Lipid mobilization in relation to body composition in man

The hepatic lipidome and HNF4α and SHBG expression in human liver

Low plasma levels of sex hormone-binding globulin (SHBG) are a marker for obesity, insulin resistance, non-alcoholic fatty liver disease (NAFLD) and type 2 diabetes. The transcription factor HNF4α is a major determinant of hepatic SHBG expression and thereby serum SHBG levels, and mediates in part the association of low SHBG with hyperinsulinemia and hepatic steatosis. We analyzed the lipidome in human liver specimens from a cohort of patients who underwent hepatic resection as a treatment for cancer, providing insight into hepatic lipids in those without extreme obesity or the clinical diagnosis of NAFLD or non-alcoholic steatohepatitis. Both steatosis and high HOMA-IR were associated with higher levels of saturated and unsaturated FA, other than arachidonic, with the most dramatic rise in 18:1 oleate, consistent with increased stearoyl-CoA desaturase activity. Individuals with low HOMA-IR had low levels of total hepatic fatty acids, while both low and high fatty acid levels characterized the high HOMA-IR group. Both insulin resistance and high levels of hepatic fat were associated with low expression levels of HNF4α and thereby SHBG, but the expression of these genes was also low in the absence of these determinants, implying additional regulatory mechanisms that remain to be determined. The relationship of all FA studied to HNFα and SHBG mRNAs was inverse, and similar to that for total triglyceride concentrations, irrespective of chain length and saturation vs unsaturation.

Toward a theory of energetically optimal body size in growing animals

Our objective was to formulate a general and useful model of the energy economy of the growing animal. We developed a theory that the respiratory energy per unit of size reaches a minimum at a particular point, when the marginal respiratory heat production rate is equal to the average rate. This occurs at what we defined as the energetically optimal size for the animal. The relationship between heat production rate and size was found to be well described by a cubic function in which heat production rate accelerates as the animal approaches and then exceeds its optimal size. Reanalysis of energetics data from the literature often detected cubic curvature in the relationship between heat production rate and body size of fish, rats, chickens, goats, sheep, swine, cattle, and horses. This finding was consistent with the theory for 13 of 17 data sets. The bias-corrected Akaike information criterion indicated that the cubic equation modeled the influence of the size of a growing animal on its heat production rate better than a power function for 11 of 17 data sets. Changes in the sizes and specific heat production rates of metabolically active internal organs, and body composition and tissue turnover rates were found to explain notable portions of the expected increase in heat production rate as animals approached and then exceeded their energetically optimum size. Accelerating maintenance costs in this region decrease net energy available for productive functions. Energetically and economically optimum size criteria were also compared.

Effects of pulsatile secretion of growth hormone (GH) on fat deposition in human GH transgenic rats

Growth hormone (GH) is an endocrine regulator of glucose and lipid metabolism as well as body growth. GH levels are decreased and a unique pulsatile secretory pattern becomes obvious after puberty particularly in males. Coincidentally with this, males tend to deposit body fat. Experimental and clinical evidence has accumulated that obesity is associated with a decrease in GH levels. A strain of transgenic rats has been generated with severe obesity but normal nose-to-tail length, which has low circulating GH levels without pulsatility (human growth hormone (hGH) transgenic rats). The present review mainly focuses on recent and current work analysing the relationship between the occurrence of obesity and low GH levels and/or the absence of GH pulsatility in this transgenic animal model. This model has elevated blood glucose, non-esterified fatty acid, insulin and leptin levels associated with hyperphagia, suggesting that these rats also carry insulin- and leptin-resistant characteristics. hGH transgenic rats were subjected to a pair-feeding treatment to normalize food intake and chronic GH replacement to normalize GH levels. While the pair-feeding for 8 weeks successfully suppressed body-weight gain, the fat pad : body weight ratio remained very similar to freely-eating control hGH transgenic rats, which indicates the hyperphagia is not the sole contributor to the excess fat accumulation in this model. However, continuous elevation of peripheral hGH levels (approximately 2-fold) for 8 weeks by means of a slow-release vehicle resulted in a significant decrease in the fat mass : body weight ratios by 30 %. This GH treatment altered neither food intake nor body-weight gain. Thus, two characteristic phenotypes observed in the hGH transgenic rats, hyperphagia and obesity, seem to be closely related to GH levels and GH secretory pattern. This relationship might be working in the regulation of changes in seasonal body composition in wild animals.

Lipid-induced beta-cell dysfunction in vivo in models of progressive beta-cell failure

We determined the effect of 48-h elevation of plasma free fatty acids (FFA) on insulin secretion during hyperglycemic clamps in control female Wistar rats (group a) and in the following female rat models of progressive beta-cell dysfunction: lean Zucker diabetic fatty (ZDF) rats, both wild-type (group b) and heterozygous for the fa mutation in the leptin receptor gene (group c); obese (fa/fa) Zucker rats (nonprediabetic; group d); obese prediabetic (fa/fa) ZDF rats (group e); and obese (fa/fa) diabetic ZDF rats (group f). FFA induced insulin resistance in all groups but increased C-peptide levels (index of absolute insulin secretion) only in obese prediabetic ZDF rats. Insulin secretion corrected for insulin sensitivity using a hyperbolic or power relationship (disposition index or compensation index, respectively, both indexes of beta-cell function) was decreased by FFA. The decrease was greater in normoglycemic heterozygous lean ZDF rats than in Wistar controls. In obese "prediabetic" ZDF rats with mild hyperglycemia, the FFA-induced decrease in beta-cell function was no greater than that in obese Zucker rats. However, in overtly diabetic obese ZDF rats, FFA further impaired beta-cell function. In conclusion, 1) the FFA-induced impairment in beta-cell function is accentuated in the presence of a single copy of a mutated leptin receptor gene, independent of hyperglycemia. 2) In prediabetic ZDF rats with mild hyperglycemia, lipotoxicity is not accentuated, as the beta-cell mounts a partial compensatory response for FFA-induced insulin resistance. 3) This compensation is lost in diabetic rats with more marked hyperglycemia and loss of glucose sensing.

