This study demonstrates that insulin sensitivity is inversely correlated with the size of the large adipose cells (cp; ) or the average cell volume (not shown) in first-degree relatives of type 2 diabetics who did not themselves have frank diabetes as assessed by fasting plasma glucose (< 7 mM). The size of the large adipose cells is largely a surrogate for BMI (), but a small effect of cp on glucose disposal remains after accounting for BMI (). These data confirm the classical view that the size of large adipose cells is an independent predictor of the degree of insulin resistance. However, taken together with other recent studies, the relationship between adipose cell size and insulin resistance is better viewed as context-dependent. For example, McLaughlin et al. (the Stanford study) (10
) did not find a statistically significant correlation between cp and insulin resistance in a group of moderately obese subjects; that study found instead a correlation between the fraction of large adipose cells and insulin sensitivity. We do not find such a correlation in the leaner Gothenburg subjects.
Evidence that large adipose cells in adipose tissue are associated with insulin resistance has accumulated for many years. With isolated adipose cells from obese human subjects, Salans et al. (5
) showed that the larger the adipose cells were, the less responsive they were to insulin in glucose utilization; Franck et al. (18
) demonstrated that large adipose cells have decreased capacity of insulin to stimulate GLUT4 translocation to the plasma membrane. Many other investigators extended this observation to the whole body in vivo
). While the fat distribution in different depots has been suggested to also play an important role in metabolic aberrations, a significant correlation was found between the mean size of the subcutaneous adipose cells and many abnormalities (e.g., hyperinsulinaemia and glucose intolerance) associated with insulin resistance (4
). Due to the confounding effect of obesity, however, it had not been very clear which of the cellularity characteristics in the adipose tissue is the most important factor for predicting insulin resistance in vivo
In this study we used the newer technology of cell counting and sizing by the Coulter Counter Multisizer III (10
). With this method, adipose cell size can be analyzed by fitting the cell size distribution to a formula, and thus the size and the number of cells can be more accurately measured. Notably, the cell size distribution as measured with the Multisizer III is either bimodal or trimodal (, ) instead of unimodal as many previous investigators have reported, using either microscopic techniques to measure adipose cell diameters or the older version of Coulter Counter to estimate the size of fat cells by measuring the cellular lipid weight (4
). By using this newer approach, we find that the classical view that large adipose cell size correlates with insulin resistance holds for our current study group.
However, the present study and the Stanford study (10
) were conducted using the same methodology, but came up with superficially opposite results. Nevertheless, despite the apparent discrepancy, we see the two studies as complementary rather than contradictory, and we believe that both fit within the paradigm of insulin resistance as a disorder of impaired adipogenesis. A key difference between the two populations is shown in : the Gothenburg subjects are leaner and for them cp increases with BMI, whereas for the more obese Stanford subjects, cp does not increase with BMI. This is consistent with the observation of Arner et al. (21
) that average adipose cell volume could be fit by a saturating, hyperbolic function of total fat mass. This is further supported by the fact that the regression slope and intercept for the Gothenburg subjects with BMI > 25 are not significantly different from those of the Stanford subjects (). Our interpretation of the common finding of a saturating relationship between fat mass and fat cell size is that it becomes harder to increase fat cell size once the diameter exceeds a certain level.
The results of this study and those of the Stanford study (10
) put the historical discussion of hypertrophy vs. hyperplasia in a new light. We suggest that the first response to an increase in body fat mass is hypertrophy, the expansion of existing adipose cells. This would appear to be more efficient and may be the preferred route for leaner subjects. For more obese subjects, the possibilities for expansion are limited and hyperplasia becomes necessary. This hypothesis is supported by a recent study from the Jensen laboratory (22
) reporting that short-term weight gain led to increased abdominal subcutaneous adipose cell size without change in cell number. Moreover, among the female subjects, the subjects who had smaller adipose cells at baseline had the greatest degree of hyperplasia and those with larger cells experienced a decrease in the average size of the large cells (diameter > 35μm). This indicates recruitment and/or expansion of previously undetected small cells.
