Sorry, ToGo, but I have to agree. These 16 days in Japan are a good opportunity to look after yourself and eat different foods and a varied diet. Your gums hurting is a classic sign of vitamin deficiency.

I would like to recommend that you try eating soba noodles as soon as you can. They are inexpensive and filling. When soba noodles (buckwheat - which has a great deal of vitamin B and other vitamins and minerals) are cooked some of those vitamins leak out into the cooking water. If you ask for Mori soba, or better still, Zaru soba (has a tasty portion of mineral packed shredded nori on top), your noodles will come with a dipping sauce. ASK FOR SOBA YU at the end of your meal, you'll get a little red teapot. Pour this into the remains of your dipping sauce and drink it like a hot drink. It tastes good (I think so, anyway) and is a great way of increasing the vitamin content of your meal. There will be vegetables available, grated mountain yam (which looks like nose snot it's called yamaimo, on the menu it is tororo soba) is perhaps the most challenging (I like it). Sansai mountain vegetables is a nice option, vegetable tempura is easy for visitors to enjoy and worth that bit of extra cost.

Most of the food you will find in Japan is rather salty, please compensate by drinking lots of water and tea. I would also recommend mugi cha as a healthy alternative (no caffeine, little if any calories, has some vitamins and minerals).

Last thing you want is kidney stones too! (Never did drink enough during my teens, twenties and thirties)