The difference in the storage capacities for carbohydrate and for fat, and its implications in the regulation of body weight

The two-compartment model presented here suggests that weight maintenance can be achieved by a regulation of food intake geared primarily toward the maintenance of stable glycogen levels, rather than toward the preservation of the overall energy balance. This concept is reminiscent of the glucostatic theory of food intake regulation proposed by Mayer. It is viewed here as being linked to changes in the body's carbohydrate stores, which represent an integration of carbohydrate and lipid fluxes, rather than to changes in blood glucose levels, whose substantial variations during the day are dependent on various circumstantial events. The model illustrates that the fat to carbohydrate ratio of the diet may have considerable potential influence on steady state body composition, even though carbohydrates and fats are both able to meet the body's energy substrate requirements. It appears that failure of appropriately reducing the range within which glycogen levels are maintained when the diet's fat content rises will require an expansion of the adipose tissue mass to raise FFA levels and fat oxidation to a rate commensurate with the proportion of fat in the diet. Maintenance of glycogen reserves below their level of saturation is made less likely by the high palatability and ubiquitous availability of foods in affluent societies. Thus, one can understand the high incidence of obesity among populations consuming mixed diets with a relatively high fat content, without having to attribute this to some defect(s) in the mechanism(s) controlling food intake.

Thematic review series: Patient-Oriented Research. Free fatty acid metabolism in human obesity

Adipose tissue lipolysis provides circulating FFAs to meet the body's lipid fuel demands. FFA release is well regulated in normal-weight individuals; however, in upper-body obesity, excess lipolysis is commonly seen. This abnormality is considered a cause for at least some of the metabolic defects (dyslipidemia, insulin resistance) associated with upper-body obesity. "Normal" lipolysis is sex-specific and largely determined by the individual's resting metabolic rate. Women have greater FFA release rates than men without higher FFA concentrations or greater fatty acid oxidation, indicating that they have greater nonoxidative FFA disposal, although the processes and tissues involved in this phenomenon are unknown. Therefore, women have the advantage of having greater FFA availability without exposing their tissues to higher and potentially harmful FFA concentrations. Upper-body fat is more lipolytically active than lower-body fat in both women and men. FFA released by the visceral fat depot contributes only a small percentage of systemic FFA delivery. Upper-body subcutaneous fat is the dominant contributor to circulating FFAs and the source of the excess FFA release in upper-body obesity. We believe that abnormalities in subcutaneous lipolysis could be more important than those in visceral lipolysis as a cause of peripheral insulin resistance. Understanding the regulation of FFA availability will help to discover new approaches to treat FFA-induced abnormalities in obesity.

Changes in beverage intake between 1977 and 2001

OBJECTIVE

To examine American beverage consumption trends and causes.

METHODS

Nationally representative data from the 1977-1978 Nationwide Food Consumption Survey, the 1989-1991 and 1994-1996 (also for children aged 2 to 9 years in 1998) Continuing Surveys of Food Intake by Individuals (CSFII), and 1999-2001 National Health and Nutrition Examination Survey were used in this study. The sample consisted of 73,345 individuals, aged >or=2 years. For each survey year, the percentage of total energy intake from meals and snacks was calculated separately for respondents aged 2 to 18 years, 19 to 39, 40 to 59, and >or=60. The percentage of energy intake by location (at home consumption or preparation, vending, store eaten out, restaurant/fast food, and school), as well as for specific beverages was computed separately for all age groups. The proportion consumed, mean portion size, and number of servings were calculated.

RESULTS

For all age groups, sweetened beverage consumption increased and milk consumption decreased. Overall, energy intake from sweetened beverages increased 135% and was reduced by 38% from milk, with a 278 total calorie increase. These trends were associated with increased proportions of Americans consuming larger portions, more servings per day of sweetened beverage, and reductions in these same measures for milk.

CONCLUSIONS

There is little research that has focused on the beneficial impacts of reduced soft drink and fruit drink intake. This would seem to be one of the simpler ways to reduce obesity in the United States.

Energy expenditure, sex, and endogenous fuel availability in humans

Adipose tissue lipolysis supplies circulating FFAs, which largely meet lipid fuel needs; however, excess FFAs, can contribute to the adverse health consequences of obesity. Because "normal" FFA release has not been well defined, average (mean of 4 days) basal FFA release and its potential regulation factors were measured in 50 lean and obese adults (25 women). Resting energy expenditure (REE), but not body composition, predicted most of the interindividual variation in FFA release. There was a significant, positive linear relationship between palmitate release and REE; however, women released approximately 40% more FFA than men relative to REE. Neither plasma palmitate concentrations nor respiratory quotient by indirect calorimetry differed between men and women. Glucose release rates were not different in men and women whether related to REE or fat free mass. These findings indicate that nonoxidative FFA clearance is greater in women than in men. This could be an advantage at times of increased fuel needs. We conclude that "normal" adipose tissue lipolysis is different in men and women and that the fuel export role of adipose tissue in obesity will need to be reassessed.

Patterns and trends in food portion sizes, 1977-1998

CONTEXT

While general consensus holds that food portion sizes are increasing, no empirical data have documented actual increases.

OBJECTIVE

To determine trends in food portion sizes consumed in the United States, by eating location and food source.

DESIGN, SETTING, AND PARTICIPANTS

Nationally representative data from the Nationwide Food Consumption Survey (1977-1978) and the Continuing Survey of Food Intake by Individuals (1989-1991, 1994-1996, and 1998). The sample consists of 63 380 individuals aged 2 years and older.

MAIN OUTCOME MEASURE

For each survey year, average portion size consumed from specific food items (salty snacks, desserts, soft drinks, fruit drinks, french fries, hamburgers, cheeseburgers, pizza, and Mexican food) by eating location (home, restaurant, or fast food).

RESULTS

Portion sizes vary by food source, with the largest portions consumed at fast food establishments and the smallest at other restaurants. Between 1977 and 1996, food portion sizes increased both inside and outside the home for all categories except pizza. The energy intake and portion size of salty snacks increased by 93 kcal (from 1.0 to 1.6 oz [28.4 to 45.4 g]), soft drinks by 49 kcal (13.1 to 19.9 fl oz [387.4 to 588.4 mL]), hamburgers by 97 kcal (5.7 to 7.0 oz [161.6 to 198.4 g]), french fries by 68 kcal (3.1 to 3.6 oz [87.9 to 102.1 g]), and Mexican food by 133 kcal (6.3 to 8.0 oz [178.6 to 226.8 g]).

