Hello and sorry to hear that you have this diagnosis - My husband (52) was diagnosed with MCL on the 11th January this year and has completed the first stage of treatment (not the same as yours) he's had six sessions of R-CHOP and is now in the process of having his cells harvested ready for an autologous transplant next month.
For the eating bit....my husband found it better to eat smaller meals more often, rather than fewer larger meals. But he just kept on eating pretty much the same as before. We did however try to eat more healthily and threw away the deep fat fryer!! We try to buy bio produce, and he has found a particular taste for organic green tea. He drinks a lot of water (particularly after and during chemotherapy). As I say, his treatment was not the same as yours but I would add that he has lost very little weight (and what he has lost is, I think, due to eating more healthily!) and he was not sick once during the whole session of chemotherapy.
After each session of chemotherapy there is a point (normally around ten days later) that the white cell count drops meaning that the immune system might be low for a while. His doctors told him to stay clear of seafood, paté, and soft cheeses and a few other things like always peel fruit and vegetables, in order to avoid any risk of infections - you should discuss this with your oncologist.
Feel free to ask anything I might be able to help with
very best of luck to you
Debbie