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

Time-course atherogenic blood lipid response to statin discontinuation in dyslipidemic adults

BACKGROUND AND AIMS

Half of dyslipidemic patients sometimes discontinue statin medication. It is unclear if blood atherogenic risk increases right after statin discontinuation or if there is a lingering protective effect. We sought to determine if a legacy effect prevented blood lipid increases during the first stages of statin cessation.

METHODS AND RESULTS

Atherogenic blood lipid profile was measured in 10 overweight (BMI 31 ± 3 kg m-2) middle-aged males (62 ± 7 years old), statin users, while fasted and postprandially. Trials were conducted before (i.e., Day 0) and after 4, 7, 15, and 30 days of statin withdrawal and 20 days after statins reloading (Day 50). Four days after statin discontinuation, blood fasting LDL-c, total cholesterol (CHOL), and triglyceride (TG) concentrations increased by 30%, 18%, and 17%, respectively (P < 0.05). The increases in LDL-c, CHOL, and TG peaked after 7-15 days at 79%, 48%, and 34% of basal levels (P < 0.001), respectively. There were no significant correlations between the increases in blood lipids and the dose or years under statin treatment (P = 0.156-0.575). Twenty days after resuming statins, blood LDL-c (2.79 ± 1.06 vs 2.20 ± 0.50 mmol L-1; P = 0.568), CHOL (4.85 ± 1.41 vs 4.25 ± 0.83 mmol L-1; P = 0.747), and TG (1.47 ± 0.60 vs 1.50 ± 0.68 mmol L-1; P = 0.782), returned to basal levels.

CONCLUSIONS

Our data does not support a statin lingering/legacy effect in blood lipids since they dangerously increased after only 4 days of statin withdrawal in every patient, regardless of dose and years under treatment. Reloading statins restored blood lipids, evidencing a reproducible biological effect at the whole-body level.

Energy metabolism of white adipose tissue and insulin resistance in humans

BACKGROUND

Insulin resistance not only occurs in obesity, but also in lipodystrophy. Although adipose tissue mass affects metabolic fluxes and participates in interorgan crosstalk, the role of energy metabolism within white adipose tissue for insulin resistance is less clear.

MATERIALS AND METHODS

A Medline search identified in vivo studies in humans on energy and lipid metabolism in subcutaneous (SAT) and visceral adipose tissue (VAT). Studies in adipocyte cultures and transgenic animal models were included for the better understanding of the link between abnormal energy metabolism in adipose tissue and insulin resistance.

RESULTS

The current literature indicates that higher lipolysis and lower lipogenesis in VAT compared to SAT enhance portal delivery of lipid metabolites (glycerol and fatty acids) to the liver. Thus, the lower lipolysis and higher lipogenesis in SAT favour storage of excess lipids and allow for protection of insulin-sensitive tissues from lipotoxic effects. In insulin-resistant humans, enhanced lipolysis and impaired lipogenesis in adipose tissue lead to release of cytokines and lipid metabolites, ultimately promoting insulin resistance. Adipose tissue of insulin-resistant humans also displays lower expression of proteins involved in mitochondrial function. In turn, this leads to lower availability of mitochondria-derived energy sources for lipogenesis in adipose tissue.

CONCLUSIONS

Abnormal mitochondrial function in human white adipose tissue likely contributes to the secretion of lipid metabolites and lactate, which are linked to insulin resistance in peripheral tissues. However, the relevance of adipose tissue energy metabolism for the regulation of human insulin sensitivity remains to be further elucidated.

Abdominal obesity and insulin resistance after an episode of acute pancreatitis

BACKGROUND

Emerging evidence indicates that individuals after an episode of acute pancreatitis (AP) are at an increased risk of developing metabolic derangements. While the link between general obesity and insulin resistance (IR) is well established, only a few studies have investigated the association between abdominal obesity and IR. The aim of this study was to investigate the associations between abdominal obesity and several indices of IR in individuals after an episode of AP.

