Relationship between fasting plasma lipid levels and adipose tissue morphology

Relationship Between Fasting Plasma Lipid Levels and Adipose Tissue Morphology Michael

P. Stern, Jerrold

Olefsky,

John W. Farquhar,

Although obesity is commonly regarded as an important contributor to the development of hypertrigiyceridemia, epidemiologic studies have typically failed to observe high degrees of correlation between triglyceride levels and various indicators of obesity. Recent evidence has indicated that human obesity is heterogeneous with respect to fat cell enlargement and increased adipose tissue cellularity. It seemed possible, therefore, that differential metabolic effects of fat cell enlargement and adipose tissue cellularity might explain the lack of correlation between triglycerides and obesity encountered in epidemiologic studies. To explore this possibility we studied 32 subjects encompassing a range of obesity similar to that typically encountered in epidemiologic studies. Fat cell size was significantly correlated with both plasma triglyceride (r = 0.51, p < 0.01) and the logarithm of plasma triglyceride (r = 0.67, p c 0.01).

and Gerald

M. Reaven

In contrast, neither triglyceride concentration nor its logarithm were strongly correlated with either total body fat or total body fat cell number. Plasma cholesterol concentration was not correlated with any of the adipose tissue characteristics. Fat cell size end degree of obesity appeared to correlate well up to a relative wefght of 1.5, after which further increases in relative weight were not accompanied by commensurate increases in fat cell size. When the analysis was confined to individuals in whom fat cell size was related to the degree of obesity, plasma triglyceride concentration was equally well correlated with both fat cell size and total body fat. Inclusion of individuals with marked obesity unaccompanied by commensurate increases in fat cell size, however, highlighted the fact that triglyceride levels were more closely related to fat cell size than to the overall degree of obesity.

P

LASMA TRIGLYCERIDE concentration has long been suspected of being a cardiovascular risk factor. Direct proof of this, however, has been lacking until the recently published studies of Carlson and Bottiger.’ It is clearly important, therefore, to attempt to elucidate the various environmental factors that modify plasma triglyceride levels, since control of these factors could significantly reduce overall cardiovascular risk. Obesity is widely regarded as one of the causes of hypertriglyceridemia,2*3 and weight reduction is generally advised as a treatment for this disorder. 2*3Nevertheless, the exact relationship

From the Department of Medicine, Stanford University School of Medicine, Stanford, Calif, and Palo Alto Veterans Administration Hospital, Palo Alto, Calif Received for publication January 18. 1973. Supported in part by grantsfrom NIH. HL 08506, HL 14174, NIH 71-2161. AM 05021. and from the Veterans Administration. Michael P. Stern, M.D.: Assistant Professor of Medicine, Stanford University School of Medicine, Stanford. Cali’ Jerrold Olefsky, M.D.: Research and Education Associate, Palo Alto Veterans Administration Hospital, Palo Alto, Calif John W. Farquhar, M.D.: Associate Professor of Medicine, Stanford University School of Medicine, Stanford, Calif: Gerald M. Reaven, M.D.: Medical Investigator, Palo Alto Veterans Administration Hospital, Palo Alto, Calif. and Professor of Medicine, Stanford University School of Medicine, Stanford, Calif: Reprint requests should be addressed to Michael P. Stern, M.D., Department of Medicine. Stanford University School of Medicine, Room S-005, Stanford, Calif 94305. 0 1973 by Grune & Stratton. Inc. Metabolism, Vol. 22, No. 10 (October). 1973

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between obesity and hypertriglyceridemia remains to be fully clarified. Thus, for example, several epidemiologic surveys, which have focused on the range of relative adiposity found in “normal” individuals, have shown only weak relationship between plasma triglyceride levels and various indicators of obesity. Both Abrams et al.4 and Carlson and LinstedP found that only about 5x-10% of the interindividual variation in plasma triglyceride concentration could be attributed to differences in relative adiposity. We have recently completed an epidemiologic study in Modesto, Calif. in which comparable results were obtained.6 A partial explanation of this seeming paradox may be provided by the suggestion of Albrink and Meigs. These authors proposed that hypertriglyceridemia was more closely related to obesity acquired during adult life than to obesity present since infancy or childhood.’ Recent studies have suggested that adult-onset obesity is due primarily to fat cell enlargement,*-” whereas lifelong obesity is due primarily to increased adipose tissue cellularity.8-‘0*‘2 It is possible, therefore, that obesity associated with fat cell enlargement might be more likely to exhibit hypertriglyceridemia than obesity associated with increased adipose tissue cellularity. If this is indeed the case, the relatively weak relationship between obesity and triglyceride levels found in most epidemiologic studies could be attributed to the fact that such studies typically do not distinguish between these two types of obesity. In the present study, this hypothesis is examined. MATERIALS

