We are beginning siman 5. The Gemara says that if a person puts flax or sesame seeds into water, the natural chemicals in the flax or sesame absorb water, and it becomes a mass by itself without any stirring. The placement of the seeds with water already makes a person chayav for losh, even according to Rabbi Yosi Ben Rabbi Yehuda.
One example of this halacha is making pudding on Shabbos. The natural elements of pudding allow for it to mass on its own, without any need for stirring. (There may be bishul issues with making pudding as well, but we will assume they have been avoided for our purposes.) The function of stirring is only to make sure that all of the powder reaches the water. There is no option for shinui, because there is only room for a shinui when the item is being stirred, and over here, no stirring is needed, so the issur starts before any stirring. Changing the stirring is too late.
Regarding the making of jello, jello is different from pudding in that all of the jello particles dissolve into the water, and it remains a liquid for some time until it later sets on its own. Some poskim argue that the fact that the jello remains in a state of liquid for some time before setting means that it does not fall within the halachos of losh, as we have learned that mixing liquids is not an issue of losh. Others disagree and equate jello and pudding
Metamucil is a type of fiber powder which can be added to water. The powder absorbs the water and becomes a mass. It helps a person have more bulk in their body, which can help a person relieve themselves, and can also help a person feel more full. The nature of Metamucil is that it masses on its own, without the need for stirring. In that sense, it is identical to pudding. However, there is a significant difference between metamucil and pudding in that the metamucil is drunk immediately, before it absorbs water and masses, and then it absorbs the liquid once it is inside the body. Some poskim suggest that since it does not mass until it is in the person’s body, there is no issue of losh. Rav Falk disagrees, and explains that the entire purpose of the Metamucil is to mass together, so it would still be an issue of losh.
There is a Mishnah Berurah which discusses that losh only applies to foods consumed in the normal derech achila; however, Rav Falk writes that it does not apply over here, since Metamucil is not food but medicine. Thus, if a person has a question about taking Metamucil on Shabbos, they should contact their Rav.
Summary
Foods which naturally absorb water without the need for stirring will create a chiyuv of losh immediately from the moment they are placed together with water. Examples include flax and sesame seeds, and may also apply to Metamucil. According to some poskim, Jello is different only because it remains in a liquid state for some time before it solidifies.
