Effect of diacylglycerol on postprandial lipid metabolism in non-diabetic subjects with and without insulin resistance

Atherosclerosis 180 (2005) 197–204

Effect of diacylglycerol on postprandial lipid metabolism in non-diabetic subjects with and without insulin resistance Hideto Takase∗ , Kentaro Shoji, Tadashi Hase, Ichiro Tokimitsu Health Care Products Research Laboratories No. 1, Kao Corporation, 2-1-3 Bunka, Sumida-ku, Tokyo 131-8501, Japan Received 23 March 2003; received in revised form 22 November 2004; accepted 22 November 2004 Available online 19 January 2005

Abstract The effects of diacylglycerol ingestion on postprandial lipid metabolism in non-diabetic subjects with and without insulin resistance were investigated. This was single dose ingestion study, in a double blind cross over manner and postprandial lipid concentrations were compared between diacylglycerol oil (DAG) and triacylglycerol oil (TAG) ingestion. The subjects were 18 male volunteers and homeostasis model assessment (HOMA-R) was used to classify them into insulin sensitive (IS, n = 10, HOMA-R < 2.0) and insulin resistant (IR, n = 8, HOMA-R ≥ 2.0) groups. Fasting serum triglycerides (TG) and remnant-like particle cholesterol (RLP-C) correlated with HOMA-R and were significantly higher in the IR as compared to the IS group. Postprandial increments of TG and RLP-C after DAG ingestion were significantly lower as compared to those after TAG ingestion. In a case of TAG ingestion, their increments positively correlated with HOMA-R and were significantly higher in the IR as compared with the IS group. In contrast, their increments remained constant after DAG ingestion in both groups. In the IR group, the postprandial lipidemia were reduced after DAG ingestion to about half of those after TAG ingestion. In conclusion, DAG reduced postprandial lipidemia especially in subjects with insulin resistance and may be beneficial in preventing atherosclerosis and related diseases. © 2004 Elsevier Ireland Ltd. All rights reserved. Keywords: Diacylglycerol; Postprandial lipidemia; Triglyceride-rich lipoproteins; Insulin resistance; HOMA-R

1. Introduction Relations between hypertriglyceridemia and coronary heart disease (CHD) have been widely investigated [1] and triglyceride-rich lipoproteins (TRL), such as chylomicrons, very low-density lipoproteins (VLDL) and their remnants are considered to be independent risk factors for CHD [2–4]. Such TRL are increased mainly in the postprandial state, and Zilversmit advanced the importance of postprandial hyperlipidemia for atherogenesis [5]. According to Zilversmit’s hypothesis, many studies have been conclusively shown the relationship between postprandial hyperlipidemia and atherosclerosis [6–9]. Hyperinsulinemia, insulin resistance and impaired glucose tolerance have been also discussed in relation with lipid ∗

Corresponding author. Tel.: +81 3 5630 7266; fax: +81 3 5630 9436. E-mail address: [email protected] (H. Takase).

0021-9150/$ – see front matter © 2004 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.atherosclerosis.2004.11.020

metabolism [10–12] and with the risk of CHD [13–15]. More recently, several studies have focused on the relation between insulin resistance and postprandial hyperlipidemia [16–18]. From these results, it may be concluded that postprandial hyperlipidemia, especially an increase in TRL levels based on insulin resistance, may cause the progression of atherosclerosis and constitute important risk factors for CHD. Diacylglycerol oil (DAG), which contains more than 80 wt.% diacylglycerol, is edible cooking oil with characteristics that are similar to conventional triacylglycerol oil (TAG). DAG has been recognized as FOSHU (Food for Special Health Use) in Japan since 1998, and also listed in the FDA GRAS (generally recognized as safe) notice of 2000. Two long-term randomized controlled trials in Japan and the US have been reported [19,20]. In these reports, the longterm ingestion of DAG as a daily cooking oil was found to suppress the accumulation of body fat, compared to TAG. DAG also affects postprandial lipid metabolism. Taguchi re-

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ported postprandial serum triglyceride (TG) and chylomicron triglyceride (Chyl-TG) levels were reduced after DAG ingestion, compared with TAG ingestion [21]. Tada reported that remnant-like particle cholesterol (RLP-C) was significantly lower after DAG ingestion, compared with TAG ingestion [22]. From these reports, DAG appears to reduce postprandial hyperlipidemia and body fat accumulation and may be effective in preventing diseases associated with lipid metabolism disorder and obesity. However, the efficacy and mechanism of DAG, as it relates to insulin resistance in humans, has not been elucidated and further investigations are still needed. To investigate the effects of DAG on postprandial lipid concentrations in relation to insulin resistance, we conducted a single dose ingestion trail for non-diabetic subjects with and without insulin resistance. The findings presented here may also contribute to a better understanding of the mechanism of DAG in human.

