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The Bare Minimum Food and Water You Need to Survive

“Extreme environmental physiologists” weigh in on what the human body really needs

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(Photo: © Marco Bottigelli/Getty)

A in the journal Experimental Physiology starts on a sober note. “It is a sad reflection of where we are in 2025,” the authors write, “that the most common question currently posed by international media to extreme environmental physiologists is: What is the minimum amount of fluid and food required daily to survive?”

Mea culpa, because this is definitely a question I’ve wondered—and written—about. It’s not just macabre curiosity. In my book , I was trying to understand the difference between the limits we perceive and actual physiological endpoints. You feel out of breath when you run, but it’s not because you’re literally going to die from lack of oxygen: the record for holding your breath is 11 minutes and 35 seconds. Similarly, endurance athletes grapple with hunger and thirst—but how closely are these warning signals tied to actually running out of fuel and fluid?

With that in mind, here’s what the researchers—Mike Tipton of the University of Portsmouth and Hugh Montgomery of University College London—had to say.

How Much Fluid You Need

Tipton and Montgomery start by describing the fluid losses for someone lounging around at a comfortable temperature. In a typical day, you lose about 500 milliliters (roughly a pint) evaporated through your skin, 500 milliliters in your breath, and 500 milliliters in urine. That means you need about 1.5 liters, or three pints, to stay hydrated.

Things get more complicated if it’s hotter, which increases sweat losses; if the air is dry, which increases breathing losses; or if you start moving around. You can lose up to two liters per hour from sweat, so your fluid needs will depend critically on how much physical activity you do and what the temperature is. Distance-running legend Haile Gebrselassie reportedly had a lab-tested maximum sweat rate of 3.6 liters per hour, one of the highest ever recorded.

You’ll also lose more water in your urine if you’re eating a high-protein diet, because your body needs to dilute the ammonia produced by protein metabolism so that it won’t be toxic to your cells. For that reason, you shouldn’t eat too much protein if you’re short of water—but avoiding protein won’t help if you’re also starving, because then your body will start breaking down muscle protein from inside the body.

On the other hand, when your body digests carbohydrates and fat, water is one of the byproducts of this metabolic reaction. That can add about 500 milliliters per day to your body’s fluid stores, reducing the minimum required fluid intake to 1 liter. This assumes you’ve got food but not water, as you might in the desert or on a lifeboat. Tipton and Montgomery quote a Second World War survivor in that situation: “It was difficult to muster a spit, and eating our hard tack (biscuit) was impossible. After a quarter of an hour of chewing, we still couldn’t swallow it and in the end simply blew the powder away like dust.”

If you don’t meet your minimum requirements, the clock is ticking. Losing more than five percent of your body weight in fluid leads to headaches and other symptoms. Ten percent impairs performance and leaves you dizzy and faint. Beyond that, your skin will start to shrivel, your blood will get dangerously salty, and eventually you’ll be at risk of critically low blood pressure and organ failure. At 15 to 20 percent, which you can reach in three days in a hot climate, you’ll die.

How Much Food You Need

The basic rule of thumb is that you need one calorie per minute to sustain your basal metabolism. That works out to just over 1,400 calories per day, which is what the average human burns at rest. Normal activity levels burn another 1,000 calories, bringing the daily total to 2,400.

The research shows that healthy people can generally maintain their normal work capacity for up to ten days, even with a severe calorie deficit. That fits with what you see in survival shows like Alone: you can spend your initial time sorting out shelter and scouting out resources in your environment, but if you’re not bringing in significant calories after a week, you’re in trouble. It also explains why some of the most successful participants end up spending weeks lying in their shelters and moving as little as possible: a calorie saved is a calorie earned.

You can handle losing about ten percent of your body weight (assuming you’re remaining hydrated, so that the loss represents actual body tissue) without serious physical repercussions. Beyond that, both strength and endurance begin to drop rapidly. If you lose 20 percent of your lean tissue (i.e. not including fat), you’re in danger of serious damage to your heart, lungs, kidneys, and liver.

Death usually results from organ failure, an infection overwhelming your weakened immune system, or your heart stopping. It takes 40 to 60 days. Kieran Doherty, an Irish Republican Army prisoner, reportedly lasted 73 days on a hunger strike in 1981 before dying. More bizarrely, reported on the case of a 27-year-old man who consumed nothing but water and vitamin supplements for 382 days under medical supervision while dropping from 456 to 180 pounds.

I don’t mean to be flippant about either of these topics. As Tipton and Montgomery note at the start of their paper, it’s awful that so many people in the world today face the very real danger of encountering these limits. But it’s also fascinating: in the human body, the warning lights go on long before the tank is empty.

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‘Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance’