What is deuterium-depleted water actually used for? I noted, flipping through the paperback Alfa Aesar catalogue today, that they sell deuterium-depleted water. Under the usage category, they list 'NMR'. I'm trying to work out what specific use this water has. Unfortunately, on the basis of very limited clinical evidence of improved cancer outcomes I've been bombarded mostly with companies recommending deuterium depleted water for drinking, which trips my snake oil sensor and is something I doubt Alfa has in mind (at US$1930 per liter, and a US RDI of 3.7 L/day for an adult male, that would seem to be a comically expensive drinking habit.)

I suspect biophysics experiments are a small part of the market for this water, but that there's some other NMR-related use I can't think of, possibly calibration related. What is it?
When you perform 1H-NMR spectroscopy in solution, there are many cases where you want to use deuterated solvent, so that signals coming from solvent hydrogen nuclei don’t interfere with the signal from your target molecule. Hence the frequent use of deuterated water, deuterated acetone, deuterated methanol, and deuterated chloroform. Aprotic solvents are also in common use, including carbon tetrachloride.

The situation is reversed when you want to perform 2D-NMR: you then need deuterium-free solvents, so their signal doesn’t mess with the molecule you're interested in. Hence, you will be interested in buying deuterium-depleted water.

(If you're wondering about real use cases for solution deuterium NMR, you can browse the literature for articles, there are plenty. One recent exemple is this one.)