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The Journals of Gerontology Series A: Biological Sciences and Medical Sciences 55:B515-B521 (2000)
© 2000 The Gerontological Society of America

Postprandial Retinyl Palmitate and Squalene Metabolism Is Age Dependent

Heikki Relasa, Helena Gyllinga, Radhakrishnan A. Rajaratnama and Tatu A. Miettinena

a Department of Medicine, Division of Internal Medicine, University of Helsinki, Finland

Tatu A. Miettinen, Division of Internal Medicine, Department of Medicine, University of Helsinki, P.O. Box 340, FIN-00029 HYKS, Helsinki, Finland E-mail: tatu.a.miettinen{at}helsinki.fi.

Decision Editor: Jay Roberts, PhD


   Abstract
Top
 Abstract
Subjects and Methods
 Results
 Discussion
 References
 
Most of our awake time is spent in a postprandial state. It has not been investigated in detail whether the postprandial clearance of triglyceride-rich lipoproteins is age dependent. In addition, postabsorptive squalene metabolism has not been studied in relation to age. Accordingly, we investigated postprandial lipid metabolism in six young (22–25 years of age) and eight old (78–79 years of age) healthy men by use of an oral fat load containing 345,000 IU of vitamin A and 0.5 g of squalene as postprandial markers. Postprandial samples were drawn after 3, 4, 6, 9, 12 and 24 hours after the fat load. The retinyl palmitate area under the incremental curve of the old subjects was higher in plasma than that of the young subjects ( p < .01). The pattern of postprandial very low density lipoprotein squalene responses differed significantly in the old compared with the young subjects ( p < .01), but the areas under the incremental curve did not differ. Postprandial retinyl palmitate and squalene concentrations correlated significantly at 3–12 hours ( p < .01). These data suggest that postprandial lipoprotein metabolism measured by retinyl palmitate and squalene is retarded with increasing age.

INTEREST in serum lipid and lipoprotein metabolism in the nonfasting state has been intense during the past years because most of our awake time is spent in a postprandial state. In addition, delayed postprandial lipoprotein clearance has been documented to occur in many different conditions, such as type III hyperlipidemia (1), hypertriglyceridemia (2)(3) and in normolipemic apolipoprotein (apo) E 2/2 carriers (4). Retarded postprandial lipoprotein clearance has also been associated with non-insulin-dependent diabetes mellitus (5) and in three inherited conditions: familial lipoprotein lipase (LPL) deficiency, familial apo C-II deficiency, and familial inhibitor to LPL (6). In 1979, Zilversmit (7) postulated that chylomicron remnants are atherogenic. Since then, the association of retarded postprandial lipid metabolism and coronary artery disease has been found in many studies (8)(9)(10)(11)(12)(13). Whether age has a consistent effect on postprandial fat clearance is not completely clear. Cohn and colleagues (14) have shown that postprandial triglyceridemia is age related, and Krasinski and associates (15) reported a higher postprandial plasma retinyl ester concentration in old versus young subjects at 5–8 hours after the test meal. However, Eriksson and coworkers (16) found no difference in serum triglycerides between young and old subjects after duodenal infusion of cholesterol and fat mixture, and Borel and colleagues (17) found increased postprandial retinol but not retinyl palmitate areas under the concentration curves in old versus young subjects. Weintraub and coworkers (18) observed that octogenarians, in fact, had lower postprandial chylomicron remnant levels than middle-aged subjects.

Chylomicrons are synthesized in enterocytes of the small intestine for the transport of absorbed dietary triglycerides, cholesterol, and fat-soluble vitamins mainly to the liver. In the circulation, chylomicron triglycerides are hydrolyzed by LPL. As triglycerides are hydrolyzed, the size of the chylomicrons is reduced, and resulting chylomicron remnants are found mainly in the very low density lipoprotein (VLDL) fraction. Chylomicron remnants are mainly cleared from the plasma by low density lipoprotein (LDL) receptors, heparan sulfate proteoglycan (HSPG)–LDL receptor related protein (LRP) pathways, and HSPG-mediated direct uptake (19).

