In Episode 2x05 Big Girls Don't Cry of The Sopranos, Dr. Melfi has the following dialogue with her therapist and former teacher, Dr. Kupferberg:
Kupferberg: You mentioned you've put on a little weight lately. Do you think the overeating has anything to do with your patient?
[Melfi shrugs reluctantly]
Kupferberg: What is it you're not telling me, Jennifer?
Melfi: I don't know. I don't know what you mean.
Kupferberg: [Unconvinced] Okay.
Melfi: I don't. What are you driving at?
Kupferberg: Just, watch your intake of sugar. And sugar substitutes.
Both characters seem to be quite uncomfortable throughout the dialogue.
What is Kupferberg hinting at with the sugar substitutes?
If possible, try to answer without spoiling too much from later episodes.