To go along with the recurring theme, it is beyond my scope of practice as well.
My strongest recommendations regarding supplementation would be to refer your clients to a registered dietician who is qualified to take an educated, scientific approach to analyzing your client’s nutritional habits and potential need for supplementation.
/agree with the out of practice, but Nick Rainey’s answer especially stuck out to me.
Weightloss must be a lifestyle, but there comes a point when a person cannot physically lift more based on their mass/weight. When an athlete or lifter reaches that, I would not hesitate to look into protein powders or other supplements.
I would only do this because I have researched them extensively and used them. I also stress the importance of studying all on the label as well as consulting a doctor/nutritionist/dietitian.
I agree with Sue and Harris. Unless you are also a registered dietician or physician it is beyond the scope of practice of a personal trainer, certified or not, to make such recommendations. If questions regarding a supplement or supplements are asked, refer your client to an RD or physician with expertise in the area.