Body fat: What is regulated?

Physiology & Behavior, Vol. 38, pp. 407-414. Copyright © Pergamon Press Ltd., 1986. Printed in the U.S.A.

0031-9384/86 $3.00 + .00

Body Fat: What is Regulated? N. M R O S O V S K Y

Departments of Zoology and Physiology, University of Toronto, Toronto, Ontario, M5S IA1, Canada R e c e i v e d 25 F e b r u a r y 1986 MROSOVSKY, N. Bodyfat: Whatis regulated? PHYSIOL BEHAV 38(3) 407-414, 1986.---This paper assumes that body fat is regulated and then reviews our ignorance about how this is accomplished. It concentrates on the challenge posed by site differences between different depots, and discusses a variety of experimental approaches that may be helpful. Adipocyte Parabiosis

Brown fat Fat cell size Fat transplants Regulation of fat Reproduction

T H E regulation of body fat is a contentious issue. Unfortunately some of the discussion has degenerated into unprofitable sniping about terminology. The term set-point has been attacked when it is perfectly possible to use it in valid and definable ways if one so chooses [45]. Insubstantial objections have been raised, such as the argument that body fat cannot be regulated because it is not constant. Anyone who has worked with incubators or environmental rooms knows that thermal inconstancy is no reason to deny the existence of the thermostat. While these unilluminating debates have been proceeding, genuine and major difficulties about the regulation of fat have arisen, almost unnoticed it seems. The conceptual and empirical challenge is that there may not be only one entity that needs to be regulated. Different fat depots are structurally, biochemically and developmentally different. SITE DIFFERENCES

Just a few out of many possible examples suffice to illustrate this point. In rats, inguinal subcutaneous fat partially regenerates after lipectomy, especially if the animals are also castrated, but epididymal fat does not regenerate after surgical removal [14, 16, 32]. In rabbits, following removal of omental, perirenal and subcutaneous fat (dorsoscapular and inguinal area), only regeneration of the perirenal fat has been detected [55]. In ground squirrels, retroperitoneal fat regenerates after lipectomy but epididymal fat does not [I 1]. The glucose metabolism of epididymal and retroperitoneal fat is more responsive to insulin than that of subcutaneous fat in rats; greater activity of lipogenic enzymes rather than increased sensitivity to insulin appear to be responsible. Cell size is probably not a factor because in this study epididymai and subcutaneous fat cells were the same size [19] although in older rats subcutaneous cells are smaller [15]; see also [8]. There are numerous other reports of cell size differences between depots. For instance, the outer parts of the popliteai fat in the hind limb contains some of the largest cells in the body. Remarkably, this finding holds across different species, even though the specimens studied differ in overall adiposity, nutritional state, and average cell size [53]. Of course fat cell size responds to variables affecting en-

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ergy balance, but the degree of response is site dependent. In obese women, subcutaneous abdominal adipocytes enlarge while adipocytes around the thighs remain more or less normal in size, indicating hyperplasia in this area. Thus upper body segment obesity is different from lower body segment obesity and this is borne out by their different metabolic profiles [28]. In female mice offered high fat diets, parametrial fat shows a greater degree of hyperplasia than other sites [33]. In rats, fat around the kidneys responds most, in terms of cell number increase, to high fat diets [15, 34, 42]. In contrast, when rats are raised in the cold, it is in the epididymal fat that cell number increases most: in the retroperitoneal and inguinal the normal age-related increase in cell number is attenuated [41]. Responses to deprivation are also site dependent. The epididymal fat of ob/ob mice is relatively unaffected during weight loss and comes to represent a greater percentage of the total body fat [26]. In contrast, in female rats without food parametrial fat cells are most reduced. As the amount of reduction is proportional to original cell size [30], the greater reduction in parametrial fat, which has larger cells to start with, does not in itself constitute evidence of regional differences. However, when fat is mobilized as a result of stimuli other than food deprivation, it is not always the largest cells that are most reduced. For instance, estrogen injections lead to greater decreases of subcutaneous than parametrial fat [30]. Nor do treatments promoting lipid deposition act equally at all sites. When insulin is given, subcutaneous fat cells increase most in size; the changes in the parametrial fat are insignificant. But when progesterone is given, there are large changes in parametrial adipocytes [31]. Lipoprotein lipase (LPL) activity is also especially increased in parametrial fat after progesterone injections [57]. One of the earliest reports of a differential response to nutritional state concerns the sucking cushion in the cheek of human and primate infants. In 1909, Shattock [56] described how this pad retained fat despite almost total absence of adipose tissue elsewhere in the body. He suggested that differences in blood supply might be the reason why this pad was spared. Certainly better documentation of this effect, and work on its mechanism, are merited, but perhaps the most important thing here is the insight it provides about the possible adaptive value of having differences between differ-