2. Materials DAG was prepared from natural plant edible oils by using the reverse reaction of an immobilized lipase [23]. The DAG used in this study contained 81.9 wt.% diacylglycerol and the ratio of 1,2-diacylglycerol to 1,3-diacylglycerol was 3:7. TAG was prepared by mixing rapeseed, safflower and perilla oils such that the fatty acid composition was similar to that of the diacylglycerol oil. Fatty acid and glyceride compositions of the experimental oils are shown in Table 1. Test oil was served as a mayonnaise type food that contained 66.3 wt.% test oil. Nutritional content of 100 g of test mayonnaise was as follows: 2.5 g proteins, 70.9 g fat and 2.2 g of carbohydrate and the energy were 656 kcal/100 g. Fat from other materials, such as egg, made up about 4.6 wt.% and, as a result, the test oil was a major source of fat in the experimental food. The taste, appearance and physical properties of the test food were similar in each experimental oils and the subject were not able to distinguish between them.

Table 1 Glyceride and fatty acid compositions of test oils DAG

TAG

Glyceride composition (wt.%) Mono 1.3 Di 81.9 Tri 16.9

– 1.6 98.4

Fatty acid composition (wt. %) C16:0 3.1 C16:1 0.3 C18:0 1.1 C18:1 38.3 C18:2 47.7 C18:3 9.0 C20:0 0.3 C20:1 0.2 C22:1 –

5.4 0.3 2.1 34.3 49.2 7.8 0.5 0.3 0.1

3. Subjects and methods Eighteen subjects were recruited from male volunteers without any history of diabetes or dyslipidemia and daily medication, which would interfere with lipid metabolism. The nature and purpose of the study was fully explained to each subject before he gave his consent to participate. All procedures of this study were approved by the Ethics Committee of Kao Corporation and carried out in accordance with the Helsinki Declaration of 1975 as revised in 1983. This was single dose ingestion study, in a double blind cross over manner. To equalize the condition of fasting state at the trial day, the subjects received a same control meal using ordinary cooking oil at 18 pm before each trial day. Fat content of control meal was 44.9 g and the energy was 805 kcal. After taking the control meal, they fasted except for water until the trial started. At the trial day, the trail started between 8:30 am and 9:00 am. The trial was performed in a temperature-controlled room and subjects were encouraged to rest in the room during the trial. After collecting blood sample at the time of fasting, the subjects were randomly divided into two groups and received the assigned test food. Amount of ingested test oil was 10 g/60 kg body weight. Blood samples were collected at 2, 3, 4 h after ingestion. After a 1-week washout period, the same procedure was repeated but the test food was reversed. Blood samples were collected in tubes without anticoagulant or with EDTA and sodium fluoride for blood glucose determinations. Serum sample was obtained by centrifugation at 3000 rpm × 15 min at 5 ◦ C. TG, total cholesterol (TC), non-esterified fatty acids (NEFA) and ketone body concentrations were measured by an automatic analyzer using standard enzymatic techniques. RLP-triglyceride (RLPTG) and RLP-C were measured by assay methods using monoclonal anti-human apolipoprotein A-1 and anti-human B-100 antibodies for measuring apo E-rich remnant-like particle [24]. Fasting insulin was assessed by an enzyme immunoassay. Fasting plasma glucose (FBS) concentrations were assessed by a glucose dehydrogenase method. Lipoprotein fractions were obtained from serum samples by ultracentrifugation [25] using a Hitachi RP65T rotor in a Hitachi SCP85H2 ultracentrifuge. Densities were adjusted by the addition of potassium bromide. The fraction separated by ultracentrifugation at 20,000 rpm for 30 min at 15 ◦ C was isolated as the chylomicron fraction. Other lipoprotein fractions, such as very low-density lipoprotein (VLDL, d < 1.006 g/mL), low-density lipoprotein (LDL, 1.006 < d < 1.063 g/mL), and high-density lipoprotein (HDL, d > 1.063 g/mL) were sequentially isolated by ultracentrifugation. The concentration of lipids in each lipoprotein fraction was measured by standard enzymatic methods. Insulin resistance was assessed by the homeostasis model assessment (HOMA-R) formula as follows [26] HOMA-R = fasting insulin (␮U/mL) × fasting glucose (mg/dL)/405.

