
Creatine Supplementation Influences Substrate Utilization At Rest
Journal of Applied Physiology, 93, 2018-2022, 2002
M. Erik Huso, Jeffrey S. Hampl, Carol S. Johnston

Scientific and anecdotal evidence have demonstrated creatine supplementation is beneficial for short-duration, high intensity exercise such as weightlifting and sprinting in most individuals.
However, while the strength gains that often accompany creatine supplementation may be beneficial for many athletes, subsequent weight gain may not; particularly for those athletes in weight-sensitive sports (e.g., wresters, lightweight rowers, etc.) or sports where physical appearance play a large role in the competition (e.g., gymnasts).
One method researchers use to determine substrate utilization (i.e., what food the body is metabolizing: carbohydrates, proteins, or fats) is the measurement of the respiratory exchange ratio (RER). Values typically range from 0.70 to 1.00.
The higher the value, the more carbohydrates that are being metabolized; on the contrary, a lower value means fat is the primary fuel.
With that said, the purpose of this 12-week, double-blind, placebo controlled weight-training trial was to determine how creatine supplementation affects substrate utilization to see if it results in more carbohydrates or more fat being "burned" for energy.
All subjects participated in a pre-determined strength training program throughout the study. In addition, the same meals were consumed 12-hours prior to RER testing to reduce potential confounders and no other food was consumed until post-measurement.
The researchers found that carbohydrate oxidation increased during creatine supplementation (there was a trend for an increase in RER). In addition, while fat mass did not change with creatine supplementation, it did decrease significantly in those taking the placebo supplementation.
The important take home message is that these results demonstrate individuals who supplement with creatine may decrease their ability to lose fat after exercise training, potentially due to the RER increase (again, which means decreased fat metabolism).
According to the results from this study only, those attempting to reduce body fat may want to steer away from supplementing with creatine as it may work against them in that regard; however, more research is necessary as this was the first study of its kind.
