Generally, it is safe to fill insulated water bottles with contents other than water. For coffee and tea, an insulated water bottle can allow you to enjoy a hot drink all through the morning.
However, it is especially important that you clean your bottle thoroughly after filling it with anything other than water. Because of the different shapes of insulated bottles, some are easier to clean than others.
For food like hot soup or oatmeal, you’re going to be better off with a food-specific thermos. These are similar to insulated water bottles, but their short and stout shape makes them easier to use and clean. A good thermos can keep hot food at a safe temperature for hours — an excellent counterpart to an insulated bottle!