The postprandial response to dietary lipid load can be measured by triglyceride or vitamin A concentrations in plasma and in triglyceride-rich lipoproteins (4)(8)(10)(12). In addition, the serum concentration of dietary squalene, a 30-hydrocarbon nonsteroid intermediate of cholesterol synthesis, has been shown to resemble the postprandial retinyl palmitate curves (20). Serum squalene originates both from endogenous synthesis and from dietary sources. Olive oil is rich in squalene (21); in populations consuming lots of olive oil, the intake of squalene can amount up to 1 g daily. The postprandial metabolism of dietary squalene has only limitedly been studied so far in humans and it has not been related to age.

The aim of the present study was to investigate postprandial intestinal lipoproteins in relation to advancing age. For this purpose, the postprandial triglyceride, squalene, and retinyl palmitate levels were measured in plasma, chylomicrons and VLDL for 24 hours in young and old men after the administration of vitamin A and squalene in a fat load.


   Subjects and Methods
Top
 Abstract
 Subjects and Methods
Results
 Discussion
 References
 
Study Group
The study group consisted of six healthy young male students, aged 22–25 years, and eight home-living old men aged 78–79 years (Table 1 ), who were volunteers from a random-age cohort of the inhabitants of Helsinki. None of the subjects had {epsilon}2 alleles, two young and four old men had apo E 4/ 3 phenotypes, and all the others had apo E 3/ 3 phenotypes. The subjects were nonsmokers and did not have renal, liver or gastrointestinal diseases. They had no medication, except for one elderly patient who had well-controlled type 2 diabetes mellitus treated with glibenclamide.


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  		Table 1. Weight, Height, Body Mass Index, Serum and Lipoprotein Lipids, and Plasma Squalene and Retinyl Palmitate Concentrations at Baseline

 
The subjects gave their written informed consent. The study protocol was accepted by the Ethics Committee of the Second Department of Medicine, University of Helsinki.

Postprandial Fat Clearance Test
After a 12-hour fast, the subjects were given a fatty meal containing 90 g of milk fat, 432 mg of cholesterol, 345,000 IU of aqueous vitamin A, and 0.5 g of squalene. The meal was given as a cream-eggshake containing 1200 kcal. After the meal, the subjects fasted for 9 hours, having at 5 PM their first actual daily meal. Blood samples were drawn before the meal and after 3, 4, 6, 9, 12, and 24 hours.

Analytical Procedures
Chylomicrons were separated from plasma after they were carefully overlayered with 1.0063 g/ml of NaCl salt solution and by ultracentrifugation in a fixed-angle Type 50 Ti rotor (Beckman Instruments, Inc, Fullerton, CA) rotor for 30 minutes, followed by density gradient separation of VLDLs (<1.006 g/ml), intermediate density lipoproteins (IDLs, 1.006–1.019 g/ml), LDLs (1.019–1.063 g/ml), and high density lipoproteins (HDLs, 1.063–1.21 g/ml) (22). The postprandial plasma samples were separated into chylomicrons and VLDL.

Commercial kits were used to analyze enzymatically serum total and lipoprotein cholesterol and triglycerides (Boehringer Diagnostica, Mannheim, Germany). Squalene was quantitated with gas-liquid chromatography on a 50-m-long capillary column (Ultra-1, Hewlett-Packard, Palo Alto, CA) from nonsaponifiable lipids (23)(24). The analyses of retinyl palmitate were carried out with high-pressure liquid chromatography (25). All retinyl palmitate procedures were completed in subdued light. Apo E phenotyping was performed by isoelectric focusing from serum (26).

In the following paragraphs, postprandial triglyceride, retinyl palmitate and squalene concentrations are given as incremental values calculated by subtracting the respective basal fasting value from each postprandial value. Postprandial retinyl palmitate and squalene responses were also quantitated by calculating the incremental area between the zero level and the 24-hour concentration curve (area under the incremental curve, or AUIC) for each subject.

As a way to evaluate the rate of lipoproteins entering the circulation, the coefficient for the increment of the ascending part of the triglyceride, retinyl palmitate, and squalene curves were calculated by determining regression lines from 3 to 6 hours in chylomicrons and from 3 to 9 hours in VLDL. The triglyceride clearance rate was evaluated by determining the coefficient for the regression line from 4 to 9 hours, which represented the descending part of the triglyceride curve.

