Palmitate and glycerol kinetics during brief starvation in normal weight young adult and elderly subjects

Glycerol production and utilization measured using stable isotopes

The rate of appearance of glycerol in the systemic circulation is determined from the enrichment of arterial blood glycerol when labelled glycerol is infused intravenously. This value provides a good measure of whole-body lipolysis during fasting, except that arterial infusion and venous sampling, if feasible, would probably give a higher more-accurate value. Lipolysis occurs primarily in adipose tissue, although other tissues contribute, notably muscle. Measurement is based on the difference in the enrichment of the glycerol entering and leaving the tissue. Lipolysis is underestimated by the extent to which glycerol released by lipolysis does not enter the systemic circulation, as occurs when lipolysis takes place in the non-hepatic tissue of the splanchnic bed. Glycerol released into the systemic circulation is utilized mainly by liver, although kidney and muscle are also major users of glycerol. Measurement of glycerol utilization is based on the amount of labelled glycerol taken up by the tissues. Other tissues probably utilize glycerol to a smaller extent, but in total this represents a significant amount. Most glycerol taken up by liver is converted to glucose. Glucose is probably the major source of glycerol-3-phosphate used in the esterification of fatty acids by adipose tissue.

Beta-adrenergically stimulated fat oxidation is diminished in middle-aged compared to young subjects

The effect of aging on beta-adrenergically mediated substrate utilization was investigated in nine young (25.2 +/- 1.7 yr old) and eight older males (52.9 +/- 2.1 yr old), matched for body weight and body composition. In a first experiment, the nonselective beta-agonist isoprenaline (ISO) was infused in increasing standardized doses, and during each infusion period energy expenditure and substrate utilization were determined by indirect calorimetry. In a second experiment, forearm skeletal muscle metabolism was studied during a standardized infusion dose of ISO (19 ng/kg fat-free mass x min). During beta-adrenergic stimulation there was an increased carbohydrate oxidation (at an ISO infusion dose of 24 ng/kg fat-free mass x min, 31% vs. 21% of total energy expenditure; P < 0.05) and a decreased fat oxidation (51 vs. 62 of total energy expenditure; P < 0.05) in older compared to young subjects. Skeletal muscle lactate release significantly increased in the older subjects (from -175 +/- 32 to -366 +/- 66 nmol/100 mL forearm tissue x min), whereas there was no change in young subjects (from -32 +/- 21 to 23 +/- 57 nmol/100 mL forearm tissue x min; interaction group x ISO, P < 0.01). Additionally, there was a tendency toward a blunted ISO-induced increase in nonesterified fatty acid uptake in the older subjects (interaction group x ISO, P = 0.062). Thus, middle-aged subjects have a blunted ability to oxidize fat during beta-adrenergic stimulation compared to young subjects. This diminished fat oxidation may be an important etiological factor in the age-related increase in body fatness and obesity by favoring fat storage above oxidation.

Systemic resistance to the antilipolytic effect of insulin in black and white women with visceral obesity

This study was designed to determine the role of visceral adipose tissue (VAT) accumulation in systemic fat metabolism and to compare this in black and white women who differ in their manifestations of upper body obesity. Systemic glycerol and free fatty acid (FFA) turnover rates (rates of appearance, Ra) were measured in the basal state and during a pancreatic euglycemic clamp in nondiabetic, premenopausal, obese black and white women with a wide range of VAT accumulation. The slopes of the regression equations predicting basal and insulin-suppressed RaGlycerol and RaFFA from VAT area, age, and fat mass or fat-free mass did not significantly differ between black and white women. VAT area was the best predictor of the %-suppressed RaGlycerol and RaFFA during the pancreatic clamp (partial r = 0.76, P < 0.0001 and partial r = 0.60, P < 0.05, respectively). Basal R(a)Glycerol, but not RaFFA, was lower in black than in white women (P < 0.05). During the clamp, black women showed greater insulin suppression of RaGlycerol than of RaFFA (P < 0.0001) and greater insulin suppression of RaGlycerol (P < 0. 05) but similar suppression of RaFFA compared with white women. These differences were independent of age, fat mass, or fat-free mass and were partly explained by a lower VAT in black women. Thus, in both races, VAT accumulation was associated with systemic resistance to the antilipolytic effect of insulin and, in obese black women, systemic lipolysis measured as glycerol turnover rate was more responsive to insulin suppression than were systemic FFA turnover rates.

Effect of short-term fasting on lipid kinetics in lean and obese women

We evaluated whole body and regional adipose tissue lipid kinetics and norepinephrine (NE) spillover during brief fasting in six lean [body mass index (BMI) 21 +/- 1 kg/m2] and six upper-body obese (UBO; BMI 36 +/- 1 kg/m2) women. At 14 h of fasting, abdominal adipose tissue glycerol and free fatty acid (FFA) release rates were lower (P = 0.07), but whole body glycerol and FFA rates of appearance (Ra) were greater (P < 0.05) in obese than in lean subjects. At 22 h of fasting, glycerol and FFA Ra increased less in obese (19.8 +/- 7.0 and 87.1 +/- 30.3 micromol/min, respectively) than in lean (44.2 +/- 6.6 and 137.4 +/- 30.4 micromol/min, respectively; P < 0.05) women. The percent increase in glycerol Ra correlated closely with the percent decline in plasma insulin in both groups (r2 = 0.85; P < 0.05). Whole body NE spillover declined in lean (P < 0.05) but not obese subjects with continued fasting, whereas regional adipose tissue NE spillover did not change in either group. We conclude that, compared with lean women, in UBO women 1) basal adipose tissue lipolysis is lower, but whole body lipid kinetics is higher because of their greater fat mass; 2) the increase in lipolysis during fasting is blunted because of an attenuated decline in circulating insulin; and 3) downregulation of whole body sympathetic nervous system activity is impaired during fasting.

Metabolism of dietary alpha-linolenic acid vs. eicosapentaenoic acid in rat immune cell phospholipids during endotoxemia

Short-term (i.e., 3 d) continuous enteral feeding of diets containing eicosapentaenoic (EPA) and gamma-linolenic (GLA) polyunsaturated fatty acids (PUFA) to endotoxemic rats reduces the levels of arachidonic acid (AA) and linoleic acid (LA) in alveolar macrophage (AM) and liver Kupffer and endothelial (K&E) cell phospholipids with attendant decreases in prostaglandin formation by these cells in vitro. Diets that contain alpha-linolenic acid (LNA) as a substrate for endogenous formation of EPA may not be as effective in facilitating these immune cell modifications given the limited activity of delta6 desaturase. In the present study we compared the effectiveness of an LNA-enriched diet vs. an (EPA + GLA)-enriched diet to displace phospholipid AA from AM and liver K&E cells in vivo in endotoxemic rats fed enterally for 3 or 6 d. We determined the fatty acid composition of AM and K&E cell phospholipids by gas chromatography. We found that AM and K&E cells from rats that had received the EPA + GLA diet for 3 d had significantly (P < 0.001) higher mole percentage of EPA and the GLA metabolite, dihomoGLA, than corresponding cells from rats given the LNA diet or a control diet enriched with LA. Rats given the LNA diet had relatively low levels of stearidonic acid, EPA and other n-3 PUFA, while rats given the LA diet had low levels of GLA and dihomoGLA. We conclude that diets enriched with LNA or LA may not be as effective as those enriched with EPA + GLA for purposes of fostering incorporation of EPA or dihomoGLA into and displacement of AA from macrophage phospholipids under pathophysiologic conditions commonly found in acutely septic patients.

