Survival skills
A human being can survive an average of three to five days without the intake of water. The issues presented by the need for water dictate that unnecessary water loss by perspiration be avoided in survival situations. The need for water increases with exercise.
Bigu (grain avoidance)
Ge Hong chronicles the effects of grain avoidance. 
I have personally observed for two or three years men, who were foregoing starches, and in general their bodies were slight and their complexions good. They could withstand wind, cold, heat, or dampness, but there was not a fat one among them. I admit that I have not yet met any who had not eaten starches in several decades, but if some people cut off from starches for only a couple of weeks die while these others look as well as they do after years, why should we doubt that the (deliberate) fasting could be prolonged still further? If those cut off from starches grow progressively weaker to death, one would normally fear that such a diet simply cannot be prolonged, but inquiry of those pursuing this practice reveals that at first all of them notice a lessening of strength, but that later they gradually get stronger month by month and year by year. Thus, there is no impediment to the possibility of prolongation. All those who have found the divine process for attaining Fullness of Life succeeded by taking medicines and swallowing breath; on this they are all in perfect agreement. A moment of crisis, however, generally occurs at an early stage when medicines are being taken and starches abandoned and it is only after forty days of progressive weakening, as one uses only holy water and feeds solely on breath, that one regains strength. (15, tr. Ware 1966:246-7) 
This "holy water" refers to a Daoist "fu" 符 "talisman" dissolved in water. Ge Hong further cites an Eastern Wu historical example to show that drinking holy water cannot prevent death. When Emperor Jing of Wu (r. 258-264) heard about Shi Chun 石春, a Daoist healer "who would not eat in order to hasten the cure when he was treating a sick person," he exclaimed, 
"In a short time this man is going to starve to death." Then he had him locked up and guarded, and all that Shih Ch'un requested was two or three quarts of water for making holy water. It went on like this for more than a year, while his complexion became ever fresher and his strength remained normal. The emperor then asked him how much longer he could continue like this, and Shih Ch'un replied that there was no limit; possibly several dozen years, his only fear being that he might die of old age, but it would not be of hunger. The emperor then discontinued the experiment and sent him away. Note that Shih Ch'un's statement shows that giving up starches cannot protract one's years. Some today possess Shih Ch'un's method. (15, tr. Ware 1966:248-9)