Note that it is necessary both to recruit new cells and to expand them, as larger adipose cells cannot expand further to store more fat. Those subjects who are unable to expand the small adipose cells, plausibly those with the smaller proportion of large adipose cells in the cross-sectional measurement of McLaughlin et al. (10
), could be more susceptible to a spillover of fat to other peripheral tissues where it promotes insulin resistance (13
). In fact, rosiglitazone treatment led to transient recruitment and expansion of small adipose cells and a sustained increase in fat cell size, thus improving insulin sensitivity both in Zucker fa/fa rats (23
) and type 2 diabetic human subjects (A. Sherman, B. Eliasson, S. Mullen, U. Smith, and SW Cushman, manuscript in preparation). On the other hand, however, non-diabetic, insulin-resistant human subjects treated with pioglitazone for 90 days showed an increase in the proportion of small adipose cells and a broadening of the distribution of large cells but no increase in large cell size (24
). Thus, improved insulin sensitivity appears to be associated with increased adipose cell size dynamism, and both recruitment and expansion of adipose cells, but which aspect predominates may depend on the metabolic status of the subjects and, perhaps, species differences. It is not so clear why having larger adipose cells is associated with insulin resistance for the subjects in the present study. We do not have enough subjects in the higher BMI/larger cp range to assess whether the capacity for recruitment and expansion of small cells correlates with insulin resistance.
Other differences are observed between the two groups: the present group is younger as well as leaner, has a higher proportion of males, and is more ethnically homogeneous. Genetic factors may also account for the discrepancy. In the study of McLaughlin et al. (10
) the subjects had no known family history of type 2 diabetes, whereas the subjects in this study were first-degree relatives of type 2 diabetics (25
). Although the genetic factors that control adipose cell size have not yet been identified, they may be involved in the association of the size of the large adipose cells with insulin resistance.
Our data do not settle whether large adipose cell size is a cause of insulin resistance or a consequence of it or both. In this study, cp is highly correlated with BMI, so it is mostly a surrogate for adiposity, but we do see a correlation between cp and glucose disposal even after taking BMI into account. Indeed, from a cell biological standpoint, cp would seem to be a more fundamental quantity than BMI. Large adipose cells are reported to have increased expression of pro-inflammatory adipokines (27
) and increased lipolysis (28
), both of which may cause insulin resistance (30
). Note, however that a recent study of healthy, moderately obese individuals did not find an association between the size of the large adipose cells and expression of inflammatory genes (33
); rather, inflammation was associated with an increased fraction of small cells, parallel to the findings on insulin resistance in a similar population (10
Another important finding of this study is that some subjects had two peaks, which can be roughly fitted with two Gaussian functions (). It is currently not clear why some subjects have two peaks, but others only one. We speculate that the left peak represents a population of intermediate size adipose cells resulting from either a recent wave of recruitment or enlargement of existing small adipose cells. Because of their relatively small size, these adipose cells have the potential to take up and store more fat; as a result the left peak would move to the right. Such shifts were seen in the adipose tissue of mice fed either a regular diet or a high-fat diet (16
). Similar double-peaked adipose cell size distributions and movement of the left peak to the right were also observed in the adipose tissue of Zucker fa/fa rats treated with the anti-diabetic drug rosiglitazone (23
Further studies are needed with lean, healthy individuals without known family histories of type 2 diabetes to assess the generality of the results. Longitudinal studies with human subjects may be useful for determining whether and under what conditions the left-shifted peaks of adipose cells size that we observed are capable of shifting to the right as seen in rodents. The main limitation of this study, however, is that the results are only associative and cannot distinguish whether the larger diameter of the large adipose cells is the cause or the consequence of insulin resistance or vice versa. It will be of great interest and importance to identify the physiological and/or genetic factors that regulate the size of adipose cells.