Despite what was some new age gurus might claim, humans aren’t light beings that can subsist on air and sunshine alone. Like all creatures, we require food and water to survive.
Typically, humans can go without food for about three weeks before the effects of starvation on the body kill a person. But since the adult body is made of 60% water, typically a person would only last three to four days without a drop of water.
These are average values, however. There are outliers who have managed to survive for longer without food or drink. How long a person can last without access to calories and liquids depends on many factors, such as environmental conditions and a person’s underlying health.
For instance, lacking access to water in the desert under the broiling sun will kill a person much faster than in the middle of the forest where it’s much cooler.
To make things easier, use this ‘rule of three’ to get an idea how long the human body can last without basics: 3 minutes without oxygen, three days without water, and three weeks without food.
How long can someone live without water?
Fluid intake has the most immediate effect on survival. While the body has fat and muscle that it can burn for energy in case food is nowhere to be found, water stockpiles are far less plentiful.
Our bodies mostly consist of water, but we lose a lot of it each day when we sweat, urinate, or even exhale. This is why water needs to be constantly replenished.
How much water a person needs on a daily basis depends on physical activity, age, body temperature (having a fever requires more water), the environment, and air humidity.
Water loss through respiration and sweating can be anywhere between 0.3 and 1 liter per 24 hours under typical conditions. However, under extreme conditions like trekking through the desert, an adult can lose as much as 1.5 liters of sweat per hour.
An adult will also lose around 1.5 liters of water through urine. If you add everything up, that’s around 2.5 liters of water lost per day. What flows in must flow out, so each adult should seek to intake at least that much water from their food and drink in order to maintain fluid balance.
Don’t worry though. You don’t have to keep your eyes on the pitcher all day, constantly measuring to make sure you’re not getting dehydrated. Each person’s water requirements vary, but the body already does a fantastic job of signaling its needs. If you don’t feel thirsty, that’s good enough.
But what happens if this delicate balance is suddenly thrown off? Within a few days without fluid intake, the kidneys lose much of their function and can collapse. Depending on how much water they lose due to physical activity, temperature, and humidity, a person can survive anywhere between 3 to 7 days without water, extreme cases notwithstanding.
An 18-year-old Austrian by the name of Andreas Mihavecz is believed to have survived the longest without water after police accidentally left him in a holding cell for 18 days in 1979. He allegedly licked condensation trails off the wall, but that doesn’t make this record any less impressive.
However, keep in mind that any dehydration that causes a loss of more than 10% of your body weight is classed as a medical emergency.
One of the most dangerous things about water loss is that it can cause blood volume to drop. With less blood circulating through the body, blood pressure can fall to levels that can be fatal.
How long can humans go without food?
The number of days a person can survive without one morsel of food has an even broader range than those suffering from water deprivation. Mahatma Gandhi, who is world-famous for his extremely long fasts, once went 21 days without food.
However, the longest a person has ever survived without food is 74 days. The record was set by Terence MacSwine, an Irish political prisoner, who went on hunger strike in protest, which eventually led to his untimely death in 1920. Generally, people who have voluntarily stopped eating during hunger strikes without ceasing their protest have died after 45 to 61 days, according to a 1997 study published in BMJ.
Hunger strike protestors are actually the most reliable evidence available that we have for how long people can go without food. Starvation experiments in controlled settings are impossible to perform, since it would be highly unethical to ask or force a person to stop eating for prolonged periods just to examine the outcome. Unfortunately, this also means that it is extremely difficult to estimate how long the average person can survive while forgoing food.
What we know for sure is that humans can survive without food for longer than without water. The body relies on calories and nutrients in food to provide cells with the energy they need to fuel vital biological processes.
When the body is deprived of food, it turns to stockpiles. First, the body turns to glycogen in the liver and muscles, converting it into sugar and amino acids.
When it runs out of glycogen, the body starts burning fat stores for energy. This is one of the reasons why fasting is excellent for weight loss. It’s not too fun when the body has to turn to proteins for energy, though. It causes significant muscle loss, including muscle tissue from the heart.
During starvation, the pulse and blood pressure drop because the heart doesn’t have enough energy to pump blood around the body as it normally would. If food isn’t ingested soon at this point, heart failure can become inevitable.
Starvation obviously interferes with the gastrointestinal system, leading to bloating, stomach pain, vomiting, nausea, and even bacterial infections.
Deprived of energy, the central nervous system is also affected. The brain consumes a fifth of a person’s energy, but with little energy to fuel its processes, starving people will have problems concentrating or sleeping.
Bottom line: it’s not clear how long the average person can go without food or water, but humans can typically survive for weeks while starving thanks to their energy stockpiles in their glycogen, fat, and muscles.
Tibi is a science journalist and co-founder of ZME Science. He writes mainly about emerging tech, physics, climate, and space. In his spare time, Tibi likes to make weird music on his computer and groom felines.