AND

METHODS

Experimental Subjects Twenty-three men and nine women were studied. Body weight had been stable for months to years in all but three of the subjects. Two men had recently lost 10 and 24 lb, respectively, and one man had recently gained 25 lb. Five men and two women gave a history of obesity which was present in six men and four developed prior to maturity. Mild glucose intolerance13 women. None of these individuals had fasting hyperglycemia or any disorders known to affect carbohydrate or lipid metabolism. Subjects were selected to encompass a broad range of relative adiposity. Relative weight was calculated as actual weight divided by ideal weight, using U.S. relative weight Department of Agriculture Weight Tables.14 In the present group of subjects, ranged from 0.81-2.22. The frequency distribution of the relative weights is shown in Fig. 1. Also shown in Fig. 1 is a comparable frequency distribution of a randomly selected group of “normal” individuals which we’recently studied in an epidemiologic survey in Modesto, Calif. Both distributions are skewed to the right. The means and standard deviations appear to be comparable, although the patient group appears to be shifted somewhat towards more obese individuals compared with the “normal” subjects.

Experimental Protocol Fifteen of the men and all nine of the women were hospitalized on a metabolic ward, and consumed a formula diet designed to simulate the average American diet. The formula contained: 15% protein, 43% carbohydrate, and 42% fat with a polyunsaturated to saturated fat ratio of 0.21. Calories were adjusted to maintain constant body weight, and the total daily ration was consumed in four equal feedings given at 8 a.m., 11 a.m., 2 p.m., and 5 p.m. Metabolic studies were carried out after 5-7 days of diet stabilization. Plasma for cholesterol and triglyceride determination was collected after an overnight fast on three occasions during the study period. The mean of the three determinations was used in subsequent data analysis. The remaining eight men were studied as outpatients on an ad-lib. diet. Subcutaneous adipose tissue specimens were ob-

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Relative

weight

weight

Frequency distribution of relative weights in patients and in “normal” subjects.

tained on each subject from the right lower quadrant of the abdomen nique.

using an

open biopsy tech-

Analytical Techniques Plasma cholesterol and triglyceride were determined in duplicate using a Technicon AutoAnalyzer-I5 Fat cell size was determined by the osmium tetroxide fixation technique of Hirsch and Gallian.16 Particle counting was accomplished with a Celloscope (Particle Data Corp., Elmhurst, Ill.) and the frequency distribution of particles (fat cells) was analyzed using a multichannel analyzer (Nuclear Data, Model 555). The average fat cell size of nonobese (relative weight less than 1.2) adults in our laboratory is 0.501 A 0.255 &g lipid/cell) (mean f SD, n = 12). This agrees well with the normal range reported by Hirsch and KnittIe’ and by Bjiirntarp and co-workersg7” Total body fat was calculated from body density which was measured by hydrostatic weighing.” The total body fat cell number was estimated by dividing the average fat cell size (expressed in micrograms lipid/cell) into the total body fat.

Statistical Techniques Correlation coefficients were calculated using standard statistical programs and an IBM 360 computer. The statistical significance of the various correlations was tested using the computer and standard statistical techniques. Epidemiologic studies have t pically found that the frequency distribution of plasma triglyceride is skewed to the right.5918-2J Several authors have recommended that this skewing be eliminated by a logarithmic transformation.5”* Therefore, in the present report, correlation coefficients were calculated using both triglyceride and the logarithm of triglyceride. RESULTS

Fat cell size was similar in subjects with and without glucose intolerance (0.764 ;t 0.179 vs. 0.702 i 0.280 rg lipid/cell) (mean * SD). Plasma triglyceride concentration, on the other hand, was higher in subjects with glucose intolerance (370 f 298 vs. 205 + 149 mg/lOO ml) (mean % SD), although this difference was of only borderline statistical significance (p = 0.05). Plasma cho-

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Table 1. Relationship Between Plasma Lipid Levels and Various Adipose Tissue Characteristics Triglyceride

Logarithm

Concentration

Fat cell size

-0.03

Total body fat

Cholesterol Concentration

0.26

0.67’

0.51 *

Fat ceil number

of Triglyceride

Concentration

0.25

0.01

-0.30

0.39t

-0.14

‘p < 0.01. t D < 0.05.