Retinyl palmitate and squalene to triglyceride ratios were calculated by dividing retinyl palmitate and squalene concentrations by the respective triglyceride concentration.

Statistical Analysis
Statistical significance was tested with an analysis of variance (ANOVA) and covariance for repeated measures, one-way and two-way ANOVA, and Student's two-sided t test and paired t test. Fasting triglycerides were used as a grouping factor in addition to age in the two-way ANOVA tests, because triglycerides are known to predict postprandial response (2)(3). Logarithmic transformations were used when the distributions were skewed. A value of p < .01 was considered statistically significant.


   Results
Top
 Abstract
 Subjects and Methods
 Results
Discussion
 References
 
The baseline characteristics of the study population are shown in Table 1 . The fasting plasma and lipoprotein cholesterol, triglyceride, retinyl palmitate, and squalene concentrations were similar in the two groups. Only LDL triglycerides of the old subjects were borderline higher.

Body mass index (BMI), fasting triglycerides, and cholesterol did not correlate with postprandial retinyl palmitate and squalene concentrations or AUICs (data not shown). Fasting triglycerides were correlated with plasma triglyceride peak hour < , and fasting cholesterol was correlated to VLDL triglyceride peak hour . A two-way ANOVA showed that postprandial retinyl palmitate and squalene concentrations or AUICs had not been affected by fasting triglycerides .

The fat load did not affect the serum total or lipoprotein cholesterol levels in either age group (data not shown). Postprandial triglyceride levels did not differ significantly between the old and the young subjects (Fig. 1). Chylomicron triglyceride peak times occurred significantly later in the old subjects (Table 2 ).



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  		Figure 1. Postprandial incremental triglyceride values in chylomicrons, A, and VLDL (very low density lipoprotein; B) in young ({circ}) and old (•) subjects. Mean ± standard error of mean.

 

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  		Table 2. Postprandial Peak Time and Peak Concentrations

 
The peak retinyl palmitate values in the chylomicrons occurred from 4 to 9 hours in the young and from 4 to 12 hours in the aged; the peak time difference was not significant (Table 2 ), even though half of the older men but none of the younger ones had the chylomicron peak at 12 hours. For VLDL, the mean peak time and concentrations were similar in the young and old. By two-way ANOVA with age and triglycerides as grouping factors, the old subjects tended to have higher plasma retinyl palmitate maximal incremental concentrations compared with the young: 537.5 ± 218.7 µg/dl, mean ± SD, versus 279.1 ± 99.8 µg/dl . The concentrations were not affected by triglycerides , and there was not a significant age by triglycerides interaction . In addition, by two-way ANOVA, the AUICs were higher in the old than in the young men in plasma, that is, 5023 ± 1749 µg/dl h versus 2671 ± 1355 µg/dl h . The AUICs were not affected by triglycerides , and there was not a significant age by triglycerides interaction . The AUICs differed less consistently in chylomicrons, that is, 2549 ± 963 µg/dl h in the old versus 1368 ± 919 µg/dl h in the young . The AUICs were not affected by triglycerides , and there was not a significant age by triglycerides interaction . In VLDL, the AUICs were 1926 ± 1101 in the old versus 937 ± 638 µg/dl h in the young . The AUICs were not affected by triglycerides , and there was not a significant age by triglycerides interaction ; Fig. 2).



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  		Figure 2. Areas under the incremental postprandial retinyl palmitate curves in young (open bars) and old (solid bars) subjects. a, p = .0095; b, p = .022; c, p = .048, young vs old. (VLDL; very low density lipoprotein.)

 
In the time by age (young vs old) ANOVA on postprandial VLDL squalene concentrations, there was a significant main effect of time (p < .0001), but not of age , and a significant time by age interaction ; this was due to increasing concentrations in the old and decreasing concentrations in the young at 9–12 hours (Fig. 3).