METHODS

Patients were eligible for this cross-sectional study if they were previously admitted with a primary diagnosis of AP based on the recent international guidelines. Fasting venous bloods were collected to measure glucose, insulin, free fatty acids, glycerol, adiponectin (AD), omentin (OM), and vaspin (VAS). The IR indices - HOMA-IR, Adipo-IR, insulin*glycerol (IG) index, HOMA-AD, HOMA-OM, and HOMA-VAS were calculated. Modified Poisson regression was conducted, with statistical model adjusting for patient-, metabolic-, and pancreatitis-related risk factors. Areas under ROC curve were calculated and Bland-Altman plots were created.

RESULTS

Of the 92 individuals recruited, 41 had abdominal obesity. HOMA-IR, IG index, HOMA-OM, and HOMA-VAS were significantly associated with abdominal obesity, both in unadjusted and adjusted models. Area under ROC curves for HOMA-IR, IG index, HOMA-OM, and HOMA-VAS were 0.698, 0.695, 0.756, and 0.735, respectively. There was a good agreement between observed HOMA-IR values and values obtained from HOMA-OM (P = 0.733) and HOMA-VAS (P = 0.595).

CONCLUSION

Individuals with abdominal obesity after AP have a significantly higher IR, independent of diabetes and other covariates. Visceral adipose tissue specific adipokines, omentin and vaspin, hold promise for future clinical investigation of tissue-specific IR.

Methods for quantifying adipose tissue insulin resistance in overweight/obese humans

BACKGROUND/OBJECTIVES

Insulin resistance of adipose tissue is an important feature of obesity-related metabolic disease. However, assessment of lipolysis in humans requires labor-intensive and expensive methods, and there is limited validation of simplified measurement methods. We aimed to validate simplified methods for the quantification of adipose tissue insulin resistance against the assessment of insulin sensitivity of lipolysis suppression during hyperinsulinemic-euglycemic clamp studies.

SUBJECTS/METHODS

We assessed the insulin-mediated suppression of lipolysis by tracer-dilution of [1,1,2,3,3-²H₅]glycerol during hyperinsulinemic-euglycemic clamp studies in 125 overweight or obese adults (85 men, 40 women; age 50±11 years; body mass index 38±7 kg m^-2). Seven indices of adipose tissue insulin resistance were validated against the reference measurement method.

RESULTS

Low-dose insulin infusion resulted in suppression of the glycerol rate of appearance ranging from 4% (most resistant) to 85% (most sensitive), indicating a good range of adipose tissue insulin sensitivity in the study population. The reference method correlated with (1) insulin-mediated suppression of plasma glycerol concentrations (r=0.960, P<0.001), (2) suppression of plasma non-esterified fatty acid (NEFA) concentrations (r=0.899, P<0.001), (3) the Adipose tissue Insulin Resistance (Adipo-IR) index (fasting plasma insulin-NEFA product; r=-0.526, P<0.001), (4) the fasting plasma insulin-glycerol product (r=-0.467, P<0.001), (5) the Adipose Tissue Insulin Resistance Index (fasting plasma insulin-basal lipolysis product; r=0.460, P<0.001), (6) the Quantitative Insulin Sensitivity Check Index (QUICKI)-NEFA index (r=0.621, P<0.001), and (7) the QUICKI-glycerol index (r=0.671, P<0.001). Bland-Altman plots showed no systematic errors for the suppression indices but proportional errors for all fasting indices. Receiver-operator characteristic curves confirmed that all indices were able to detect adipose tissue insulin resistance (area under the curve ⩾0.801, P<0.001).

CONCLUSIONS

Adipose tissue insulin sensitivity (that is, the antilipolytic action of insulin) can be reliably quantified in overweight and obese humans by simplified index methods. The sensitivity and specificity of the Adipo-IR index and the fasting plasma insulin-glycerol product, combined with their simplicity and acceptable agreement, suggest that these may be most useful in clinical practice.

Stable isotope-labeled tracers for the investigation of fatty acid and triglyceride metabolism in humans in vivo

Understanding lipid metabolism and its regulation requires information on the rates at which lipids are produced within the body, absorbed (dietary lipids) into the body, transported within the body, and utilized by various tissues. This article focuses on the use of stable isotope-labeled tracers for the quantitative evaluation of major pathways of fatty acid and triglyceride metabolism in humans in vivo. Adipose tissue lipolysis and free fatty acid appearance in plasma, fatty acid tissue uptake and oxidation, and hepatic very low-density lipoprotein triglyceride secretion are among the metabolic pathways that can be studied by using stable isotope labeled tracers, and will be discussed in detail. The methodology has been in use for many years and is constantly being refined. A variety of tracers and analytical approaches are available and can be used; knowing the advantages, assumptions, and limitations of each is essential for the planning of studies and the interpretation of data, which can provide unique insights into human lipid metabolism.

