I do not automatically ask clients to keep a food diary as part of their training with me.
However, if they are asking me to assist them with weight management, I point out to them that keeping a food diary and sticking with it is one of the best predictors of successful weight management.
I do not make it a mandatory task because some clients are very resistant to the idea, and I simply respect their decision to opt out after I have laid out the pros of using one. I do, however, point out that the reluctance of keeping a food diary may warrant further reflection on their part.
If they are keeping one, I am always open to reviewing it with them, pointing out areas where a change can bring a good return on investment.