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  		Figure 3. Postprandial incremental squalene values in chylomicrons, A, and VLDL (very low density lipoprotein; B) in young ({circ}) and old (•) subjects. Mean ± standard error of mean. p = .0096 between the VLDL curves, analysis of variance (ANOVA) for repeated measures. *p = .03, young vs old at 12 hours, one-way ANOVA.

 
In the follow-up comparisons by one-way ANOVA, 12-hour VLDL squalene tended to be higher in the old (185.5 ± 135.4 µg/dl) compared with the young (57.4 ± 43.7 µg/dl; . Half of the older men, but none of the younger ones, had the chylomicron and VLDL peaks at 12 hours (not significant). However, the peak concentrations of chylomicrons and VLDL were similar, and the AUICs did not differ between the two groups (data not shown). In the chylomicrons and VLDL, the regression lines for the ascending part of the triglyceride, retinyl palmitate, and squalene curves were similar in both age groups (data not shown).

Postprandial retinyl palmitate values were significantly correlated with those of squalene in chylomicrons and VLDL of the two combined age groups at every time point (p < .01), except at 24 hours; chylomicron and VLDL 12-hour values are shown in Fig. 4 as an example. The respective correlations with those of triglycerides were less frequently significant (data not shown). Fig. 5



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  		Figure 4. Correlation between postprandial 12-hour retinyl palmitate and squalene concentrations on log scales in chylomicrons, A, and VLDL (very low density protein; B) of young ({circ}) and old (•) subjects. The r values were calculated from combined groups.

 


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  		Figure 5. Postprandial retinyl palmitate to triglyceride, A, and squalene to triglyceride, B, ratios in chylomicrons ({blacksquare}) and VLDL (very low density lipoprotein; {square}). Mean ± standard error of mean with analysis of variance (ANOVA) for repeated measures; *p < .01 for single time points, one-way ANOVA.

 
In the time by lipoproteins (chylomicrons vs VLDL) ANOVA on retinyl palmitate to triglyceride ratios, there were significant main effects of time (p < .0001) and lipoprotein fractions (p < .0001), and a significant time by lipoprotein fraction interaction (p < .0001). Follow-up comparisons with one-way ANOVA at each time revealed that the ratio was higher in chylomicrons at 6 and 9 hours (p < .01 for both). In the time by lipoproteins (chylomicrons versus VLDL) ANOVA on squalene to triglyceride ratios, there were significant main effects of time (p < .0001) and lipoprotein fractions (p < .0001), and a significant time by lipoprotein fraction interaction (p < .0001). Follow-up comparisons with one-way ANOVA at each time revealed that the ratio was higher in chylomicrons at 0, 9, and 24 hours (p < .01 for each).


   Discussion
Top
 Abstract
 Subjects and Methods
 Results
 Discussion
References
 
The major observation in the present study was that postprandial intestinal lipoprotein metabolism, measured by retinyl palmitate and squalene in plasma and lipoproteins, was age dependent and more effective in the young than old men.

It is well known that postprandial lipids are closely related to fasting triglyceride levels (2)(3), whereas BMI does not consistently predict the postprandial response (14)(27). The lack of correlation or interaction of baseline triglycerides to postprandial retinyl palmitate and squalene levels in our study suggests that the slightly higher baseline triglycerides of the old subjects did not confound the results. In addition, the fasting triglyceride values of the elderly subjects were in the normal range. Although one of our old subjects had type 2 diabetes mellitus, we did not omit him because his postprandial results did not differ from the other old men. In addition, the apo E phenotype distribution was similar between the two age groups, thus eliminating this possibly confounding factor (4).

Because the study population was relatively small and several statistical comparisons were made, only values of p < .01 were considered statistically significant. However, as there were many borderline significancies, values of p < .05 were also presented in the Results section.

In concordance with the earlier study by Borel and colleagues (17), chylomicron triglyceride peaks were reached later by the old subjects in the present study. The possible explanation for these later peak values might be related to a reduced gastric emptying rate (28)(29). However, the delay in chylomicrons was 1.5 hours, which is barely enough to explain the different retinyl palmitate and squalene responses at 12–24 hours between the groups. We did not measure the intestinal transit time, but in previous studies no consistent evidence has been found for reduced intestinal function with increasing age (29)(30). In addition, we calculated the regression lines for the ascending postprandial curves for both chylomicrons and VLDL, and we observed that the respective coefficients were practically similar in the two age groups for triglycerides, retinyl palmitate, and squalene. This finding suggests, similarly to earlier studies (17)(31), that the absorptive activities were not age dependent.

