Your body can survive for days without food. Just because you can survive for long periods of time without eating, however, doesn’t mean you should. Fasting comes with some negative -- even serious -- side effects. Aim to eat a safe number of calories daily, even if you’re overweight or obese, to prevent malnutrition and related health problems.
Going for long periods of time without eating causes you to go into starvation mode, a condition where your body breaks down stored body fat -- and eventually other body tissues -- for energy. The number of days you can survive without food varies from person to person. A 2009 review in “Archiv Fur Kriminologie” reports that humans can generally survive with no food or drink for eight to 21 days and may be able to survive without food, if they have water, for up to two months.
Drawbacks of Fasting
Fasting and subsequent starvation can cause malnutrition, fatigue, nausea and dizziness. Weight-Control Information Network reports that eating fewer than 800 calories a day increases your risk for developing gallstones. While you’ll shed body fat when you're in starvation mode, you’ll also lose valuable lean muscle tissue. Fasting can also cause decreases in your body’s metabolism, which is a side effect of starvation.
Medically Supervised Programs
Many medically supervised, very-low-calorie weight-loss programs contain just 500 to 800 calories daily. These programs often use nutrient-dense medical nutrition shakes or bars as replacements for meals to help obese individuals lose weight rapidly. Weight-Control Information Network reports that very-low-calorie programs often result in losing up to 5 pounds weekly but suggests individuals following such a regimen only do so for periods of up to 12 weeks under medical supervision.
If you’re not supervised by a medical professional, avoid dipping below 1,200 calories daily. Harvard Health Publications suggests women eat at least 1,200 calories daily and men get a minimum of 1,500 calories a day unless supervised by their doctors. Women who eat 1,200 calories and men who consume 1,500 calories a day will likely lose weight. To maintain current body weights, women often require 1,600 to 2,400 calories daily, while many adult men need 2,000 to 3,000 calories a day, according to the publication "Dietary Guidelines for Americans, 2010."
Basal Metabolic Rate
Your basal metabolic rate is the number of calories your body requires daily to maintain its weight when in a completely rested state. According to the American Council on Exercise, an effective way to calculate basal metabolic rate is using the Mifflin-St Jeor Equation -- (9.99 x weight) + (6.25 x height) – (4.92 x age) + 5 for men and (9.99 x weight) + (6.25 x height) – (4.92 x age) – 161 for women. In this equation, weight is in kilograms, height is in centimeters and age is in years.