It is well known that creatine leads to water retention. An interesting question is where this water is contained in the body: inside (intracellular) or outside (extracellular) the (muscle) cells?
Creatine is predominately stored in muscle cells and is an osmotically active substance. Thus it increases the osmolality of the cells. As a result, the cells will attract water by osmosis. Logically, it seems very reasonably to assume that the water is stored together with creatine inside the (muscle) cells.
Some research has tested this logic. After all, you need to test your hypotheses. There are several techniques which can be used to put this logic to the test. In the literature you'll find the following three techniques being used:
- Multifrequency bioimpedance analysis (MBIA)
- Magnetic resonance (MR)
- Isotope dilution analysis (IDA)
So let's dive into the literature and see what these techniques have to say about our logical reasoning.
Multifrequency bioimpedance analysis (MBIA): creatine leads to intracellular water retention
The human body consists out of various types of tissues, such as adipose tissue, bone tissue, muscle tissue, etc. These tissues differ considerably in the amount of fluid they contain as well as they electrical conductivity. MBIA is based on this last principle, the differences in conductivity.
Briefly, small electrical currents with varying frequencies are sent through the body. It is assumed you can estimate the extracellular fluid volume with the low frequency and the total fluid volume with the high frequency. By subtracting the extracellular volume from the total volume you obtain the intracellular fluid volume. There are a lot of catches with this method though, but I will not cover these here.
MBIA has been used by Ziegenfuss et al. . Subjects were given 0.07 g creatine per kg fat free mass for three days. For a 70 kg individual with 15% fat this would amount to 21 g daily. So they basically gave them a regular loading dosage. They found a non-statistical significant (P=0.07) increase in total body fluid of 2%. Intracellular fluid volume increased significantly by 3% and extracellular fluid volume remained unchanged. These results suggest that creatine water retention is indeed limited to the intracellular fluid compartment.
Magnetic resonance (MR): creatine retains water inside the cells
MBIA (isotope dilution analysis) estimate the water contents of the entire body. MR techniques enable a researcher to check out one particular area. MR exploits the fact that protons have a magnetic dipole moment. These dipole moments of the protons align with a magnetic field when present. This alignment can either be in the direction of the magnetic field (spin up) or against it (spin down). When you release a radiofrequency pulse on these protons, these dipole moments will change direction. When the radiofrequency pulse is gone again, they fall back in their old direction (that of the magnetic field). This 'falling back' takes some time and this varies per tissue. These times are also called relaxation times. It is assumed that these relaxation times differ between the extracellular and intracellular compartment. As a consequence measurements of these relaxation times can thus provide information about the extracellular and intracellular compartment volumes.
This technique has been used by Saab et al. . They gave subjects 20 g creatine daily for 5 days and took a look at the flexor digitorum profundus muscle (a muscle in the forearm). The authors found an increase in the proton concentration which matches the relaxation time that fits the intracellular compartment, but not the extracellular compartment. As such, the authors conclude that creatine supplementation retains water inside the cells.
Isotope dilution analysis (IDA): creatine leads to water retention inside and outside of cellsIsotopes of a particular element have the same number of protons but differ in their number of neutrons. As a result, they largely behave the same physiologically, but you can distinguish them in your measurements because of the different number of neutrons.
So what does this have to do with body fluid measurements? For example, take an isotope of hydrogen: deuterium. When two molecules of deuterium combine with oxygen, you get heavy water (dideuterium oxide). If you drink a glass of heavy water, you'll be able to distinguish these heavy water molecules from the regular water molecules. You let them drink it and wait 2 to 6 hours for it to spread evenly throughout the body. Then you tap of some blood and you measure the concentration in it. You then determine the concentration of heavy water in it with a scintillation counter, and voila. With a little calculating you'll know the total amount of water in the body!
You can do the same thing with a different isotope which can not enter cells, so you can calculate the extracellular water volume. One such isotope is sodium bromide. And then by subtracting the extracellular water volume from the total water volume, you'll know the intracellular water volume.
This is a very reliable method and has been applied by Powers et al. to investigate creatine water retention . The results were somewhat unexpected to me. They measured an increase in total body fluid 7 and 28 days after creatine intake. No surprise there. However, they measured no alteration in fluid distribution. Which means both the extracellular and intracellular fluid compartments increased in proportion to each other.
This seems at odds with the two previous studies I mentioned above. A notable difference with the MR study is that that one looked locally to just a single muscle. IDA measures fluid distribution of hte whole body, just like MBIA. MBIA on the other hand is definitely less reliable than IDA. But still, I have no sensible explanation for this.
Creatine intake leads to fluid retention. Logic dictates that the fluid will be contained within the (muscle) cells and not outside of the cells (extracellularly). MBIA and MR measurements support this view. However, IDA, which could be viewed as a golden standard measurement for fluid distribution, suggests that fluid retention occurs proportionally in both the extracellular and intracellular compartment. There does not seem to be a good explanation for this. I'm inclined to go with the results of the MBIA and MR measurements. Further research could further clarify this discrepancy.
- Ziegenfuss, Tim N., Lonnie M. Lowery, and Peter WR Lemon. “Acute fluid volume changes in men during three days of creatine supplementation.” J Exerc Physiol 1.3 (1998): 1-9. APA
- Saab, George, et al. “Changes in Human Muscle Transverse Relaxation Following Short‐Term Creatine Supplementation.” Experimental physiology 87.3 (2002): 383-389.
- Powers, Michael E., et al. “Creatine supplementation increases total body water without altering fluid distribution.” Journal of athletic training 38.1 (2003): 44.