Hewvl,
That depends upon what you mean by ‘bitmap’. In Photoshop Bitmap has a very specific meaning – it refers to a one-bit, bi-level image with only black and white – no tones at all. In that case, obviously all tonal and colour data would be lost.
If you mean the .bmp format, then when saving as .bmp you have a number of choices as to bit-depth. Provided you choose 24-bit then no data will be lost when saving an RGB file previously in JPEG format