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cedures described above. In all analyses, significance was set at P < 0.05.

4. Statistical analyses All data are presented as the means ± S.E.M. Statistical analyses were performed using SPSS (version 11.0; SPSS Inc., Chicago). Differences in the baseline characteristics of subjects between two trial days were assessed by a paired t-test. Differences between the IR and the IS groups were determined using the student’s t-test. Correlations between baseline characteristics of subjects were assessed by non-parametric Spearman’s rank test and a linear multiple regression analysis in a stepwise manner. A two factor doubly repeated-measures analysis of variance (ANOVA) was used to investigate change in blood variables over time. Treatment and time were included in the model as repeated factors within-subject. A statistical model was constructed using the effects of treatment, time and interaction of treatment × time. If the interaction of treatment × time and/or treatment effect was significant, a post hoc paired t-test with a Bonferonni correction for multiple comparison was performed at each collection time. Postprandial lipidemia was assessed by maximum incremental concentrations from baseline (Cmax ) in each subject. Cmax differences between the two test oils were assessed by the paired t-test and the student’s t-test was used for the difference between the IR and the IS groups. Correlation between Cmax and the characteristics of subjects were assessed by using same pro-

5. Results No significant differences were found in fasting blood parameters between individual trail days (data not shown) and the average values for each trial are listed in Table 2. The mean BMI of the subjects was 25.2 ± 2.6 kg/m2 (21.0–31.4) and eight subjects were overweight (BMI ≥ 25). The mean fasting TG was 122 ± 72 mg/dL (42–293) and five subjects showed evidence of hypertriglyceridemia (TG ≥ 150). The greatest FBS was 105 mg/dL and no subject appeared to be diabetic. The mean HOMA-R was 2.07 ± 0.76 (0.95–3.45). In this study, the subjects were divided into two groups, insulin sensitive (IS, n = 10, HOMA-R < 2.0) and insulin resistance (IR, n = 8, HOMA-R ≥ 2.0) groups. The baseline characteristics in each group are also shown in Table 2. Correlations between BMI, age, HOMA-R and lipid profiles are listed in Table 3. BMI and age did not correlate with any lipid parameters, but HOMA-R significantly correlated with TG, Chyl-TG, and VLDL-TG (P = 0.025, P = 0.048 and P = 0.030 respectively). From the multiple regression analysis in a stepwise manner, only HOMA-R was recognized as a significant explanatory variable for fasting TG and RLP-C.

Table 2 Baseline characteristics and fasting blood parameters All

ISa

IRb

P-valuec

Number of subjects Age (year) Body mass index (kg/m2 ) Insulin (␮U/mL) Glucose (mg/dL) HOMA-Rd Triglyceride (mg/dL) RLP-triglyceride (mg/dL) Total cholesterol (mg/dL) RLP-cholesterol (mg/dL)

18 37 ± 1 25.2 ± 0.6 9.1 ± 0.7 92 ± 2 2.1 ± 0.2 122 ± 16 22.8 ± 3.8 201 ± 7 5.1 ± 0.6

10 38 ± 1 24.0 ± 0.6 7.2 ± 0.5 88 ± 2 1.6 ± 0.1 87 ± 12 15.9 ± 2.8 188 ± 8 3.8 ± 0.4

8 37 ± 2 26.8 ± 0.9 11.4 ± 0.8 96 ± 2 2.7 ± 0.2 165 ± 27 31.3 ± 6.8 217 ± 11 6.7 ± 1.2

