palmitic acid - Nature

European Journal of Clinical Nutrition (1998) 52, 22±28 ß 1998 Stockton Press. All rights reserved 0954±3007/98 $12.00

The effect of age and gender on the metabolic disposal of [1-13C]palmitic acid AE Jones, JL Murphy, M Stolinski and SA Wootton Institute of Human Nutrition, University of Southampton, Southampton

Objective: To examine the effect of age and gender on the metabolic disposal of [1-13C] palmitic acid. Design: Cross-sectional. Setting: Clinical Nutrition and Metabolism Unit at Southampton General Hospital, Institute of Human Nutrition, University of Southampton. Subjects and measurements: Twelve children (5 boys and 7 girls; aged 5±10 y) and six men (BMI 23.3 2.6 kg=m2; aged 20±30 y) were recruited. Following oral administration of a bolus dose of [1-13C]palmitic acid (10 mg=kg body weight) consumed with a test meal (1667 kJ) the excretion of 13C-label was measured on breath as 13CO2 over 24 h and in stool over 5 d to account for differences in absorption of [1-13C]palmitic acid. The 13C-enrichment of samples was determined by continuous ¯ow-isotope ratio mass spectrometry. Net substrate oxidation was estimated from gaseous exchange measurements in the postabsorptive state and over 6 h postprandially. Results: The excretion of 13CO2 on breath varied between subjects both in the pattern and amount excreted over 24 h. Breath 13CO2 was not different between boys (61.0 22.4% of absorbed dose) and girls (54.2 17.9% of absorbed dose). The excretion of breath 13CO2 was less in the men (35.1 9.3% of absorbed dose; P 0.005) and that observed previously by our group in women (30.7 6.7% of absorbed dose; P 0.005) than in the children. Net fat oxidation was greater in the children in both the postabsorptive (2.43 0.78 g=h) and postprandial (11.89 3.13 g=6 h) states than in the men (0.93 g=h 1.50; P 0.016; 9.86 10.53 g=6 h; NS) and women studied previously (0.53 0.68 g=h; P 0.003; 0.03 3.21 g=6 h; P 0.001). Conclusions: Our observations that children oxidised nearly twice the amount of [1-13C] palmitic acid than adults in conjunction with greater net fat oxidation in children than adults in both the postabsorptive and postprandial states should be considered before current UK dietary recommendations for fat and saturated fats, developed for adults, are applied to growing children. For dietary recommendations to be developed further more information is required, particularly in groups of infants and the elderly, about the factors that in¯uence the postprandial handling of dietary fat. Sponsorships: This work was supported by The Ministry of Agriculture, Fisheries and Food, Scienti®c Hospital Supplies, UK Ltd. and The Wessex Medical Trust. Descriptors: metabolism; fat oxidation; palmitic acid; stable isotope; dietary recommendations

Introduction The way in which fat from the diet is handled within the body is directly related to both the levels of circulating lipids and lipoprotein pro®les and the development of atherosclerosis, insulin-resistance and obesity. Based principally on this association, current UK recommendations for dietary fat advise a reduction in both total fat intake to about 35% of dietary energy and in saturated fatty acid (SFA) intake to no more than 10% of dietary energy (Department of Health, 1994). In developing these guidelines it would appear that several issues have been overlooked. Firstly, based on measurements of gross faecal lipid losses in balance studies in adults, it is generally accepted that the apparent absorption of dietary fat is very ef®cient, with over 96% of that ingested absorbed (Wollaeger et al, 1947; Carey et al, 1983). Such an approach assumes that all dietary fatty acids are handled equally across the gastrointestinal (GI) tract and fails to take into account the Correspondence: Dr SA Wootton, Clinical Nutrition and Metabolism Unit, Mailpoint 113, Level C West Wing, Southampton General Hospital, Tremona Road, Southampton, SO16 6YD, UK. Received 16 June 1997; revised 20 August 1997; accepted 30 August 1997