The acetate recovery factor to correct tracer-derived dietary fat oxidation in humans

When using (13)C tracer to measure plasma fat oxidation, an acetate recovery factor should be determined in every subject to correct for label sequestration. Less is known regarding the acetate recovery factor for dietary fatty acid oxidation. We compiled data from six studies to investigate the determinants of the dietary acetate recovery factor (dARF) at rest and after physical activity interventions and compared the effects of different methods of dARF calculation on both the fat oxidation and its variability. In healthy lean subjects, dARF was 50.6 +/- 5.4% dose (n = 56) with an interindividual coefficient of variation of 10.6% at rest and 9.2% after physical activity modifications. The physical activity interventions did not impact dARF, and the intraindividual coefficient of variation was 4.6%. No major anthropological or physiological determinants were detected except for resting metabolic rate, which explains 7.4% of the dARF variability. Applying an individual or an average group dARF did not affect the mean and the variability of the derived dietary lipid oxidation at rest or after physical activity interventions. Using a mean dARF for a group leads to over- or underestimation of fat oxidation of less than 10% in individual subjects. Moreover, the use of a group or individual correction did not affect the significant relationship found between fasting respiratory exchange ratio and dietary fat oxidation. These data indicate that an average dARF can be applied for longitudinal and cross-sectional studies investigating dietary lipid metabolism.

Regional fat deposition as a factor in FFA metabolism

Humans have a large variability in body fat distribution, which has tremendous implications for metabolic health. Obese individuals with an upper-body-fat distribution have increased health complications such as dyslipidemia, hypertension, insulin resistance, and type 2 diabetes in comparison with lower-body-obese individuals. Additionally, females have more body fat, a greater proportion of fat in their lower body, and much less visceral fat than do lean males at the same body mass index. The reasons for these differences in body fat distribution have not been clearly identified but could be important. Herein we review what has been learned about regional differences in triglyceride storage capacity and lipolysis as they relate to the causes and consequences of regional fat accumulation. Both sex and site differences in regional fat storage have been described. In contrast, with the exception of variations between men and women in the contribution of visceral adipose tissue to hepatic FFA delivery, most studies have failed to show important sex differences in regional lipolysis in vivo.

Fat metabolism in exercise - with special reference to training and growth hormone administration

Despite abundance of fat, exclusive dependency on fat oxidation can only sustain a metabolic rate corresponding to 50-60% of VO(2max) in humans. This puzzling finding has been subject to intense research for many years. Lately, it has gained renewed interest as a consequence of increased obesity and physical inactivity imposed by Western lifestyle. Why are humans so poor at metabolizing fat? Can fat metabolism be manipulated by exercise, training, diet and hormones? And why is fat stored in specialized adipose tissue and not just as lipid droplets inside muscle cells? In the present review, human fat metabolism is discussed in relation to how human fat metabolism is designed. Limitations in this design are explored and examples of different designs for fat metabolism from animal physiology are included to illustrate these limitations. Various means of manipulating fat metabolism are discussed with special emphasis on exercise, training, growth hormone (GH) physiology and GH administration. It is concluded that fat stores, non-esterified fatty acids (NEFAs) availability and enzymes for fat oxidation can be increased substantially. However, it is almost impossible to increase fat oxidation during endurance exercise at higher intensities. It seems that, for some reason, the human being is far from optimally designed for fat oxidation during exercise. Acute GH administration has several unexpected effects on fat and carbohydrate metabolism during aerobic exercise, and future research in this area is likely to provide valuable information with respect to GH physiology and the regulation of fat and carbohydrate metabolism during aerobic exercise.

