When someone says "I'm starving," he or she usually means it as an exaggeration of being hungry. Interestingly, hunger and starvation have two completely different meanings, however. Starvation occurs when a person cannot supply his or her body with adequate nutrition over a longer timeframe, usually weeks or months. Hunger is the body's natural urge to replenish its food stores over the course of a day or two. Starvation causes numerous physical and mental side effects due to a prolonged lack of nutrition — some of them permanent even if the person is able to eat regularly again — and starvation will eventually kill the victim.
One main difference between everyday hunger and starvation is that the latter will eventually cause severe weight loss, first by burning the body's stores of fat, then moving to muscle. In extreme cases of starvation such as eating disorders, the body may directly consume muscle first in a phenomenon known as catabolism. Muscle has more energy than fat, so the body will use it to sustain its vital processes in the absence of food. Along with fat and muscle, the body's stores of electrolytes such as calcium, magnesium, and sodium diminish. Without electrolytes, the nervous system cannot effectively transmit the electrochemical impulses and communicate with other parts of the body.
Other problems can occur due to continued hunger and starvation; for example, a malnourished woman's reproductive system will shut down to conserve energy. This leads to amenorrhea, or the absence of menstrual cycles for at least three months in succession. In turn, amenorrhea causes calcium deficiencies and osteoporosis in later life. Women may also develop lanugo, a fine coat of hair over the entire body.
Hunger and starvation can both impact mental functioning, though starvation predictably has greater effects. The brain uses the most energy of any organ in the body; therefore, any drop in food intake will affect its operation. Sluggishness, lethargy and difficulty maintaining focus are signs of malnourishment. One of the most noticeable effects of starvation is the loss of short-term memory. Chronic irritability and depression can also occur because the brain lacks the neurotransmitters serotonin and dopamine, which are believed to regulate feelings of happiness and well-being.
The body enters starvation mode when less than 1,200 calories are consumed daily for a prolonged period. All non-vital functions shut down and the body attempts to conserve all incoming food. Refeeding has to be done slowly due to shrinking of the stomach; too much food can cause the stomach to rupture and stomach acid will leak into the abdominal cavity.