VAT is often considered to be a labile fat depot based on the preferential loss of VAT with weight loss [5
], along with the in vitro observations that visceral adipocytes are more lipolytically active than subcutaneous adipocytes [49
], and are therefore believed to have a higher fat turnover rate. Others have hypothesized that VAT is a secondary fat storage pool that accumulates during positive energy balance only after subcutaneous stores are full [51
]. Some have suggested that exercise specifically mobilizes VAT as a result of preferential stimulation of VAT lipolysis [52
]. Here, we demonstrated that VAT and FM change in parallel and that the magnitude of VAT change is primarily determined by FM change according to an allometric relationship independent of the weight loss method.
Allometric equations have historically been used to describe relationships between the relative growth of various body parts of an organism [6
]. To see this, equation 1
can be rearranged as follows:
thereby indicating that the relative growth rates of VAT and FM are proportional to each other. The fact that the best fit value of the constant k was greater than 1 (i.e., k = 1.3 ± 0.1) corresponds to the observation of preferential VAT changes versus FM changes. The fact that this same value for k adequately describes both genders as well as a wide variety of weight loss interventions suggests that differences between men and women can be explained by the initial VAT to FM ratio and there is no preferential benefit of one weight loss intervention over another.
The allometric model also suggests a way to determine whether or not an obesity therapy preferentially targets visceral adipose tissue. To show that the treatment in question has a special benefit for VAT reduction, one needs to show that the plot of ΔVAT/ΔFM versus the initial VAT/FM lies above the best fit line described by a control group of subjects undergoing a standard obesity treatment (e.g., caloric restriction). Based on the results of the present study, we expect that the data from such a control group will be well described by a line with a slope of approximately 1.3. While the data from a treatment group targeted against visceral adiposity may not be well-described by the allometric relationship, the data must lie above the control group’s line to claim that the treatment preferentially reduces visceral adiposity.
We also found data matching our inclusion criteria from two weight gain interventions, one examining recovery of anorexic women [46
] and the other was an overfeeding study of healthy young men [47
]. These weight gain data also appeared to follow the allometric relationship as depicted by asterisks (*) in Figures and , but they were not used in the model fitting procedure. Future work should investigate the applicability of the allometric relationship in weight gain studies.
In their 1999 review of VAT changes with weight loss, Smith et al. introduced a selectivity index to facilitate comparisons between weight loss interventions and quantify their ability to selectively target VAT [5
]. The selectivity index was defined as the percent change of VAT mass divided by the percent change of FM which is mathematically identical to the allometric constant k. Our observation that the same value of k adequately represented various types of weight loss interventions conforms to Smith et al.’s conclusion that no clear pattern was detected for the selectivity index across the interventions [5
]. However, Smith et al. claimed that the selectivity index depended on the initial proportion of visceral fat in a subset of studies that reported single slice area ratios of initial VAT to subcutaneous adipose tissue (SAT). In this subset of data, we also found weak positive correlations between the selectivity index with both the initial VAT/FM and VAT/SAT (r2
= 0.26 and 0.29, respectively). However, there is a high probability that these correlations were spurious since shuffled data produced higher r2
values than the un-shuffled data 15% of the time. Furthermore, we found no significant correlations of the selectivity index as a function of initial VAT/FM in the full dataset (r2
= 0.03). Thus, the data are consistent with a constant selectivity index identical to the allometric constant k which is independent of the initial proportion of VAT.
The allometric equation 1
can be integrated to give a power law relationship:
where b is a parameter that sets the baseline amount of VAT for a given initial FM. Unlike many reported allometric relationships, the value of b is not a universal constant in our case. Rather, b depends on gender, ethnicity, as well as other potential factors that contribute to determining the initial VAT. Thus, the typical log-log plots often used to assess allometric relationships in our case produces a scatter of points since the values of the parameter b vary widely across ethnicity and gender groups (not shown). However, since our analysis used the initial VAT and FM as model variables, the dependence of the parameter b on gender and ethnicity was automatically accounted for.
The most obvious limitation of the present analysis was that the calculated uncertainties of the data points were quite large due to the fact that we only had access to the reported average values from the published studies. Future studies should investigate these relationships using data on body composition change in individual subjects. Despite this limitation, a clear relationship was apparent in the data and the allometric model described this relationship remarkably well. Our analysis therefore suggests that changes of VAT mass are determined primarily by FM changes as well as the initial ratio of VAT to FM and is independent of gender or the method of weight loss intervention.