Most healthy and well-nourished adults can probably survive and remain healthy for 30-40 days without food, drinking only water.
The researcher Elia M. has estimated that lean adults could survive for 60-70 days and the obese ones for 200-300 days without food . Such extreme fasting would require supplementation with vitamins and minerals and supervision by a doctor.
The longest well-documented survival without food is 382 days by a 207 kg (456 pounds) man .
Without food and water, healthy individuals could survive for 3-5 days in average, but possibly only 1 day in hot weather or up to 8 days in ideal ambient conditions that would allow no or minimal sweating.
There is no scientific evidence that humans can naturally survive just from air and sunlight, without any food and water, as suggested by the belief called breatharianism or inedia . To live, we need to get energy from caloric nutrients, such as carbohydrates, proteins and fats, and regularly replace the lost water, vitamins and minerals .
The survival time without food depends on the body fat and muscle mass, calories burnt (physical activity, generation of heat), the speed of metabolism (basal metabolic rate), hydration, vitamin and mineral supplementation, overall health and mental state .
Death usually occurs after loss of 40-50% of the body weight [2,9]. The lowest body mass index (BMI) compatible with life is 10-12 kg/m-2 . The lowest BMI that is considered normal is 18.5 kg/m -2, for example, in an 180/60 kg (5.9 feet/123 pounds) person.
Who can survive longer without food: men or women?
According to famine studies, in general, “women withstand semi-starvation better than men, which could be explained by the larger percentage of body fat in women .
How long had been adults fasting and survived?
According to Postgraduate Medical Journal, in 1965, a 27-year-old Scotsman weighing 207 kilograms (456 pounds) was fasting for 382 days (a year and 17 days) to lose weight . He was monitored by the doctors who were giving him yeast and vitamin supplements. During the fast:
- He lost 115 kg (253.5 pounds) or 60% of his initial body weight.
- His blood glucose levels fell to 2 mmol/L (the lowest normal value is 4 mmol/L) but he was not experiencing any disabling symptoms.
- The 4th month in his fast his potassium levels dropped; after taking potassium supplements for 67 days, his potassium levels normalized and remained normal to the end of the fast without further supplementation.
- He had low blood magnesium levels.
- He had high blood uric acid levels but no gout or kidney stones.
- He was feeling well for the entire period of fasting.
In the medical journals, there are reports of other obese individuals who have been fasting for 350, 256, 249, 236, 139 and 117 days [2,14].
There are anecdotal online reports of people who were water fasting for 55 days (a man, age 35, lost 35 pounds) , 50 days (a woman, weighing 145 pounds, lost 50 pounds)  or 40 days (a man weighing 290 pounds, lost 40 pounds) .
According to Daily Mail, in 2013, a 65 years old woman, 162 cm/72 kg from Redmond, Washington was fasting for 47 days and then stopped for fear of encouraging other people to starve to death . She lost 15 kg (33 pounds) or 20% of her initial body weight .
According to The Western Journal of Medicine, in 1982, a 41 years old monk, 172 cm/68.6 kg, was fasting for religious reasons for 36 days, eating nothing and drinking 2 liters of water per day, under medical supervision and then stopped due to disabling fatigue and dizziness upon standing . He lost 15.7 kg (34.6 pounds), that is 22% of his initial body weight. He was a healthy ovo-lacto vegetarian, but he had a slow heart rate (40-50 beats/min) before starting fasting.
How long had been adults fasting and died?
A 49-year-old man died of bronchial pneumonia after being 30 days on a diet consuming no food and drinking only distilled water .
How long had been people hunger striking and survived?
According to The New York Times, in 1920, 9 men in prison in Ireland survived after being 94 days on a hunger strike . It is not known if they have suffered from any permanent health damage.
In 1998, a 46 years old man in prison started a hunger strike and ended it after 68 days, which left him with liver damage and partial vision loss .
How long had been people hunger striking and died?
In 1920, 2 Irish men died after being 74 and 76 days on a hunger strike [2,3].
In 1981, in prison in Ireland, 10 men died after 46 to 73 days after the beginning of a hunger strike .
How long had been children fasting and survived?
According to (potentially unreliable) anecdotal reports, some adolescents with anorexia nervosa have not been eating for up to a month . Children and adolescents need some energy for growth, so they can probably survive a shorter time without food than adults.
How long had been children fasting and died?
A 3.5-year-old girl died of malnutrition and pneumonia after 27-day distilled water fast introduced by her parents who believed the fasting would clear her body of toxins .
