In some dog groups on Facebook, I see people advising each other about how to help a dog through separation related problems.
Almost invariably, someone will suggest giving the dog a kong before you leave, in order to distract them or keep them occupied. And/or someone will suggest giving your dogs treats before you leave.
If you have a dog with true separation distress of some kind, then you will have noticed that they will tend not to eat while you’re out.
They don’t feel safe enough to eat.

(Picture of Stevie’s handsome head, with his gentle eyes. Stevie was my SA dog and he recovered fully from it within one year. My SA work is dedicated to him.)
Let me recap briefly on why I don’t use food in this scenario:
Firstly, it will not take the dog long to learn that the treats/kong predict something happening to them which causes them intense fear and distress. They can become wary of taking treats or kongs, as they learn to expect you will leave. I have even heard of a dog becoming fearful of accepting food from her caregiver at all. That must be ghastly.
Secondly, if the dog is distracted by the kong and you leave in that time, they will at some point suddenly realise you’re gone and go into full panic mode.
Furthermore, they will learn you have tricked them. So you have lost a bit of their trust, and they will become more vigilant to your every movement.
To learn a new life skill, your dog must be aware of what you are doing and always be under threshold – that is, not being in a state of fear but instead able to process what you’re doing and feel safe.
The successful way of helping dogs recover reliably from separation related issues is by a carefully guided process of desensitisation.