CONCLUSION

Portion sizes and energy intake for specific food types have increased markedly with greatest increases for food consumed at fast food establishments and in the home.

Aromatase, adiposity, aging and disease. The hypogonadal-metabolic-atherogenic-disease and aging connection

In males, aging, health and disease are processes that occur over physiologic time and involve a cascade of hormonal, biochemical and physiological changes that accompany the down-regulation of the hypothalamic-anterior pituitary-testicular axis. As aging progresses there are relative increases of body fat and decreases in muscle mass. The increased adipose tissue mass is associated with the production of a number of newly generated factors. These include aromatase, leptin, PAI-1, insulin resistance, and the dyslipidemias, all of which can lead to tissue damage. Fatty tissue becomes the focal point for study as it represents the intersection between energy storage and mobilization. The increase in adipose tissue is associated with an increase in the enzyme aromatase that converts testosterone to estradiol and leads to diminished testosterone levels that favor the preferential deposition of visceral fat. As the total body fat mass increases, hormone resistance develops for leptin and insulin. Increasing leptin fails to prevent weight gain and the hypogonadal-obesity cycle ensues causing further visceral obesity and insulin resistance. The progressive insulin resistance leads to a high triglyceride-low HDL pattern of dyslipidemia and increased cardiovascular risk. All of these factors eventually contribute to the CHAOS Complex: coronary disease, hypertension, adult-onset diabetes mellitus, obesity and/or stroke as permanent changes unfold. Other consequences of the chronic hypogonadal state include osteopenia, extreme fatigue, depression, insomnia, loss of aggressiveness and erectile dysfunction all of which develop over variable periods of time.

Significant increase in young adults' snacking between 1977-1978 and 1994-1996 represents a cause for concern!

BACKGROUND

Studies on children and adolescents suggest a large increase in the role of snacking; however, little is know about changes in the snacking behavior of young adults.

METHODS

USDA's nationally representative surveys from 1977-1978 to 1994-1996 are used to study snacking trends among 8,493 persons 19-29 years old.

RESULTS

Snacking prevalence increased from 77 to 84% between 1977-1978 and 1994-1996. The nutritional contribution of snacks to total daily energy intake went from 20 to 23%, primarily because energy consumed per snacking occasion increased by 26% and the number of snacks per day increased 14%. The mean daily caloric density (calorie per gram of food) of snacks increased from 1.05 to 1.32 calories. The energy contribution of high-fat desserts to the total calories from snacking decreased (22 to 14%), however, this food group remained the most important source of energy. The energy contribution of high-fat salty snacks doubled. Sweetened and alcoholic beverages remained important energy contributors.

CONCLUSION

This large increase in total energy and energy density of snacks among young adults in the United States may be contributing to our obesity epidemic.

Glucose infusion does not suppress increased lipolysis after abdominal surgery

The purpose of this study was to investigate the effect of glucose infusion on lipid metabolism after abdominal surgery. Patients (n = 6) with non-metastasized colorectal carcinoma were investigated on the second day after surgery and healthy volunteers were studied after an overnight fast. The rates of glycerol appearance (R(a) glycerol), i.e., lipolysis rates, were assessed by primed continuous infusion of [1,1,2,3,3,-5H2]glycerol before and after 3 h of glucose infusion (4 mg x kg(-1) x min(-1)). Plasma concentrations of glycerol, free fatty acids, glucose, lactate, insulin, and glucagon were determined. Fasting R(a) glycerol was higher in patients than in volunteers (7.7 +/- 1.8 versus 1.9 +/- 0.3 micromol x kg(-1) x min(-1), P < 0.05). Glucose infusion suppressed the R(a) glycerol in volunteers to 1.0 +/- 0.2 micromol x kg(-1) x min(-1) (P < 0.05), whereas lipolysis was not affected in patients. Plasma concentrations of glycerol and free fatty acids similarly decreased during glucose administration by 50% in both groups (P < 0.05). In contrast to the patients, a significant correlation (r = 0.78, P < 0.05) between the R(a) glycerol and plasma glycerol concentration was observed in normal subjects. The hyperglycemic response to glucose infusion was significantly more pronounced (P < 0.05) in patients (10.7 +/- 0.7 mmol/L) than in volunteers (7.1 +/- 0.4 mmol/L), whereas the plasma insulin increased to the same extent in the two groups (P < 0.001). In conclusion, lipolysis rates are increased after abdominal surgery and glucose administration, most likely due to insulin resistance, and fail to inhibit stimulated whole-body lipolysis.

Glycerol and nonesterified fatty acid metabolism in human muscle and adipose tissue in vivo

To determine the relationship between glycerol and nonesterified fatty acid (NEFA) release from adipose tissue, and to test whether forearm muscle and abdominal adipose tissue are capable of extracting these two lipolytic products from the circulation, 13 male subjects were studied after an overnight fast during combined infusion of radiolabeled palmitate and glycerol. Blood samples were taken from a radial artery, a deep forearm vein, and a superficial abdominal vein before and during a 2-h infusion of glucose at approximately 7 mg. kg-1. min-1. The ratio of the appearance rates of total NEFA to glycerol was approximately 3/1 during the baseline period but decreased to 1.3/1 during glucose infusion. There was significant extraction of both glycerol and NEFA by forearm muscle. In contrast, there was no apparent uptake of glycerol by adipose tissue. Adipose tissue NEFA uptake was undetectable during the baseline period but became significant during glucose infusion. These data indicate that there is very little to no in situ reesterification of NEFA in adipose tissue after an overnight fast. During glucose infusion, there was apparently a relative increase in the fraction of glycerol derived from the action of lipoprotein lipase and an increase in reesterification in situ.

