International Journal of Obesity (2000) 24, 508±513 ß 2000 Macmillan Publishers Ltd All rights reserved 0307±0565/00 $15.00 www.nature.com/ijo The postprandial rates of glycogen and lipid synthesis of lean and obese female Zucker rats

OA Obeid1, J Powell-Tuck1 and PW Emery2*

1Department of Human Nutrition, St Bartholomew's and London Hospital School of Medicine and Dentistry, London, UK; and 2Department of Nutrition and Dietetics, King's College London, London, UK

OBJECTIVE: To determine the relative rates of glycogenesis and lipogenesis following administration of a test meal in lean and obese Zucker rats. PROTOCOL: Nine-week-old lean and obese Zucker rats were fasted overnight, then tube-fed a test meal of balanced composition amounting to 16 kJ (lean rats and one group of obese rats) or 24 kJ (one group of obese rats) and containing 200 mg 1-13C glucose. Immediately after the meal the rats were injected intraperitoneally with 5 mCi of 3 H2O and killed 1 h later. METHODS: Glycogenesis was calculated from the incorporation of 3H into liver glycogen divided by the speci®c activity of plasma water. Lipogenesis was calculated similarly from the incorporation of 3H into saponi®able lipids in liver and perirenal adipose tissue. The proportion of glycogen synthesized by the indirect pathway via pyruvate was determined from the ratio of 3H labelling at positions C6 and C2 in the glycogen glucose residues. Glycogen synthesis from glucose was determined from the ratio of 13C enrichment in liver glycogen to that in plasma glucose. RESULTS: The rate of synthesis of glycogen was considerably lower in the livers of obese rats than those of lean controls, with the larger meal causing a small but signi®cant increase in glycogenesis. The proportion of glycogen synthesized via pyruvate showed a non-signi®cant increase in the obese rats, while the amount of glycogen synthesized from glucose was signi®cantly decreased. Hepatic lipogenic rates were about ®ve times higher in both groups of obese rats than the lean controls. In adipose tissue, lipogenesis per g tissue was slightly reduced in the obese rats, although there was clearly an increase in adipose tissue lipogenic activity per whole animal. The larger meal caused a greater rise in plasma glucose and insulin concentrations but did not affect lipogenic rates, although it did cause a greater suppression of lipolysis, as indicated by a lower plasma glycerol concentration. CONCLUSION: Ingested carbohydrate is partitioned predominantly into hepatic fatty acid synthesis in obese Zucker rats. Hepatic glycogen synthesis is suppressed and comes mainly from precursors other than glucose. The suppres- sion of hepatic glycogen synthesis may contribute to the increased energetic ef®ciency of obese Zucker rats. International Journal of Obesity (2000) 24, 508±513

Keywords: glycogen synthesis; fat synthesis; indirect pathway; Zucker rats; insulin

