStudent.com
Fat can only be mobilized into energy so fast. Converting from kj to kCal that’s about 60kCal/kg of fat per day. Hence in higher demand times we mostly use glucose floating about and make more from glycogen.
So take a lean person with maybe 10 kg of fat … that’s 600 kCal from fat a day and the rest needs to come from fat free mass, initially primarily glycogen stores but then more muscle, then more organs, then finally, brain … if they last that long. Note also that comes to 25kCal an hour … reasonably intense activity entailed in survivial can easily burn 600 kCal an hour.
Now take an obese person with maybe 40 kg of fat. That is theoretically a max of 2400 kCal/d that can come from fat. They expend more energy moving that weight around on the one hand but they also have more fat free mass as well from having carried the weight around. So from fat theoretically at least the obese person can mobilize 100 kCal/hr and has more other fat free mass to call upon before getting to essential organs or brain.
Scurvy (vitamin C deficiency) would be the first concern, and more long-term undernutrition will lead to beriberi (thiamine deficiency) and pellagra (niacin deficiency). Vitamin D deficiency severe enough to cause rickets will take a while longer because that vitamin is fat-soluble.
One of those TV shows about morbidly obese people featured a man whose immobility was caused in part by severe chronic pain that seemed to defy diagnosis. He was finally seen by a doctor who tested his vitamin D level, and he did indeed have rickets, because he never went outdoors and didn’t consume enough to meet his body’s needs. :eek: He was given megadoses for a while, and within days, he felt much better.
When I worked at the nursing home pharmacy, we could always tell who never went outside because they were on vitamin D, usually a 50,000 unit capsule once a month. There was one NH where almost everyone was on prenatal vitamins, too. 
We wondered what someone might think about Dad or Grandpa taking this, but it made sense; they’re good vitamins, and inexpensive too.
A woman on another website has told the story many times about her stepdaughter who was rejected by the military because she was considered obese per her BMI. She was very athletic and muscular; that weight was NOT because she was “fat”.
That is an interesting concept, 60 cal/kg is about what I’ve heard (I’ve heard 25-30 calories per day per pound of fat.
I’ve heard when getting fat for every 2-3 pounds of fat you gain you gain a pound of fat free mass to carry it around. I’m sure that rule of thumb breaks down at the extremes (I doubt someone who weighs 900 pounds has 400 pounds of ffm) but for the majority of people I wonder if that rule of thumb is mostly right. If so, the obese will have a higher resting metabolism. But probably not that much higher since during periods of starvation RMR and physical activity both decline.
I’m not fat. I’m just prepping for the apocalypse.
This is a common problem with BMI - high lean mass gives a high BMI. Of course, this won’t help in a starvation situation - the body will start destroying muscle sooner, which is really bad for the kidneys and other parts of the body - you can get runaway muscle breakdown that will kill you very rapidly
The concept that the obese have a higher rest metabolism got discussed before. Yes, other than extreme obesiy (where people just don’t move any more) resting metabolism does increase with obesity but not linearly. The obese tend to move around as much as the non-obese do and that requires more muscle mass and more energy.
The most precise calculator for that requires knowing percent body fat.
Still the extra fat and fat free mass will feed that for a while and the obese will also downshift resting metabolic rate aftersome modest weight loss more than the non-obese will.
That 60kCal/kg number? Ends up (if accurate) coming out that someone with 25kg (55 pounds) of fat, can really only lose about 3 pounds of fat per week theoretical maximum. And that it really is very hard to lose those “last few pounds” even theoretically.
It’s pretty well known that people with a lot of muscle will have high BMIs. It’s kinda silly to think the military, which has a high proportion of very muscular people applying, would be unaware of the fact.
The army standards for obesity are based on body-fat percentage, not BMI, for exactly this reason.
I wonder what people’s mental state is like during starvation. If I go 24 hours without eating I get pretty irritable, and have trouble concentrating. I can’t imagine that continues to get worse, otherwise after a year your Scotsman would have the personality of a very whimsical axe-murderer.
But does it get better as your body adjusts to living off ketones and the like? Or do you just stay angry and stupid the whole time.