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ent fat depots. As Shattock wrote, "Its persistence during emaciation serves the purpose of assisting the cheeks to functionate, and in this degree aids in the preservation or prolongation of life.'" Even more remarkable than inter-depot differences in the degree of response of nutritional and physiological variables is the possibility that different depots can alter in opposite directions. In the first few weeks post partum, the suprailiac fat of human mothers decreases while the triceps fat increases slightly [60]. The statistical significance of the effect remains to be analysed and the results depend on measurements of skinfold thickness which have a bad reputation. However, in this case measurements were made by a single experienced practitioner, and the association of decreasing suprailiac fat with constant or increasing triceps fat in the post-partum period has been reported in another study [36]. Post-partum decreases in the thickness of suprailiac fat with no consistent changes in the triceps are reported by Naismith and Ritchie I481. In human pregnancy fat is put on more around or near the trunk than the limbs [36]. Rebuffe-Scrive et al. [54] have evidence that women deposit fat especially around the upper thigh (femoral site) for mobilization in late pregnancy and lactation. In rats, subscapular and retroperitoneal depots increase during pregnancy [58]. Parametrial fat does not increase, an odd finding, since it is this depot that hypertrophies most in response to progesterone injections [31,57]. ADAPTIVE V A L U E OF DIFFERENT KINDS OF FAT

Why should particular depots bear the brunt of supporting reproduction'? Why not have all the depots in the body share the task? To understand this we must discard the idea of fat as a unitary tissue. We know already that white and brown fat, whether qualitatively distinct or occupying different positions along a continuum [1], are morphologically, biochemically and functionally different. Once we know that brown fat has a thermogenic role, some of its other characteristics make sense. For instance, during food restriction while LPL activity in white fat declines, it actually increases in brown fat; lipid in this tissue is presumably needed to support thermogenesis [18]. Following successful adaptation to the severe thermal demands of repeated rewarming from deep hypothermia, fat in the abdominal cavity of rats is almost absent while brown fat retains a remarkable amount of lipid [44]. If brown and white fat serve different purposes, it is not so large a step to suggest that different kinds of white fat have their own specialized functions. It is just that we do not yet know what those functions are. But the case of the sucking pad, with its obvious mechanical role and resistance to food deprivation, should give us encouragement to speculate. Perhaps particular biochemical properties would be suitable for supplying substrates in an optimal form and rate for milk production. It has been suggested that poor lactational performance may be a consequence of failure to build reserves during pregnancy [48], Perhaps fat depots with special proliferative abilities have evolved precisely because they can compensate for inadequate or damaged total complements of lipid storing cells. Elaboration of these ideas, for instance comment on multiple functions and the question of why fat with particular properties should be located in a particular part of the body, will not be undertaken here. The mechanistic problem is how does the body control the levels of fat in the different depots as well as the overall level.