Body fat distribution, insulin resistance, and metabolic diseases

Obesity has now developed into a world-wide epidemic and is associated with large economic costs and prevalent diseases, particularly with central body fat distribution. Insulin resistance almost invariably occurs, and might be a major trigger for disease-generating mechanisms either directly or via generation of other disease precursors ("risk factors"). The hypothalamo-pituitary-adrenal (HPA) axis seems to be hypersensitive in abdominal obesity, a statement supported by increased responses to challenges from the adrenals to central regulatory centers. Furthermore, the feedback control by central glucocorticoid receptors, probably a secondary, functional consequence of an elevated HPA axis activity, because the receptor gene appears normal. Secretion of sex steroid and growth hormones is diminished, which might be consequence of elevated HPA axis activity. Hyperandrogenicity in women is probably of adrenal origin and another consequence of the sensitivity of the HPA axis. The endocrine abnormalities thus are periodically elevated cortisol and androgen (women) concentrations, as well as low secretions of gender-specific steroid and growth hormones. Since elevated cortisol, and low sex-steroid and growth hormone secretions, probably direct storage fat to visceral depots, the multiple endocrine abnormalities probably cause enlargement of these depots. Furthermore, these hormonal abnormalities most likely at least contribute to the creation of insulin resistance with additional effects of elevated fatty acids from central fat depots, which are sensitive to lipid mobilization agents. This chain of events indicates the central role of the hypersensitive HPA axis. Known causes of sensitization of this axis have been identified in subjects with abdominal obesity, including depression, anxiety, alcohol, and smoking. A common cause of HPA axis activation is perceived stress, with a depressive, defeatist, or "helplessness" reaction. In subjects with abdominal preponderance of body fat stores a number of psychosocial and socioeconomics handicaps have been identified, hypothetically predisposing to such reactions. In a primate model (monkeys), mild psychosocial stress is followed by identical psychological, endocrine, anthropometric, and metabolic abnormalities as in humans with abdominal preponderance of body fat stores, including early signs of diabetes and cardiovascular disease. These findings strongly support the interpretation that a stress reaction activating the HPA axis is involved also in the human syndrome. Interventions with normalization of the endocrine perturbations are followed by clear improvements of the multiple abnormalities in both clinical, experimental, cellular and molecular studies, suggesting that the pathogenesis of abdominal preponderance of body fat and its endocrine, anthropometric and metabolic abnormalities are indeed consequences of the endocrine abnormalities identified.

A review of dietary and environmental correlates of obesity with emphasis on developing countries

The relationships between dietary and environmental factors and obesity are reviewed. Findings from selected population studies of diet and body weight are presented. In general, the results from population studies of diet and obesity have been inconsistent and marked with methodological weaknesses, especially the use of cross-sectional study design. Apart from the diet, several social and economic factors appear to be important correlates of obesity in the population. However, most studies have focused on the socioeconomic status as a broad, composite measure. The relationships between income, education, occupation, place of residence, and obesity are reviewed here, with emphasis on the developing countries. In many developing countries, the changing dietary pattern, along with rising life expectancy and changing socioeconomic environment, has contributed to the increasing problems of obesity and other diet-related chronic diseases that will have an enormous impact on the health and health care resource of these countries in the near future.

Changes in various plasma lipid components, glucose, and insulin in Spermophilus lateralis during hibernation

This study evaluated fat mobilization as related to gluconeogenesis in two age groups of hibernating golden-mantled ground squirrels (Spermophilus lateralis). Our experimental group consisted of a total of 16 male and 15 female squirrels. Plasma samples were collected from selected animals being killed weekly from January to March, and the concentration of triglycerides, glycerol (GY), free fatty acids (FFA), total cholesterol, lipase activity, glucose, and insulin, were determined by biochemical assays and radioimmunoassay. Our results showed a mean FFA/GY ratio of five, which was higher than the predicted value of three, suggesting a significant depletion of GY and an enhanced rate of gluconeogenesis via GY to maintain glucose homeostasis in the hibernating animals. The factor of age did not significantly affect plasma lipid components. However, in the male group, plasma glucose levels were significantly higher for adults than for juveniles. Overall, females had significantly higher plasma glucose levels than males (150 +/- 11 vs. 110 +/- 8 mg%, P < 0.05). In the adult group, a gender influence was also seen on plasma insulin levels, with females' being higher than males' (66 +/- 13 vs. 25 +/- 3 microIU/ml, P < 0.01). We suggest that during hibernation, female squirrels may have a higher rate of lipolysis and gluconeogenesis along with a lower glucose utilization than their male counterparts. Additionally, adult females may exhibit a higher peripheral insulin resistance during hibernation than adult males, a possibility which merits further study.

Glycerol production in subcutaneous adipose tissue in lean and obese humans

To estimate the regional subcutaneous glycerol production rate in normal and obese humans, the venous arterialized plasma glycerol, interstitial glycerol in the subcutaneous adipose tissue together with adipose tissue blood flow (ATBF, ml/100 g.min) were measured in the postabsorptive state and for 2 h after ingestion of 100 g of oral glucose. Eight lean and eight obese men with normal oral glucose tolerance tests were investigated with the subcutaneous microdialysis technique and 133Xe clearance. In the postabsorptive state, the interstitial glycerol concentrations in lean and obese subjects were 170 +/- 21 vs. 282 +/- 28 microM (P less than 0.01) and 156 +/- 23 vs. 225 +/- 12 microM (P less than 0.05) in the abdominal and femoral subcutaneous adipose tissue, respectively. The corresponding arterial glycerol levels were 54 +/- 4 vs. 75 +/- 14 microM (NS). Abdominal ATBF was greater in lean subjects (3.2 +/- 0.6 vs. 1.6 +/- 0.3; P less than 0.05), whereas femoral ATBF was similar in both groups (2.7 +/- 0.4 vs. 2.4 +/- 0.7). Estimated mean local glycerol release (mumol/100 g.min) was similar in the lean and obese group (0.16 +/- 0.03 vs. 0.20 +/- 0.05 and 0.18 +/- 0.02 vs. 0.17 +/- 0.04) in the abdominal and femoral site, respectively. We conclude that glycerol production from the subcutaneous tissue is increased in obesity, irrespective of adipose tissue distribution. This enhancement is due to the increased adipose tissue mass.