0.830 0.019 <0.001 0.016 <0.001 0.012 0.039 0.047 0.021

Lipoprotein fractione (mg/dL) Chyl-triglyceride (mg/dL) VLDL-triglyceride (mg/dL) LDL-triglyceride (mg/dL) HDL-triglyceride (mg/dL) Chyl-cholesterol (mg/dL) VLDL-cholesterol (mg/dL) LDL-cholesterol (mg/dL) HDL-cholesterol (mg/dL) Free fatty acid (meq/L) Total ketone body (␮mol/L) Acetoacetic acid (␮mol/L) 3-Hydroxybutyric acid (␮mol/L)

5±1 73 ± 11 28 ± 2 15 ± 1 1±0 20 ± 3 127 ± 6 49 ± 2 0.40 ± 0.02 77 ± 9 26 ± 2 51 ± 7

3±1 48 ± 9 23 ± 2 14 ± 1 0±0 14 ± 3 120 ± 7 52 ± 2 0.35 ± 0.02 79 ± 14 27 ± 3 53 ± 11

7±2 103 ± 18 34 ± 4 16 ± 1 1±0 28 ± 4 136 ± 10 46 ± 2 0.47 ± 0.03 74 ± 8 25 ± 3 49 ± 6

0.064 0.011 0.010 0.422 0.017 0.013 0.177 0.071 0.006 0.772 0.671 0.810

Values represent mean ± S.E.M. a Insulin sensitive group, subject with HOMA-R < 2.0. b Insulin resistance group, subject with HOMA-R ≥ 2.0. c Student’s t-test between IS and IR groups. d HOMA-R = Insulin (nU/mL) × Glucose (mg/dL)/405. e Separated by ultracentrifugation.

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Table 3 Correlation between fasting lipids and characteristics of subject BMI

Age

HOMA-R

r

P

r

P

r

P

Triglyceride RLP-triglyceride Total cholesterol RLP-cholesterol

−0.026 −0.036 0.030 0.001

0.919 0.887 0.906 0.997

−0.018 −0.065 0.017 0.146

0.944 0.799 0.948 0.564

0.525 0.534 0.456 0.449

0.025 0.023 0.057 0.062

Lopoprotein fraction Chyl-trigkyceride VLDL-trigkyceride LDL-trigkyceride HDL-trigkyceride Chyl-cholesterol VLDL-cholesterol LDL-cholesterol HDL-cholesterol

−0.100 −0.042 0.026 −0.314 0.055 −0.055 0.132 −0.197

0.694 0.868 0.919 0.204 0.829 0.829 0.601 0.433

0.013 −0.014 0.034 −0.195 0.068 −0.043 −0.016 −0.019

0.961 0.958 0.892 0.439 0.789 0.866 0.949 0.941

0.444 0.513 0.464 0.120 0.484 0.518 0.360 −0.298

0.065 0.030 0.053 0.636 0.042 0.028 0.142 0.229

Correlation coefficient and P-value by Spearman’s rank test.

Changes in TG, Chyl-TG, RLP-TG, and RLP-C from the baseline after ingestion of the test oils are shown in Fig. 1. Postprandial change in TG levels was significantly lower after DAG ingestion, compared with TAG ingestion (treatment P = 0.008, treatment × time P = 0.012). The concentration of RLP-TG was also significantly lower after DAG ingestion (treatment P = 0.012, treatment × time P = 0.011). The treatment effect for Chyl-TG was P = 0.050. Results of effects on the postprandial changes and Cmax in lipids and insulin

are summarized in Table 4. DAG reduced postprandial lipidemia, as evaluated by Cmax in TG and RLP-TG significantly. In Chyl-TG, the P-value was small (P = 0.067) but not significant. Insulin secretion tended to be lower after DAG ingestion (treatment P = 0.092 by ANOVA). A treatment effect was observed for HDL-TG (P = 0.028) and a treatment × time interaction was observed for LDL-TG (P = 0.042). The concentration of LDL-TG and HDL-TG after ingestion were relatively low in the case of DAG ingestion, however, the mean

Fig. 1. Postprandial change from baseline for TG (TG), Chylomicron-TG (Chyl-TG), VLDL-TG (VLDL-TG) and RLP-C (RLP-C) after DAG () and TAG (䊉) ingestion. Two way repeated-measures ANOVA showed significant difference between the test oils in TG (P = 0.008). In Chyl-TG, P-value was P = 0.050. Post hoc paired t-test with a Bonferonni correction for multiple comparison showed a significant difference at 4 h in TG (P = 0.020) and Chyl-TG (P = 0.045).