contribution made by endogenous losses and bacteria to faecal lipid (Lusk, 1906). However, balance studies in children (Fernandes et al, 1962) and adults (Pinter et al, 1964) with steatorrhoea and labelling studies in rodents (Bloom et al, 1951; Kirschner & Harris, 1961; Ockner et al, 1972; Clarke et al, 1977) and one study in man (Jones et al, 1985a) have shown that fatty acid absorption is in¯uenced by chain length, degree of unsaturation and position within dietary triacylglycerols (TAG). Secondly, labelling studies in rodents (Mead et al, 1956; Lynn & Brown, 1959; Kirschner & Harris, 1961; Goransson, 1965; Dupont, 1966; Leyton et al, 1987) and man (Forsgren, 1969; Newcomer et al, 1979; Jones et al, 1985b) have shown that once absorbed the metabolic disposal of dietary fat, in terms of the rate and amount of oxidation, depends upon fatty acid chain length and degree of unsaturation. Thirdly, what is generally ignored, is the extent to which individuals within the population may handle dietary fat in different ways. This is re¯ected in the continued presentation of results as mean values as a convenient mathematical description of a response by a group, which may mask important variations of biological interest. In accepting that the metabolic and physiological capabilities for handling dietary fat is the

Metabolic disposal of [1-13C]palmitic acid AE Jones et al

same for all individuals, current recommendations are applied to the population as a whole, irrespective of age and gender (Department of Health, 1994). In other words the same recommendations for fat and in particular for SFA are used for children from 5 y of age, men and women, the elderly and the overweight and obese. If dietary guidelines and food policy in relation to fat and SFA are to be developed further a better understanding of the factors that determine the capability of a given individual to handle dietary fat is required. We have previously employed innovative stable isotope tracer methodologies to examine both the GI handling and metabolic disposal of [1-13C]palmitic acid in healthy women (Murphy et al, 1995)a. The present study extended these observations to healthy children and men. Palmitic acid was chosen as it is the most predominant SFA in the UK diet present in the form of mixed TAG (Gregory et al, 1990). Combining the results of these studies has enabled us to determine the extent to which the metabolic disposal of [1-13C] palmitic acid maybe in¯uenced by age and gender. Methods Subjects The subjects were twelve healthy children (5 boys and 7 girls) aged 5±10 y and six healthy, normal-weight men (BMI 23.3 2.6 kg=m2) aged 21±30 y. The adult subjects were members of staff at the University of Southampton and the children were recruited from local schools. Informed consent was obtained from the adult subjects and from the parents of the children. The study was approved by the Ethical Committee of Southampton and South West Hampshire Health Commission. Study protocol The same protocol was used as described previously in women (Murphy et al, 1995)a. After an overnight fast subjects consumed [1-13C]palmitic acid at a dose of 10 mg=kg body weight (99 atom % excess; Masstrace, Woburn, USA) as part of a controlled standard test meal (natural 13C abundance 7 25.5% Table 1). Breath samples were collected before test meal consumption to provide a measure of baseline 13C excretion on breath, and were then collected at hourly intervals for 10 h, and again at 15 and 24 h. Whole body breath CO2 excretion was measured by indirect calorimetry (Deltatrac, Datex, Helsinki, Finland) at the same time points as breath sampling. Subjects were rested for the duration of the test and were given constant supervision at the Clinical Nutrition and Metabolism Unit at Southampton General Hospital. No additional food or liquids, except for bottled mineral water, were permitted until 6 h after consumption of the test meal in the children and until 10 h in the men which was the same as the previous study in women. The subjects were instructed to avoid consuming foods naturally enriched in 13C (for example corn or maize products, cane sugar) for at least 2 d prior to and during the study period. A weighed food intake was recorded over a 5 d period. The food records were coded according to the McCance and Widdowson food tables (Holland et al, 1989) and analysed using a computerised food composition database (Compeat; Lifeline Nutritional Services Ltd., London, UK).