Fatty acids in the portal vein of the rat regulate hepatic insulin clearance

The effects of FFA on hepatic insulin clearance were studied in the in situ perfused rat liver. Clearance decreased with increasing body weight (age) of the rats. When FFA were added to the perfusate a 40% reduction of hepatic removal of insulin was found over the normal, physiological range (less than 1,000 mumol/liter), less pronounced in heavier rats. When perfusion was started with high concentrations of FFA, inhibition was rapidly reversible, a phenomenon again blunted in heavier rats. In contrast to FFA, different glucose concentrations in the perfusate did not affect the hepatic insulin uptake in the presence of FFA within physiological concentrations. Thus, hepatic clearance of insulin is proportional to rat weight (age) and portal FFA concentrations. Other studies have recently shown that fatty acids inhibit insulin binding, degradation, and function in isolated rat hepatocytes, and that hepatic clearance is inversely dependent on hepatic triglyceride concentrations, both inhibitions reversible by prevention of fatty acid oxidation. It is suggested that the diminished hepatic clearance of insulin in heavier (older) rats is at least partly due to their relative obesity and increased hepatic triglyceride contents. This effect as well as that of portal FFA is probably mediated via fatty acid oxidation in the liver. This mechanism may have implications for the regulation of hepatic metabolism, and peripheral insulin concentrations.

Preferential loss of body fat during starvation in dietary obese rats

This study was undertaken to examine whether diet-induced obesity alters the amount and/or composition of weight lost during starvation. The amount and composition of weight lost during a 4-day period of starvation was determined before and at 17, 30 and 42 weeks after rats (350 g of body weight) were given a high fat diet (HFD). To control for effects of aging, a second group of rats, fed standard laboratory chow, was also subjected to similar periods of starvation. Although total weight loss during starvation was never greater for HFD rats than for chow-fed rats, the former group showed a clear patter of increasing loss of body fat and total energy and conservation of fat-free tissues with periods of starvation later in life. In addition, chow-fed rats showed substantial energy conservation during each period of starvation (i.e. they lost less energy each day than their pre-starvation energy requirements). In contrast, HFD rats demonstrated substantial energy conservation only at 17 weeks and not at 30 or 42 weeks; during the last period of starvation, their average daily loss of carcass energy exceeded their pre-starvation energy requirements. This suggests the increased fat mass of these rats may have led to increased fuel availability and to an increased metabolic rate during starvation. If these results are applicable to humans, the more obese subjects are likely to show greater total loss of energy than lean subjects, but show a lesser loss of lean body mass, at least initially. If protein requirements are reflected by the ability to mobilize protein during food restriction, protein requirements would be substantially lower in the dietary obese rats than in controls. In summary, diet-induced obesity leads to preferential loss of body fat and conservation of lean mass during starvation.

Interstitial glycerol concentration measured by microdialysis in two subcutaneous regions in humans

Interstitial glycerol concentrations were measured in the abdominal and femoral subcutaneous tissue in 10 lean and 6 obese subjects. In the lean subjects [waist-hip circumference ratio (WHR) 0.89 +/- 0.01] the fasting interstitial glycerol concentrations in the abdominal and femoral regions were 185 +/- 20 and 160 +/- 15 (SE) microM, respectively, whereas glycerol in venous plasma was considerably lower (66 +/- 5 microM). After an oral glucose load (100 g) the interstitial glycerol concentrations declined but remained higher than in plasma. In obese subjects (WHR 1.1 +/- 0.03) the fasting interstitial glycerol levels were higher, in both regions, than in lean individuals. Furthermore, interstitial glycerol in the abdominal site (291 +/- 32 microM) was significantly higher than in the femoral fat (210 +/- 13 microM; P less than 0.05), while plasma glycerol was similar to that in lean subjects (80 +/- 12 microM). Interstitial glycerol remained higher in the obese than in the lean subjects also after an oral glucose load, but the regional differences were less apparent. Thus interstitial glycerol is higher in the subcutaneous tissue than in venous blood. The data also suggest that lipolysis is enhanced in the abdominal subcutaneous tissue as compared with the femoral site in obese subjects and that lipolysis is exaggerated in both regions in obese as compared with lean individuals.

Microdialysis--a novel technique for clinical investigations

Microdialysis is a novel, non-traumatic technique which, when properly performed, allows continuous direct measurements of substances in the interstitial space of a tissue or organ. It also makes it possible to obtain protein-free fluid for analyses, while contaminating bacteria or viruses are excluded. The measurements provide us with an insight into metabolic and pharmacological events at the cellular level in a target organ. Apart from its usefulness for experimental work, it is likely to become an important tool for defined areas in clinical practice.

Effects of cimaterol, a beta-adrenergic agonist, on lipid metabolism in rats

Rats fed a high carbohydrate diet containing 10 or 100 ppm cimaterol for 4 weeks gained 41% to 59% less fat and 70% to 76% more protein than controls, with no major changes in either energy gain or efficiency of energy retention. Effects of cimaterol on lipid metabolism in these rats were assessed. Cimaterol stimulated lipolysis in vivo and in vitro, but failed to influence rates of de novo fatty acid synthesis in either liver or white adipose tissue. Activities of fatty acid synthetase and malic enzyme in these tissues were also unaffected by cimaterol. Cimaterol administered in vivo failed to affect lipoprotein lipase activity in white adipose tissue, but elevated enzyme activity 67% to 75% in the extensor digitorium longus muscle. Lipoprotein lipase activity in the extensor digitorum longus muscle was also elevated by 66% during a 2 hour incubation with 1 mmol/L cimaterol. We conclude that cimaterol selectively stimulates both lipolysis in white adipose tissue and lipoprotein lipase activity in skeletal muscle, to direct energy away from adipose tissue deposition toward skeletal muscle accretion.

Influence of body fat distribution on free fatty acid metabolism in obesity

In order to determine whether differences in body fat distribution result in specific abnormalities of free fatty acid (FFA) metabolism, palmitate turnover, a measure of systemic adipose tissue lipolysis, was measured in 10 women with upper body obesity, 9 women with lower body obesity, and 8 nonobese women under overnight postabsorptive (basal), epinephrine stimulated and insulin suppressed conditions.