I H E I)II,EMMA POSED B~ T R A N S P L A N q E X P E R I M E N I S

Local differences do not in themselves negate the idea of a single and central regulator. A house with a single thermostat is not uniformly warm or cool: some rooms have double glazing and several radiators, others are poorly insulated and lack sufficient heaters. In the same way, site differences between fat depots lead to the idea of intrinsic differences between adipocytes in different regions. But this idea immediately encounters a barrier: the results from transplant experiments. When fat is exchanged between obese and lean mice, the host environment is more important in determining cell size than the strain of origin 13]. This work "'supports the concept of regulation of fat cell mass occurring not at the level of the individual cell, but at the level of the whole organism [38]." When gonadal fat (large cells) and subcutaneous fat (small cells) fl'om New Zealand Obese (NZO) mice are both transplanted to a third site, the kidney, the differences in cell size largely disappear 137]. How then does one reconcile local differences with the evidence that these differences--at least for cell size--are not intrinsic to the fat cells'.) Several possibilities will be considered.

Neural Di[ferences Perhaps there is a separate regulation of each fat depot. Noting, but setting aside, the unlikely possibility that there are biochemically different feedbacks controlling different stimulatory or inhibitory hormones, separate regulation would probably be accomplished by primarily neural means, with a series of relatively independent feedback systems each with their own brain area in control. The notion should not be dismissed out of hand. Even with the regulation of reproductive h o r m o n e s - - s o much better understood than that of adipose tissue--there have been some recent surprises: unilateral gonadectomy results in compensatory increases in FSH which can be blocked by ipsilateral but not by contralateral hypothalamic lesions or islands [43,49]. So it seems that in this case there is some neural lateralization in regulation. Returning to fat, it had been reported that ventromedial hypothalamic lesions result mainly in abdominal fat deposition while midbrain lesions lead to selective increases on the shoulders and interscapular regions [7]. There may be some degree of separation in the regulation of different depots but this line of explanation becomes absurd if carried too far. Not only does cell size differ between depots, it differs also within depots. The work of Pond [53] on the popliteal fat again provides a nice example. The large cells come from the end nearer the surface while near the femur the cells are much smaller, and this is true across a variety of species. Another example: in rats the cells in the distal part of the epididymal fat are smaller than in the proximal part but enlarge more when high fat diets are offered [6]. However, neural differences between depots do not necessitate independent regulatory systems. There could be some common feedback signal, contributed to by different depots, with local differences in innervation determining the response participation by different depots or parts of depots. An analogy would be a house with a single thermostat but differential heat distribution to different rooms when the heating came on.

Vascular Differences Another possibility is that intra-depot differences result

R E G U L A T I O N O F BODY F A T from differences in vasculature. The same might be true of inter-depot differences. F o r instance, in NZO mice the blood content of gonadal fat is only 45 percent that of the subcutaneous fat [37]. The gonadal cells are larger; perhaps cells with relatively poor blood supply are larger. This fits with the work of Crandall et al. [10] who found that blood flow in the mesenteric fat in rats tends to be high but that cells there are small. However, they concluded that cell volume is not directly related to blood flow because, per unit of surface area, the blood flow in certain depots (retroperitoneai and epididymal) is quite similar in young and old rats but cells sizes are very different. In rabbits higher blood flow, indexed to fat cell number, seems to be associated with larger rather than smaller cells [12]. Evidently there are major differences in blood supply to different depots but it is not yet clear how they relate to cell size or other characteristics of adipocytes. On the basis of present information, it is just as likely that differences in size or metabolic activity of adipocytes determine the degree of vascularization and vice versa. Undiscovered Intrinsic Differences