Determination of glycerol concentrations and glycerol isotopic enrichments in human plasma by gas chromatography/mass spectrometry

An analytical method is presented to determine glycerol concentrations and stable isotope tracer enrichments in human plasma after intravenous tracer infusion in a single analytical run, using gas chromatography coupled to mass spectrometry. The method uses an internal standard, which is also a stable isotope labeled form of glycerol. Three substances were tested as model compounds viz. [2-13C]glycerol, and [1,2,3-13C3]glycerol, and [1,1,2,3, 3-2H5]glycerol. Any combination of two can be used (one as internal standard, one as tracer), even if overlapping of the mass spectra occurs. The method is precise (recovery of spiked glycerol and tracer are, respectively, 99.7 and 99.8%) and reproducible (intraassay variation <1.5%, interassay variation <6%) and needs only a small amount of plasma (100 microl).

Role of human liver lipogenesis and reesterification in triglycerides secretion and in FFA reesterification

To measure 1) the contribution of hepatic de novo lipogenesis (DNL) and plasma free fatty acid (FFA) reesterification to plasma triglyceride (TG) secretion and 2) the role of oxidation and hepatic and extrahepatic reesterification in FFA utilization, five normal subjects drank deuterate water and were infused (postabsorptive state) with [1-13C]palmitate and [1,2,3-2H5]glycerol. Total lipid oxidation (Lox) was measured by indirect calorimetry. FFA oxidation (2.76 +/- 0.65 mumol.kg.-1.min-1) accounted for 45% of FFA turnover rate (Rt) (1.04 mumol.kg-1.min-1) and 91% of Lox; FFA reesterification was 3.27 +/- 0.54 mumol.kg-1.min-1. Fractional and absolute TG Rt were 0.21 +/- 0.02 h-1 and 0.11 +/- 0.05 mumol.kg-1.min-1. DNL accounted for 3.9 +/- 0.9% of TG secretion, and hepatic FFA reesterification accounted for 49.4 +/- 5.7%; this last process represented a utilization of FFA of 0.16 +/- 0.02 mumol.kg-1.min-1. We conclude that, in the postabsorptive state, 1) DNL and FFA reesterification account for only 50-55% of TG secretion, the remaining presumably being provided by stored lipids or lipoproteins taken up by liver, 2) most reesterification occurs in extrahepatic tissues, and 3) oxidation and reesterification each contribute about one-half to FFA utilization; FFA oxidation accounts for almost all Lox.

Lipolysis in elderly postmenopausal women

The rate of fat oxidation at rest decreases with age in women. The mechanisms for this decrease are not clear. Theoretically, a decrease in the availability of fatty acids could explain the decline in fat oxidation. In consequence, the in vivo rate of production of fatty acids as a proxy for lipolysis was measured in 21 healthy women. Eleven of the volunteers were elderly (> 65 years) and 10 were young (< 24 years), and all were characterized for body composition. The nonadjusted rate of delivery of fatty acids into the systemic circulation was similar among elderly and young individuals (609 +/- 80.3 v 597 +/- 69.9 mumol/min, respectively, P > .1). When lipolysis was adjusted for the differences in fat-free mass using analysis of covariance (ANCOVA), rates were slightly increased in the elderly group (626 +/- 80 mumol/min) and decreased in the young group (578 +/- 84 mumol/min), but remained nonstatistically significant. It is concluded that mechanisms other than lipolysis must explain the decrease of fat oxidation in aging women, i.e., a decrease in the capacity of muscle to oxidize fat and/or a decrease in its capacity for transport of long-chain fatty acids.

Cyclic vs continuous enteral feeding with omega-3 and gamma-linolenic fatty acids: effects on modulation of phospholipid fatty acids in rat lung and liver immune cells

Arachidonic acid (AA) present in lung and liver immune cell phospholipids is the precursor of eicosanoids that promote neutrophil margination, leading to tissue injury and inflammation. Administration of novel enteral formulations low in linoleic acid (LA) and containing eicosapentaenoic acid (EPA) from fish oil and gamma-linolenic acid (GLA) from borage oil displaces AA and promotes cell formation of eicosanoids with reduced inflammatory potential. The present study was undertaken to determine whether or not short-term provision of enteral diets containing GLA, EPA, or both in a cyclic fashion modulated the fatty acid composition of rat alveolar macrophage (AM) and liver Kupffer and endothelial (K&E) cell phospholipids in vivo to the extent achieved during continuous feeding.

METHODS

Rats were isocalorically fed through a gastrostomy catheter for 3 or 6 days with high-fat, low-carbohydrate diets that were enriched with either LA (diet A), EPA (diet B), or EPA + GLA (diet C). The rats were randomized by infusion modality, ie, continuous vs cyclic (14 hours feeding with 10 hours fasting daily) feeding. AM and K&E were isolated and phospholipid fatty acid profiles were determined by gas chromatography.

RESULTS

The dietary effects on AM and K&E cell phospholipid fatty acids for a given feeding period were not significantly influenced by the infusion modality. AM and K&E cells from rats receiving either diet B or diet C for 3 days had significantly lower AA and LA and higher EPA and dihomo-GLA (DHGLA), respectively, than rats given diet A regardless of the infusion modality. The mole % of EPA and DHGLA in K&E cells were higher after 6 vs 3 days of cyclic feeding with diet C. Using the eicosanoid precursor ratio (EPA + DHGLA/AA), the potential for generation of AA-derived eicosanoids was lower in rats given die B or C vs diet A regardless of infusion modality.

DISCUSSION

Given the rapid changes in lung and liver immune cell phospholipid fatty acids, short-term provision of EPA and GLA-enriched diets cyclically or continuously may prove clinically relevant for modulating the fatty acid composition and potential eicosanoid formation by these cells.

Simultaneous measurement of the rates of appearance of palmitic and linoleic acid in critically ill infants

Lipolysis has been measured in humans by means of stable isotope techniques using labeled palmitic acid (PA) or glycerol as tracers. If other fatty acids (FA) such as linoleic acid (LLA) have the same rate of appearance (Ra) as PA and therefore contribute equally to oxidative and nonoxidative metabolism is unknown. We infused albumin-bound [U-13C]PA and [U-13C]LLA in seven critically ill infants (weight 3.6 +/- 1.3 kg, age 57 +/- 64 d) receiving 20.9 +/- 5.4 kcal. kg-1.d-1 of i.v. glucose only, and measured simultaneously the Ra of PA and LLA from the isotopic enrichment of plasma FFA by mass spectrometry. A needle biopsy of the s.c. adipose tissue was obtained for FA composition. PA in adipose tissue was higher than LLA (40 +/- 6.7 versus 5.4 +/- 3.2 mol %, p < 0.001). The Ra values of PA and LLA were 5.73 +/- 2.79 and 1.34 +/- 0.92 mumol.kg-1.min-1, respectively (p = 0.005). However, the ratio of the FA's Ra to their respective mol% values in adipose tissue was lower for PA than for LLA (0.15 +/- 0.06 versus 0.25 +/- 0.06, p = 0.02). The Ra of LLA acid was higher than could be expected from the FA composition of adipose tissue, thus indicating a preferential release of LLA during lipolysis. In critically ill infants receiving only i.v. glucose, the contribution of LLA to the oxidative and nonoxidative metabolism may be larger than what assumed from the FA composition of plasma and adipose tissue.