Protein turnover, lipolysis, and endogenous hormonal secretion in critically ill children

OBJECTIVES

The catabolic state is a major contributor to morbidity and mortality of critical illness and may be related to endocrine changes. We studied whether protein and lipid turnover correlate with insulin and growth and thyroid hormone plasma levels in critically ill infants.

DESIGN

Prospective clinical study.

SETTING

Pediatric intensive care unit.

PATIENTS

Twelve critically ill children and ten age-matched controls.

MEASUREMENTS

We measured lipolysis and protein turnover by infusing albumin-bound uniformly 13C palmitic acid and 2H3-leucine for 3 hrs and 2H5-glycerol for 5 hrs to critically ill infants. Simultaneously, we measured serum growth hormones, insulin, C-peptide, thyroid-stimulating hormone, T4, T3, albumin, retinol binding protein (RBP), and prealbumin. Hormone and serum protein levels were also measured in six children when recovered from critical illness. Ten healthy age-matched children served as controls for hormone serum levels comparison.

RESULTS

Palmitic acid and glycerol turnover were 5.6 +/- 2.2 micromol/kg/min and 12.2 +/- 7.3 micromol/kg/min, respectively, whereas alpha-ketoisocaproic turnover was 4.9 +/- 2.8 micromol/kg/min. Alpha-ketoisocaproic turnover positively correlated (R = 0.7, p = .03) with duration of pediatric intensive care unit admission and with prealbumin and RBP serum levels (R = 0.9, p = .001). Insulin-like growth factor binding protein (IGFBP)-2 was significantly higher and IGFBP-3 was significantly lower in critically ill children (p = .03 and p = .04 vs. recovery phase, respectively). No other hormonal differences were found. Serum albumin was significantly lower in sick children. We found a significant correlation between prealbumin and RBP and IGFBP-3 (R = 0.6, p = 0.03 and R = 0.6, p = .04, respectively). Alpha-ketoisocaproic turnover positively correlated with IGFBP-1 (R = 0.79, p = .01) and did not correlate with insulin-like growth factor I (R = -0.5, p = .15 [not significant]) No other correlations were found. Lipid turnover measurements did not correlate with any endogenous hormone levels or with duration of critical illness.

CONCLUSION

Protein turnover but not lipolysis correlated with a persisting critically ill condition, serum prealbumin, RBP, and plasma IGFBP-1.

Stable isotope approaches, applications, and issues related to polyunsaturated fatty acid metabolism studies

The use of stable isotope tracers for investigating fatty acid metabolism in human subjects has increased substantially over the last decade. Advances in analytical instrumentation, commercial availability of labeled substrates, and safety considerations are major reasons for this increased use of stable isotope tracers. Several experimental design options are available for using either deuterium or carbon-13 as tracers for fatty acid and lipid studies. Options include feeding a pulse dose of labeled fat or a mixture containing two or more labeled fats. Multiple doses of the labeled fat can be fed at timed intervals to increase enrichments. Administration by injection or continuous intravenous infusion is an alternative. Another option is to use diets containing foods from plants that have slightly higher natural carbon-13 enrichment. Each basic experimental design has its specific strengths, and the best choice of experimental design depends on the study objectives. Stable isotope studies have been used to address a variety of questions related to unsaturated fatty acid metabolism in humans. Examples are provided that illustrate the use of stable isotopes to investigate oxidation of docosahexaenoic acid, desaturation of linoleic and linolenic acids in infants and adults, incorporation of long-chain n-6 and n-3 fatty acids, bioequivalency of linolenic acid in primates, 13C nuclear magnetic resonance spectra of arachidonic acid in living rat brain, and effect of triacylglycerol structure on absorption. Radioisotope and stable isotope tracer studies in animals and humans are responsible for much of our understanding of fatty acid and lipid metabolism. However, tracer studies have limitations, and there are some unresolved issues associated with isotope studies. Examples of unresolved issues are quantification of isotope data, validity of in vivo fatty acid metabolite results, kinetic modeling, subject variability, and use of blood lipid data as a reflection of tissue lipid metabolism. Resolving these issues, developing novel methodology, and applying stable isotope tracer methods to questions related to PUFA metabolism are broad areas of interesting and challenging research opportunities.