Transplant experiments so far have only investigated cell size and fatty acid composition [13]. Although these studies have not produced evidence for instrinsic differences, Meade et al. [38] were careful to note that differences at the cellular level might still be found in some kinds of fat cells, or in some species. Indeed human fat cells transplanted into nude mice retain their relatively large size [39]. Perhaps this is because the human cells do not have appropriate receptors for murine lipolytic hormones; if so, this would be an intrinsic species difference between the adipocytes. Differences in the responsiveness to various mediators of lipid mobilization might exist. It remains possible that further work with transplants will reveal characteristics that do not depend on the host environment, and so help explain differences between different depots. Already it is clear that, even if not their permanent characteristics, at least the actual instrinsic state of adipocytes exerts effects on lipogenesis capable of modifying the influence of the host environment. Consider the following experiment by Ashwell and Meade [2]. Goldthioglucose (GTG) obese mice received two transplants, one in each kidney, at the same time: one transplant was gonadal fat from other GTG obese mice, the other was gonadal fat from lean mice. The host mice were then rationed and reduced in weight by 36 percent over three weeks. At the end of that time cells in the lean fat transplant had enlarged while those from the GTG donor had become smaller. So in the same environment cells from one source can enlarge while those from another source can shrink. What seems to be happening is that the host environment determines the final steady state size of the cell. The host provides some signal about this size; in this particular case GTG damage to the CNS results in specification of a large size. But this is not something merely passively imposed on the cell by the host environment. The intrinsic state of the cell with reference to that final steady state is also part of the process of attaining the specified size. Cells that are small (e.g., cells from a lean mouse) in comparison to the cells normal for that host environment are hungry cells, and have their own mechanisms for snatching more than their share of substrate. The host environment sets the goal, but local regulatory processes participate in attaining it. Once one admits of such local processes, it is a small step to suppose that

409 inter-depot differences in the effectiveness or autonomy of these processes might account for some of the regional differences. W H A T IS T H E R E G U L A T E D V A R I A B L E ? W H A T IS T H E PROPER

QUESTION?

Significant contributions to knowledge about body weight regulation may well occur by studying the effects of specific candidate feedback substances, insulin and glycerol for example. But for a full comprehension it also seems advisable to address some more general questions about the kind of system that is operating. This is because in trying to answer the question " w h a t is the regulated variable," one finds that this question is replaced by a tangle of inter-related questions. Are there separate regulatory systems for different depots, or is there some overall regulation with a common feedback signal receiving contributions from all the depots, but with some local factors accounting for site differences? A n d - - w h e t h e r there are separate regulatory systems or one system with local differences--what is the relative importance of the CNS and the local factors? Does the CNS specify fatness, or does some intrinsic or local specification of fatness provide signals to the brain until it is met? Does the tail wag the dog? Which is the tail and which is the dog? We may not be able to say what is the regulated variable, but perhaps we might be able to discover whether it correlates with total fat mass or with cell size. If the CNS receives information about either total mass or cell size, how is it informed? Is this by some factor(s) produced by the fat itself, or by something produced elsewhere that is correlated with adiposity? If cell size is regulated, what determines cell number? If we could pull one or two of these threads clear, perhaps it would help unravel the rest of the tangle. We should not be overly pessimistic about this because there are a variety of available techniques that might resolve some of these general issues. These will now be discussed, together with some suggestions for possible experiments. LIPECTOMY

This seems like the obvious way to discover whether fat cell size or total mass is regulated. Lipectomy should not be followed by compensatory reactions that restore adiposity if cell size (or a correlate) is the regulated variable. Site differences in regeneration after lipectomy have already been mentioned. Even though regeneration does not occur with every site, the fact that it occurs at all argues against the view that cell size is being monitored exclusively (cf. [9]). But is this critical evidence? Perhaps excision of fat in some areas disrupts the blood supply and the surrounding matrix so severely that regeneration is impossible. And perhaps where regeneration is still possible, it represents a local propensity of the tissue rather than a response to regulatory system that has detected a deficit. More telling evidence for regulation of total fat mass would be increases in lipid levels of other depots. The experiments of Reyne et al. [55] demonstrating that in rabbits only one out of the four sites excised regenerated, need to be followed up: the site with the capability of regenerating should be left in place and it should be assessed if more lipid is deposited here after removal of the other depots. In rats a significant increase in mesenteric fat has been reported after removal of the epididymal pads and the right inguinal pad [32]. This may be a case of compensatory hypertrophy because it was seen 95 days post-

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FIG. I. Subcutaneous white rid from the scapular are~t 15 days after transplantation into the hypothalamus of the same rat. Inset sho~, body weight.