Glycerol production and utilization in humans: sites and quantitation

Liver is assumed to be the major site of glycerol uptake and fatty acid reesterification. [U-13C]glycerol was infused into ten 60 h-fasted healthy subjects. Measured were 1) blood glycerol concentrations and 13C enrichments in brachial and pulmonary arteries and in hepatic, renal, superficial, and deep forearm veins; 2) glycerol appearance rates in systemic circulation; and 3) splanchnic bed and kidney glycerol uptakes with use of balance and tracer methodology. Glycerol concentrations were one-fifth in hepatic, one-half in renal, 40% more in superficial, and the same in deep vein and pulmonary artery as in brachial artery blood. Glycerol enrichments were one-fifth in hepatic, two-thirds to three-quarters in renal and superficial veins, and the same in pulmonary as in brachial artery blood. Splanchnic glycerol uptake was 29% and kidney glycerol uptake was 17% of glycerol's rate of appearance, 5.11 mumol.min-1.kg-1. Splanchnic fatty acid uptake was 25% of calculated fatty acid release. Glycerol contributed 15% to glucose production. Most of the [13C]glycerol uptake by splanchnic bed and kidneys was incorporated into glucose. Thus, in 60 h-fasted individuals, most glycerol uptake does not occur in liver, and the extent of fatty acid reesterification in liver is in doubt.

Contributions of liver and kidneys to glycerol production and utilization in the dog

The classical concept holds that liver and kidneys are the main sinks of glycerol released by adipose tissue. However, rates of glycerol appearance (Ra) exceed the rate of glycerol delivery to liver and kidneys. We measured the hepatic and renal contributions to glycerol production and utilization in anesthetized dogs that were fasted either overnight or for 24 h after 3 days on a carbohydrate-free diet. Dogs were infused with [2H5]glycerol, and the concentration and 2H enrichment of glycerol were measured across liver and kidney. After a baseline period, either norepinephrine or glucose plus insulin was infused to alter the rate of glycerol production. Our study shows that the production of glycerol by liver and kidneys amounted to 4-9% and 4-7% of the Ra of glycerol, respectively. Uptake of glycerol by liver and kidneys amounted to 26-30 and 10-19% of the Ra of glycerol, respectively. Thus, contrary to the classical concept, the bulk of glycerol utilization occurs in nonhepatic, nonrenal tissues that have very low glycerol kinase activity per gram.

Metabolic response to radiation therapy in patients with cancer

The effect of radiation therapy on substrate metabolism was evaluated in five patients with head and neck or lung cancer. Stable isotope tracer methodology was used to determine urea, amino acid, glucose, and lipid kinetics during postabsorptive conditions before initiation, near the midpoint (after receiving 2,672 +/- 36 rads), and at completion (after receiving 6,072 +/- 307 rad) of a 6- to 8-week course of radiation therapy. Nutritional status was maintained throughout the treatment period by providing supplemental enteral feedings as needed. Postabsorptive plasma insulin, catecholamine, and amino acid concentrations did not change during the course of treatment. Before radiation therapy was initiated, values for the plasma rate of appearance (Ra) of urea (3.35 +/- 0.33 micromol x kg(-1) x min(-1)), alpha-ketoisocaproate ([alpha-KIC] 2.16 +/- 0.19 micromol x kg(-1) x min(-1)), phenylalanine (0.59 +/- 0.052 micromol x kg(-1) x min(-1)), and glucose (10.56 +/- 1.31 micromol x kg(-1) x min(-1)) were in the normal range. However, glycerol and palmitate Ra values (3.11 +/- 0.30 and 2.01 +/- 0.33 micromol x kg(-1) x min(-1), respectively) were 25% higher than values observed previously in normal subjects. Substrate flux did not change during radiation therapy, and measurements obtained during the midpoint and at completion of treatment were similar to initial values. These results demonstrate that large doses of radiation therapy, administered over 6 to 8 weeks to the upper body, do not cause significant metabolic stress.

Substrate metabolism in humans: 1995 A.S.P.E.N. research workshop

BACKGROUND

The 1995 A.S.P.E.N. Research Workshop was held at the annual meeting in Miami Beach, Florida, on January 15, 1995. The workshop focused on substrate metabolism in humans.

METHODS

State-of-the-art presentations on the regulation of energy, carbohydrate, lipid, and protein metabolism during health and disease were made by the preeminant leaders in the field. The presentations concentrated on in vivo studies performed in humans and included both recently published and unpublished data.

RESULTS

Using sophisticated research methodology, such as nuclear magnetic resonance spectroscopy, compartmental modeling, stable isotope tracers, microdialysis, and abdominal vein catheterization, the investigators presented data that clarified unresolved issues, challenged many previously held dogmas, and raised new questions for future investigations in human intermediary metabolism.

CONCLUSIONS

This workshop demonstrated that in vivo investigation remains the best approach for providing physiologically relevant data in humans. An understanding of normal human physiology and the metabolic alterations caused by disease is critical for optimal nutritional and metabolic management of patients.

A limitation in the use of mass isotopomer distributions to measure gluconeogenesis in fasting humans

The use of distributions of mass isotopomers in glucose from [U-13C]glycerol to estimate fractional rates of gluconeogenesis was examined. [U-13C]glycerol was infused into normal subjects who ingested acetaminophen and fasted for 60 h. Isotopomer distributions were measured by mass spectrometry in blood glucose and in glucuronic acid from urinary acetaminophen glucuronide. The distributions are incompatible with glucose production solely via gluconeogenesis from a single pool of triose phosphates. Rather, with the assumption of a single enriched triose phosphate pool, the distributions indicate, despite the 60 h of fasting, about as much glucose formation from an unlabeled glucose source as from that pool. Therefore the data indicate cellular heterogeneity in glycerol's metabolism, so that two or more pools with significantly different enrichments were the source of the glucose and glucuronic acid. This heterogeneity is related to much greater concentrations of glycerol in periportal than in pericentral zones of the liver lobule. Beyond evidence for heterogeneity, the findings emphasize a limitation in applying analyses of mass isotopomer distributions to measure polymer biosynthesis in the presence of heterogeneity in the precursor pool.

Effects of caffeine ingestion on NE kinetics, fat oxidation, and energy expenditure in younger and older men

Age-related differences in energy expenditure, fat oxidation, and norepinephrine (NE) kinetics after caffeine ingestion were examined using a placebo-controlled double-blind study in 10 older (O, 65-80 yr) and 10 younger (Y, 19-26 yr) men who were moderate consumers of caffeine. Caffeine ingestion resulted in similar increases in Y and O men for plasma caffeine levels (Y = 89 +/- 100 to 6,340 +/- 1,938 ng/ml, P < 0.05; O = 124 +/- 38 to 7,066 +/- 2,366 ng/ml, P < 0.05) and energy expenditure (Y = 11%, 1.38 +/- 0.15 to 1.52 +/- 0.22 kcal/min, P < 0.05; O = 9.5%, 1.15 +/- 0.13 to 1.26 +/- 0.20 kcal/min, P < 0.05). However, caffeine ingestion increased fatty acid concentrations (362 +/- 159 to 803 +/- 253 mumol/l, P < 0.05) and tended to increase rate of appearance of fatty acids (624 +/- 376 to 1,394 +/- 1,331 mumol/l, P = 0.07) in younger but not older men. Rates of fat oxidation and NE appearance and clearance did not significantly differ from baseline values in either group. In conclusion, older and younger men show a similar thermogenic response to caffeine ingestion, whereas older men show a smaller increase in fatty acid availability after a caffeine challenge. These metabolic differences are not related to alterations in NE kinetics or fat oxidation.