Table 1 Fatty acid composition of the test meal (120 g white bread, 10 g butter, 100 g orange juice; 1667 kJ; 24% energy from fat, 11% energy from protein; 65% energy from carbohydrate) Fatty acid C4:0 C6:0 C8:0 C10:0 C12:0 C14:0 C16:0 C18:0 C14:1 C16:1 C18:1 (cis) C18:1 (trans) C18:2 C18:3

Amount (g) 0.25 0.16 0.09 0.22 0.27 0.90 2.49 1.04 0.07 0.14 1.91 0.35 0.82 0.09

Fatty acid composition analysed using Foodbase (Institute of Brain Chemistry and Human Nutrition, London, UK).

Measurement of 13C-enrichment on breath Enrichment of 13C on breath as 13CO2 was analysed by continuous ¯ow-isotope ratio mass spectrometry (CFIRMS) (20-20 stable isotope analyser, Europa Scienti®c Ltd., Crewe, UK). The enrichment of 13C in each sample was calculated from the increase in the ratio of 13CO2 to 12 CO2. This sample ratio was compared with that obtained from a working reference standard, being 5% CO2. The results were then expressed as the `del per mil' (parts per thousand) difference between 13CO2=12CO2 ratios of the samples and the reference standard Pee Dee Belemnite (PDB) as de®ned by Craig, 1957 (13CPDB, %). The proportion of 13C-label excreted on breath as 13CO2 was expressed as a percent of administered 13C-label per hour and as the cumulative percentage dose excreted over 24 h. Correction for excretion of 13C-label in stool It is more appropriate to express the excretion of 13CO2 on breath as a proportion of absorbed dose rather than as administered dose as this accounts for any differences in the GI handling of [1-13C]palmitic acid between individuals. To achieve this the excretion of 13C-label in stool must be determined and used as a correction factor for breath 13CO2 excretion. The methodology for collecting and processing of stools has been described previously (Murphy et al, 1995).a A baseline stool sample was collected on the day before the breath test study period to measure baseline 13C excretion. Thereafter, all stools passed were collected for up to 5 d following administration of the label. The 13C-enrichment of stool samples was analysed by CF-IRMS and the proportion of administered 13 C-label in stool was calculated according to the formula presented by Schoeller et al, 1981. In addition the nature of the 13C-label in stool was determined by gas chromatography-isotope ratio mass spectrometry (GC-IRMS) (Orchid Gas Chromatograph Interface Module and 20-20 stable isotope analyser, Europa Scienti®c Ltd., Crewe, UK) as described previously (Stolinski et al, 1997). Lipids were extracted from baseline and 13C-enriched stools by modi®cation of the method of Folch et al, 1957, saponi®ed, methylated, evaporated to dryness and taken up in hexane for analysis by GC-IRMS. The excretion of 13C-label in stool was 18.5 7.8% of administered dose in the children

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(Murphy et al, 1997), 14.3 9.8% of administered dose in the women (Murphy et al, 1995) and 36.4 9.9% of administered dose in the men. The 13C-label within all stool samples was characterised as 13C-palmitic acid, namely as the species that was consumed (Murphy et al, 1997). Substrate oxidation Net substrate oxidation was estimated from gaseous exchange measurements made by indirect calorimetry (Frayn, 1983). Net substrate oxidation was estimated in the postabsorptive state using indirect calorimetry measurements before test meal consumption and over 6 h postprandially using hourly indirect calorimetry measurements. An estimate of protein oxidation was assumed based on measurements of the protein intake of individual subjects as inaccuracies in assumed nitrogen excretion of up to 30% would have no signi®cant effect on fat and carbohydrate oxidation values. Presentation of results and statistical analysis The results are reported as means s.d. in the text with individual data represented graphically to illustrate interindividual variability. Statistical comparisons between the data were performed using ANOVA with subsequent Tukey's post-hoc tests. Differences between means were considered statistically signi®cant where P < 0.05.