surgery, that is after the time of transient lipid deposition associated with testicular damage when the epididymal pads are excised [14]. However, it may be that lipectomy experiments of this kind would best be performed on species where major abdominal surgery is not needed and risk of confounding factors is reduced. Fat-tailed sheep or lizards, for instance, might be useful. In one experiment on fat-tailed Kellakui lambs, preventing 2.37 kg of fat from developing by docking at 2-3 weeks of age was partially compensated for by deposition of l . l l kg of fat elsewhere within about 8 months [51]. This is very much of a half answer, and symbolizes the failure so far of lipectomy experiments to provide unambiguous answers. Perhaps allowing longer for full compensation to occur is necessary. It may, nevertheless, be worth thinking about other ways to investigate whether total mass or cell size is regulated.

size appropriate for the host. The animal should end up much fatter than a control and food intake should, if anything, increase after the transplant. Such an experiment might present technical problems. It might be troublesome to get sufficiently large fat transplants to take. On the other hand, the chances might be improved by transplanting cells almost devoid of lipid and this is a reason for food restricting the donor. Liebelt et al. [35] and Meade et al. [38] have suggested that acceptance of transplants is better the less the actual total fat mass exceeds what is normal for the animal at that age. Experiments just reported [29] show that lipid depleted fat can sometimes take and enlarge when transplanted into an undeprived recipient. No changes in the recipient's food intake were detected. Data on total body fat were not given. USE OF CYCLES IN HIBERNATORS

TRANSPLANTATION OF HUNGRY CELLS Food restrict the donor animal so that its adipocytes are small. Transplant a large number of its small cells into an unrestricted conspecific. The host animal now has its own normal cells plus additional fat in the form of hungry cells. If total fat mass is regulated, then its own cells should decrease in size, and if food intake changes, then it should decrease. If cell size is regulated, then the hungry cells should signal their depleted state, either by withdrawing substrates from the blood stream or by some other signal, until they reached a

Cycles of fat gain and loss in mammalian hibernators represent adjustments in the regulated levels of fat because the changing values are continuously defended. The set-points for fat are sliding [47]. When hibernators gain or lose weight, it is largely through changes in cell size [46,61]. This by itself does not tell one if it is defended level of total fat mass or of cell size that is adjusted when regulated levels change. However, if hyperplasia can be induced in hibernators by feeding high fat diets, and if it is the size of existing fat cells that is adjusted when defended levels change, then the amplitudes

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of the cycle should be larger in animals with more fat cells. This experiment is being tried at present in collaboration with Faust. It is conceptually related to the production of hyperplastic nonhypertrophic obese rats [15] but with the hibernator's changes in defended levels added. An approach to the question of fat cell size versus total mass regulation opposite to augmenting cell number by feeding high fats diets is to reduce cell number by lipectomy. The high fat diet method is preferred because it avoids the complications of surgery and because compensation after lipectomy in hibernators may involve hyperplasia [11]. Discovering whether cell size or total fat mass is regulated would be a useful advance but would not clarify whether site differences depended on differences intrinsic to the cells or on the local environment or different neural inputs. To address these possibilities other experiments are needed.