RESULTS

Upper body obese women had greater (P less than 0.005) basal palmitate turnover than lower body obese or nonobese women (2.8 +/- 0.2 vs. 2.1 +/- 0.2 vs. 1.8 +/- 0.2 mumol.kg lean body mass (LBM)-1.min-1, respectively), but a reduced (P less than 0.05) net lipolytic response to epinephrine (59 +/- 7 vs. 79 +/- 5 vs. 81 +/- 7 mumol palmitate/kg LBM, respectively). Both types of obesity were associated with impaired suppression of FFA turnover in response to euglycemic hyperinsulinemia compared to nonobese women (P less than 0.005). These specific differences in FFA metabolism may reflect adipocyte heterogeneity, which may in turn affect the metabolic aberrations associated with different types of obesity. These findings emphasize the need to characterize obese subjects before studies.

Free fatty acid turnover and lipolysis in septic mechanically ventilated cancer-bearing humans

Loss of body fat and lean body mass are features of critical illness, and anabolism is difficult to achieve despite parenteral nutrition. Resting energy expenditure (REE), free fatty acid turnover (FFT) and glycerol turnover (glyTO) were measured in septic, mechanically ventilated cancer-bearing patients, both fasting and immediately following or during a glucose infusion providing 87% of REE. No patient was in septic shock nor required pressor support. In the fasting state, REE was greatly elevated compared with basal energy expenditure calculated using the Harris-Benedict equations. Fasting FFT (14.2 +/- 0.9 mumol/kg/min) and glyTO (4.7 +/- 0.5 mumol/kg/min) were elevated compared with normal humans. Fasting respiratory quotient (RQ) was 0.68 +/- 0.02 and did not rise significantly with glucose. Fat appears to be the preferred calorie source in septic, cancer-bearing humans even in the presence of glucose. As similar, but less pronounced, changes have been seen in septic and injured humans without cancer, it is likely that these changes are not cancer-specific.

Glucose uptake in human adipose tissue

One hundred grams of glucose with 50 microCi U-14C-glucose were given orally to 17 women with widely varying amounts of body fat. Radioactivity and glucose metabolism in vitro were then measured in adipose tissue obtained by needle biopsies in the abdominal and femoral regions after four hours. Radioactivity in triglycerides was then measured in repeated biopsies 1 day, 1 week, and monthly up to 7 months after glucose administration. Glucose label in triglycerides after four hours was higher in abdominal than femoral adipocytes in obese women. It increased slightly during the following week, and then decreased exponentially with a half-life of 12 months in the abdominal region and 19 months in the femoral region. Uptake of glucose carbon in total body fat was estimated from the triglyceride label measured and determinations of body fat mass, and found to be in the order of less than 4% of given glucose. The studies in vitro suggested that much of the glucose taken up in adipose tissue is converted to lactate. If this is the case in vivo, then glucose uptake in adipose tissue might well be of significance for total body glucose homeostasis, particularly in obese subjects, amounting to maximally perhaps one third to one half of the oral glucose given. The majority of this glucose uptake would then, however, leave adipose tissue again as lactate. The shorter half-life of label in abdominal adipocytes is in agreement with findings of increased lipolysis in these adipocytes in vitro.

Parameters for evaluation of lipid metabolism

The influence of the clinical condition and the intravenous intake on parameters of fat metabolism has been analyzed. Compared to normal subjects, the correlation between plasma concentrations and the turnover rate of glycerol and free fatty acids varies in the opposite direction in nutritionally depleted and severely injured patients. The significance of plasma concentrations as an index of fat mobilization should be interpreted in relation to the clinical condition. Kinetic measurements are particularly interesting in hypermetabolic patients. Plasma triglyceride and cholesterol concentrations are markedly affected by surgical procedures. Any delay (in treatment) following the injury and the type of intravenous regimen used have an important influence on plasma lipid levels and should be taken into account when groups of patients are studied. The infusion of exogenous fat emulsions significantly affects not only plasma triglyceride levels but also phospholipid and cholesterol concentrations and will modify the plasma lipoprotein pattern. Measurements of plasma clearance and oxidation of fat can be used to evaluate individual tolerance and the metabolic utilization of lipids, but these procedures cannot be easily applied in routine clinical practice. Regular monitoring of plasma triglyceride, cholesterol, and phospholipid concentrations during and after cessation of fat infusion is recommended for each patient who is receiving daily fat infusions so that the safe rate of infusion for that individual can be determined.

The energy cost of triglyceride-fatty acid recycling in nonobese subjects after an overnight fast and four days of starvation

The basal blood glycerol concentration was determined and the rate of glycerol turnover was assessed by a nonradioactive infusion technique in six healthy nonobese adults after an overnight fast and again after four days of total starvation. Simultaneously, estimates of total energy expenditure and net fat oxidation were made from measurements of oxygen consumption, carbon dioxide production, and urinary nitrogen excretion. The data were combined to provide quantitative estimates of the activity of the triglyceride/fatty acid cycle. The basal concentration of glycerol in venous blood rose from a mean value of 54 +/- 8 mumol/L (SEM) before starvation to 154 +/- 5 mumol/L on day 4 of starvation. Glycerol turnover rates correlated well with the basal blood glycerol concentration (r = .95) and increased from a mean value of 115 +/- 17 mumol/min before starvation (equivalent to mobilization of about 3.95 kJ triglyceride/min) to 304 +/- 20 mumol/min (equivalent to mobilization of about 18.41 kJ/min). The estimated rate of net fat oxidation was 3.00 +/- 0.47 kJ/min before starvation and 4.00 +/- 0.14 kJ/min on day +4 of starvation. The rate of triglyceride energy recycling or rate of deposition of triglyceride energy into fat stores was calculated from the difference in the rate of fat energy mobilization and the rate of energy released during net fat oxidation. The values were found to be 0.94 +/- 0.26 kJ/min before starvation and 6.29 +/- 0.54 kJ/min on day +4 of starvation.(ABSTRACT TRUNCATED AT 250 WORDS)

Bezafibrate and fenofibrate in type II diabetics with hyperlipoproteinaemia

A double-blind study was carried out in 64 Type II diabetic patients, controlled with diet therapy and sulphonylureas (glibenclamide or chlorpropamide), who had associated hyperlipoproteinaemia which could not be normalized during a run-in period of 2 months by hygiene and dietary measures alone. The patients were allocated at random into 4 groups and each group was treated over 4 months with bezafibrate (600 mg/day) or fenofibrate (300 mg/day) in addition to their glibenclamide (5 to 15 mg/day) or chlorpropamide (250 to 500 mg/day) therapy plus diet. The results of monthly laboratory investigations showed that both hypolipaemic agents produced significant reductions in serum triglycerides, total cholesterol and LDL cholesterol and an increase in HDL cholesterol. There was a significantly greater increase in HDL cholesterol in patients treated with bezafibrate and glibenclamide, particularly in women, and greater control of blood glucose in those on bezafibrate. Both hypolipaemic drugs were well tolerated. Slight increases were recorded in serum transaminases in fenofibrate patients and in bleeding time in bezafibrate patients. It is concluded that, because of its greater effect on blood glucose and HDL cholesterol, the combination of bezafibrate with glibenclamide would appear to be the best of the regimens studied for the treatment of non-insulin-dependent diabetics with associated hyperlipoproteinaemia.