Results Breath 13CO2 excretion The time course and pattern of 13CO2 excretion on breath over the 24 h study period varied between subjects. In general the pattern of breath 13CO2 showed a single-peaked curve with peak values occurring between 3 and 9 h after label administration. However, in six of the twelve children the pattern of breath 13CO2 excretion differed exhibiting a double-peaked curve (Figure 1). In the remaining children the pattern of 13CO2 excretion was similar to that observed in men. In all subjects excretion of 13CO2 on breath by 24 h was negligible with 13C-enrichment values similar to baseline values (ranging from 727% to 724%).

The total excretion of 13CO2 on breath over 24 h differed between the groups, but also showed large variation within each group. This was particularly notable in the children where an almost three fold range in breath 13CO2 excretion was observed. There was no difference in total breath 13CO2 excretion between boys (48.1 18.7% of administered dose) and girls (44.7 14.8% of administered dose; NS). Compared to the children breath 13CO2 excretion was much lower in the men (22.2 6.7% of administered dose; P 0.001), which was similar to that observed previously in women (26.5 7.8% of administered dose; NS). Excretion of absorbed 13C on breath as 13CO2 The absorption of 13C-label was determined from the difference between the amount of label administered and that excreted in stool. The proportion of label excreted on breath as 13CO2 was then expressed as a proportion of label that was absorbed. Correcting the excretion of 13C-label on breath for differences in absorption did not markedly alter the variability observed between subjects or alter the differences between groups (Figure 2). When expressed as a proportion of absorbed dose rather than administered dose breath 13CO2 excretion remained greater in the children (57.0 19.3% of absorbed dose) than in the men (35.1 9.3 % of absorbed dose; P 0.005) and than that observed previously in women (30.7 6.7% of absorbed dose; P 0.005). The difference in breath 13CO2 excretion between children and men was observed from 8 h following label administration (27.6 7.9% of absorbed dose vs 19.4 6.7% of absorbed dose; P 0.04) whilst the difference between children and women was observed from 3 h (5.4 3.4% of absorbed dose vs 2.2 1.0% absorbed dose; P 0.04). Substrate oxidation Net substrate oxidation, which is the sum of endogenous and exogenous oxidation and synthesis, was estimated in the postabsorptive state and over 6 h postprandially (Table 2). The pattern of net fat and carbohydrate oxidation from the postabsorptive state over the postprandial period is shown in Figure 3. Net fat oxidation decreased following

Figure 1 Time courses for the excretion of 13C-label on breath as 13CO2 (% administered dose=hour) over 24 h following oral administration of [1-13C]palmitic acid showing a single-peaked curve in a typical man and a double-peaked curved in a typical child.


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Figure 2 Total excretion of 13 C-label on breath as 13 CO2 (% absorbed dose) over 24 h following oral administration of [1-13 C]palmitic acid. The bars represent mean values for each group and the circles individual values. The data in women is taken from Murphy et al, 1995.

Figure 3 Time courses for (a) net fat oxidation (g/h) and (b) net carbohydrate oxidation (g/h) over 6 h following consumption of a test meal. The mean value for each group is shown. Net substrate oxidation rates are estimated from gaseous exchange measurements made by indirect calorimetry (Frayn, 1983). Table 2 Net substrate oxidation rates (mean s.d.) estimated from gaseous exchange measurements made by indirect calorimetry (Frayn, 1983) in the postabsorptive and postprandial states. Fat oxidation Group

Postabsorptive (g=h)

Children Men Women

2.43 0.78 0.93 1.50* 0.53 0.68*

Carbohydrate oxidation

Postprandial (g=6 h)

Postabsorptive (g=h)

Postprandial (g=6 h)

11.89 3.13 9.86 10.53 0.03 3.21*{

5.55 1.15 15.46 3.61* 9.26 1.29*{

42.06 8.17 85.01 21.10* 73.10 1.32*

*Signi®cant difference compared to children (P < 0.05). { Signi®cant difference compared to men (P < 0.05).