Validation of deuterium labeled fatty acids for the measurement of dietary fat oxidation: a method for measuring fat-oxidation in free-living subjects

BACKGROUND

Fatty acid oxidation has frequently been measured using (13)C or (14)C labeled fatty acids. While providing valuable data, the breath test method is hindered by the need for a controlled environment to measure VCO(2) and collect frequent breath samples. Additionally, the CO(2) breath tests require the use of (13)C- or (14)C-acetate to correct for isotope exchange in the TCA cycle. We validated d(31)-palmitic acid for measuring dietary fat oxidation. When oxidized, the deuterium appears as water and mixes with the body water pool providing a cumulative record of fat oxidation.

METHODS

The recovery of deuterium from d(31)-palmitic acid at 10 h post-dose was compared to that of (13)CO(2) from [1-(13)C]-palmitic acid in nine subjects (body mass index (BMI)=23.6+/-2.8; percentage body fat (%BF)=22.6+/-5.3; mean+/-s.d.). Subjects were studied at rest. [1-(13)C]-acetate (2 mg/kg) was dosed in a liquid breakfast. On a second day, [1-(13)C]-palmitic acid (10 mg/kg) and d(31)-palmitic acid (15 mg/kg) were dosed with the same liquid breakfast.

RESULTS

Recovery of (13)CO(2) from [1-(13)C]-acetate at 10 h post-dose was 53.7+/-10.4%. Recovery of d(31)-palmitic acid was 13.2+/-7.7% (mean+/-s.d.) and [1-(13)C]-palmitic acid recovery was 6.4+/-3.6%. When the (13)C data was corrected for [1-(13)C]-acetate (Na salt) recovery, the mean difference in percentage recovery between the two tracers was 0.5+/-2.8% and cumulative recoveries through 10 h post-dose were highly correlated (y=1.045x - 0.47; r(2)=0.88, P<0.0002). Our data shows both labels to be equivalent in their ability to measure dietary fat oxidation in resting subjects.

CONCLUSIONS

The use of deuterium labeled palmitic acid eliminates the need for rigid control over the subjects' environment. Frequent sampling and measurement of VCO(2) are not needed for accurate calculation of percentage recovery of the deuterium label. In addition, the deuterium label has a decreased potential for isotopic exchange compared to (13)C or (14)C, so a recovery correction factor is probably not required.

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.

Nutritional management in cystic fibrosis--an alternative perspective in gastrointestinal function

The gastrointestinal problems in cystic fibrosis (CF) may limit energy and nutrient availability and also cause symptoms such as abdominal pain and disturbed bowel habit which may further suppress appetite or alter the diet. Taken together this may lead to an inadequate supply of energy and nutrients to meet the nutritional requirements of the individual resulting in restricted growth or weight loss. A failure to optimize the digestive and absorptive capacity of the gastrointestinal tract places greater emphasis upon nutritional management by food intake alone. Practitioners need to focus more on gastrointestinal dysfunction in CF and its impact upon food intake in order to improve the efficacy of nutritional management. Refined stable isotopic tracers allow further exploration of the pathophysiology of the gastrointestinal tract in terms of nutrient availability. In clinical practice, a closer assessment of gastrointestinal function is supported by the use of simple, noninvasive tools which, both objectively and systematically, characterize those patients who have problems.

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.

Aging, fat oxidation and exercise

Aging is characterized by deleterious changes in body composition and in fat distribution. The mechanisms that determine the aging-associated changes in body composition are not well defined, but the evidence suggests that the loss of fat-free mass is at least partially attributable to physical inactivity. The increase in fat mass may be the result of alterations in fatty acid metabolism. Indeed, fat oxidation is decreased in elderly individuals in several physiological conditions: a) at rest, b) during exercise, and c) in response to meal ingestion (after weight loss). These defects are related in part to loss of fat-free mass, but may also be the consequence of estrogen loss (in women) and/or a decrease in the intrinsic capacity of muscle for fat oxidation, and are amenable to partial correction by exercise training. Special emphasis should be placed in future studies upon the role of steroid hormone in the regulation of fatty acid metabolism in elderly individuals (especially women), as well as therapeutic interventions that may increase the quantity of the fat-free mass and/or fat oxidation.