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Table 4 Postprandial change of lipid profiles and insulin ANOVAa

Change from the baseline 0h

2h

Triglyceride (mg/dL) Total DAG TAG RLP DAG TAG Chyromicron DAG TAG VLDL DAG TAG

0 0 0 0 0 0 0 0

17 25 12.5 17.5 10 12 7 9

± ± ± ± ± ± ± ±

4 4 3.4 3.0 2 2 2 2

27 42 15.4 27.9 14 20 10 14

± ± ± ± ± ± ± ±

5 7 3.1 5.6 2 4 2 2

18 36 8.9 21.3 6 12 9 15

± ± ± ± ± ± ± ±

5 8 2.7 5.0 1 3 3 4

Cholesterol (mg/dL) Total DAG TAG RLP DAG TAG Chyromicron DAG TAG VLDL DAG TAG

0 0 0 0 0 0 0 0

−4 −3 0.9 1.4 1 1 0 0

± ± ± ± ± ± ± ±

4 1 0.4 0.4 0 0 1 1

1 2 1.4 2.6 1 1 1 1

± ± ± ± ± ± ± ±

1 1 0.3 0.6 0 0 1 1

2 1 1.0 1.9 1 1 1 2

± ± ± ± ± ± ± ±

2 2 0 0.3 0.6 0 1 1

Insulin (␮U/mL)

DAG TAG

0 0

3h

−2.5 ± 0.6 −1.2 ± 0.6

4h

−3.3 ± 0.6 −1.9 ± 0.5

−4.1 ± 0.6 −3.2 ± 0.6

Cmax

Group

Time

g×t

mg/dL

0.008

< 0.001

0.012

< 0.001

0.011

0.050

< 0.001

0.131

0.109

< 0.001

0.127

± ± ± ± ± ± ± ±

4 7 3.0 5.2 2 4 2 3

0.010

0.012

33 50 20.1 34.0 17 24 15 20

0.810

0.005

0.926

< 0.001

0.133

0.012

< 0.001

0.020

0.607

0.004

0.696

± ± ± ± ± ± ± ±

1 1 0.3 0.6 0 0 1 1

0.870

0.094

5 4 2.0 3.2 1 2 2 3

0.092

< 0.001

0.196

P-valueb

0.0 ± 0.0 0.6 ± 0.2

0.011 0.067 0.084

0.044 0.042 0.215 0.017

Values represent mean ± S.E.M. a ANOVA: two-way repeated-measures analysis of variance. b Paired t-test between DAG and TAG.

difference at each time was very small. There were no significant differences in T-C; however, Chyl-C and RLP-C were significantly lower in the case of DAG ingestion. There were no significant differences in free fatty acid and ketone bodies (data not shown). Subclass analyses were performed and the results are summarized in Table 5. In the IS group, postprandial TG did not differ significantly for either of the test oils. The IR group had

a significantly greater postprandial TG after TAG ingestion, compared with the IS group, however no significant difference was noted after DAG ingestion. A remarkable reducing effect of postprandial hyperlipidemia by DAG was observed in the IR group. Multiple regression analyses for Cmax of lipids after TAG ingestion were performed using BMI, age and HOMA-R as explanatory variables, but as in the case of the fasting state,

Table 5 Cmax of lipids in IS and IR groups IS Mean ± S.E.M. DAG ingestion

Triglyceride

Cholesterol

TAG ingestion

Triglyceride

Cholesterol

a b c

Total RLP Chylomicron VLDL Total RLP Chylomicron VLDL Total RLP Chylomicron VLDL Total RLP Chylomicron VLDL

Paired t-test between DAG and TAG in IS group. Paired t-test between DAG and TAG in IR group. Student’s t-test between IS and IR group.

32 19.7 18 13 4 1.8 1 2 35 23.5 17 13 5 2.2 1 2

P vs. ISc

IR

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

7 4.0 4 3 1 .4 0 1 6 4.1 3 3 2 .5 0 1

P vs. TAGa

Mean ± S.E.M.