Relationship between insulin-mediated glucose disposal and lipid metabolism in man

To assess the possible effects of lipid metabolism on insulin-mediated glucose disposal, 18 nondiabetic Pima Indian women (age 18-35 yr) were studied using 1-14C-palmitate infusion to measure free fatty acid turnover rate followed by a euglycemic clamp (clamp) to measure in vivo insulin-mediated glucose disposal (M). Indirect calorimetry was performed in the basal state and during the clamp. This was used to assess glucose oxidation rate, lipid oxidation rate, and to calculate nonoxidative glucose disposal (storage). Basal and clamp lipid oxidation rate correlated with basal plasma free fatty acid concentration (r = 0.81, P less than or equal to 0.0001, r = 0.67, P less than 0.003, respectively). The fall in lipid oxidation was highly correlated with the increase in glucose oxidation during the insulin infusion (r = 0.96, P less than or equal to 0.0001). The clamp lipid oxidation rate negatively correlated with the glucose oxidation rate (r = -0.85, P less than 0.0001) and with the M value (r = -0.60, P less than 0.01) but was not correlated with the clamp glucose storage (r = -0.2, P = 0.4). On the other hand, glucose storage appeared to make a greater contribution to the difference in M value between the upper and lower extremes of M than did glucose oxidation, as evidenced by an increase in glucose storage of 0.59 mg/kg fat-free mass times minute per 1 mg/kg fat-free mass times minute increase in glucose disposal. The M value was negatively correlated with obesity as measured by percent body fat (r = -0.64, P less than 0.004), but neither basal free fatty acid concentration, basal free fatty acid turnover, basal lipid oxidation, nor clamp lipid oxidation correlated with percent body fat. We conclude that an interaction of lipid and glucose metabolism in a glucose fatty acid cycle, as proposed by Randle et al. (1), may be operative in the regulation of glucose oxidation in man. The disposal of glucose however has two components. The storage component does not appear to be associated with lipid oxidation in the way that the oxidative component is and may be regulated by a different mechanism. Since the results show that the glucose storage component plays a significant role in distinguishing between those with low and high M values, we suggest that the glucose fatty acid cycle can, at best, only partially explain impaired in vivo insulin-mediated glucose disposal. Furthermore, the data suggest that the impact of obesity on in vivo insulin resistance appears to be mediated by factors other than changes in lipid availability or metabolism.

Measurement of glycerol turnover by infusion of nonisotopic glycerol in normal and injured subjects

A new technique for measuring glycerol turnover in human subjects is presented using a staged infusion of unlabeled glycerol. A total of 105 measurements was made in normal subjects and postoperative or acutely ill patients. The relation between glycerol turnover rate (R tau) and concentration (C) (R tau = 31 + 2.07C) from 80 staged infusion measurements in 21 normal subjects did not differ significantly from observations of others made by isotope dilution. In 11 patients, glycerol turnover was also measured by dilution of isotopic glycerol. Mean values did not differ significantly between the two methods. The average difference, regardless of sign, for individual values was 17%. This agreement tends to validate the assumptions of both methods. Advantages of the staged infusion method are that it is quicker and easier to perform and does not entail a risk due to administration of radioactive materials. Evidence obtained in this study from a three-stage procedure indicates that satisfactory information may be obtained from a two-stage infusion at successive rates of 250 and 600 mumol/min for 70 min each, with a prime of 2 mmol given as a bolus at the start of each stage.

Total and individual free fatty acid concentrations in liver cirrhosis

The finding of high plasma free fatty acid (FFA) levels in cirrhotic patients has been attributed either to decreased hepatic clearance or to enhanced fat mobilization. To better clarify these hypotheses, total and individual FFA and glycerol levels were determined in 21 cirrhotic patients with different degrees of hepatocellular damage (evaluated by liver function tests), portal hypertension (evaluated by endoscopy and clinical signs), and nutritional status (evaluated by anthropometric and biohumoral parameters) and in 10 age- and sex-matched healthy subjects. Glucose tolerance and insulin and glucagon levels were determined in all individuals. Well-nourished and malnourished patients were identified within the cirrhotic group. Plasma FFA and glycerol concentrations were well correlated (r = 0.47, P less than 0.05), levels being significantly higher in cirrhotic individuals than in controls (746.6 +/- 46.29 SE v 359.22 +/- 40.82 mumol/L, P less than 0.001 for plasma FFA; 150.1 +/- 3.12 v 82.5 +/- 9.2 mumol/L, P less than 0.01 for glycerol). Plasma FFA and glycerol showed no correlation with the liver function test results or portal hypertension parameters. Interestingly, plasma levels of FFA and glycerol were influenced by the nutritional status, significantly higher FFA levels being observed in the well-nourished than in the malnourished patients (842.5 +/- 47.5 v 563.4 +/- 78 mumol/L, P less than 0.005). Furthermore, a positive correlation was found between plasma glycerol level and percentage of triceps skinfold (r = 0.45, P less than 0.05). No correlation was found between plasma levels of FFA or glycerol and glucose tolerance, insulin and glucagon.(ABSTRACT TRUNCATED AT 250 WORDS)

Increases in plasma glycerol levels precede the hypophagia following subcutaneous glycerol injection in rats

Cumulative food intake and plasma glycerol levels following subcutaneous glycerol injections were investigated in rats, because increases in plasma glycerol have been suspected to contribute to glycerol-induced hypophagia. Besides plasma glycerol, plasma non-esterified fatty acids (NEFA), plasma D-(-)-3-hydroxybutyrate (3-HB), blood glucose, and liver glycogen content were also measured. Two daily injections of 80 mg/kg body wt. glycerol did not affect food intake and failed to increase plasma glycerol 30 min after the injection. A single injection of 660 mg/kg body wt. glycerol reduced food intake and increased plasma glycerol 1-2 hours after the injection. This glycerol dose also increased liver glycogen content 1-2 hours after the injection but did not affect plasma NEFA and 3-HB. Rats injected with 660 mg/kg body wt. glycerol did not reduce feeding within the first 2 hours following the injection, when plasma glycerol levels were increased. Inhibition of feeding began at about 3 hours and continued up to 6 hours although plasma glycerol levels had declined to control values 5-6 hours after the injection. It is concluded that metabolic consequences of elevated plasma glycerol levels rather than increases in plasma glycerol levels per se elicit the food intake reduction following exogenous glycerol loads in rats.