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MANIPULATING BLOOD SUPPLY AND FURTHER TRANSPLANT EXPERIMENTS So far, investigations of the relationships between cell size and blood supply have been correlational. They need to be followed up by manipulations. Would reducing the blood flow to a fat depot affect cell size? Alternatively, fat could be transplanted to a number of vascularly different sites. The latter approach might help provide a better empirical and theoretical framework for understanding a puzzling point about cell size in transplant experiments. When relatively large (gonadal) and relatively small (subcutaneous) cells were transplanted to the kidney capsules in the same strain of mice, the large cells shrunk and the small cells expanded, and both ended up at an intermediate size [2]. The disappearance of size differences supports the view that local environmental factors are important, but why should the cells from both depots have ended up at an intermediate size? Why not both at a relatively large or small size? More experiments are needed to discover if all sites that do not normally contain fat impose an intermediate size on transplanted cells. Such investigations could be combined with measures o f intra-depot differences other than cell size. For instance, parametrial fat in rats hypertrophies in response to progesterone, perirenal fat proliferates in response to high fat diets. Would these differences be maintained if both kinds of fat were transplanted to an area like the kidney or the brain of the same animal, and then challenged with high fat diets or progesterone? If the parametrial fat in the transplant still responded most to progesterone, that would constitute evidence for an intrinsic difference. If it did not, then it would leave the awkward problem of explaining the greater number of estrogen-induced progestin receptors that are normally found in this depot [20]. PARABIOSIS Overeating by one partner of a pair of parabiotic rats, whether as a result of VMH lesions, LH stimulation or force feeding, depresses intake in the other partner [24, 50, 52]. Some people wonder if the behaviour of the fed partner prevents the other from eating. However, the thin partner displays active aversion to food [52]. Also in experiments by Harris and Martin [23], the partner to a tube fed rat was thinner even though its food intake was only slightly decreased and its protein content normal. Therefore, there may be a lipid-depleting factor independent of food intake inhibition. Insulin, glucagon and glucose do not rise in the thin partners [23,52] and cross-circulation is too sluggish to im-

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FIG. 2. Body weights (mean and standard error) and food intakes of rats with VMH lesions (solid circles) and controls (open circles). Adapted from Hallonquist and Brandes [22].

plicate any of the short-lived peptides. We still await the results of a "systematic attempt to track down the inhibiting agents in the blood of obese hyperphagic rats [52]." INTRACEREBRAL TRANSPLANTS Parabiosis experiments suggest that there is some blood borne substance from a fat animal that is capable of decreasing the fatness of its partner. If this signal comes from the adipocytes themselves, rather than from something often associated with fatness (e.g., hyperinsulinemia), then magnifying the feedback by implanting fat into the brain should result in weight loss. I have looked at the effects of a few unilateral intrahypothalamic transplants of interscapular subcutaneous white fat in rats. The aim was to place the implants near the paraventricular or ventromedial nuclei so that any mechanical damage would result in weight gain, if anything. No effects on weight other than in the immediate postoperative period were detected (Fig. 1). My students were quick to call this Project Fathead. The real problem perhaps, is not that it is ill-conceived but that it has not been pursued beyond a few preliminary attempts. A positive effect, such as sustained weight loss, and defense of a lower weight, would be instructive, but the present lack of effects do not rule out a feedback direct from the fat. Not enough tissue may have been transplanted or its microscopically healthy appearance may conceal biochemical malaise. Also the transplant may have been in the wrong area; circumventricular sites where the blood brain barrier is not so developed might be more appropriate. Extensive work would be needed to study these possibilities. It could, however, be enormously useful to know if the blood borne signal is something manufactured in the fat or elsewhere. BRAIN LESIONS The claim of Box eta/. [7] that VMH and midbrain lesions

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resulted in fat deposition in different areas was based on impression rather than measurement. These observations are of sufficient potential importance to merit confirmation. In general, experiments with lesions affecting body weight would be enriched by quantitative assessments of the changes in different depots. These have been made in a few studies on VMH and LH lesions. No strong evidence was produced that such lesions affected just a limited number of depots but there were some interesting findings. LH lesions in obese Zucker rats reduce epididymal, retroperitoneal and subcutaneous fat. However, while the subcutaneous and epididymal pads had smaller cells, the retroperitoneal cells in the lesioned rats were actually larger than those of obese controls [40]. To understand this we must make use of the idea that once adipocytes reach a certain size, proliferation of further cells is likely to occur [15]. When this proliferation sets in, cell size may actually decrease somewhat. In obese Zucker rats the proliferation in the retroperitoneal pad continues well into adulthood. The LH lesions inhibited this hyperplasia. The greater cell size in LH-lesion animals can then be interpreted as a result of preventing the size decrease that accompanied hyperplasia in the controls [40]. (Why the subcutaneous fat of the LHlesion obese group did not have larger cells than the unlesioned remains unexplained; hyperplasia in this fat was also inhibited by the lesion.) So this study, while it does not provide evidence for separate neural controls for different depots, does illustrate how brain lesions can affect both cell size and number. The LH lesion decreases cell size in some depots and stems hyperplasia in others. Dual effects on hypertrophy and hyperplasia are also evident in experiments with VMH lesions. To understand these it is necessary, following Hallonquist and Brandes [21,22], to realize that VMH lesions have two different effects on weight. First, there is an immediate elevation of the set point (see also [5]), accounting for the dynamic phase; then comes a further slower but sustained elevation of the defended level. Hallonquist and Brandes [21,22] showed that there is no plateau phase: weight gain continues slowly but at a greater rate than in controls up to at least 10 months, post lesion. They suggested that filling of existing cells occurred in the dynamic phase and hyperplasia during the sustained second phase. These speculations are reasonable because previous work has shown that there is no hyperplasia for at least 12 weeks after VMH lesions [25] while more recent work has shown that hyperplasia is observable by around 18 weeks [17]. The latter experiments were not continued long enough to separate the slow sustained weight-gain phase convincingly from the dynamic phase, while in the work of Hallonquist and Brandes [21,22] this was done, but fat cells were not studied. Putting these two sets of experiments together, there is a rough coincidence between the onset of hyperplasia and the start of the sustained phase. So VMH lesions stimulate hyperplasia. The effects are

