Barbara A. Fielding

Fatty acid composition of adipose tissue and blood in humans and its use as a biomarker of dietary intake.

Accurate assessment of fat intake is essential to examine the relationships between diet and disease risk but the process of estimating individual intakes of fat quality by dietary assessment is difficult. Tissue and blood fatty acids, because they are mainly derived from the diet, have been used as biomarkers of dietary intake for a number of years. We review evidence from a wide variety of cross-sectional and intervention studies and summarise typical values for fatty acid composition in adipose tissue and blood lipids and changes that can be expected in response to varying dietary intake. Studies in which dietary intake was strictly controlled confirm that fatty acid biomarkers can complement dietary assessment methodologies and have the potential to be used more quantitatively. Factors affecting adipose tissue and blood lipid composition are discussed, such as the physical properties of triacylglycerol, total dietary fat intake and endogenous fatty acid synthesis. The relationship between plasma lipoprotein concentrations and total plasma fatty acid composition, and the use of fatty acid ratios as indices of enzyme activity are also addressed.

Intramuscular triglyceride and muscle insulin sensitivity: Evidence for a relationship in nondiabetic subjects

Intracellular triglyceride (TG) is an important energy source for skeletal muscle. However, recent evidence suggests that if muscle contains abnormally high TG stores its sensitivity to insulin may be reduced, and this could predispose to type II diabetes. To test this hypothesis, we measured muscle lipid content in 27 women aged 47 to 55 years (mean, 52) and related it to their glucose tolerance, insulin resistance, and muscle insulin sensitivity as measured by insulin activation of glycogen synthase, an insulin-regulated enzyme that is rate-limiting for insulin action in muscle. Both muscle TG content and intracellular lipid determined by Oil red O staining of muscle fibers were negatively associated with glycogen synthase activation (r = .43, P = .03 and r = -.47, P = .02, respectively). In addition, intracellular lipid correlated with features of the insulin resistance syndrome, including an increased waist to hip ratio (r = .47, P = .01) and fasting nonesterified fatty acids ([NEFA] r = .44, P = .04). These data demonstrate that increased muscle TG stores are associated with decreased insulin-stimulated glycogen synthase activity. Intracellular fat may underlie a major part of the insulin resistance in normal subjects, as well as type II diabetics.

Integrative physiology of human adipose tissue

Adipose tissue is now recognised as a highly active metabolic and endocrine organ. Great strides have been made in uncovering the multiple functions of the adipocyte in cellular and molecular detail, but it is essential to remember that adipose tissue normally operates as a structured whole. Its functions are regulated by multiple external influences such as autonomic nervous system activity, the rate of blood flow and the delivery of a complex mix of substrates and hormones in the plasma. Attempting to understand how all these factors converge and regulate adipose tissue function is a prime example of integrative physiology. Adipose tissue metabolism is extremely dynamic, and the supply of and removal of substrates in the blood is acutely regulated according to the nutritional state. Adipose tissue possesses the ability to a very large extent to modulate its own metabolic activities, including differentiation of new adipocytes and production of blood vessels as necessary to accommodate increasing fat stores. At the same time, adipocytes signal to other tissues to regulate their energy metabolism in accordance with the bodys nutritional state. Ultimately adipocyte fat stores have to match the bodys overall surplus or deficit of energy. This implies the existence of one (or more) signal(s) to the adipose tissue that reflects the bodys energy status, and points once again to the need for an integrative view of adipose tissue function.

Substituting dietary saturated fat with polyunsaturated fat changes abdominal fat distribution and improves insulin sensitivity.