A 9-year-old girl died after being 40 days on a water fast introduced by her grandmother who operated “fasting clinic” at home .
In 2016, a 13-year-old girl from Hyderabad, India died of dehydration and kidney failure, 2 days after completing 68-day water fast for religious reasons .
Body Changes During Fasting
After stopping eating, your blood glucose levels tend to fall. The hormone changes involve a drop of the insulin levels and a rise of epinephrine (adrenaline), cortisol, glucagon and growth hormone levels, which results in a breakdown of the body glycogen, fat and protein.
The following are expected feelings and body changes during fasting:
Feelings. 4 to 6 hours after your last meal you become hungry and increasingly anxious and your heart starts to beat faster.
Energy sources. The first day of fasting (from several hours after your last meal to 24-36 hours), glycogen in the liver and muscles breaks down into glucose, which covers most of the body energy needs.
Body changes. The first day, you may lose 1-2.5 kilograms of body weight, mainly because of the release and excretion of water that has been bound to glycogen (Chart 1).
Day 2 to 10
Feelings. After 3 to 4 days, you may lose appetite and become euphoric (“fasting high”) but more likely you just continue to be hungry [2,9]. You start to experience fatigue and exercise intolerance .
Energy sources. Your body fat starts to break down into fatty acids and glycerol. Fatty acids are either break down in the process called beta-oxidation or converted into ketones, which can nourish the brain and muscles. Protein breaks down into amino acids. The amino acids and glycerol convert, in the process called gluconeogenesis, into glucose, which is necessary for the brain and red blood cells.
Body changes. Between the days 3 and 6 you can expect to have your last bowel movement. Your breath and urine have a sweet odor (from ketones). Your resting heart rate can drop to as low as 35/min . Your blood pressure can also fall, especially upon standing; you can prevent this by taking sodium supplements or drinking mineral water high in sodium . Your blood glucose levels can drop, which can cause lightheadedness . You can expect to lose 700-1,000 g (1.7-2 pounds) of body weight per day, from this 150-300 g fat and 150-350 g muscle mass [2,14,24, chart 1].
Day 11 to 35
Feelings. Many people who fasted voluntarily for spiritual reasons experienced the sense of well-being, peace and clarity after 2 or 3 weeks of fasting . Fatigue and exercise intolerance continue for the entire period of fasting. During involuntary fasting (starving), apathy and irritability may be the main feelings .
Energy sources. The main source of energy is fat (fatty acids and ketones) and, to a much lower extent, muscle protein .
Body changes. You can expect to lose about 300-450 grams of body weight per day, from this 150-300 g fat and 100-150 g muscle mass [2, chart 1].
Day 36 to 45
Feelings. After the day 35 (or sooner or later), you may start to experience severe symptoms, such as strong hunger pangs, nausea, profound weakness, muscle cramps and lightheadedness . These signs suggest that you should consider stopping fasting.
Energy sources. When your body fat stores are nearly depleted, the main source of energy becomes muscle protein.
Body changes. You will continuously lose body weight .
During this stage, you should consider taking multivitamin supplements, especially vitamin B1, and minerals potassium and magnesium.
Beyond the Day 45
For lean persons, it is nor recommended to fast beyond the day 45 because of rapid loss of muscle mass and increased risk of severe health problems (see complications).
For overweight persons with abundant fat stores, it is possible to fast beyond the day 45 without severe side effects [2,14]. At this stage, everyone should be monitored by a doctor and taking supplements as prescribed. The expected weight loss is 300 g per day, from this 150-250 g fat and 25 g muscle mass [8, chart 1].
Chart 1. Composition of Weight Loss During Fasting
|Total weight loss||1,000-2,500 g||700-1,000 g/day||300-450 g/day||200-330 g/day|
|Glycogen loss||500 g||Negligible||No||No|
|Fat loss||Negligible||150-300 g/day||150-300 g/day||150-250 g/day|
|Protein loss||Negligible||30-70 g/day||20-30 g/day||5 g/day|
|Muscle mass loss||1,600 g||150-350 g/day||100-150 g/day||25 g/day|
|Water loss||1,000-2,000 g||400-600 g/day||100-200 g/day||50-100 g/day|
Chart 1. Modified from the studies (2,9,14,15,21,23,24,25,26,27), mostly in obese individuals who were sedentary during fasting. Chart explanation:
- Each gram of glycogen loss is accompanied by at least 3 grams of water loss .