Endocrinologic and metabolic effects of interleukin-6 in humans

Interleukin-6 (IL-6) is one of the major circulating cytokines in catabolic states. To investigate its endocrinologic and metabolic actions in vivo, we studied eight patients with metastatic renal cell cancer two times, once during infusion of saline (control) and once during a 4-h infusion of 150 micrograms recombinant human IL-6 (rhIL-6). Rates of appearance (Ra) of glucose and free fatty acids (FFA) in plasma were measured by using the isotope dilution method. Energy expenditure and substrate oxidation were determined by indirect calorimetry. rhIL-6 induced increases in plasma norepinephrine (+261 +/- 97%, P < 0.001), cortisol (+210 +/- 48%, P < 0.001), and glucagon (+70 +/- 18%, P < 0.001), in resting energy expenditure (+25 +/- 2%, P < 0.001 vs. control), and in plasma FFA concentration (+60 +/- 30%, P < 0.001), FFA Ra (+105 +/- 18%, P < 0.001), and fat oxidation (+38 +/- 16%, P < 0.001). Glucose Ra increased by 20 +/- 5% (P < 0.01) during rhIL-6 infusion with a concomitant increase in the metabolic clearance rate of glucose. In conclusion, our data demonstrate that rhIL-6 induces many of the endocrinologic and metabolic changes found in catabolic states and thus may mediate some of the metabolic effects previously ascribed to other cytokines.

Rate of glucose entry into hepatic uridine diphosphoglucose by the direct pathway in fasted and fed states in normal humans

We used the glucuronate (GlcUA) probe technique to measure the rate of glucose entry into hepatic uridine diphosphoglucose (UDP-glc) by the direct pathway, to quantify the rate of appearance (Ra) of hepatic UDP-glc, and to calculate hepatic glucose cycling in vivo in normal humans. The direct pathway contribution to UDP-glc as determined by the ratio of [1-d1]-GlcUA to plasma [LD1]-glucose enrichments was minor (15% to 20%) in normal men after an overnight fast. After 9 hours of refeeding with intravenous (IV) glucose or an oral liquid formula meal each at a rate of 7 mg carbohydrate/kg/min, the direct pathway increased to 66.3% +/- 6.7% and 61.6% +/- 6.0% (mean +/- SE), respectively. Plasma glucose concentrations remained below 7.8 mmol/L and could not account for most of the variability in direct pathway contribution. The dilution of labeled [L-D1]-galactose in excreted acetaminophen-GlcUA was used to measure Ra UDP-glc, on the assumption that labeled galactose passes through the liver during its assimilation. Ra UDP-glc was 1.1 +/- 0.1 mg/kg/min after an overnight fast and increased to 2.0 +/- 0.1 with i.v. glucose and 2.6 +/- 0.2 with the oral liquid mixed meal. By combining the fractional glucose contribution with the Ra of hepatic UDP-glc, the rate of direct glucose entry into hepatic UDP-glc was 0.2 mg/kg/min (fasted) and increased to 1.3 to 1.6 (fed). This represented approximately 18% to 21% of systemic glucose disposal or 19% to 23% of the administered carbohydrate load during i.v. or oral refeeding.(ABSTRACT TRUNCATED AT 250 WORDS)

Use of 2H2O for estimating rates of gluconeogenesis. Application to the fasted state

A method is introduced for estimating the contribution of gluconeogenesis to glucose production. 2H2O is administered orally to achieve 0.5% deuterium enrichment in body water. Enrichments are determined in the hydrogens bound to carbons 2 and 6 of blood glucose and in urinary water. Enrichment at carbon 6 of glucose is assayed in hexamethylenetetramine, formed from formaldehyde produced by periodate oxidation of the glucose. Enrichment at carbon 2 is assayed in lactate formed by enzymatic transfer of the hydrogen from glucose via sorbitol to pyruvate. The fraction gluconeogenesis contributes to glucose production equals the ratio of the enrichment at carbon 6 to that at carbon 2 or in urinary water. Applying the method, the contribution of gluconeogenesis in healthy subjects was 23-42% after fasting 14 h, increasing to 59-84% after fasting 42 h. Enrichment at carbon 2 to that in urinary water was 1.12 +/- 0.13. Therefore, the assumption that hydrogen equilibrated during hexose-6-P isomerization was fulfilled. The 3H/14C ratio in glucose formed from [3-3H,3-14C]lactate given to healthy subjects was 0.1 to 0.2 of that in the lactate. Therefore equilibration during gluconeogenesis of the hydrogen bound to carbon 6 with that in body water was 80-90% complete, so that gluconeogenesis is underestimated by 10-20%. Glycerol's contribution to gluconeogenesis is not included in these estimates. The method is applicable to studies in humans of gluconeogenesis at safe doses of 2H2O.

Glycerol metabolism in severe falciparum malaria

Gluconeogenesis and liver blood flow (LBF) in severe falciparum malaria were assessed from the clearance and metabolic response to intravenously administered glycerol (0.3 g/kg) and Indocyanine Green ([ICG] 0.4 mg/kg), respectively. Fasting baseline blood glycerol concentrations (mean +/- SD) were significantly higher in acute malaria (133 +/- 65 mumol/L, n = 14), than in convalescence (65 +/- 31 mumol/L, n = 9, P = .01), but basal triacylglycerol concentrations were similar. Estimated glycerol turnover was also more than twice as high in acute malaria compared with convalescence (1.36 +/- 0.87 v 0.54 +/- 0.15 mumol.min-1.kg-1, P = .015). The increment in plasma glucose (AUC0-55 min) following glycerol infusion was greater during acute malaria compared with convalescence (median [range], +31.6 [-0.9 to +107.6] v +14.5 [-103 to +27.1] mmol.min-L-1, P < .05), but the insulin increments were similar (P = .9), indicating reduced tissue insulin sensitivity. The increment in venous lactate (AUC0-55 min) was higher in severely ill patients (17.2 [-7.8 to +53.4] mmol.min.L-1, n = 10) compared with patients with moderately severe malaria (-3.1 [-8.7 to 3.2] mmol.min-L-1, n = 4, P = .01). LBF estimated from ICG clearance was lower during acute illness than in convalescence (mean +/- SD, 15.5 +/- 2.3 v 18.6 +/- 2.9 mL.min-1.kg-1, P = .007) and correlated inversely with the basal venous lactate concentration (rs = .53, P < .05). LBFs less than 15 mL.min-1.kg-1 were associated with hyperlactatemia, and all four fatal cases had LBFs of less than 12 mL.min-1.kg-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Fast gas chromatographic-mass spectrometric method for the evaluation of plasma fatty acid turnover using [1-13C]palmitate

Turnover of plasma free fatty acids (FFAs) can be determined from the palmitate enrichment of plasma after administration of analogues labeled with stable isotopes. We studied the conditions to measure both the concentration and the 13C enrichment of plasma palmitate by gas chromatography-mass spectrometry (GC-MS) using crude extracts. The method used plasma extraction after addition of heptadecanoic acid as internal standard and methylation with diazomethane. Subsequently the samples were analyzed by GC-MS. Plasma palmitate levels determined with this simplified method did not differ statistically from those obtained by a more "classical" procedure using FFA separation from other plasma lipids. Palmitic acid turnover rates (Ra) were evaluated in the steady-state period, in two normal subjects after 90 min infusion with [1-13C]palmitate bound to human albumin. The rate of appearance (Ra) was found to be 0.92 and 1.08 mmol kg-1 min-1, which is in good agreement with the turnover rate previously reported for normal subjects. Sample preparation and GC-MS analysis by the proposed procedure are simple and rapid and thus the method appears to be particularly useful in clinical studies where numerous samples have to be analyzed.