0.582 0.378 0.550 0.911 0.589 0.392 1.000 0.922 – – – – – – – –

36 20.5 15 17 6 2.2 1 3 69 47.2 32 29 3 4.5 2 4

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

5 5.0 2 3 1 .4 0 1 10 8.7 6 4 1 1.0 0 1

P vs. TAGb 0.003 0.012 0.010 0.009 0.375 0.074 0.007 0.069 – – – – – – – –

0.664 0.904 0.505 0.427 0.410 0.461 0.214 0.725 0.037 0.063 0.171 0.012 0.700 0.043 0.503 0.141

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Correlations between Cmax and HOMA-R are also listed in Table 6.

6. Discussion Possible relations between effect of DAG ingestion on postprandial lipidemia and insulin resistance were examined. In the fasting state, the concentrations of blood lipids were strongly correlated with insulin resistance, as assessed by HOMA-R. Free fatty acid levels were also significantly higher in the IR group, and an enhanced synthesis of VLDL might be a dominant cause of hypertriglyceridemia in IR subjects. RLP-C was also significantly higher in the IR group (Table 2). These findings are consistent with previously reported results [10–12]. In this study, age was well balanced between the IS and IR groups, but BMI was significantly higher in the IR group. It is likely that BMI was significantly related to the fasting blood lipids levels, but multiple regression analysis in a stepwise manner revealed that only HOMA-R was a significant explanatory variable for fasting lipid concentrations and BMI was independent (Table 3). In all subject group (n = 18), postprandial changes in the concentration of blood lipids were significantly lower after DAG ingestion, compared with TAG ingestion (Fig. 1, Table 4). Such reducing effects on postprandial TG and RLPC have been reported previously by Taguchi [21] and Tada [22] in healthy male volunteers. In those studies, the fat load were 10 g to 44 g/60 kg body weight (Taguchi) and 30 g/m2 body surface area (Tada). In present study, the fat load was relatively low (10 g/60 kg body weight). Average total fat intake in Japanese was reported about 54.4 g/day and 10.9 g/day from oil, such as edible vegetable oils (The National Nutrition Survey in Japan, 2002, Ministry of Health, Labour and Welfare, Japan). In this study, the significance of a reducing effect on postprandial lipidemia was reconfirmed using practical fat loads in daily use. The relationship between the efficacy of DAG ingestion on postprandial lipidemia and insulin resistance was also in-

Fig. 2. Correlation between HOMA-R and postprandial increment (Cmax ) of TG after DAG () and TAG (䊉) ingestion. After TAG ingestion, Cmax correlated HOMA-R after TAG ingestion (r = 0.710, P = 0.001, n = 18), but not after DAG ingestion (r = 0.071, P = 0.779, n = 18).

only HOMA-R was found to be a significant explanatory variable for Cmax . The correlation between Cmax of TG after ingestion of each of the test oils and HOMA-R are shown in Fig. 2. The postprandial increment in TG after TAG ingestion was strongly correlated with the HOMA-R (r = 0.710, P = 0.003). In contrast, in the case of DAG ingestion, no correlation with HOMA-R was observed (r = −0.085, P = 0.727). Correlations between HOMA-R and Cmax in lipid are listed in Table 6. The Cmax of RLP-TG, Chyl-TG and VLDL-TG after TAG ingestion had a significant positive correlation with HOMA-R, but not after DAG ingestion. The reducing effect of DAG (Cmax ) was defined by the following equation for each individual: ∆Cmax = Cmax (TAG) − Cmax (DAG) Table 6 Correlation between HOMA-R and Cmax of lipids DAG (n = 18)

Cmax (n = 18)a

TAG (n = 18)

r

P

Triglyceride Total RLP Chylomicron VLDL

0.071 −0.077 −0.282 0.204

0.779 0.760 0.256 0.418

0.710 0.624 0.534 0.722

0.001 0.006 0.022 0.001

0.778 0.670 0.820 0.688

0.001 0.006 0.001 0.005

Cholesterol Total RLP Chylomicron VLDL

0.252 0.047 −0.390 0.277

0.313 0.855 0.110 0.267

−0.026 0.521 0.452 0.565

0.918 0.026 0.060 0.014

−0.193 0.553 0.669 0.470

0.427 0.023 0.006 0.053

Correlation coefficient and P-value by Spearman’s rank test. a C max = Cmax (TAG) − Cmax (DAG).