Introduction a ®xed amount of substrate. The reasons for these apparently con¯icting results may relate to the amount and composition of the substrates administered and to Ingested carbohydrate has three possible fates Ð the timing of the measurements. Thus in one study the immediate oxidation, storage as glycogen or conver- incorporation of 14C from a 14C-alanine load into sion to fat Ð and the balance between these processes hepatic glycogen was found to be higher in obese may be disturbed in obesity. Studies in genetically Zucker rats in both the fed and 24 h fasted state,2 obese Zucker rats have shown that the rates at which whereas in another the incorporation of 14C-alanine glucose, glycerol and alanine are incorporated into into hepatic glycogen was lower in obese rats 3 ± 4 h liver lipids are all greater in obese rats than in lean after a meal while the incorporation of 14C-glycerol controls.1,2 This suggests that the ¯ux of these sub- and 3H-glucose into hepatic glycogen was similar strates through the hepatic glycolytic and lipogenic between obese and lean rats.1 Hence it is necessary pathways is increased in obese rats. On the other hand to make simultaneous measurements of glycogen and hepatic glycogen content and glycogen synthetase lipid synthesis in response to a ®xed test meal in order activities have been shown to be increased in obese to determine whether the partitioning of substrates Zucker rats.3,4 between these two processes is different from normal Clearly it is not possible for there to be increased in obese Zucker rats. amounts of both glycogen and lipid synthesized from It is known that glycogen can be synthesized in the liver by two different pathways, the direct pathway (glucose?glucose-6-phosphate?glucose-1-phosphate *Correspondence: PW Emery, Department of Nutrition and Dietetics, King's College London, Franklin-Wilkins Building, ?glycogen) and the indirect pathway, which involves 150 Stamford St, London SE1 8WA, UK. the synthesis of glucose-6-phosphate via gluconeo- E-mail:[email protected] genic reactions.5,6 The precursors for the gluconeo- Received 27 March 1999; revised 18 August 1999; accepted 12 November 1999 genic reactions may be either pyruvate and lactate Glycogenesis and lipogenesis in Zucker rats OA Obeid et al 509 formed by glycolysis in peripheral tissues or in vitamin mix 12 and 2 DL-methionine. Thus the energy different zones of the liver, or amino acids, glycerol content of the diet was 16 kJ=g with 20% of energy as or fructose from the diet or from endogenous sources.6 protein, 23% as fat and 57% as carbohydrate. Since the indirect pathway is energetically less ef®- cient than the direct pathway,5 it might be expected that suppression of the indirect pathway could con- Experimental protocol Rats were maintained for 11 days on the semisynthetic tribute to the increased energetic ef®ciency of obese diet and food intake and body weight were monitored Zucker rats. However, studies have shown that gluco- every 2 ± 3 days. Obese rats were randomly divided neogenic activity in obese Zucker rats may actually be into two groups. On the eleventh day food pots were increased.1 This occurs in spite of the very high removed at 5:00 p.m. The next day (between 9:00 and insulin levels,3 indicating that the liver is resistant to 11:00 a.m.) all rats were fed a 3 ml liquid meal, the normal action of insulin in suppressing gluconeo- followed by 0.5 ml water, by gavage. For the lean rats genesis, and this may be important in the development (group L1) and seven of the obese rats (group O1) the of diabetes in these animals. meal contained 1.0 g of their normal diet, while for The rates of glycogen and lipid synthesis are known the other seven obese rats (group O1.5) the meal to change rapidly after a meal.7 We have developed a contained 1.5 g of the same diet. In all cases 200 mg protocol for simultaneously measuring the rates of of starch in the meal was replaced by 200 mg of 1-13C- hepatic glycogen and lipid synthesis in vivo in rats 1 h glucose. Immediately after the meal the rats were after the ingestion of a standard test meal, a time when injected intraperitoneally with 5mCi of 3H O in 0.3 both processes are near their maximal rates.7 The 2 ml of saline. One hour later the rats were killed by method is based on the work of Postle and Bloxham.8 decapitation and blood was collected from the neck Overall rates of glycogen and fatty acid synthesis vessels in EDTA tubes, plasma was separated and from all precursors are determined from the amount stored at 20C until analysis, and liver and adipose of 3H incorporated following injection of 3H O. The 2 tissue from around the kidneys were rapidly removed relative activities of the direct and indirect pathways and immediately frozen in liquid nitrogen then stored can be estimated from the relative enrichment of 3Hat at 20C until analysis. different positions within the glycogen glucose resi- dues, while the rate of glycogen synthesis speci®cally from glucose may be determined by giving 1-13C Analytical methods glucose and measuring its incorporation into glyco- Hepatic glycogen content, plasma glucose and gly- gen. Hence the present experiment was designed to cerol concentrations, the speci®c radioactivities of determine the rates of postprandial glycogen and lipid plasma water, hepatic glycogen, hepatic fatty acids synthesis and to investigate whether there is a differ- and adipose tissue fatty acids, and the amounts of ence in the contribution of the two pathways of radioactivity at positions C2 and C6 in the hepatic postprandial glycogen synthesis between lean and glycogen glucose residues were all measured by obese Zucker rats. Since the size of the test meal is standard methods which have been described in known to affect both the rate and the pathway of detail previously.13 glycogen synthesis,9 two groups of obese rats were 13C enrichment of plasma glucose and liver glyco- used. In one group the size of the test meal was the gen was measured as follows. Plasma (50 ml) was same as that given to the lean controls, which was deproteinized with 0.5 ml of ethanol and the super- based on the normal intake of lean rats at a single natant after centrifugation was dried under nitrogen. A meal, while in the other group the meal size was 10 ml aliquot of the extracted glycogen glucose or increased in proportion to the increase in customary enriched glucose standard was also dried under nitro- meal size observed in obese Zucker rats.10 gen. The dried samples were mixed with 100 mlof butyl boronic acid=pyridine solution (12.5 g=l), heated at 90C for 30 min and cooled to room temperature. Methods Acetic anhydride (100 ml) was added and kept for 1 h at room temperature, then samples were dried under nitrogen and reconstituted in 0.5 ml of decane. The Animal housing and diet enrichment of the glucose acetate boronate derivatives Seven-week-old female lean (Fa= , n ˆ 7) and obese was determined by gas chromatography ± electron ( fa=fa, n ˆ 14) Zucker rats were obtained from Harlan ionization ± mass spectrometry with selected ion mon- UK Ltd, Bicester, Oxon. Rats were housed individu- itoring at m=z 297 and 29814 using a HP5971A MSD ally in wire bottomed cages to facilitate collection of (Hewlett Packard, Woking, Surrey). Results were spilled food. Room temperature was maintained at calculated as atoms percentage excess (APE). 22Æ 1C with a 12:12 dark ± light cycle (light on at Plasma triglyceride concentration was measured 8:00 a.m.). The rats had free access to water and were using a GPO-PAP kit from Boehringer (BCL, fed ad libitum on a semisynthetic diet consisting of Lewes, Sussex). Plasma insulin concentration was (g=kg): 300 sucrose, 300 maize starch, 198 casein, measured using a Phadeseph Insulin RIA kit from 100 maize oil, 40 cellulose, 40 mineral mix,11 20 Pharmacia & Upjohn Diagnostics (Uppsala, Sweden).