Free fatty acid and glucose metabolism in human aging: evidence for operation of the Randle cycle

We assessed insulin effects on plasma free fatty acid (FFA) and glucose metabolism in seven elderly (71 +/- 2 yr) and in seven younger (21 +/- 1 yr) subjects matched for body weight and body mass index but not for percent body fat (32.4 +/- 3.8% in elderly vs. 20.4 +/- 3.5% in young, P < 0.05), by performing sequential euglycemic clamps at five insulin doses (0.6, 1.5, 3, 6, and 15 pmol.min-1.kg-1) in combination with indirect calorimetry and [1-14C]palmitate plus [3-3H]glucose infusion. At baseline, plasma FFA concentration, turnover infusion. At baseline, plasma FFA concentration, turnover and oxidation, and total lipid oxidation were all increased in the elderly (897 +/- 107 vs. 412 +/- 50 mumol/l and 11.2 +/- 1.4 vs. 5.14 +/- 0.86, 3.45 +/- 0.65 vs. 1.37 +/- 0.25, and 4.63 +/- 0.72 vs. 3.01 +/- 0.33 mumol.min-1.kg-1 lean body mass, P < 0.05 for all comparisons), whereas glucose turnover was similar as a result of decreased glucose oxidation (8.2 +/- 1.4 vs. 13 +/- 1.9 mumol.min-1.kg-1 lean body mass, P < 0.05) and increased glucose storage (6.6 +/- 1.4 vs. 1.7 +/- 1.3 mmol.min-1.kg-1 lean body mass, P < 0.05). At all insulin infusions, plasma FFA concentration, turnover and oxidation, and total lipid oxidation were higher in the elderly than in the younger group (P < 0.05). However, if normalized per fat mass, all FFA and lipid metabolic fluxes, both in the postabsorptive state and during hyperinsulinemia, were comparable in the two groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of cigarette smoking and its cessation on lipid metabolism and energy expenditure in heavy smokers

The relationship between thermogenic and potentially atherogenic effects of cigarette smoking (CS) and its cessation was investigated. Heavy smokers (n = 7, serum cotinine > 200 ng/ml, > 20 cigarettes/d) were maintained on isoenergetic, constant diets for 2 wk, 1 wk with and 1 wk without CS. Stable isotope infusions with indirect calorimetry were performed on day 7 of each phase, after an overnight fast. CS after overnight abstention increased resting energy expenditure by 5% (not significant vs. non-CS phase; P = 0.18). CS increased the flux of FFA by 77%, flux of glycerol by 82%, and serum FFA concentrations by 73% (P < 0.02 for each), but did not significantly affect fat oxidation. Hepatic reesterification of FFA increased more than threefold (P < 0.03) and adipocyte recycling increased nonsignificantly (P = 0.10). CS-induced lipid substrate cycles represented only 15% (estimated 11 kcal/d) of observed changes in energy expenditure. De novo hepatic lipogenesis was low (< 1-2 g/d) and unaffected by either acute CS or its chronic cessation. Hepatic glucose production was not affected by CS, despite increased serum glycerol and FFA fluxes. Cessation of CS caused no rebound effects on basal metabolic fluxes. In conclusion, a metabolic mechanism for the atherogenic effects of CS on serum lipids (increased hepatic reesterification of FFA) has been documented. Increased entry of FFA accounts for CS-induced increases in serum FFA concentrations. The thermogenic effect of CS is small or absent in heavy smokers while the potentially atherogenic effect is maintained, and cessation of CS does not induce a rebound lipogenic milieu that specifically favors accrual of body fat in the absence of increased food intake.

Model for measuring absolute rates of hepatic de novo lipogenesis and reesterification of free fatty acids

We have previously presented a precursor-product stable isotopic technique for measuring in vivo the fraction of very low-density lipoprotein-fatty acids (VLDL-FA) derived from de novo lipogenesis (fractional DNL). Here, we propose a technique for converting fractional DNL into absolute rates of DNL and describe its explicit underlying assumptions. The technique combines the fractional DNL method with a modification of the method of S. Klein, V. R. Young, G. L. A. Blackburn, B. R. Bistrain, and R. R. Wolfe (J. Clin. Invest. 78: 928-933, 1986), for estimating hepatic reesterification of free fatty acids (FFA). Infusions of [1,2,3,4-13C]palmitate and [1-13C]acetate are performed concurrently with indirect calorimetry in human subjects. Fractional DNL (based on mass isotopomer distribution analysis of VLDL-FA), the rate of appearance of plasma FFA (Ra of FFA), and net fat oxidation in the whole body are measured. Equations from the hepatic reesterification model, modified to include the contribution from DNL, allow calculation of absolute DNL (= fractional DNL x [Ra of FFA - net whole body fat oxidation], when respiratory quotient < 1.0). Sample results from human subjects with different dietary energy intakes are presented, with calculations of absolute DNL, absolute reesterification, and absolute fat oxidation rates. The assumptions of this technique (in particular, that all fat oxidized is derived at steady state from circulating FFA and that DNL and reesterification of FFA both occur exclusively in liver) are discussed.(ABSTRACT TRUNCATED AT 250 WORDS)

Progressive alterations in lipid and glucose metabolism during short-term fasting in young adult men

Stable isotope tracers and indirect calorimetry were used to evaluate the progressive alterations in lipid and glucose metabolism after 12, 18, 24, 30, 42, 54, and 72 h of fasting in six healthy male volunteers. The rates of appearance (Ra) of glycerol and palmitic acid in plasma doubled from 2.08 +/- 0.22 and 1.63 +/- 0.20 mumol.kg-1 x min-1, respectively, after 12 h to 4.36 +/- 0.36 and 3.26 +/- 0.40 mumol.kg-1 x min-1, respectively, after 72 h of fasting (P < 0.01). Of the total increase in lipid kinetics, 60% occurred between 12 and 24 h of fasting; the greatest interval change occurred between 18 and 24 h of fasting. Glucose Ra and plasma concentration decreased by approximately 25% between 12 h (11.0 +/- 0.4 mumol.kg-1 x min-1 and 5.58 +/- 0.08 mmol/l, respectively) and 72 h (8.3 +/- 0.3 mumol.kg-1 x min-1 and 4.14 +/- 0.10 mmol/l, respectively) of fasting (P < 0.01), but no statistically significant changes occurred between 18 and 24 h of fasting. Plasma insulin decreased by approximately 50% between 12 h (64.6 +/- 12.9 pmol/l) and 72 h (30.1 +/- 7.9 pmol/l) of fasting (P < 0.001). Of the total decline in plasma insulin, 70% occurred within the first 24 h of fasting. These results demonstrate that the mobilization of adipose tissue triglycerides increases markedly between 18 and 24 h of fasting in young adult men. The early alterations in lipid metabolism are associated with a decline in circulating insulin but do not seem to be regulated by changes in glucose kinetics or plasma glucose concentrations.