most prominent in the retroperitoneal pad; they occur also in the inguinal area but are absent in the epididymal fat l lTr. Since the retroperitoneal depot even in intact rats continues proliferating into adult life more than other depots [4,15], this supports the suggestion that VMH lesions augment or accelerate a normal process [22]. Yet there is something most curious about this effect: its long latency. In the initial dynamic phase, cells enlarge in a variety of depots: epididymal, parametrial, dorsoscapular and inguinal subcutaneous, and retroperitoneal [17, 25, 59]. So great is this hypertrophy that cell sizes can reach very large values, e.g., 2 /zg lipid/cell [25]; when high fat diets are offered, hyperplasia sets in before such values are attained [15]. It almost seems as if initially the VMH lesion inhibits hyperplasia and then enhances it when if finally occurs I I 7]. Another remarkable point about the sustained weightgain phase is that its progress is not retarded by temporary food restriction ([21,22]; Fig. 2). The VMH lesion seems to alter the developmental programme for cell number directly. not just as a result of overeating, or even continued fattening which is, of course, prevented by the food restriction. This phenomenon is reminiscent of the sliding set-point changes in hibernators where the defended weight continues to change despite food restriction [5,47]. Yet there are differences because during the hibernator cycle cell size changes [46,61] while the post-dynamic weight gain of VMH rats depends on hyperplasia. Possibly, however, all the hyperplasia after a VMH lesion occurs in a burst and the subsequent sustained weight-gain depends on enlarging these cells. The results from lesion experiments and comparisons with hibernators highlight some further major questions. Are hypertrophy and hyperplasia two separate options in the service of total fat mass regulation'? Or are there separate regulatory systems tbr cell size and cell number, with total fat mass as the resultant': CONCLUSIONS We used to know little about the factors influencing body fat levels but had a simple theory, essentially the lipostatic theory of Kennedy [27]. Today we have a great many more facts but lack the theoretical support to make them comprehensible. We now need more experiments that have interpretable outcomes with respect to some of the broader issues about regulation of adipose tissue. This paper has reviewed some of the approaches that might help in distinguishing between different systems that could be involved, namely individualized depot regulation, overall fat mass regulation, cell size regulation, factors intrinsic to the adipocytes, neural, vascular or other environmental influences, blood-borne feedbacks and CNS controls. However, if the final state of the fat depends to some significant degree on a combination of all these influences, and if there is a multiplicity of interacting regulatory systems, then the brew may become much more murky before anything clear begins to distill out.

ACKNOWLEDGEMENTS I thank M. A. Ashwell, I. M. Faust, J. D. Hallonquist, P. Herman and R. E. Keesey for comments and the Natural Sciences and Engineering Research Council of Canada for Support.

REGULATION

OF BODY FAT

413 REFERENCES

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