International Journal of Obesity Glycogenesis and lipogenesis in Zucker rats OA Obeid et al 510 Calculations between each of the obese groups and the lean controls using Fisher's least signi®cant difference procedure. A probability of less than 0.05 was con- Glycogenesis. The rate of glycogenesis was calcu- 3 sidered signi®cant. lated as micromoles of H2O incorporated into glyco- gen per hour per gram liver by dividing the speci®c radioactivity of glycogen (dpm=g liver) by the speci®c radioactivity of plasma water (dpm=mmol). Results

Lipogenesis. The rate of lipogenesis was calculated Body weight and food intake (Table 1) 3 as micromoles of H2O incorporated into saponi®able The obese rats were already more than 60% heavier lipid per hour per gram tissue by dividing the speci®c than the lean controls at the beginning of the experi- radioactivity of fatty acids (dpm=g tissue) by the ment, and during the 10 day period of ad libitum speci®c radioactivity of plasma water (dpm=mmol). feeding on the semisynthetic diet each lean rat gained about 30 g while each obese rat gained more than Percentage of glycogen synthesized via pyruvate. The 70 g. This was associated with 80% greater food two hydrogen nuclei attached to the carbon at position intake in the obese rats than in the lean rats. Liver 6 in the glucose residues become labelled when weight was considerably greater in the obese than the glycogen is synthesized from pyruvate because the lean rats. There was no difference between the two two hydrogens attached to C3 of malate will have obese groups in food intake, body weight or liver become labelled. The hydrogen attached to position 2 weight. in the glucose residues becomes labelled during the glucose phosphate isomerase reaction, and this will affect glycogen synthesized by both the direct and Plasma metabolite and hormone concentrations indirect pathways equally. Thus the percentage of (Table 2) glycogen synthesized via pyruvate was estimated by Plasma glucose concentration 1 h after the test meal measuring 3H at C2 and C6 of glucose glycogen and was similar in the obese rats given 1 g of diet to that in using the following equation:15 lean rats given the same sized test meal. However increasing the size of the test meal increased the percentage glycogen synthesized via pyruvate ˆ subsequent plasma glucose concentration, as 3HatC6 100 expected. Plasma glycerol concentration in the obese 3HatC2 2 was considerably higher than that in the lean rats. The larger meal caused a decrease in plasma glycerol concentration in group 01.5 compared with group Statistics. Results were analysed by analysis of 01. This probably re¯ects greater suppression of variance, and speci®c comparisons were made lipolysis, although it may also indicate greater

Table 1 Body weight, food intake and liver weight

Lean rats (L1) Obese rats (O1) Obese rats (O1.5)

Mean s.e. Mean s.e. Mean s.e.

