After running or cycling when it's not all that warm outside the area around my kidneys (just above waistline, at the sides, towards the back) is particularly cold and stays cold for longer than the rest of my body. Is this normal? Is it part of the kidney function or just my 'lovehandles' radiating heat?
I very much doubt you are experiencing coldness coming from as far inside your body as your kidneys are. It is most likely just your skin. If the air is cold I guess the sides of your body would experience an increased flow of cold air over them making them cooler than the front of back of your body.
I'm only making an informed guess but I would think if there was something wrong with your kidneys you would know about it.