> 1000g (ml)

Water has a density of 1 g/mL, most other fluids we ingest are close, but gallium weighs 5g per milliliter and platinum over 20 g/ml.

In purely mechanical considerations, 1000 ml of platinum would weigh 21.4 kg or almost 50 lbs. I'm not a doctor, but I imagine the stomach is not able to hold that much weight!