Food and water consumption are essential to human life. Your body needs energy from food sources and hydration from water to function properly. The many systems in your body work optimally with a varied diet and adequate water intake daily.
But our bodies are also able to survive for days without water. We can go days or sometimes weeks without food because of adjustments to our metabolism and energy consumption.
Why the time period varies
When a person’s body does not receive enough calories to carry out its usual life-supporting functions, this is known as starvation. This can happen if food intake is severely restricted, or if a person’s body cannot digest food to absorb nutrients.
When the human body is severely deprived of calories, it starts functioning differently to reduce the amount of energy it burns. If nutrition is not restored, starvation leads to loss of life.
There is no hard and fast “rule of thumb” for how long you can live without food. To some extent, the answer depends on individual differences and circumstances. Information about life expectancy is based on real-world contexts, such as hunger strikes and serious medical conditions.
With no food and no water, the maximum time the body can survive is thought to be about one week. With water only, but no food, survival time may extend up to 2 to 3 months.
Over time, a severely restricted food intake can reduce lifespan.
Being underweight, defined as having a body mass index (BMI) below 18.5, is associated with malnutrition and a range of health conditions that can lower life expectancy. These include reduced immune system function, digestive conditions, and cancer.
A 2018 study found that having a BMI below 18.5 could shorten your life by an average of 4.3 years for males and 4.5 for females.
How is this possible?
Being able to live for days and weeks with no food and water seems inconceivable to many of us. After all, a daylong fast or even an hours-long stretch without food and water can make many of us irritable and low on energy.
Your body actually adjusts itself if you engage in a short-term fast or are unable to access food and water for very long stretches of time.
For this reason, adults without health conditions are usually able to engage in religious fasts and even try “fasting” diets like the eat-stop-eat approach while continuing their regular daily activities.
It takes about 24 hours without eating for your body to change how it produces energy.
Under normal circumstances, your body breaks down food into glucose. Glucose provides energy to the body.
During the first 24 hours without food, as your glucose storage is depleted, your body will begin to convert glycogen from your liver and muscles into glucose.
By the second day without food, your glucose and glycogen are depleted. Your body will begin to break down muscle tissue to provide energy. However, your body is designed to conserve muscle, not break it down. So this phase provides temporary energy while your metabolism is making a major shift.
To prevent excessive muscle loss, the body begins to rely on fat stores to create ketones for energy, a process known as ketosis.
During the first 5 days without food, a person may lose 1 to 2 kilograms (2.2 to 4.4 pounds) of body weight each day. Most of this weight loss is related to dehydration and electrolyte imbalance. Over several weeks of starvation, changes in the body usually cause weight loss to slow down to an average of 0.3 kilograms (0.7 pounds) per day.
The more fat stores available, the longer a person can typically survive during starvation. Once the fat stores have been completely metabolized, the body then reverts back to muscle breakdown for energy, since it’s the only remaining fuel source in the body.
You’ll begin to experience severe adverse symptoms during the stage of starvation where your body is using its muscle reserves for energy.
A study in the British Medical Journal states that those undergoing a hunger strike should be monitored closely for severe side effects of starvation after losing 10 percent of their body weight. It also says that very serious conditions will occur when an individual loses 18 percent of their body weight.
Why does water intake affect this?
With adequate water intake, some people have survived with no food for weeks or even several months. Survival time is longer with water intake because the body has much more in its reserves to replace food than fluid. Your kidney function will diminish within a few days without proper hydration.
One 1997 commentaryTrusted Source that looked at hunger strikes suggested that a person needs to drink about 1.5 liters of water a day to survive starvation for a longer period of time. The author also suggested adding up to half a teaspoon of salt a day to the water.
Side effects and risks of restricted eating
Depending on the specifics, reports suggest that some people can survive starvation for days or weeksTrusted Source. However, during starvation, your body’s many systems will begin to deteriorate.
The effects of starvation may include:
1. faintness
2. dizziness
3. cognitive changes
4. low blood pressure
5. slow heartbeat
6. weakness
7. dehydration
8. changes in thyroid function
9. abdominal pain
10. electrolyte imbalances
11. heart attack
12. organ failure
People who have experienced starvation or very restricted food intake may have long-term effects including:
1. stunted growth
2. poor bone health or osteoporosis
3. post-traumatic stress or depression
Those who experience starvation for a prolonged time can’t begin to consume normal amounts of food right away. The body needs to be very slowly eased into eating again to avoid adverse reactions, known as refeeding syndrome. The effects of the refeeding syndrome may include:
1. heart conditions
2. neurological conditions
3. swelling
To reintroduce food safely, people who have experienced starvation or malnourishment need to be cared for in a hospital or specialized medical facility.
The bottom line
Human bodies are fairly resilient. In some circumstances, the body can function for days or weeks without proper food and water. This isn’t to say that going without food for a prolonged period is healthy or comes without risk.
It’s reported that some people have been able to survive up to a week with no food and no water. In some situations, people may be able to survive longer if they consume water.
People who have experienced starvation may have long-term health effects. Starvation eventually becomes fatal, if food is not reintroduced.
People who have experienced starvation or malnourishment will need to be monitored closely by a doctor and medical team to avoid refeeding syndrome.
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