The duration of survival without food is greatly influenced by factors such as body weight, genetic variation, other health consideration and, most importantly, the presence or absence of dehydration.
For total starvation in healthy individuals receiving adequate hydration, reliable data on survival are hard to obtain. At the age of 74 and already slight of build, Mahatma Gandhi, the famous nonviolent campaigner for India's independence, survived 21 days of total starvation while only allowing himself sips of water. In a 1997 article in the British Medical Journal, Michael Peel, senior medical examiner at the Medical Foundation for the Care of Victims of Torture, cites well-documented studies reporting survivals of other hunger strikers for 28, 26, 38 and 40 days. Most other reports of long-term survival of total starvation, however, have been poorly substantiated.
Unlike total starvation, nera-total starvation with continued hydration has occurred frequently, both in history and in patients under medical supervision. Survival for many months to years is common in concentration camps and during famines, but the unknown caloric intake during these times makes it impossible to predict survival. What is evident is that the body can moderate metabolism to conserve energy and that individual survival varies markedly. The body's ability to alter its metabolism is poorly understood, but it occurs at least in part through changes in thyroid function. This may help explain the evolutionary persistence of genes causing diabetes, which in the past could have allowed individuals to survive periods of starvation by enabling more economical use of energy.