British Dietetic Association (BDA) says that high levels of additional protein can cause side effects like nausea, kidney and liver damage. It now demands clearer warnings about what is in gym supplements that can come in the form of powders and bars, the BBC reported.
The body needs protein for muscle growth and many gym goers use it to try to get bigger quickly.
Gym supplements come under food law so although they have to be labelled properly what is in them can vary.
They are different to medicines, which legally have to ensure contents are more specific.
There have been warnings before, most recently from the Food Standards Agency, which advises people not to take gym supplements containing DMAA.
The BDA argues there is now evidence to show excess levels of additional protein taken over a long time can cause health problems.
"The more protein in your diet the more you have to get rid of," the BDA.
"People who have these high protein diets are now running into problems with their kidneys because of the amount of protein they must get rid of," it added.
The association believes people can get enough protein naturally from things like chicken and milk.
However, the Health Food Manufacturers' Association, which represents the supplement industry, says compared to other foods or medicines, gym supplements have an enviable record.
Many people commenting on Newsbeat's Facebook page also defended the reputation of gym supplements.