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test meal consumption with the lowest rates of fat oxidation occurring at about 3 h postprandially. Fat oxidation then increased so that by 6 h postprandial fat oxidation was greater than that observed in the postabsorptive state. In the women net lipogenesis (respiratory quotient > 1.0) was indicated over the initial 3 h of the postprandial period. Net carbohydrate oxidation mirrored fat oxidation, increasing following test meal consumption with a peak oxidation rate at about 1 h postprandially and returning to postabsorptive values by 6 h. The children showed greater net fat oxidation in both the postabsorptive and postprandial states than the adults. In contrast net carbohydrate oxidation was lower in the children in both the postabsorptive and postprandial states than in the adults. Net fat oxidation was similar in the men and women in the postabsorptive state, however postprandially fat oxidation was greater in the men than the women. Net carbohydrate oxidation was greater in the men than in the women in both the postabsorptive and postprandial states. Discussion The present study examined the metabolic disposal of [1-13C]palmitic acid in healthy children and men by measuring both the total 13C-enrichment in stool and on breath as 13CO2 and the nature of the species bearing the 13C-label in stool. Together with results from our previous study in women we have been able to determine the extent to which the metabolic disposal of [1-13C]palmitic acid maybe in¯uenced by age and gender. Oxidation of [1-13C]palmitic acid was determined by measuring the excretion of 13C-label on breath as 13CO2. The time course and pattern of breath 13CO2 excretion varied between groups. Most subjects exhibited a singlepeaked curve, however half of the children showed a double-peaked curve. There was no obvious physical difference between the children who exhibited a doublepeaked curve and those who exhibited a single-peaked curve. In those subjects showing a double-peaked curve the second peak in breath 13CO2 excretion appeared to follow the consumption of a second meal and was attributable to an increase in both breath 13C-enrichment and whole body breath CO2 excretion, as opposed to simply CO2 excretion alone. This increase was not the result of consuming foods naturally enriched with 13C, as the subjects avoided these foods during the study period. The increase in 13CO2 excretion may have simply re¯ected an overall increase in metabolic rate associated with the digestion, absorption and assimilation of food. Alternatively the meal induced rise in 13CO2 excretion may re¯ect the release of chylomicrons containing [1-13C]palmitic acid which were retained temporarily within enterocytes. Similar observations in postprandial lipid metabolism have been reported previously with a biphasic plasma triacylglycerol (TAG) response (Williams et al, 1992; Zampelas et al, 1994) and the fatty acid pro®le of chylomicron-TAG re¯ecting that of a preceding meal rather than the fatty acid composition of a meal just ingested (Fielding et al, 1996). In order to compare the present observations with the results of other published studies the excretion of 13C-label on breath must ®rstly be expressed as a proportion of administered dose. In the present study the children showed the greatest levels of breath 13CO2 excretion, with no differences between boys and girls. Breath 13CO2 excretion in the men was approximately half of that