lesterol concentration was not significantly different in the two groups (282 i 78 vs. 236 f 64 mg/lOO ml) (mean f SD). The relationships between plasma lipid levels and various quantitative adipose tissue characteristics are summarized in Table 1. Both triglyceride and the logarithm of triglyceride appear to be more closely related to fat cell size than to either fat cell number or total body fat. A graphical representation of the relationship between triglyderide and fat cell size is shown in Fig. 2. Also shown in Table 1 is the lack of any significant correlations between plasma cholesterol and any of the adipose tissue characteristics. Since triglyceride levels appeared to be closely related to fat cell size, but not to total body fat, the relationship between fat cell size and obesity in general was examined. This relationship is depicted in Fig. 3. It is seen that fat cell size increases up to a relative weight of about 1.5. Beyond this point, however, a plateau is reached and further increases in fat cell size are no longer observed. Thus, presumably, beyond this point, further increases in obesity are associated with increasing numbers of fat cells. The relationships between plasma lipid levels and various adipose tissue characteristics were therefore examined in individuals in whom fat cell size and the degree of obesity appeared well correlated-i.e., individuals with relative weight less than 1.5. These results are summarized in Table 2. Once again, a significant relationship was observed between fat cell size and plasma triglyceride concentration (r = 0.55, p < 0.01) 1000

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Relationship

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and between fat cell size and the logarithm of triglyceride concentration (r = 0.71,~ < 0.01). However, in this group of subjects, an equally good relationship was observed between total body fat and triglyceride levels. Thus, by excluding individuals with marked obesity unaccompanied by commensurate increases in fat cell size, it was possible to show a relationship between triglyceride level and overall degree of obesity. It should be emphasized that in neither group was there a significant relationship between triglyceride level and fat cell number. DISCUSSION

The present results are in general agreement with those of Bjiirntiirp and coworkers who reported that subjects with endogenous hypertriglyceridemia had larger fat cells than controls. 2’ In that study, however, the hypertriglyceridemic subjects also had greater amounts of total body fat than controls, so that the study did not specifically discriminate between the effects of fat cell enlargement, per se, and those of increased body fat. The present study, on the other hand, indicates that plasma triglyceride levels are more closely related to fat cell size than to either fat cell number or total body fat. The results of this study also provide a possible explanation of the paradox Table 2.

Relationship

Between

Characteristics

Plasma Lipid Levels and Various Adipose Tissue

in Subjects With Relative Weight

Triglyceride

Logarithm

Concentration

Concentratv3n

Fat cell size

0.55

0.71 *

Fat cell number

0.03

0.04

Total body fat

0.56’

0.73 *

‘p < 0.01.

l

Less Than 1.5

of Triglyceride

Cholesterol Concentration

0.37 -0.08 0.28

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noted earlier. It is proposed that plasma triglyceride concentration is primarily related to fat cell size rather than to obesity, per se. However, fat cell size is related to obesity over part of its range. Specifically, increasing degrees of obesity are accompanied by increases in fat cell size up to a relative weight of about 1.5. Beyond this point, however, commensurate increases in fat cell size are no longer observed. Presumably, in this range of obesity, adipose tissue cellularity becomes an increasingly important factor. However, plasma triglyceride levels show no relationship to fat cell number. Thus, in epidemiologic studies, where triglyceride levels are related to the degree of obesity without reference to the interplay of fat cell enlargement and adipose tissue cellularity, only weak relationships are observed. The present data suggest that only about 26:/, (or, if one accepts the logarithmic transformation, 45%) of the variability in plasma triglyceride can be accounted for by fat cell size. Thus, obviously, other factors must be involved in the genesis of hypertriglyceridemia. The fact that subjects with glucose intolerance had higher triglycerides than subjects without this abnormality, despite similar fat cell sizes, suggests that glucose intolerance makes a contribution to hypertriglyceridemia independent from that of fat cell size. The results also offer a plausible explanation for the salutory effects of weight reduction in hypertriglyceridemic individuals. It is now well documented, both in experimental animals and in man, that weight reduction is associated with decreases in fat cell size without accompanying changes in fat cell number.8*‘2.22 Since hypertriglyceridemic individuals presumably have predominantly fat cell enlargement rather than increased adipose tissue cellularity, fat cell shrinkage would seem to be the specific therapy indicated. Interpretation of the present data is subject to the reservation that fat cells from only a single fat depot have been sampled. It is known that fat cells vary in size from one fat depot to another. 8*9~”However, an analysis of the data of Salans et al.” and of Bjijrntiirp and Sjostriim9 indicates that, within individuals, cell sizes from different sites are highly correlated. Thus, it seems likely that valid conclusions can be based on cells taken from a single site. Nevertheless, final acceptance of our interpretations should await confirmation of the results using cells obtained from different fat depots. ACKNOWLEDGMENT The authors

gratefully

acknowledge

the superior

technical

assistance

of Fred Conrad.

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