Abstract.Aims/hypothesis: British dietary recommendations are to decrease total fat intake to less than 30 % of daily energy intake and saturated fat to less than 10 %. In practice, it is difficult for people to make these changes. It may be easier to encourage people to switch from a diet rich in saturated fatty acids to one rich in polyunsaturated fatty acids. Methods: A total of 17 subjects – six people with Type II (non-insulin-dependent) diabetes mellitus, six non-obese and five obese people without diabetes – were randomised to spend two 5-week periods on a diet rich in saturated or in polyunsaturated fatty acids, in a crossover design. At the start of the study and after each dietary period, we assessed abdominal fat distribution using magnetic resonance imaging, insulin sensitivity using hyperinsulinaemic-euglycaemic clamps and fasting lipid parameters. Results: Dietary compliance, assessed by weekly 3-day dietary records and measurement of biochemical markers, was good. Energy and fat intake appeared to be reduced on the diet rich in polyunsaturated fatty acids although body weights did not change. Insulin sensitivity and plasma low density lipoprotein cholesterol concentrations improved with the diet rich in polyunsaturated fatty acids compared with the diet rich in saturated fatty acids. There was also a decrease in abdominal subcutaneous fat area. Conclusion/interpretation: If this result is confirmed in longer-term studies, this dietary manipulation would be more readily achieved by the general population than the current recommendations and could result in considerable improvement in insulin sensitivity, reducing the risk of developing Type II diabetes. [Diabetologia (2002) 45: 369–377]

Activation of Peroxisome Proliferator–Activated Receptor (PPAR)δ Promotes Reversal of Multiple Metabolic Abnormalities, Reduces Oxidative Stress, and Increases Fatty Acid Oxidation in Moderately Obese Men

OBJECTIVE— Pharmacological use of peroxisome proliferator–activated receptor (PPAR)δ agonists and transgenic overexpression of PPARδ in mice suggest amelioration of features of the metabolic syndrome through enhanced fat oxidation in skeletal muscle. We hypothesize a similar mechanism operates in humans. RESEARCH DESIGN AND METHODS— The PPARδ agonist (10 mg o.d. GW501516), a comparator PPARα agonist (20 μg o.d. GW590735), and placebo were given in a double-blind, randomized, three-parallel group, 2-week study to six healthy moderately overweight subjects in each group. Metabolic evaluation was made before and after treatment including liver fat quantification, fasting blood samples, a 6-h meal tolerance test with stable isotope fatty acids, skeletal muscle biopsy for gene expression, and urinary isoprostanes for global oxidative stress. RESULTS— Treatment with GW501516 showed statistically significant reductions in fasting plasma triglycerides (−30%), apolipoprotein B (−26%), LDL cholesterol (−23%), and insulin (−11%), whereas HDL cholesterol was unchanged. A 20% reduction in liver fat content (P < 0.05) and 30% reduction in urinary isoprostanes (P = 0.01) were also observed. Except for a lowering of triglycerides (−30%, P < 0.05), none of these changes were observed in response to GW590735. The relative proportion of exhaled CO2 directly originating from the fat content of the meal was increased (P < 0.05) in response to GW501516, and skeletal muscle expression of carnitine palmitoyl-transferase 1b (CPT1b) was also significantly increased. CONCLUSIONS— The PPARδ agonist GW501516 reverses multiple abnormalities associated with the metabolic syndrome without increasing oxidative stress. The effect is probably caused by increased fat oxidation in skeletal muscle.

Specialized hepatocyte-like cells regulate Drosophila lipid metabolism

Lipid metabolism is essential for growth and generates much of the energy needed during periods of starvation. In Drosophila, fasting larvae release large quantities of lipid from the fat body but it is unclear how and where this is processed. Here we identify the oenocyte as the principal cell type accumulating lipid droplets during starvation. Tissue-specific manipulations of the Slimfast amino-acid channel, the Lsd2 fat-storage regulator and the Brummer lipase indicate that oenocytes act downstream of the fat body. In turn, oenocytes are required for depleting stored lipid from the fat body during fasting. Hence, lipid-metabolic coupling between the fat body and oenocytes is bidirectional. When food is plentiful, oenocytes have critical roles in regulating growth, development and feeding behaviour. In addition, they specifically express many different lipid-metabolizing proteins, including Cyp4g1, an ω-hydroxylase regulating triacylglycerol composition. These findings provide evidence that some lipid-processing functions of the mammalian liver are performed in insects by oenocytes.