Number of rats 7 7 7 Body weight (g): day 1 113.1 2.8 182.4* 6.5 189.3* 8.5 day 11 144.0 4.1 258.0* 7.5 260.1* 8.9 Food intake (g=day)a 16.09 0.79 29.13* 0.84 28.30* 0.68 Liver weight (g) 4.68 0.22 10.81* 0.62 10.58* 0.74

aMean of 10 days. *Signi®cantly different from lean controls, P < 0.05.

Table 2 Plasma metabolite and hormone concentrations

Lean rats (L1) Obese rats (O1) Obese rats (O1.5)

Mean s.e. Mean s.e. Mean s.e.

Number of rats 7 7 7 Plasma glucose concentration (mmol=l) 7.79 0.33 7.73 0.46 8.99{ 0.42 Plasma glycerol concentration (mmol=l) 121 27 500* 40 379*{ 23 Plasma triacylglycerol concentration (mmol=l) 0.30 0.03 2.65* 0.15 3.19* 0.47 Plasma insulin concentration (mU=ml) 8.8 0.5 31.6* 4.1 49.0*{ 4.9

*Signi®cantly different from lean controls, P < 0.05. { Signi®cantly different from group O1, P < 0.05.

International Journal of Obesity Glycogenesis and lipogenesis in Zucker rats OA Obeid et al 511 uptake of glycerol by the liver for gluconeogenesis The synthesis of glycogen from glucose was inves- and triglyceride synthesis. Plasma triglyceride con- tigated by giving 1-13C glucose with the test meal. centration was also much greater in the obese than the The 13C enrichment of liver glycogen was signi®- lean rats, and there was no signi®cant difference cantly lower in both groups of obese rats than in the between the two obese groups. The obese rats also lean controls (Table 4). The 13C enrichment of plasma showed the expected hyperinsulinaemia, with the glucose is also shown in Table 4, and this was larger meal causing a greater rise in plasma insulin signi®cantly lower in group O1 than both the other than the smaller meal. groups. Thus the ratio of enrichment of liver glycogen to plasma glucose, which is indicative of the rate of synthesis of glycogen from glucose, was signi®cantly Hepatic glycogen synthesis lower in both obese groups than the lean controls. Table 3 shows that the overall rate of incorporation of 3H into glycogen was signi®cantly lower in both obese groups than in the lean rats. The same result is Lipogenesis (Table 5) apparent whether the data are expressed per g liver 3 or per whole liver, although in the latter case there is The rate of hepatic lipogenesis, calculated from H evidence that glycogen synthesis increased somewhat incorporation into saponi®able lipid in the liver, was in response to the larger meal. Glycogen content per g about ®ve times higher in both obese groups than in liver 1 h after the meal was also lower in the obese the lean rats. Clearly if hepatic lipogenic rates were rats than the controls, although the difference was not calculated per whole liver the difference between lean statistically signi®cant in the case of group O1.5. and obese rats would be even more marked (data not Glycogen content per whole liver was not signi®- shown). In contrast, the rate of lipogenesis in adipose cantly different between any of the groups. tissue was signi®cantly lower in both the obese groups Table 3 also shows the results of the positional than in the lean controls when the data are expressed analysis of the 3H incorporated into glycogen glucose. per g tissue. We did not measure adipose tissue mass, but it is known to be about four times greater in obese There were no signi®cant differences in the proportion 16 of glycogen synthesized via the indirect pathway from Zucker rats of this age than in their lean littermates, pyruvate. so that it is likely that adipose tissue lipogenesis per whole rat would be greater in the obese than the lean rats.