observed in children but was similar to that observed previously by our group in women. In a previous study (Watkins et al, 1982) the oxidation of [1-13C] palmitic acid over 6 h in normal children aged 2.5±8 y was reported to be approximately 7% of administered dose with no correction made for apparent absorption of 13C-label as they did not measure 13C excretion in stool. In the present study almost twice as much of the 13C-labelled palmitic acid was oxidised over the same 6 h period (approximately 13% of administered dose) in children. The reasons for the differences between the studies are unclear. Part of the difference in oxidation of [1-13C]palmitic acid may be attributed to the approach used to determine total CO2 excretion on breath. Watkins and coworkers assumed a value of 300 mmoles=h=m2 (Shreeve et al, 1976) whereas we directly measured the excretion of breath CO2 by indirect calorimetry. However it is unlikely that the value employed for CO2 excretion could explain all of the difference in oxidation of 13C-labelled palmitic acid between the studies as there are other contributory factors such as age and body composition of the subjects. It is more appropriate to correct the oxidation values for differences in absorption of the labelled substrate and report breath 13CO2 excretion as a proportion of absorbed dose. This had the effect of increasing breath 13CO2 excretion values but the differences between groups did not alter. Children still showed the greatest levels of breath 13 CO2 excretion with no differences between boys and girls whilst the men showed a tendency for greater breath 13CO2 excretion than that observed previously in women. Oxidation of [1-13C]palmitic acid was measured over a 24 h period in the present study and was not measured at any subsequent time points. It may be that further oxidation of 13 C-labelled palmitic acid occurred after the 24 h period although the 13C-enrichment of breath at 24 h was at baseline levels (within 1%) in all subjects. Furthermore it is likely that not all of the 13CO2 generated from the oxidation of [1-13C]palmitic acid would have been excreted on breath but may have been retained within body pools or excreted via urine, skin or stool. Previous studies have either presented uncorrected data (Vantrappen et al, 1989) or have employed correction factors to more accurately predict oxidation of the labelled substrate (Jones et al, 1985b; Paust et al, 1990). The values reported in the present study for oxidation of absorbed label have not been corrected for retention of 13CO2 and as such are underestimates. Preliminary results from a study administering an oral bolus dose of NaH13CO3 to a subset of subjects in the present study have shown that over the 24 h period approximately one third of the 13CO2 generated by label oxidation may not be excreted on breath (Jones, 1996). Applying this correction factor to breath 13CO2 values in the present study would result in values for breath 13CO2 excretion of approximately 87% of absorbed dose in children and 50% of absorbed dose in men compared with 47% of absorbed dose in women previously. Oxidation of [1-13C]palmitic acid in children in the present study was almost twice that observed in adults over the same time period suggesting differences in the metabolic partitioning of dietary fat between children and adults. In conjunction with [1-13C]palmitic acid oxidation an indication of overall fat oxidation can be gained from gaseous exchange measurements which estimate the sum of endogenous and exogenous, oxidation and synthesis. Net fat oxidation was greater in the children in both the postabsorptive and postprandial states than in the men


and women whilst carbohydrate oxidation was less in the children than in the adults. This indicates that a greater proportion of energy expenditure was attributable to fat oxidation and less to carbohydrate oxidation in the children than the adults both postabsorptively and following consumption of a meal. Although net substrate oxidation rates were expressed in absolute terms as an amount per unit of time, when differences in body size and body composition were accounted for by covariate analysis with lean body mass the differences in net fat oxidation between children and adults were further emphasised with an 8 fold difference between children and adults whilst carbohydrate oxidation became similar. These observations for net fat oxidation support the oxidation data for [1-13C]palmitic acid suggesting greater oxidation of dietary fat in children than adults. However what should also be considered is the extent to which [1-13C]palmitic acid oxidation re¯ects the oxidation of total palmitic acid within the test meal, all SFA within the test meal, other classes of fatty acids within the test meal and dietary fat per se. Using data for [1-13C]palmitic acid oxidation in conjunction with net fat oxidation it is possible to estimate the proportion of net fat oxidation accounted for by oxidation of palmitic acid within the test meal, all SFA within the test meal as well as total fatty acids within the test meal over the 6 h postprandial period. Assuming that [1-13C]palmitic acid oxidation re¯ects oxidation of palmitic acid within the test meal palmitic acid oxidation would account for 4% of net fat oxidation in the children and 3% in the men. If [1-13C]palmitic acid oxidation re¯ects oxidation of all SFA then SFA oxidation would account for 8% of net fat oxidation in children and 7% in men. Alternatively if [1-13C]palmitic acid oxidation re¯ects oxidation of all fatty acids then fatty acid oxidation would account for 16% of net fat oxidation in children and 13% in men. These estimates cannot be repeated for the women as net lipogenesis was indicated during the initial postprandial period resulting in negligible net fat oxidation over the 6 h postprandial period although oxidation of [1-13C]palmitic acid was observed during this period. In addition what is also not known is the extent to which the oxidation of fatty acids, other than palmitic acid, may be different in children compared with adults. In children the oxidation of SFA may be greater than in adults because other fatty acids such as linoleic acid are spared for synthetic processes. Further studies using other 13 C-labelled fatty acids administered within the same test meal are required to resolve these issues. Taken together the results for [1-13C]palmitic acid oxidation and net fat oxidation estimated from gaseous exchange measurements would indicate that children are not simply small adults but that they differ from adults in the way in which dietary fat is metabolically partitioned towards oxidation or retention with greater oxidation in both the postabsorptive and postprandial states. These observations add support to the debate about the appropriateness of applying current dietary recommendations relating to total fat and SFA developed for adults to children from 5 y of age (Lifshitz & Torim, 1996; Zlotkin, 1996). Differences in the postprandial partitioning of SFA towards either oxidation or retention may not only occur between children and adults as large variation in [1-13C]palmitic acid oxidation was also observed within each group. Other factors such as body size, preceding diet or nutritional state and physical activity may also in¯uence the extent to which SFA are partitioned towards oxidation