Postprandial plasma triglycerides reached their maximum earlier and disappeared faster than retinyl palmitate and squalene in both age groups. This indicates that both the chylomicron and VLDL particles have low retinyl palmitate and squalene to triglyceride ratios in the early stage of absorption, with the values of the particles increasing markedly in the later samples. However, the resulting ratio should be higher for the remnant VLDL than for chylomicrons if VLDL is derived from chylomicrons. Both the retinyl palmitate and squalene to triglyceride ratios were, however, higher in chylomicrons. Thus, we can assume that both chylomicron and VLDL particles were released from the intestine. The accumulation of particles containing apo B-100 during the postprandial state (3)(32) is not supposed to alter the ratio, because it has been shown that the triglyceride content of VLDL containing apo B-100 is essentially unchanged during alimentary lipemia (33). Recent findings using apo B-48 as the intestinal lipoprotein marker have indicated that, during digestion the intestine produces lipoproteins with high and low triglyceride contents, possibly into the density grade of 1.019 g/ml (34).

Decreased activity of LPL but not that of hepatic lipase (35) and increased production of VLDL apo B-100 (36) have been associated with increasing age. In addition, the removal of LDL apo B was similar in 75-year-old and 50-year-old men (37), suggesting that LDL receptor activity was at least normal. Thus, it can be suggested that the significantly higher plasma retinyl palmitate AUIC, the trend of the higher 24-hour incremental concentration of plasma retinyl palmitate, and the different postprandial VLDL squalene responses in the old subjects compared with the young ones are caused by a retarded hepatic uptake of postprandial lipoproteins. This retardation could be secondary to the age-associated increase in the VLDL pool (36) or to primary defects in HSPG–LRP pathways or HSPG-mediated direct uptake (19).

The postprandial correlations between squalene and retinyl palmitate at 3–12 hours confirm our previous results (20) that serum squalene measurement offers a reliable means to study postprandial chylomicron remnant clearance. Postprandial peak times were similarly detectable by squalene and retinyl palmitate. Plasma 24-hour retinyl palmitate but not squalene concentrations tended to be higher in the old subjects, and the AUIC for VLDL squalene only tended to be higher in the old versus the young. This can be explained by the fact that the AUIC is largely determined by the 24-hour value. It has been shown that the transfer of retinyl palmitate from the remnant to the LDL particle is less than 5% in in vitro incubation studies (38) or after an oral fat load test (39). The transfer of squalene between different lipoprotein particles has yet to be studied in humans.

In spite of retarded postprandial lipid metabolism, the old subjects in our study did not suffer from symptoms of coronary artery disease. However, our cohort of healthy home-living 79-year-old men is highly selected. Those who have died from complications of atherosclerosis might have had defects in lipid metabolism already at a younger age. Our finding of retarded removal of postprandial lipids in the elderly subjects can be interpreted to indicate that postprandial lipids are not life-span limiting. It is possible that the retarded removal of postprandial lipids is actually an old-age-induced phenomenon. The cross-sectional study, however, does not give the answer to whether the retarded removal of postprandial lipids is truly an age-related process.


   Acknowledgments
 
This study was supported by grants from the Research Foundation of Orion Corporation, the Clinical Research Institute of the Helsinki University Central Hospital, the Finnish Heart Research Foundation, the Juho Vainio Foundation, and the Finnish Academy, Council of Medical Sciences. The excellent technical assistance of Leena Kaipiainen, Pia Hoffström, Anja Salolainen, Orvokki Ahlroos, and Ritva Nissilä is greatly acknowledged. This article was partially presented as an abstract at the Xth International Symposium on Atherosclerosis, Montreal, Canada, October 1994.

Received November 26, 1999

Accepted March 24, 2000


   References
Top
 Abstract
 Subjects and Methods
 Results
 Discussion
 References
 

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