(The original language of this article is French, and the graphs are shown from actual screenshots in the original language)
Water is the most abundant element in the human body, where it plays a major role in the normal functioning of the organism. In fact, it constitutes the main liquid in which biological molecules are dissolved, ensuring their function, and it also participates in maintaining proper cellular structure. In a normal individual, water represents about 60% of body weight, distributed between extracellular compartments (interstitial fluid, blood, etc.) and inside cells¹. The hydration rate varies between different organs, ranging from 76% for skeletal muscle² to about 15% for adipose tissue³. Additionally, water exchange and regulation mainly occur at the level of the fat-free mass (FFM), so it is important to specifically measure this compartment. Besides the hydration rate, the water distribution within adipose tissue is different from that of the rest of the fat-free mass: 80% of the water is located in the extracellular space³, compared to 38-42% of the water in the FFM. The main consequence of these differences is that an excess of adipose tissue can artificially mask dehydration and/or a water imbalance in the fat-free mass.
The “Hydration outside fat” and “Water balance outside fat” indicators aim to precisely study water movements within the FFM, where the organs ensuring normal body function are located. To do this, the two parameters use a value of water outside fat that is equal to the total water volume minus the water in adipose tissue, which is equal to 15% of the total water volume.
In this case study, we will present the results of 3 subjects with different hydration states where the use of these indicators allows for a detailed analysis of the hydration state.
Case No. 1 – First example of intracellular overhydration
| Sex | Male |
| Age | 73 years |
| Height | 166 cm |
| Weight | 88,80 kg |
| BMI | 31,97 kg/m2 |
Quick Analysis
If we focus on a quick analysis of the results, we can see that this patient has a very good body composition considering his age. Indeed, despite the slight excess of fat mass, this subject has significant muscle mass, as well as normal fat-free hydration and water balance. Nevertheless, it is interesting to monitor the results on the relevant indicators.
Hydration
The “Total Water” indicator shows good overall hydration of the body: the theoretical difference is only 400 mL, and we can see that the hydration rate of the FFM is very close to the reference value. Similarly, the “Hydration outside fat” indicator shows good overall hydration of the FFM, showing a difference of 140 mL. If we focus on total and fat-free water balance, we can observe that the total water balance is maintained despite a slight cellular overhydration; however, the “Water balance outside fat” indicator shows an imbalance in water distribution within the FFM. More precisely, the patient presents an intracellular overhydration of approximately 1L, which translates to an extracellular dehydration of the same volume. The difference between the indicators measuring total water balance and fat-free water balance can be explained by the subject’s slight excess of fat mass. Indeed, as previously explained, 80% of the water in adipose tissue is extracellular, and when studying total water balance, this increases the total extracellular water volume enough to result in a normal balance.
Case No. 2 – Second example of intracellular overhydration
| Sex | Female |
| Age | 42 ans |
| Height | 160 cm |
| Weight | 48 kg |
| BMI | 18,75 kg/m2 |
Quick Analysis
During the quick analysis of this patient, we can observe a low BMI of 18.75 kg/m², which is associated with reduced fat mass (-2.66 kg) and muscle mass (-1.11 kg) compared to their references. Conversely, we can observe a slight overhydration (+0.97 L) with a maintained water balance.
Hydration
As observed during the quick analysis, we can see a slight overall overhydration of 930 mL and fat-free overhydration of 970 mL, giving respectively a fat-free hydration of 75.68% (observed on the “Total Water” indicator) and a fat-free hydration of 74.79%.
If we consider the overall water balance, we can observe that it is maintained, but the results indicate that this overhydration is slight and would be mainly at the intracellular level. However, if we consider the fat-free water balance, we can see that the overhydration is present in both the intracellular (+520 mL) and extracellular (+460 mL) compartments, but the balance between the two compartments is maintained.
Case No. 3 – Example of water imbalances present in overweight and obesity
| Sex | Female |
| Age | 63 years |
| Height | 172 cm |
| Weight | 71 kg |
| BMI | 24 kg/m2 |
Quick Analysis
The quick analysis shows that despite a BMI of 24 kg/m², this patient can be considered overweight as she presents a fat mass surplus of 4.08 kg compared to her reference. The skeletal muscle mass is normal and at the reference level; however, we can observe that this patient has significant dehydration (-2.72 L) despite a maintained water balance.
Hydration
The “Total Water” and “Hydration outside fat” indicators confirm the dehydration of this patient with a difference of -2.21 L and -2.72 L compared to their reference, translating to a fat-free mass hydration of 68.55% and a fat-free hydration of 65.86%. The significant difference between these two values is related to compartment modeling and the calculation method of FFM hydration. Indeed, the two-compartment modeling (FFM/fat mass) only separates the lipids of the adipose tissue from the rest of the human body, including the water in the adipose tissue, so a surplus of water associated with a surplus of fat mass can artificially increase the hydration of the FFM. Thus, as shown in this case, it is useful to use the hydration and water balance indicators outside fat when measuring a person with a higher fat mass than the reference.
If we consider the overall water balance, it seems that it is maintained despite the dehydration. However, in the FFM, this fat-free balance is not maintained with significant cellular dehydration. More specifically, this patient shows a deficit of 1.79 L and 0.93 L of extracellular and intracellular water, respectively, indicating that physiological mechanisms may have been activated to maintain some intracellular hydration.
Conclusion
In conclusion, these 3 cases show that it is necessary to study all available hydration parameters to characterize an individual’s hydration status as precisely as possible. More specifically, it seems useful to study the fat-free parameters to check for dehydration and/or a water imbalance within the FFM that could be masked by excess fat mass.
