Most people are familiar with the concept that if you eat less, you'll lose weight; however, you don't want to take the idea too far. While it's possible to survive for days without actually eating as long as you continue to drink water, a fast this long can damage your body; plus, it will slow your metabolism. And to participate in your normal daily activities, you need to eat more calories than the bare minimum for survival to avoid going into a semi-starvation state.
Survival Time Without Food
You can survive without food and water from eight to 21 days, and without food for up to two months, according to an article published in 2009 in the German journal Archives of Criminology. This would not be pleasant, however, and you potentially can do a lot of damage to your body and your health. A fast this long means that first your body loses any extra water weight; then it uses up its glycogen -- sugars stored in the muscles. Afterwards, a combination of fat and muscle breaks down to meet your energy needs, causing a higher percentage of muscle loss than would occur if you followed a reduced-calorie diet.
Minimum Calorie Requirements
While survival may be possible on fewer calories, women should eat at least 1,200 calories per day, and men should eat at least 1,800 calories a day, even when trying to lose weight, to avoid a metabolism slowdown.
Calculate your basal metabolic rate, or BMR -- the number of calories you burn just lying in bed all day -- using an online calculator. Men usually need more calories than women; larger people need more calories than those who are smaller; and younger people usually need more calories than older individuals. Use your height, weight and age for the calculation. For example, a 5-foot, 4-inch tall, 30-year-old woman who weighs 140 pounds has a BMR of about 1,415 calories per day, and a 5-foot, 10-inch tall, 40-year-old man who weighs 190 pounds has a BMR of about 1,992 calories per day.
Increases Due to Activity Level
Most people do more than just lie in bed while they're awake, making it necessary to multiply the BMR by an activity factor to determine how many calories a person should eat each day to maintain her weight. If you're sedentary, multiply your BMR by 1.2; if you do some light activity, use 1.375; and if you're moderately active, multiply by 1.55. Use 1.725 as an activity factor if you're active and 1.9 if you're very active. So the 30-year-old, 5-foot, 4-inch tall woman who weighs 140 pounds needs about 1,698 calories per day to stay the same weight if she's sedentary, or approximately 2,193 calories if she's moderately active.
Effects of Too Few Calories
Following a very-low-calorie diet, such as a crash diet, detox or cleanse, can cause serious health issues, because it doesn't provide all of the nutrients essential for good health. It may weaken your immune system and cause an irregular heartbeat or even a heart attack, especially if you have underlying health problems. These fad diets put you at risk for protein-energy malnutrition if you stay on them for long periods, which can cause you to lose muscle, bring on anemia, lower your body temperature, decrease your pulse, give you skin problems, make you irritable and cause diarrhea or constipation.