In vivo assessment of the metabolic alterations in glucagonoma syndrome

Stable-isotope methodology and indirect calorimetry were used to evaluate metabolic abnormalities in a patient with glucagonoma syndrome manifested by 17% body weight loss, hypoaminoacidemia, and hyperglycemia. Energy expenditure (26 kcal/kg) was the same as that predicted by the Harris-Benedict equation. The rate of appearance (Ra) of intracellular leucine (2.70 mumol/kg/min), an index of protein breakdown, was normal, although the percentage of leucine flux oxidized (31%), an index of amino acid catabolism, was 50% greater than the normal mean value. Glucose Ra in plasma (12.9 mumol/kg/min), representing hepatic glucose production, and glycerol Ra in plasma (3.04 mumol/kg/min), a measurement of whole-body lipolysis, were 15% and 25% greater, respectively, than mean values found in normal volunteers. These results suggest that long-term alterations in energy, leucine, glucose, and lipid metabolism in patients with glucagonoma are minimal. However, small long-term metabolic alterations caused by glucagon excess, in conjunction with chronic negative energy balance, could be responsible for the weight loss, hypoaminoacidemia, and hyperglycemia observed in this patient population.

A mass spectrometric method for measuring glycerol levels and enrichments in plasma using 13C and 2H stable isotopic tracers

The stable isotope tracer [1,1,2,3,3,-2H5]glycerol has been commonly used as a tracer to measure glycerol kinetics and lipolysis in vivo. The method for measuring samples using the trimethylsilyl derivative and electron impact gas chromatograph-mass spectrometry retains only three of the five deuteriums, resulting in the possibility of incorrectly identifying the whole glycerol tracer molecule. This reports preparation of glycerol as the heptafluorobutyrl derivative and measurement by negative ion chemical ionization gas chromatography-mass spectrometry to produce a derivative with an intense molecular ion that retains all five deuterium labels. Thus the heptafluorobutyrl derivative analyzed by negative ion mass spectrometry overcomes the problems associated with fragmentation and loss of the isotopic label. Glycerol concentration using a labeled internal standard can be determined in plasma with a precision of 3%. Nanomole amounts of glycerol can be analyzed for 13C or 2H enrichments with a precision of +/- 0.14 mol% excess isotope. This simple, sensitive method for measuring glycerol levels and stable isotopic enrichment in plasma uses a simple extraction procedure and requires a minimal volume of plasma (less than 300 microliters).

Generalized lipodystrophy: in vivo evidence for hypermetabolism and insulin-resistant lipid, glucose, and amino acid kinetics

Stable isotope tracers and indirect calorimetry were used to evaluate whole-body energy, glucose, lipid, and amino acid metabolism in a patient with generalized lipodystrophy during basal conditions and in response to insulin therapy. The results were compared with those obtained in previous studies in normal volunteers. The basal rate of glucose production (33.7 mumol/kg.min) was three times higher than normal. The basal rate of glycerol appearance in blood, an index of lipolysis, was 60% greater than normal when expressed per kilogram body weight (3.82 mumol/kg.min), but was more than 10 times normal when expressed per kilogram body fat mass (123.2 mumol/kg.min) because of the marked decrease in body fat in our patient (3% of total body weight). Leucine rate of appearance, an index of protein breakdown, and nonoxidative leucine disposal, an index of protein synthesis, were also greater than normal. Resting energy expenditure (REE) was 30% greater than normal. The effect of insulin infusion on these metabolic parameters was markedly blunted. These metabolic abnormalities help explain many of the clinical findings such as hyperglycemia, hypertriglyceridemia, fat depletion, hepatomegaly, and steatosis observed in patients with lipodystrophy. Ineffective insulin function in many tissues appears to be an important factor in the pathophysiology of lipodystrophy.

Energy requirements in the elderly

The major components of daily energy expenditure may change during aging. A review of current estimates of energy requirements leads to the conclusion that direct estimates of daily energy expenditure are desirable to confirm the validity of the present recommendations. A more extensive examination of the metabolic and nutritional significance of the major food fuel sources is necessary to further define human energy needs.

Effect of reduced dietary intake on energy expenditure, protein turnover, and glucose cycling in man

The effect of a 50% reduction in food intake on energy expenditure, protein metabolism, glucose cycling, and body composition was investigated in eight moderately overweight men. The prestudy mean calorie and protein intake was determined for eight subjects. They were then maintained on this diet for 6 weeks (mean +/- SEM, 3,269 +/- 75 kcal/d, 20.0 +/- 0.5 g N/d, period I), after which the diet was reduced uniformly in the major foodstuffs by 50% for the next 4 weeks (1,555 +/- 38 kcal/d, 9.6 +/- 5 g N/d, period II). At the end of each period we measured (1) body fat and fat free mass by underwater weighing, (2) 24-hour energy expenditure by indirect calorimetry in a calorimeter, (3) whole body protein synthesis and breakdown rates with 15N glycine, and (4) glucose cycling between glucose and glucose-6-phosphate and fructose cycling between fructose-6-phosphate and fructose-1,6 bisphosphate with 6,6-D2- and 2-D1-labeled glucose. The results were subjects lost 4.0 +/- 0.1 kg fat (by underwater weighing) during the 4 weeks on the reduced-energy regimen. Protein turnover and glucose cycling were reduced by 20% and 15%, respectively. Twenty-four-hour energy expenditure was 2,553 +/- 166 kcal/d for period I and 2,369 +/- 69 kcal/d for period II, giving a difference of 184 +/- 34 kcal/d between the two periods. In conclusion, (1) although energy intake was reduced by 50%, the decrease in energy expenditure was small due to the buffering effect of body fat.(ABSTRACT TRUNCATED AT 250 WORDS)

Determination of plasma-free fatty acid kinetics with tracers: methodologic considerations

Plasma-free fatty acids (FFA) are an important source of energy for a variety of tissues. Recently, there has been an increased interest in the measurement of FFA kinetics in vivo, using radiolabeled or stable isotopic tracers. Standard techniques for measurement of FFA-specific activity are relatively imprecise and have limited sensitivity. We have developed a method for determination of the concentration and specific activity of individual plasma FFA that is precise (coefficient of variation less than 2%) and sensitive (detection limit in the high femptomolar to low picomolar range). Using this method, one can measure the kinetics of three or more long-chain fatty acids simultaneously. Its sensitivity is a particular advantage if one wishes to measure low rates of FFA turnover such as are encountered during hyperinsulinemia. It has been suggested that, for optimal accuracy in the determination of substrate kinetics, the tracer should be administered in the left ventricle and mixed venous blood samples should be obtained from the right heart. We have conducted experiments in dogs which demonstrate that peripheral tracer infusion and more conventional arterial (or arterialized venous) sampling actually provide more accurate estimates of FFA turnover; this is fortunate, since intracardiac infusion and sampling are not practical for human studies.

Possible role of adrenergic mechanism in starvation-induced reduction in circulating thyroxine and triiodothyronine in rats

To elucidate the possible role of adrenergic mechanism in thyroid hormone metabolism during starvation, serum thyrotropin (TSH), thyroxine (T4), and 3,5,3'-triiodothyronine (T3) levels and conversion of T4 to T3 in perfused liver were investigated in fasting (60 h) and fed rats. Propranolol (0.5 mg/kg), yohimbine (0.3 mg/kg) or phentolamine (5.0 mg/kg) was subcutaneously injected to the rat every 12 h. Serum levels of TSH, T4, and T3 were significantly lower in fasting rats than in fed rats. Although propranolol, yohimbine, and phentolamine administration did not significantly alter circulating TSH, T4 and T3 levels in fed rats, phentolamine partly inhibited the starvation-induced reduction in circulating TSH, T4, and T3. Thyroxine uptake and T3 production in perfused liver were significantly lower in fasting rats than in fed rats. Phentolamine treatment did not alter the T4 uptake and T3 production in perfused liver of fasting rats. These results suggest that alpha-adrenergic mechanism may have some role in starvation-induced reduction in circulating T4 and T3, and that phentolamine partly inhibited this phenomenon probably through the inhibitory effect on reduction in circulating TSH during starvation.