Downregulation of Adipose Tissue Fatty Acid Trafficking in Obesity A Driver for Ectopic Fat Deposition

OBJECTIVE Lipotoxicity and ectopic fat deposition reduce insulin signaling. It is not clear whether excess fat deposition in nonadipose tissue arises from excessive fatty acid delivery from adipose tissue or from impaired adipose tissue storage of ingested fat. RESEARCH DESIGN AND METHODS To investigate this we used a whole-body integrative physiological approach with multiple and simultaneous stable-isotope fatty acid tracers to assess delivery and transport of endogenous and exogenous fatty acid in adipose tissue over a diurnal cycle in lean (n = 9) and abdominally obese men (n = 10). RESULTS Abdominally obese men had substantially (2.5-fold) greater adipose tissue mass than lean control subjects, but the rates of delivery of nonesterified fatty acids (NEFA) were downregulated, resulting in normal systemic NEFA concentrations over a 24-h period. However, adipose tissue fat storage after meals was substantially depressed in the obese men. This was especially so for chylomicron-derived fatty acids, representing the direct storage pathway for dietary fat. Adipose tissue from the obese men showed a transcriptional signature consistent with this impaired fat storage function. CONCLUSIONS Enlargement of adipose tissue mass leads to an appropriate downregulation of systemic NEFA delivery with maintained plasma NEFA concentrations. However the implicit reduction in adipose tissue fatty acid uptake goes beyond this and shows a maladaptive response with a severely impaired pathway for direct dietary fat storage. This adipose tissue response to obesity may provide the pathophysiological basis for ectopic fat deposition and lipotoxicity.

Lipoprotein lipase and the disposition of dietary fatty acids

Lipoprotein lipase (EC 3.1.1.34; LPL) is a key enzyme regulating the disposal of lipid fuels in the body. It is expressed in a number of peripheral tissues including adipose tissue, skeletal and cardiac muscle and mammary gland. Its role is to hydrolyse triacylglycerol (TG) circulating in the TG-rich lipoprotein particles in order to deliver fatty acids to the tissue. It appears to act preferentially on chylomicron-TG, and therefore may play a particularly important role in regulating the disposition of dietary fatty acids. LPL activity is regulated according to nutritional state in a tissue-specific manner according to the needs of the tissue for fatty acids. For instance, it is highly active in lactating mammary gland; in white adipose tissue it is activated in the fed state and suppressed during fasting, whereas the reverse is true in muscle. Such observations have led to the view of LPL as a metabolic gatekeeper, especially for dietary fatty acids. However, closer inspection of its action in white adipose tissue reveals that this picture is only partially true. Normal fat deposition in adipose tissue can occur in the complete absence of LPL, and conversely, if LPL activity is increased by pharmacological means, increased fat storage does not necessarily follow. LPL appears to act as one member of a series of metabolic steps which are regulated in a highly coordinated manner. In white adipose tissue, it is clear that there is a major locus of control of fatty acid disposition downstream from LPL. This involves regulation of the pathway of fatty acid uptake and esterification, and appears to be regulated by a number of factors including insulin, acylation-stimulating protein and possibly leptin.

Preferential Uptake of Dietary Fatty Acids in Adipose Tissue and Muscle in the Postprandial Period

Despite consistent evidence that abnormalities of fatty acid delivery and storage underlie the metabolic defects of insulin resistance, physiological pathways by which fat is stored in adipose tissue and skeletal muscle are not clear. We used a combination of stable isotope labeling and arteriovenous difference measurements to elucidate pathways of postprandial fat deposition in adipose tissue and skeletal muscle in healthy humans. A test meal containing [U-13C]palmitate was combined with intravenous infusion of [2H2]palmitate to label plasma fatty acids and VLDL-triglyceride. Both dietary (chylomicron) and VLDL-triglyceride were cleared across adipose tissue and muscle, though with greater fractional extraction of the chylomicron-triglyceride. In adipose tissue there was significant uptake of plasma nonesterified fatty acids (NEFAs) in the postprandial but not the fasting state. However, this was minor in comparison with chylomicron-triglyceride fatty acids. We modeled the fate of fatty acids released by lipoprotein lipase (LPL). There was clear preferential uptake of these fatty acids compared with plasma NEFAs. In muscle, there was unexpected evidence for release of LPL-derived fatty acids into the plasma. With this integrative physiological approach, we have revealed hidden complexities in pathways of fatty acid uptake in adipose tissue and skeletal muscle.

Age of appearance of circadian rhythm in salivary cortisol values in infancy.

Samples of saliva (4 in 24 hours), collected at monthly intervals for the first 6 months of life in 8 term infants by their mothers, were analysed for cortisol by radioimmunoassay. Values in the first month were more variable, daily mean values were greater, and amplitudes of variation were greater than in subsequent months. The circadian rhythm appeared by the third month.