Lipid transport in the human newborn. Palmitate and glycerol turnover and the contribution of glycerol to neonatal hepatic glucose output

Free fatty acid (FFA) transport was measured in 11 and glycerol turnover in 5 newborns with continuous tracer infusion of [1-(13)C]palmitate or [2-(13)C]glycerol, respectively. In addition, simultaneous determination of glucose production in the latter group with [6,6-(2)H(2)]glucose tracer and measurement of the appearance rate of [(13)C]glucose derived from [(13)C]glycerol allowed calculation of gluconeogenesis from glycerol.The average FFA inflow rate was 11.5+/-1.7 mumol kg(-1)min(-1), 2.5-4.5 h after the last feeding, and 16.7+/-2.8 mumol kg(-1)min(-1), 5-12 h after the last meal. These rates are comparable to those found in adults only after 8-16 h and approximately 72 h of fasting, respectively. FFA inflow in the newborn was directly correlated with time of fasting, plasma FFA level, and plasma glycerol level. Palmitate clearance and fractional removal were inversely related to palmitate level. Glycerol flux averaged 4.4+/-0.5 mumol kg(-1)min(-1), a value three- to fourfold that of the postabsorptive adult. Approximately 75% of transported glycerol was converted to glucose and represented 5.0+/-0.6% of hepatic glucose production. Furthermore, there was a direct relationship between glycerol turnover and the fraction of glucose coming from glycerol. Despite the absolutely elevated neonatal FFA and glycerol transport rates, these were quantitatively similar to values found in adults with comparable elevated substrate levels. Furthermore, other similarities with the adult in the relationships between inflow transport and substrate values, and between transport and fractional removal suggest that the regulatory aspects of lipid transport in man are already well developed by the first day of life.

Epinephrine plasma thresholds for lipolytic effects in man: measurements of fatty acid transport with [l-13C]palmitic acid

To determine the plasma epinephrine thresholds for its lipolytic effect, 60-min epinephrine infusions at nominal rates of 0.1, 0.5, 1.0, 2.5, and 5.0 micrograms/min were performed in each of four normal young adult men while they also received a simultaneous infusion of [1-13C]palmitic acid to estimate inflow transport of plasma free fatty acids. These 20 infusions resulted in steady-state plasma epinephrine concentrations ranging from 12 to 870 pg/ml. Plasma epinephrine thresholds for changes in blood glucose, lactate, and beta-hydroxybutyrate were in the 150--200-pg/ml range reported by us previously (Clutter, W. E., D. M. Bier, S. D. Shah, and P. E. Cryer. 1980. J. Clin. Invest. 66: 94--101.). Increments in plasma glycerol and free fatty acids and in the inflow and outflow transport of palmitate, however, occurred at lower plasma epinephrine thresholds in the range of 75 to 125 pg/ml. Palmitate clearance was unaffected at any steady-state epinephrine level produced. These data indicate that (a) the lipolytic effects of epinephrine occur at plasma levels approximately threefold basal values and (b) lipolysis is more sensitive than glycogenolysis to increments in plasma epinephrine.

The effect of carbohydrate intake on the lipolytic rate in depleted patients

The effect of intravenous carbohydrate intake on glycerol turnover and fat metabolism was estimated in six nutritionally depleted surgical patients requiring total parenteral nutrition. Two diets were given. Nitrogen intake was the same in both diets. The calorie intake, adjusted by varying glucose intake, provided either 72% or 128% of the measured resting energy expenditure. Glycerol turnover was measured during administration of 5% dextrose solutions before starting total parenteral nutrition, and again after 4 days on each diet. Turnover rates of glycerol were closely correlated with plasma concentrations. However, fractional turnover rates were only two-thirds of normal values, indicating decreased clearance possibly due to decreased hepatic blood flow. Glycerol turnover, plasma free fatty acid concentrations, and rate of fat oxidation declined progressively with increased glucose intake. When compared with these results, previous studies of injured and septic patients showed: higher values for glycerol turnover, FFA concentrations, and fat oxidation; poor corrlation between glycerol turnover and concentration; inhibition of lipogenesis at high glucose intake; and high rates of norepinephrine excretion. The data suggest that in severe injury, counter regulatory hormones may almost completely block the effects of insulin on hormone sensitive lipase but have less influence on insulin stimulation of FFA esterification and inhibition of ketone body synthesis.

Effects of fat mass reduction by dieting and by lipectomy on carbohydrate metabolism in obese patients

The interrelation of enlarged body fat mass (BFM) with reduced carbohydrate tolerance and hyperinsulinemia was studied in obese subjects with chemical diabetes. These patients were subjected to lipectomy following weight loss induced by a low-calorie, low-carbohydrate diet. An improvement in glucose tolerance and in insulin sensitivity and a reduction in insulin release during OGTT was observed after a diet-induced BFM loss of 9.9 +/-1.2 kg. Subsequent surgical reduction of BFM by 6.0 +/- 0.5 kg had no further effect upon carbohydrate tolerance, insulin release or insulin sensitivity though a marked decrease in basal plasma FFA values was observed. These findings suggest that fat mass enlargement per se has no effect on blood glucose homeostasis after oral or i.v. loading. The improvement in carbohydrate tolerance and in insulin resistance usually observed following diet-induced loss of BFM seems to be due to the reduction in calorie and carbohydrate intake rather than to decrease of BFM.