Glycerol metabolism in humans: validation of 2H- and 13C-labelled tracers

For a tracer to be valid it must follow the metabolism of the tracee without distortion. Especially when the tracer contains several deuterium substitutions, the tracer can be altered or degraded differently from the metabolite it is to trace or be subject to distorting isotope effects. To determine whether 2H5-glycerol is a valid tracer for following glycerol kinetics, 2H5-glycerol and [2-13C]glycerol tracers were infused simultaneously in six healthy postabsorptive adult subjects. After 90 min of tracer infusion, epinephrine was also infused for 60 min to stimulate lipolysis and increase glycerol flux. Glycerol flux increased from 2.2 +/- 0.3 to 6.7 +/- 0.4 mumol/kg per minute (with the 13C tracer) and from 2.2 +/- 0.3 to 6.7 +/- 0.3 mumol/kg per minute (with the 2H tracer) when epinephrine was infused. There was no significant difference in glycerol flux measured with the 2H tracer compared to the 13C tracer either under basal or a stimulated flux condition. These results indicate that 2H5-glycerol is a valid tracer for measuring glycerol metabolism in humans.

Whole-body lipolysis and triglyceride-fatty acid cycling in cachectic patients with esophageal cancer

Whole-body lipolytic rates and the rate of triglyceride-fatty acid cycling (reesterification of fatty acids released during lipolysis) were measured with stable isotopic tracers in the basal state and during beta-adrenergic blockade with propranolol infusion in five cachectic patients with squamous cell carcinoma of the esophagus, five cachectic cancer-free, nutritionally-matched control patients, and 10 healthy volunteers. Resting energy expenditure and plasma catecholamines were normal in all three groups. The basal rate of glycerol appearance in blood in the patients with cancer (2.96 +/- 0.45 mumol.kg-1.min-1) was similar to that in the nutritionally matched controls (3.07 +/- 0.28 mumol.kg-1.min-1), but 48% greater than in the normal-weight volunteers (2.00 +/- 0.16 mumol.kg-1.min-1) (P = 0.028). The antilipolytic effect of propranolol and the rate of triglyceride-fatty acid cycling in the patients with cancer were also similar in the cachectic control group and approximately 50% greater than in the normal-weight volunteers, but the differences were not statistically significant because of the variability in the data. We conclude that the increase in lipolysis and triglyceride-fatty acid cycling in "unstressed" cachectic patients with esophageal cancer is due to alterations in their nutritional status rather than the presence of tumor itself. Increased beta-adrenergic activity may be an important contributor to the stimulation of lipolysis.

Selective reduction of alpha 2-adrenergic responsiveness in hamster adipose tissue during prolonged starvation

The influence of fasting on the dual adrenergic control of adipose tissue lipolysis was investigated in hamsters because in this species the adipocytes exhibit both beta-stimulatory and alpha 2-inhibitory adrenergic responses. In adipocytes from fed animals, the number of alpha 2-receptors (identified with [3H]clonidine and [3H]RX 821002) was greater than that of beta-receptors. As in humans, the alpha 2-adrenoceptor number was greater in adipocyte membranes from subcutaneous (inguinal and popliteal) than from internal (perirenal and epididymal) adipose tissues. Despite this difference in alpha 2-adrenoceptor number, the antilipolytic responses to the alpha 2-agonists clonidine and UK 14304 were similar in the two tissues. Food deprivation for a period of 1-6 days induced a net depletion of both adipose tissues. In 6-day starved animals the number of adipocyte alpha 2-adrenoceptors and the maximal antilipolytic effect of UK 14304 were less than 50% of those in fed controls. In contrast, the antilipolytic responses to phenylisopropyladenosine or prostaglandin E1 remained unchanged. Starvation induced a decrease in alpha 2-adrenoceptor number and an increase in beta-adrenergic sensitivity that were greater in adipocytes from subcutaneous than from internal fad pads. The data suggest that the adipocyte beta- and alpha 2-adrenoceptors are independently regulated during starvation. In the adipocyte, the alpha 2-antilipolytic responses and the alpha 2-adrenoceptor levels are dependent on the extent of the adipose mass; they are particularly reduced in emaciated hamsters.

The physiology of nutritional assessment and therapy in protein-calorie malnutrition

Protein-calorie malnutrition (PCM), in the purest sense, is the result of depleted body protein stores due to semistarvation. A review of the hormonal response to simple semistarvation illustrates the elegant adaptive ability of the body to respond to an inadequate diet. By contrast, the body's metabolic response to an injury or illness stimulus is a dynamic process orchestrated by monokines and hormones. Although the injury response, strictly speaking, is not synonymous with PCM, the resultant increased energy expenditure, anorexia, and potential for skeletal muscle breakdown can result in an even more rapid depletion of body protein stores. Ultimately, the need for nutritional support depends on the amount of recent weight loss, anticipated time of insufficient oral intake, and the degree of stress. A discussion of basic concepts of anthropometry precedes examples of advantages and disadvantages of a given anthropometric parameter for selected disease states. The effects of PCM on visceral structure and function are discussed in detail so that the reader can appreciate why the metabolic response to injury may have a very different impact on the nourished compared with the malnourished patient. Particular attention is paid to the adverse effects of PCM on immune function and its antithesis, the beneficial impact of nutritional repletion on the immune system. An approach to refeeding discusses indications for initiation of nutritional support, choice of route, design of a macronutrient and micronutrient regimen, and guidelines for monitoring. Familiarity with the metabolic alterations of refeeding is key to the mitigation of potentially life-threatening complications of sudden refeeding. Appreciation of the anticipated response to nutrition is important, as the response will vary with the degree of stress. A nearly optimal response can be expected with appropriate nutrition in the nonstressed semistarved patient, whereas inefficient repletion is to be expected in the severely stressed patient. The review concludes with a discussion of the role of nutrition as a modifier of the body's metabolic response to injury.

Energy cost of fat-fuel mobilization in geriatric trauma

Age-related changes in body composition may result in varied responses to acute accidental injury. Gaining fat as age advances is common and therefore the mobilization of fat fuel resources in traumatized geriatric patients needs closer examination. We have measured in six elderly (age, 60 to 74 years) and seven young (age, 18 to 46) traumatized, hypermetabolic, and highly catabolic patients, in the "flow phase" of the metabolic response to injury, the rates of whole-body lipolysis and net fat oxidation. This enabled us to calculate the rate of triglyceride/free fatty acid (TG/FFA) cycling in the whole body and to assess its contribution to energy expenditure. Energy metabolism in general and the fat metabolism in particular were found to be somewhat slowed in elderly trauma patients compared with equally injured young individuals, although the aged patients had more total body fat. The energy cost of TG/FFA cycling is significantly (P less than .025) lower in elderly trauma victims (0.28 +/- 0.06 kcal/kg/d) compared with young patients (0.63 +/- 0.1 kcal/kg/d). This can account for approximately 3% to 4% of the elevation in metabolic rate over that predicted in the uninjured state.

