Why can people last longer without food than they can without water? Food has water in it. Does this not factor in to survival?
In: Biology
Food doesn’t have enough water in it. The thing about calories are that they’re pretty fungible. If you don’t have enough of one particular energy source, the body has metabolic pathways to provide at least reasonable facsimiles. Our bodies are incredibly energy dense. If you’re 180 pounds and 10% fat you have 18 pounds of fat.
18 pounds of fat at about 3500 calories per day means you have over 60,000 calories in just your fat tissue. On a zero calorie input you could probably survive a month on that alone. Of course, other things start to break down and go haywire as you lose more adipose tissue but that’s just a really fit person. Someone with larger fat stores could go even longer.
Water on the other hand we go through about 2/3 of a gallon of water per day. We get about 40% of that through metabolism (some of the energy generation generates water) and water in food. The rest comes from water we drink.
We need water for a whole heap of biochemical processes because a lot of our enzymes that do things that let us live use hydrolysis. When ATP, your body’s molecular energy currency, splits to release energy it is hydrolyzed, or broken apart by water. Without water, we can’t even consume the energy we manufacture.
Water is needed for most of your body processes. Getting dehydrated (depleting water in the body) will shut down those processes and ultimately lead to death.
As for food I think it is more of like a storage of energy and if you run out of food your body still has body fat/muscles to get energy from.
There is water in food. Both as water itself in the various foods we eat but it is also a byproduct of metabolizing foods or stored up fat reserves. However the water from the metabolism is not enough and your body needs more. Depending on what kind of food you eat it is possible to live without drinking any liquids itself as there may be enough water in the food. The thing is that while the human body can store up energy reserves in form of fat there is no way for the human body to store up water for as long. And the human body is not good at conserving water either. So we need a fairly constant source of water for when we are thirsty.
It does – for example, you’d last longer eating a bunch of grapes than you would stale bread.
If you were entirely sedentary and a 154lb man, you’d need to intake at a bare minimum 32 ounces of water per day to survive. Note that this is far beneath anything resembling a healthy level and you’d likely have major health complications, and die if you actually did much of anything.
Getting a quarter gallon worth of water from food in a day is a tall order, and typically if you’re in a situation where you have absolutely no water, you’re not going to have the benefit of a surplus of food or to just lounge around in a comfortable environment.
Meanwhile, the body has much more reliable methods for storing large amounts of energy in the form of fat. And if that fails, the body can cannibalize muscle.
After oxygen, water is the most critical chemical for us to live. ALL life exists in water. The chemical reactions that compose biology take place in water. So we have to maintain plenty of it inside. Unfortunately, a bunch of things our bodies do tend to leak water out of it. Our liver kidneys filter junk out of our bodies and and use water to transport it away via urine. When we breath, water leaks out of our lungs and airways as vapor. When our bodies need to cool down, they leak water through our skin so it cools us as it evaporates. All these things mean we have to keep drinking it to keep our insides wet enough for biochemistry to happen.
Food on the other hand does a lot of work compared to how much we have to eat. We store lots of it short term as sugar in our blood and glycogen stores. We store even more of it long term as fat. Since we don’t constantly need that much of it we can get by on these reserved much longer than we can get by on our water storage.
Edit:
It’s “liver and kidneys” not “liver kidneys”
I’m not a scientist or whatsoever, so if I said something wrong, feel free to correct me politely.
We exhale like one litre a day of water just by breathing, so we need to “get back” that amount, plus all the liquids we lose when we sweat and urinate. Anyway, we can live longer without eating because our bodies can slow down their metabolism a little, so that they use a little less energy and can go on longer. Plus, the human body stores the unused energy as fat, that will eventually be used when needed
Anyway yes, food contains water, and obviously the amount of water contained is different for every food, that means that we actually take water even by eating certain foods.
We store sugars in our liver, and we store fat all over the body. We don’t really have the same storage mechanisms for water (all water is basically used up or peed out). Most of your body is water, everything in your body is bathed in water, and we get rid of water when we pee and sweat and make saliva, etc.
So basically we need a lot of water and being dehydrated is going to kill you faster than malnutrition because we require more water to survive than we do food to survive.
Latest Answers