We reported the association between PHA and body compositions in a sample of overweight-to-obese patients stratified according to BMI categories in this cross-sectional observational study. The study confirmed that a higher mean level of PHA was observed in the obese graded III group, but in this group, there was a negative correlation between PHA and BMI. Similar results were found in a study based on a large German database with a 230,337 sample population(39). As we mentioned above, PHA was calculated by reactance and resistance values which prompted changes in both hydrations of soft tissue and cell membrane permeability(23–26). Thus, results could be explained as follows: First, an increase in soft tissue hydration led to a decrease in PHA. Compared with non-obese people, obese people were often accompanied by an increase in fluid volumes(42). Pathological fluid overload occurred in patients with severe obesity(43, 44). Second, the abnormal membrane permeability also resulted in a decrease in PHA value. An increased PHA value indicated a greater number of intact cell membranes, and a smaller PhA value indicated cell death or a decrease in cell integrity(45). Inflammation is strongly associated with cellular health(46), obesity contributes to meta-inflammation that the levels of inflammatory mediators will increase, which could induce cellular damage and death(47, 48). Besides, obese people often had higher BCV, according to the current flow through physiological fluids, the higher BCV was, the lower resistance was(25, 39). This also led to a reduction in PHA values. BCV was an independent predictor of PHA in three obese groups.
PHA could reflect the water distribution between the intracellular and extracellular water compartments(42). Previous studies had shown an inverse relationship between BMI and water balance(49), due to greater hydration and fluids in the extracellular compartments, obese people had a higher ECW/ICW ratio than non-obese people(42, 50), and in healthy people, ECW/ICW ratio was an important determinant of the PHA(26). However, in our study, we didn’t find a significant change in the value of ECW/ICW with BMI increased. This ratio was influenced by two factors, ECW and ICW. The two factors increased with the rise of BMI, when the two factors rose by a similar degree, the ratio would stay similar. In previous studies, obese patients didn’t distinguish further according to BMI(51). However, this ratio had negative correlations with PHA in all four groups, this result was consistent with a previous study in other study populations(26). Similar to the above reasons, higher ECW/ICW ratio of the adipose tissue in obese patients reflected that the expansion was relatively greater for the extracellular compartment of which had greater hydration and fluids, and the overload fluids had negative effects on PHA(44, 50). There exists a study that confirmed that for obese people, even after weight reduction or weight maintenance, the value of ECW/ICW didn’t reverse(51). Obesity may be associated with primary (irreversible) changes in hemodynamics and humoral regulation(51–54). Mazariegos et al. found after weight loss elevated exchangeable Na + still reminded, this might be a reason to explain irreversible effects on this phenomenon(53). On the other hand, obesity is a state of chronic inflammation with raised circulating levels of inflammatory markers(55–57). White adipose tissue is a major endocrine organ that releases a range of protein signals and adipokines(55), this may be an influencing factor.
FM and FFM were often used to determine nutrient requirements and energy expenditure(58, 59). In our study, FFM had positive sides, the results were similar in previous studies conducted in other study samples(32, 60). The association between PHA and FFM can be explained by the percentage of water volume in FFM, that was to say the muscle tissue(60). Adipose tissue consisted of ~ 14% of water whereas the FFM consists of ~ 72% of water(61). Consider the nature of resistance and reactance mentioned above, the reactance is due to the electrostatic storage of cellular membranes, tissue interfaces, and nonionic tissues whereas the resistance refer to the pure proportional to the number of electrolytes due to extracellular and intracellular water(23, 24). Resistance was inversely proportional to the fluid volume of the human body(21). Therefore, when the integrity of the cell membrane is better, the value of reactance value will be higher. Because of the higher hydration of FFM, the amount of intracellular and extracellular water will also be higher(61). Then the resistance value of the body will be smaller. According to the calculation formula(23), PHA will be higher. In addition, FM had negative effects with PHA among all groups. Low hydration of FM results in high resistance, with the volume of FM increased, the reactance will decrease(21). Thus, the higher FM was, the lower PHA was.
Although all the body composition variables were included in this study, only 42% of PHA variability could be explained in the overweight group whereas 85.8% in the obese graded I group,82.3% in the obese graded II group, and 87.6% in the obese graded III group. Previously published studies suggested PHA was associated with metabolic parameters like uric acid levels(40), vitamin D(62), transthyretin levels(63), hs-CRP(64, 65). All the metabolic parameters had different correlations with PHA. Besides, PHA might be related to both muscle mass and muscle strength(66). Didn’t conform to the consistent results, we verified that skeletal muscle mass had a positive correlation with PHA. Thus, metabolic parameters and muscle function are also important aspects to explain PHA variability.