or retention. It may be that the amount of fat and=or fatty acid composition of the preceding diet is an important factor. Studies examining postprandial lipaemia have demonstrated that both the amount and type of dietary fat consumed in¯uence plasma TAG in both postabsorptive and postprandial states (Zampelas et al, 1994; Murphy et al, 1995).b In addition, there may be an inherent component (genetic or early life programming) to an individual's capability for handling dietary fat and in particular SFA such that the capability for oxidising dietary fat remains constant throughout life. Alternatively can we intervene through lifestyle changes such as diet and physical activity to alter oxidation of dietary fat? We would like to hypothesise that the postprandial partitioning of dietary fat is regulated by the interaction of intrinsic, metabolic and lifestyle factors. Intrinsic factors such as genetics or programming, age and gender, cannot be modi®ed and may set an individual towards being a `fat burner' or `fat storer'. Metabolic characteristics such as body size and metabolic rate can be altered over a period of time and may in¯uence the ability to oxidise fat on a relatively long term basis. Acquired lifestyle factors such as diet and physical activity will change regularly and may set the pattern of fat oxidation or retention on a short term basis. Further work, in particular longitudinal studies, are required to resolve these issues. Conclusions The present study examined the metabolic disposal of [1-13C] palmitic acid in healthy children and men. Together with observations from our previous study in women we have been able to determine the extent to which the metabolic disposal of [1-13C]palmitic acid may be in¯uenced by age and gender. Oxidation of [1-13C]palmitic acid differed between groups with the children showing almost twice the excretion of 13CO2 on breath than adults over the same time period. Whilst there no differences between boys and girls in the excretion of 13CO2 on breath the men tended to show greater levels of breath 13CO2 excretion than observed previously in women. These differences in oxidation of [1-13C]palmitic acid in conjunction with our observations of greater net fat oxidation estimated from gaseous exchange measurements in children than adults in both the postabsorptive and postprandial states demonstrate that age and possibly gender are factors which determine an individual's capability for handling dietary fat. These observations question the appropriateness of current recommendations for dietary fat and in particular for SFA which are applied to the population as a whole, including children from 5 y of age. For dietary recommendations to be developed further more information, particularly in groups of infants and the elderly, about the factors that in¯uence an individual's capability for handling dietary fat is required. AcknowledgementsÐWe thank the subjects and their families whose cooperation was essential for the completion of this study and for the assistance from Mrs A. Hounslow and Miss R. Treneer. The support of The Ministry of Agriculture, Fisheries and Food, Scienti®c Hospital Supplies UK Ltd. and The Wessex Medical Trust is gratefully acknowledged.

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