Use a small, sharp knife to slice the skin and white pith off the grapefruit. Slice the fruit segments out of the white membrane and add the fruit to the blender. Squeeze the membrane over the top to wring out any remaining juice. If there is any flesh clinging onto the skin, press that over the blender as well to squeeze in any juice.
Scrub and trim the carrots, then roughly chop them. Add them to the blender. Peel and break in the banana. Add 1 tsp turmeric and pour in 250ml cold water. Add a few ice cubes and blitz till smooth. Serve straight away.