Nedarim 80b - Laundry before Life

The gemara quotes from a tosefta that if there would be a spring in one city from which water flows down to another city, all agree that the upper city can stop the flow to the second city if they need it for drinking water in order to live. even at the cost of the lives of the second city. The rationale is "chayecha kodmin" - your life comes before others, and assumes that the spring belongs to the upper city. Based on the girsa of the ya'avetz in rashi (mefaresh), this halacha is only true if the actual spring is inside the upper city, so that the water belongs to them and the only question is whether they have to risk their lives to share it with others. But, if the spring was located above both cities and merely flowed through one city to another, the upper city is not considered the owner of the water and would not have rights to damn the river from flowing to the lower city.

In the Tosefta, there is a dispute whether the laundry of the upper city comes before the life of the lower city. Rabbanan say that the life of the lower city comes before the laundry of the upper city, but R' Yossi argues that the laundry of the upper city comes before the life of the lower city because the upper city is not required to undergo tza'ar haguf even at the cost of the lives of the lower city. Based on R' Yossi's opinion the Beis Shmuel (Even Ha'ezer 80:15) explains that a nursing mother is entitled to eat foods that may cause harm to the baby if by refraining from these foods she will have suffer physical pain, because R' Yossi entitles one to wash their clothing to alleviate tza'ar haguf even at the expense of the lives of the lower city. The Beis Shmuel seems to understand that this would only be when there is a definite tza'ar haguf to the upper city, and a safek sakana to the lower city, but if it would be a definite sakana to the lower city, all would agree that the sakana of the lower city takes precedence.

The B'er Heitev points out two difficulties with the Beis Shmuel's comparison: 1. Why would we follow R' Yossi, rather than following the Rabbonon who disagree with R' Yossi? 2. The laundry of the upper city can also lead to sakana and only under those circumstances would R' Yossi give precedence to the upper city. Regarding the second point, it does not seem that way from the language of the gemara; rather the gemara clearly indicates that the issur of "lo ta'amod al dam rei'echa" does not apply when it will cost you tza'ar haguf. But the first question is a strong question (as the Beis Shmuel himself points out). Another distinction that can be made is that in the case of the gemara they are holding back the water from the lower city which is indirectly causing harm, but when the mother eats foods that are bad for the nursing child, she is directly causing harm. Perhaps withholding water also qualifies as a "direct harm"?