I talked to my daughter last night, taking her Registered Dietician exam this month. She works as a part-time nutrition consultant at 2 different fitness centers as well as a clinical nutritionist at 2 area hospitals.
All I can say she is not a 'fan' of any of the low-carb diets. Eventually you can run into ketosis start burning protein instead of fat and have a negative impact on your kidneys.
I asked her if trainers are educated in nutrition and she kind of shook her head 'no'. I guess some are but not to the extent that she is. For a trainer to match her education they would need to attend a 4 year college graduating with a B.S in Nutrition then work (1 year) as an intern in dietitics for a program approved by the American Dietitic Association then take the certification exam (Registered Dietician).
So I would follow the recommendations of someone like my daughter over a fitness trainer or even one of the diet doctors. Most MD courses and doctors have had less nutrition education than my daughter. This assessment is not from my daughter being an RD but from what her duties were during internship. As an example, while a doctor is the one that orders a 'feeding' tube or 'particular' diet, it is the RD who actually develops the contents of that feeding as doing a nutrition assessment. An RD intern does 3-week rotations thru just about every department in a hospital from pediatrics to cardialogy to oncology to behavior(pysch) thru geriatrics.
While this thread was originally on just getting ready for a wardrobe, it seemed a good place to tout the role of an RD in dieting. That anyone wishing to loss weight is better off visiting a licensed nutritionis, preferably an RD rather than reading a book like Atkins.