Table 3 Glycogen synthesis and content and positional analysis of 3H incorporation into liver glycogen glucose Lean rats (L1) Obese rats (O1) Obese rats (O1.5) Discussion Mean s.e. Mean s.e. Mean s.e. Number of rats 7 7 5 These results show that in the postprandial period the Hepatic glycogen synthesis 3 rate of glycogen synthesis is lower than normal in the (mmol H2O incorporated=h=g liver) 67.3 6.9 7.18* 1.44 17.2* 4.0 livers of obese Zucker rats, but the rate of fatty acid 3 (mmol H2O incorporated=h=liver) synthesis is much higher than normal. This alteration 307 22 79.6* 17.4 162*{ 33 Hepatic glycogen content: in the partitioning of ingested carbohydrate may (mg=g) 9.17 0.28 5.50* 0.64 7.05 1.21 account for their tendency to deposit more fat than (mg=liver) 42.6 1.3 60.5 9.4 68.6 11.0 lean controls when fed the same amount of food, or it 3H at C2 (%) 32.2 1.6 38.7 4.2 35.9 2.3 3H at C6 (%) 5.83 0.28 11.3 2.0 12.2 3.6 may represent a long-term adaptation to chronic Glycogen from pyruvate (%) hyperphagia. Since the response of both groups of 9.22 0.71 16.0 3.6 17.1 4.7 obese rats was broadly similar it is unlikely that *Signi®cantly different from lean controls, P < 0.05. it represents an acute response to consuming a smaller { Signi®cantly different from group O1, P < 0.05. meal than that to which they were accustomed.

Table 4 13C enrichment of plasma glucose and liver glycogen glucose

Lean rats (L1) Obese rats (O1) Obese rats (O1.5)

Mean s.e. Mean s.e. Mean s.e.

Number of rats 7 7 7 Liver glycogen glucose 13C enrichment (atoms percentage excess) 4.68 0.42 1.05* 0.26 1.67* 0.23 Plasma glucose 13C enrichment (atoms percentage excess) 35.4 1.8 23.2* 2.3 37.3{ 0.5 Ratio (liver  103=plasma) 131 9 46.1* 12.8 44.6* 5.9

*Signi®cantly different from lean controls, P < 0.05. { Signi®cantly different from group O1, P < 0.05.

International Journal of Obesity Glycogenesis and lipogenesis in Zucker rats OA Obeid et al 512 Table 5 Lipogenesis in liver and adipose tissue

Lean rats (L1) Obese rats (O1) Obese rats (O1.5)

Mean s.e. Mean s.e. Mean s.e.

Number of rats 7 7 6 3 Liver lipogenesis (mmol H2O incorporated=h=g liver) 5.68 0.39 27.8* 2.4 28.7* 2.8 3 Adipose tissue lipogenesis (mmol H2O incorporated=h=g tissue) 7.21 0.55 5.04* 0.63 4.62* 0.61

*Signi®cantly different from lean controls, P < 0.05.