Effect of short- and long-term beta-adrenergic blockade on lipolysis during fasting in humans

Stable isotope tracers and indirect calorimetry were used to evaluate the importance of beta-adrenergic stimulation of lipolysis and triglyceride-fatty acid cycling during fasting in healthy human volunteers. Each subject was studied after 12 and 84 h of fasting both with and without propranolol infusion (protocol 1) and when oral propranolol treatment was given throughout fasting (protocol 2). In protocol 1, the rates of appearance of glycerol and palmitic acid increased from 3.04 +/- 0.19 and 1.78 +/- 0.17 mumol.kg lean body mass-1.min-1, respectively, after 12 h of fasting to 5.28 +/- 0.31 and 3.47 +/- 0.15 mumol.kg lean body mass-1.min-1, respectively, after 84 h of fasting (P less than 0.005). The rate of triglyceride-fatty acid cycling increased from 97 +/- 8 to 169 +/- 5 mumol/min (P less than 0.005). Intravenous propranolol infusion decreased the rate of lipolysis after both 12 and 84 h of fasting, but the magnitude of the antilipolytic effect was much greater after 84 h (P less than 0.005). In protocol 2, the rate of lipolysis and triglyceride-fatty acid cycling was still increased by fasting despite beta-adrenergic blockade with oral propranolol. This study demonstrates that beta-adrenergic stimulation contributes to the mobilization of fat during fasting. However, other mechanism(s) can increase lipolysis and triglyceride-fatty acid cycling when beta-adrenergic receptors are continuously blocked.

Hypoglycemia: a pathophysiologic approach

An exploration of the factors that sustain glucose levels in the normal fasting subject reveals that the single major component is conservation of glucose rather than gluconeogenesis. Conservation is achieved by recycling of glucose carbon as lactate, pyruvate and alanine, and a profound decrease in the oxidation of glucose by the brain brought about by the provision and use of ketones. What glucose continues to be oxidized is for the most part formed from glycerol. Gluconeogenesis from protein plays little part in the process. Fasting hypoglycemia results from disorders affecting either one of the two critical sustaining factors--the recycling process or the availability and use of ketones. Individual hypoglycemic entities are examined against this background.

Effect of carbohydrate refeeding on free fatty acids after a fast in obese diabetic and obese non-diabetic females

The metabolic effects of refeeding with oral or intravenous carbohydrate were studied in obese women after ten or 14 days of fasting. Seven patients were refed with protein-free fruit juice for a total of 250 g of carbohydrate (1,000 kcal) over ten hours. The juice was sipped continuously throughout this time, causing a drop in free fatty acids (FFA) from 1.07 +/- 0.08 to 0.61 +/- 0.05 mmol/L (P less than .01) over the first four hours. Over the next four hours, despite continuous ingestion of the carbohydrate and elevated plasma glucose (132 +/- 9 mg/dL) and insulin (2.81 +/- 0.86 ng/mL) (1 ng/mL = 25 microU/mL), FFA rose to 0.99 mmol/L (P less than .01). Similar results were obtained in five patients refed with similar amounts of oral glucose and four patients who received an equivalent amount of glucose intravenously (IV). Refeeding with carbohydrate of obese diabetic and non-diabetic women after a two-week fast caused an abrupt decrease in FFA that was followed after four hours by an increase in FFA and glycerol, despite continued ingestion of carbohydrate glucose and insulin.

Differential catecholamine responses to dietary intake: effects of macronutrients on dopamine and epinephrine excretion in the rat

Sympathetic nervous system (SNS) function responds to changes in diet in animals and humans; whether alterations in peripheral dopaminergic activity or in adrenal medullary secretion also occur with dietary manipulation is unclear. The present studies in rats demonstrate that casein supplementation of a lab chow diet raised urinary excretion of dopamine (DA) and epinephrine (E); both sucrose and lard feeding suppressed urinary DA, though only lard appeared to exert any effect on E excretion (reduction). Addition of tyrosine to the chow diet in an amount equivalent to the tyrosine content of casein increased DA output comparably to that seen in casein-fed rats, but did not reproduce the effects of casein on E excretion. Oral administration of carbidopa, an inhibitor of 3,4-dihydroxyphenylalanine (DOPA) decarboxylation in kidney reduced the DA response to casein, but chemical sympathectomy, which lowered urinary norepinephrine (NE), and adrenal denervation, which diminished E excretion, did not. Thus, the patterns of response of the peripheral dopaminergic system in kidney and of the adrenal medulla to short-term nutrient and tyrosine ingestion are distinct from those observed for the SNS and for each other, suggesting that all three peripheral catecholamine systems may be governed by separate regulatory mechanisms.

Thermogenic response to an oral glucose load in man: comparison between young and elderly subjects

To investigate the effect of age and change in body composition on the increase in energy expenditure consecutive to the ingestion of a 75-g glucose load, respiratory exchange measurements were performed on 24 subjects, 12 elderly (mean +/- SEM, 73 +/- 1 yr) and 12 young (25 +/- 1 yr). The body weight was comparable, 62 +/- 2 kg in the elderly group vs 61 +/- 3 in the young, but the body fat content of the elderly group was significantly greater than that of the young (29 +/- 2% vs 19 +/- 2%, p less than 0.001). The elderly group presented a slight glucose intolerance according to the World Health Organization (WHO) criteria, with a 120-min plasma glucose of 149 +/- 9 mg/dl (p less than 0.005 vs young). The postabsorptive resting energy expenditure (REE) was 0.83 +/- 0.03 kcal/min in the elderly group vs 0.98 +/- 0.04 in the young (p less than 0.02); this decrease of 15% was mainly related to the decrease in fat free mass (FFM) in the elderly group, which averaged 14%. The difference was not significant when REE was expressed per kg FFM. The glucose-induced thermogenesis (GIT) expressed as percent of energy content of the load was 6.2 +/- 0.6% in the elderly group and 8.9 +/- 0.9% in the young (p less than 0.05). It is concluded that the glucose-induced thermogenesis is decreased in elderly subjects. However, when expressed per kg FFM, the increment in energy expenditure (EE), in response to the glucose load, is not different in elderly subjects, suggesting that the decrease of thermogenesis may be attributed to the age-related decrease in FFM.

The impact of body composition on the regulation of lipolysis during short-term fasting

Lipolytic rates were determined in normal-weight young adult (low body fat), normal-weight elderly (moderate body fat), and obese young adult (high body fat) subjects after an overnight (12-15 hr) and short-term (60-87 hr) fasting period. A simultaneous infusion of [1-13C] palmitate acid and [2H5]glycerol was used to measure the inflow of palmitate and glycerol into the bloodstream, and H2 18O dilution was used to measure body composition. The amount of body fat differed between the young adult (11 kg), elderly (23 kg) and obese (49 kg) subjects. Total lean body mass was similar in the normal-weight young adult (60 kg) and obese (63 kg) groups, but was lower in the elderly (46 kg). Lipolytic rates per unit of fat mass decreased in each group relative to the increase in the amount of body fat. Lipolytic rates, expressed as a function of lean body mass, however, were the same in all three groups. These results suggest that body composition contributes to the regulation of lipolysis during fasting. The decreased rate of lipolysis in subjects who have excess body fat may reflect the decreased need, per unit fat mass, for lipolysis to meet the energy requirements of the lean body mass.

The use of isotopic tracers in studying lipid metabolism in human subjects

We have attempted to evaluate some of the tracer methodologies involved in studying lipid metabolism in humans. The magnitude of this subject prohibits a comprehensive review of all areas. Since the major function of adipose tissue appears to be to supply the body with energy, we have particularly emphasized the approaches used to study the mobilization and oxidation of fat. The importance of these issues, as well as the increasing availability of non-radioactive tracers, suggest an optimistic future for this area of research.