- Each gram of muscle protein loss is accompanied by 4 grams of muscle water loss, so 1 g of protein loss is accompanied by 5 grams of muscle mass loss [21,p.401]. In the obese individuals, muscle mass loss is slower than in the lean ones and can decrease to as low as 25 grams per day after 6 weeks of fasting [7,p.1234]; in the lean individuals with exhausted fat stores, it could increase to more than 1 kg per day, though.
- Some of the water loss is due to sodium loss .
- Energy expenditure (calories burnt) during fasting across the studies was 1,700-3,000 Calories/day [24,26].
- Death usually occurs after loss of 40-50% body weight or 30-50% of lean body mass [2,9]. For example, an average person (lean or obese) with 30 kg of muscle mass  would die after the loss of 9-15 kg of muscle mass.
Precautions While Fasting
- Drink as much water as needed to maintain good hydration (colorless or straw yellow urine, no thirst, no dry mouth); if you are sedentary and live in a moderate climate, probably 1.5-2.5 liters and not more than 4 liters per day.
- Avoid heavy physical exercise.
- If you fast for more than 35 days, start to take vitamin B1, potassium and magnesium supplements.
Complications of Fasting
Long-term fasting can cause :
- Unpleasant symptoms: a headache, nausea, abdominal pain, cramps, weakness, hair loss, cessation of menstruation, body swelling (edema)
- Dehydration (in case of insufficient water drinking)
- Anemia (in individuals with low iron stores)
- Vitamin deficiency, mainly of vitamin B1 and C (not likely in the first month)
- Drop in blood glucose levels (hypoglycemia)
- Drop in blood pressure, especially upon standing (postural hypotension)
- Drop in blood potassium levels or hypokalemia (rare, even after 2 months fast)
- Polyneuropathy (nerve damage)
- Urate kidney stones (mainly in obese persons)
Life-threatening complications of fasting :
- Small intestinal obstruction (ileus)
- Infections, such as pneumonia
- Heart failure
- Kidney failure
- Lactic acidosis
- Abnormal heart rhythm (ventricular arrhythmia) and sudden death (especially after water fasting supplemented by proteins) [2,19]
During fasting, the blood levels of the hormone insulin drop and so do the body stores of potassium, magnesium and vitamin B1. If you consume large amount of carbohydrates (sugar, soda, bread, rice, potatoes) shortly after stopping fasting, insulin levels will suddenly rise and cause a movement of potassium, magnesium and phosphate from the blood into the cells and therefore a drop in their blood levels (hypokalemia, hypomagnesemia and hypophosphatemia) . At the same time, large amounts of vitamin B1 will be used for carbohydrate metabolism, eventually resulting in vitamin B1 deficiency. A combination of these events, known as refeeding syndrome, can cause sudden nausea, vomiting, diarrhea, swelling (edema), brain damage, abnormal heart rhythm, heart failure, coma or death [4,5].
Refeeding syndrome may develop after a fast as short as 5-10 days. Some of the risk factors include low BMI (<16), chronic alcoholism, eating disorders, critical illnesses, starvation (especially in children) and prolonged fasting (>14 days) .
Breaking a Fast
If you are experiencing severe symptoms, such as profound weakness, dizziness upon standing, muscle cramps, chest or abdominal pain, you should break a fast, preferably under a doctor’s supervision. When stopping fasting, you should reintroduce food gradually to avoid refeeding syndrome .
2 days before stopping fasting, consider taking high-potency vitamin B1, potassium and magnesium supplements.
During the first 2 days after stopping fasting :
- Eat small non-sugary liquid or soft meals (soup, vegetable juice, gravy, salads), which in total should not exceed 10 Cal/kg body weight/day (700 Cal in a 70 kg person). The recommended sources of calories are 50–60% carbohydrates, 15–25% fats and 20–30% protein.
- Drink only as much water as needed to excrete 3-6 full bladders of colorless or straw yellow urine per day.
- Avoid consuming large amounts of carbohydrates (bread, rice, potatoes, soda, fruit juice, sweets, sugar) for at least 2 days.
Within the next few days, gradually increase your calorie intake to your usual level.
Who should not fast?
You should not fast if you:
- Have no clear goal why to fast
- Are a young child or very old
- Are pregnant or nursing
- Have any disease, especially insulin-dependent diabetes mellitus, heart or kidney disease, anemia, low blood pressure, impaired immunity or those who recently (in last 6 months) had a surgery
- Have an eating disorder
- Want to lose weight, because it is very likely you will regain weight shortly after the fast 
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