It has previously been suggested that the increased signi®cant trend towards greater indirect pathway rate of lipogenesis is caused by hyperinsulinaemia, activity in the obese rats. Clearly the lower total 3H since it can be prevented by prior administration of incorporation in these rats cannot be explained by a streptozotocin.17 However in the present study hepatic shift from indirect to direct pathway activity. Rather it lipogenesis was no greater in group O1.5 than in must re¯ect a real decrease in glycogen synthesis rate. group O1 despite the difference in plasma insulin The fractional rate of glycogen synthesis from concentration between these obese groups. This may glucose was estimated from the ratio of 13C labelling have been because lipogenesis was already maximally in liver glycogen to that in plasma glucose. This stimulated, or it may represent a manifestation of the calculation assumes that the labelling of the precursor insulin resistance that obese Zucker rats are known to glucose was relatively constant through the incorpora- exhibit. tion period, and that it is adequately represented by Insulin is known to stimulate glycogen synthesis sampling peripheral blood, although it is likely that in vitro in cultured hepatocytes,18 although the same portal blood glucose enrichment would be somewhat effect is not always observed in vivo.19 Thus the low higher. Absolutes rates of glycogen synthesis from rates of glycogen synthesis in hyperinsulinaemic glucose could also be calculated by multiplying by the obese Zucker rats in the present experiment may be glycogen pool size, but since this did not differ a further manifestation of insulin resistance. Similarly, signi®cantly between the groups the conclusion leptin has also been shown to stimulate glycogen would be the same. The rate of glycogen synthesis deposition in cultured hepatocytes,20 so that the low from glucose, which represents the sum of direct rate of glycogen synthesis may be due to the defective pathway activity plus recycling of glucose to glu- response to leptin which is now believed to be the cose-6-phosphate via pyruvate and lactate, was lower main genetically determined metabolic defect in obese in the obese rats than the lean controls Since the Zucker rats. proportion of glycogen synthesized via pyruvate The rate of glycogen synthesis was estimated in the tended to be greater in the obese rats it is likely that present experiment by measuring the incorporation of the precursors for indirect pathway synthesis of gly- 3 3 H from H2O into glycogen. However the amount of cogen in the obese rats included a signi®cant con- 3H per glycogen glucose residue is also affected by tribution from non-carbohydrate substrates such as the pathway by which the glycogen has been synthe- amino acids and glycerol. Previous studies have sized, since the gluconeogenic pathway allows 3H shown increased gluconeogenesis from glycerol, incorporation at all six positions while direct synthesis though not from alanine, in obese Zucker rats.1 of glycogen from glucose only allows 3H incorpora- In a previous study using similar methodology we tion at position 2 of the glycogen glucose residues. found that increasing the size of the test meal We therefore explored the extent to which the differ- decreased the proportion of glycogen synthesized by ent pathways were operating by positional analysis. the indirect pathway in normal and tumour bearing The two hydrogen nuclei attached to the carbon at rats.9 In the present study increasing the size of the position 6 in the glucose residues become labelled meal had no effect on the proportion of glycogen when glycogen is synthesized from pyruvate because synthesized by the indirect pathway in obese Zucker the two hydrogens attached to C3 of malate will have rats. This may have been because the increased insulin become labelled. The hydrogen attached to position 2 secretion failed to suppress the gluconeogenic path- in the glucose residues becomes labelled during the way, again emphasizing the insensitivity of this path- glucose phosphate isomerase reaction, and this will way to insulin action in obese Zucker rats. affect glycogen synthesized by both the direct and The difference in glycogen synthesis rates between indirect pathways equally. Thus the ratio of labelling obese and lean rats was considerably larger than the at C6 to that at C2 (corrected by a factor of two difference in the net amount of glycogen present in the because of the two hydrogens attached to C6) indi- liver 1 h after the meal. We did not measure hepatic cates the proportion of glycogen being synthesised via glycogen content before the meal, so this may have pyruvate. The results of this experiment show a non- been greater in the obese rats than the lean controls.