r

P

r

P

H. Takase et al. / Atherosclerosis 180 (2005) 197–204

vestigated (Table 5). In the IS group, postprandial increments of lipids were generally low, even after TAG ingestion, and no significant differences were observed between the two test oils. In the IR group, postprandial lipidemia was significantly higher, compared with the IS group, after TAG ingestion. This is consistent with previously reported results [16–18]. In contrast, there were no significant differences between IS and IR group after DAG ingestion. As a consequence, postprandial lipidemia was significantly lower after DAG ingestion in the IR subjects to about half of those after those after TAG ingestion. A correlation analysis revealed the relation between postprandial lipidemia and insulin resistance (Table 6, Fig. 2). After TAG ingestion, the Cmax of lipids were positively correlated with HOMA-R, but no correlations were observed after DAG ingestion. Differences in Cmax between the two test oils (Cmax ) were highly correlated with HOMA-R except T-C. From these analyses, it can be concluded the reducing effect of DAG on postpandial lipidemia is closely associated with insulin resistance and would be preferable for individuals with insulin resistance. A possible mechanism for the reducing effect of DAG on postprandial lipidemia has been investigated in several animal studies. Kondo et al. reported the that the main digestive product of DAG is 1-monoacylglycerol, which is poorly reesterified into TG in the cells of the intestinal lining, and it is thought to be one of the probable causes of the discriminative postprandial response of DAG [27]. Murase et al. reported DAG stimulated ␤-oxidation and lipid metabolismrelated gene expression, including acyl-coenzyme A oxidase, medium-chain acyl-coenzyme A dehydrogenase, and uncoupling protein-2 in the small intestine within the first 10 days of 8-month ingestion in C57BL/6J mice [28]. This enhancement of lipid metabolism in small intestine was observed after repeated intake of DAG and it is still not obvious if same enhancement could be found in single dose intake. However, such enhanced lipid oxidation in the small intestine may cause a lesser delivery of substrate for chylomicron production than with TAG. In present study, the postprandial increment of endgeneous Chyl-TG was significantly reduced after DAG ingestion. Additionally, the weight ratio between cholesterol and triglycerides in chylomicron particles at the maximum concentration of Chyl-TG were essentially same after TAG and DAG ingestion (0.080 ± 0.005 in TAG and 0.085 ± 0.010 in DAG, P = 0.627). This suggests that there was no significant difference in the composition of lipids in chylomicron particles, but that the total secretion of chylomicron particle might be reduced after DAG ingestion. Hyperlipidemia in the insulin resistance state has been attributed mainly to hyperinsulinemia and suppressed lipoprotein lipase (LPL) activity and enhanced TG-rich lipoprotein secretion [16]. In this study, effect of DAG on hyperinsulinemia was not clearly observed, because ingested test food contained only a small quantity of carbohydrate and insulin was lower than baseline from 2 h after ingestion. However, insulin secretion tended to be lower after DAG ingestion com-

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pared with TAG, it may suggest the possibility of effect of DAG on hyperinsulinemia. On the other hand, recent studies have shown that expression of microsomal triglyceride transfer protein (MTP) activity, which facilitate the assembly and secretion of apoB-containing lipoprotein particles, was enhanced in an animal model of insulin resistance [29,30]. Very limited reports suggested the effect of DAG on LPL and MTP activities. Tada et al. reported that no difference was observed in serum LPL mass after loading of DAG and TAG in human and the differences in postprandial remnants concentration were considered to be a result of the differences in the concentrations of chylomicron and VLDL secretion [22]. Taguchi et al. reported that a high-fat diet-induced TG concentration in the liver and hepatic microsomal triglyceride transfer protein activity were significantly decreased by a DAG diet in rats [31]. This result showed that a DAG diet might suppress hepatic MTP activity, which was enhanced in insulin resistance state, and therefore reduced postprandial lipidemia. However, effect of single dose DAG ingestion on lipid synthesis in the liver and intestine remains incompletely understood in human and further investigation will be needed. In conclusion, DAG reduced postprandial lipidemia, especially in subjects with insulin resistance compared to conventional TAG. The postprandial increments of TG-rich lipoproteins is a major risk factor for atherogenesis and DAG might be beneficial in preventing athroscleosis and related cardiovascular disease.

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