A popular gym supplement could be doing squat.

While some supplements have scientifically-backed benefits, not all do the heavy lifting they claim to by building muscle, enhancing power output and boosting brain performance.

And the impacts of one especially popular addition to any gym-goer’s routine actually depend on three major factors.

A naturally occurring substance in muscle cells, creatine is found in foods like red meat, fish and poultry, though many take it as a supplement.

However, research published Sunday in the Handbook of Creatine and Creatinine In Vivo Kinetics looked into the hype behind the supplement and found some limitations.

While the findings, led by pharmaceutical researcher and former professor Dr. Mehdi Boroujerdi, highlight some positive effects of creatine, there are other aspects that can influence its efficiency.

First, creatine’s impact can depend on an individual’s baseline creatine levels.

Men and women respond to the chemical differently, as women tend to have lower natural creatine stores and therefore, may respond better to supplementation.

Those with lower baseline levels, such as older adults, may also experience more of the cognitive improvements, including memory, mood and processing speed, as well as age-related decline in muscle mass.

Vegans and vegetarians may also notice more benefits, as they receive very little creatine from their diets and have lower baseline levels.

The second factor that can affect how creatine works is the dose, as the standard dosing recommendation is 3 to 5 grams daily for maintenance.

But while orally ingested creatine is the most widely used form, not all of the supplement is absorbed due to gastrointestinal stability and individual muscle capacity.

Taking creatine with carbohydrates has been shown to improve absorption, though.

Additionally, higher doses don’t always mean bigger gains.

“The belief that larger doses yield greater benefits is unfounded, as muscle creatine stores have a saturation limit,” Boroujerdi said in a press release. “Excess creatine is simply excreted as creatinine, offering no additional advantage.”

And the third factor of how well creatine works for someone is individual physiology.

In addition to how it’s digested in the body and differences in muscle mass between genders, tissue can only hold a certain amount of creatine and everyone’s individual levels are different.

Despite a strong safety profile and some evidence supporting muscle growth, the benefits of creatine are not universal.

“Despite its many benefits, creatine is not a magic bullet,” Boroujerdi said. “It does not directly build muscle or replace the need for proper training and nutrition.”