International Journal of Obesity Glycogenesis and lipogenesis in Zucker rats OA Obeid et al 513 This would suggest that obese Zucker rats show much 7 Emery PW, Carpenter TTA, Obeid OA. Alterations in post- smaller oscillations in liver glycogen content during prandial glycogen and lipid synthesis in cachectic tumor- bearing rats. Nutr Cancer 1993; 20: 231 ± 240. the feeding and fasting cycle, in line with previous 8 Postle AD, Bloxham DP. The use of tritiated water to measure observations made in Zucker rats that have been absolute rates of hepatic glycogen synthesis. Biochem J 1980; fasted for up to 48 h.21,22 Obese Zucker rats may be 192: 65 ± 73. less reliant on storing and mobilizing liver glycogen 9 Obeid OA, Khayatt JA, Emery PW. The effect of meal size on to maintain blood glucose during a short-term fast, postprandial carbohydrate metabolism in normal and tumor bearing rats. Nutrition 1998; 14: 191 ± 196. perhaps because they are better adapted to mobilize 10 Becker EE, Grinker JA. Meal patterns in the genetically obese stored fat more rapidly. Alternatively the higher rate Zucker rat. Physiol Behav 1977; 18: 685 ± 692. of glycogen synthesis in lean rats may be matched by 11 Bernhart F, Tomarelli R. A salt mixture supplying the a higher rate of concurrent glycogen breakdown, National Research Council estimates of the mineral require- thereby reducing the net rate of glycogen deposition. ment of the rat. J Nutr 1966; 89: 495 ± 500. 12 Naismith DJ, Akinyanju P, Yudkin J. In¯uence of caffeine Since this futile cycle would be energetically inef®- containing beverages on the growth, food utilisation and cient a reduction in such activity may represent one plasma lipid of the rat. J Nutr 1969; 97: 375 ± 381. mechanism by which obese Zucker rats become more 13 Obeid OA, Emery PW. The effect of acute Acipimox admin- energetically ef®cient. istration on the rates of lipid and glycogen synthesis We are not aware of any measurements of hepatic in cachectic tumour bearing rats. Nutr Cancer 1997; 28: 100 ± 106. glycogen synthesis in lean and obese human subjects, 14 Wieko J, Sherman WR. Boroacetylation of carbohydrates. 23 but reduced rates of muscle glycogen synthesis and Correlations between structure and mass spectral behavior in reduced rates of hepatic glycogenolysis in the post monoacetylhexose cyclic boronic esters. J Am Chem Soc 1976; absorptive state24 have been reported in obese sub- 98: 7631 ± 7637. jects. Thus it is possible that a reduction in the 15 Kuwajima M, Golden S, Katz J, Unger RH, Foster DW, McGarry JD. Active hepatic glycogen synthesis from gluco- amounts of glycogen stored and mobilised during neogenesis precursors despite high tissue levels of fructose 2,6 the normal cycle of feeding and fasting may contri- biphosphate. J Biol Chem 1986; 261: 2632 ± 2637. bute to increased energetic ef®ciency and a tendency 16 Radcliffe JD, Webster AJF. The effect of varying the quality to lipid deposition in some obese human subjects. of dietary protein and energy on food intake and growth in the Zucker rat. Br J Nutr 1979; 41: 111 ± 124. 17 Godbole V, York DA. Lipogenesis in situ in the genetically Acknowledgements obese Zucker fatty rat ( fa=fa): role of hyperphagia and We are grateful to Professor A I Mallet, St John's hyperinsulinaemia. Diabetologia 1978; 14: 191 ± 197. 13 18 Tosh D, Beresford G, Agius L. Glycogen synthesis from Institute of Dermatology, UMDS, London for the C glucose by direct and indirect pathways in hepatocyte cultures analysis. from different nutritional states. Biochim Biophys Acta 1994; 1224: 205 ± 212. 19 Powles J, Obeid OA, Emery PW. The effect of exogenous insulin administration on postprandial glycogen synthesis in References the rat (Abstract). Proc Nutr Soc 1994; 53: 58A. 1 Terrettaz J, Jeanrenaud B. Contribution of glycerol and alanine 20 Aiston I, Agius L. Leptin enhances glycogen storage in to basal hepatic glucose production in the genetically obese hepatocytes by inhibition of phosphorylase and exerts an ( fa=fa) rat. Biochem J 1990; 270: 803 ± 807. additive effect with insulin. Diabetes 1999; 48: 15 ± 20. 2 DomeÁnech M, LoÂpez-Soriano FJ, Carbo N, ArgileÂs JM. The 21 Wade AJ. Reduced liver glycogen depletion in fasted genetic- metabolic fate of an oral 14C alanine load in the obese Zucker ally obese (Zucker) rats (Abstract). J Physiol 1979; 293: 33P. rat. Int J Obes 1992; 16: 213 ± 218. 22 Koubi H, Duchamp C, Geloen A, Freminet A, Minaire Y. 3 Lemonnier D. Hyperinsulinism in genetically obese rats. Resistance of hepatic glycogen to depletion in obese Zucker Horm Metab Res 1971; 3: 287 ± 288. rats. Can J Physiol Pharmac 1991; 69: 841 ± 845. 4 Wade AJ. Glycogen and lipid metabolism in the liver of the 23 Damsbo P, Vaag A, Hother-Nielsen O, Beck-Nielsen H. genetically obese (Zucker) rat (Abstract). J Physiol 1981; 319: Reduced glycogen synthase activity in skeletal muscle from 74 ± 75P. obese patients with and without Type 2 (non-insulin depen- 5 McGarry JD, Kuwajima M, Newgard CB, Foster DW, Katz J. dent) diabetes mellitus. Diabetologia 1991; 34: 239 ± 245. From dietary glucose to glycogen: the full circle round. Ann 24 MuÈller C, Assimacopoulos-Jeannet F, Mosimann F, Schneiter Rev Nutr 1987; 7: 51 ± 73. P, Riou JP, Pachiaudi C, Felber JP, JeÂquier E, Jeanrenaud B, 6 Radziuk J. Sources of carbon in hepatic glycogen synthesis Tappy L. Endogenous glucose production, gluconeogenesis during absorption of an oral glucose load in humans. Fed Proc and liver glycogen concentration in obese non-diabetic 1982; 41: 110 ± 116. patients. Diabetologia 1997; 40: 463 ± 468.

International Journal of Obesity