Pikuach Nefesh - MiMa'amakim 1:2

There was an airfield near the Kovno Ghetto and each day the Nazis asked for 1,000 Jews to come and work at the airfield. As a ration, the Jews were given some form of soup that was most decidely not kosher (and probably not so edible either). A group of the Jews who were on the work detail asked Rav Oshry if they were allowed to eat the soup, since at the present moment their failure to eat the soup would not cause them to starve, and thus perhaps the general rules of piku'ach nefesh would not apply.

Rav Oshry began his reply by basing himself on the case of someone who is ill on Yom Kippur who either feels that he needs to eat or is told by a doctor to eat. In both instances, we allow the individual to eat, even if his failure to eat will cause his condition to worsen down the road and not at the present moment. Based on that, Rav Oshry held that it would be permitted for the people on work detail to eat the soup.

However, Rav Oshry presses the case further, noting that in the Yom Kippur case the individual is already sick, whereas in the question posed to him the people were not sick at the time, and thus perhaps they cannot claim to fall under the rubric of piku'ach nefesh. To answer this concern, Rav Oshry cites the case of a person lost in the wilderness who does not know which day is Shabbat. The law in such a case is that the individual should count off six days and then hold the 7th day as Shabbat. However, there is a discussion as to whether he should actually rest on that day, or whether he should behave as normal and simply designate that day as unique through kiddush and havdala. According to the Bigdei Yesha, he can treat the day normally in terms of food preparation and does not have to fast, since he is trying to hasten his exit from the wilderness. From this, Rav Oshry derives that a person can take certain liberties to stave off a harmful situation, even if he is not currently in that situation. And thus, Rav Oshry permitted the workers to have the soup even though they were currently in a healthy condition.