Abstract
In this mini review, we have summarized our results about the effect of habitual diet on n-3 polyunsaturated fatty acid (n-3 PUFA) status and clinical complication in healthy Australian men and women. Compared with omnivores, vegetarians, especially vegans have lower n-3 PUFA levels in the tissue membrane phospholipids. Which is associated with increased collagen and ADP stimulated ex vivo whole blood platelet aggregation, plasma 11-dehydro thromboxane B2 and homocysteine levels, and decreased plasma HDL-C. Those may be associated with an increased thrombotic and atherosclerotic risk. However, meat-eaters had a significantly higher cluster of cardiovascular risk factors compared with vegetarians, including increased body mass index, waist to hip ratio, blood pressure, plasma total cholesterol (TC), triacylglycerols and LDL-C levels, serum lipoprotein(a) concentration, plasma factor VII activity, ratio of TC/HDL-C, LDL-C/HDL-C and TAG/HDL-C, and serum ferritin levels. Based on the present data, we do not suggest meat eaters to alter their diet to a vegetarian diet. However, we are suggesting that meat eaters and ovolacto vegetarians reduce their total fat, saturated fat and sodium intake, and increase their fiber, carbohydrate and antioxidant consumption. Vegetarians, especially vegans should increase their dietary n-3 to n-6 fatty acids ratio, vitamin B12 and zinc intake.
Introduction
The predominant polyunsaturated fatty acid (PUFA) in the Western diet is linoleic acid (LA, 18:2n-6) which is commonly found in vegetable seed oils. This is the parent fatty acid of n-6 series PUFA which can be converted in vivo to C20 and C22 n-6 long chain (LC) PUFA. Alpha-linolenic acid (ALA, 18:3n-3) is less abundant than LA, however it is also present in vegetable oils and is the precursor of C20 and C22 n-3 LC PUFA. Omnivores can obtain their C20 and C22 n-3 LC PUFA either from dietary ALA or direct from the consumption of fish, eggs or animal products (Mann et al 1995). Ovolacto vegetarians can gain a limited amount of C20 and C22 n-3 LC PUFA from milk, dairy products and eggs. In contrast, vegans must rely totally on endogenous synthesis from ALA by desaturation and elongation. Because animals can convert ALA to C20 and C22 n-3 LC PUFA, and plants cannot, there are no C20 and C22 n-3 LC PUFA in plant-based vegan diets. A diet with a low n-3 to n-6 PUFA ratio can cause a decreased tissue n-3 to n-6 PUFA ratio (ie. increased AA to EPA ratio) which may promote production of TXA2 (a potent platelet aggregating agent) leading to increased thrombosis tendency (Kinsella et al 1990).
In general, vegetarians have lower blood pressure, lower fat intake, lower body mass index, and higher antioxidant intake than omnivores. These factors are known to reduce the risk of cardiovascular disease (CVD). However, vegetarians also have a relatively high intake of LA which is a precursor of arachidonic acid (AA, 20:4n-6), and low eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3) intakes (Sanders & Roshanai, 1992). Is there a decreased n-3 PUFA status in vegetarian populations? Is there any associated between n-3 PUFA status and clinical complication? Therefore, we have conducted two cross-sectional studies in healthy Australian men and women, respectively.
The effect of habitual diet on the thrombotic risk factors in healthy male subjects
Dietary habits are believed to be related with the incidence of CVD (Thorogood et al 2003) and it is of interest whether habitual meat-eaters have a greater potential risk of thrombosis compared with vegetarians. In this study the hypothesis was that meat consumption does not increase the levels of blood indicators of an increased risk of thrombosis. Therefore, the aim of this study was to examine the thrombosis potential risk factors of subjects who are habitual meat-eaters compared with those who are habitual vegetarians. To achieve this aim, we have conducted a cross-sectional study in free-living healthy male subjects. One hundred and thirty nine male healthy subjects aged 20-55 years were recruited in the metropolitan area of Melbourne. Each volunteer completed a semi-quantitative Food Frequency Questionnaire (FFQ) and gave a blood sample. According to their habitual dietary intake (based on the semi-quantitative FFQ), the subjects were divided into vegan (n=18), ovolacto vegetarian (n=43), moderate meat eater (n=60) and high meat eater (n=18) four groups (Li et al 1999a).
Daily dietary protein, total saturated fat, cholesterol, sodium and zinc intake were significantly lower in both vegetarian groups, particularly in the vegan group compared with both high- and moderate-meat-eater groups. In contrast vitamin C, carbohydrate, fiber, iron and polyunsaturated fat intakes were higher in both vegetarian groups than in meat-eater groups. Compared with both the high- and moderate-meat-eater groups, both vegetarian groups have a significantly lower body mass index (BMI), waist to hip ratio, blood pressure, plasma total cholesterol (TC), LDL-C, ratio of TC/HDL-C and LDL-C/HDL-C, serum ferritin and vitamin B12 levels, haemoglobin and red blood cell count.
The proportion of total n-3 PUFA, 20:5 n-3, 22:5 n-3, and 22:6 n-3, and n-3 to n-6 ratio were significantly higher and the AA to EPA ratio were significantly lower in both the high- and the moderate-meat-eater groups than in both the ovolacto vegetarian and the vegan groups in the plasma PL.
The proportions of 20:5n-3, 22:5n-3, 22:6n-3 and total n-3 PUFA, and ratio of n-3 to n-6 were significantly lower in the vegans group than the ovolacto vegetarian, high- and moderate-meat-eater groups in the platelet PL.
Plasma homocysteine (Hcy) concentrations, mean platelet volume (MPV), and collagen and ADP stimulated ex vivo whole blood platelet aggregation were significantly higher in both vegetarian groups than in both high- and moderate-meat-eater groups. The vegan group had a significantly lower HDL-C and plasma factor VII activity than moderate-meat-eater group, whole blood platelet count than ovolacto vegetarian group, triacylglycerol (TAG), mean corpuscular volume and mean corpuscular haemoglobin levels than high-meat-eater group. There was no statistical difference between the four dietary groups with respect to white blood cell count, mean cell haemoglobin concentration, plasma fibrinogen, plasminogen concentrations, prothrombin time, activated partial thromboplastin time, antithrombin III, plasma 11-dehydro thromboxane B2 (TXB2), serum a-tocopherol, platelet phospholipid total saturated and n-6 PUFA levels, and AA induced whole blood platelet aggregation (Li et al 1999a).
Most acute clinical cases of cardiovascular disease are caused by the formation of a thrombus (Cahill & Newland, 1993) with platelet aggregation being the initial step in these events (Benditt & Schwartz, 1994). Increased platelet aggregability is significantly associated with CHD mortality (Thaulow et al 1991). Previously, only one study (Fisher et al. 1986) has investigated platelet function in vegetarians. They found no significant difference between the vegetarians and the omnivores ex vivo collagen-, AA-, ADP- and epinephrine-stimulated platelet aggregation. However, Fisher et al. (1986) used different methodology to that used in the present study. They used the traditional optical method for platelet aggregation test (using plasma) and they also adjusted the platelet count. It has been reported that dilution of platelet rich plasma can cause change in platelet responsiveness (Thaulow et al 1991). In the present study population, compared with both high- and moderate-meat-eater groups, vegetarians have a higher plasma Hcy level and ex vivo collagen and ADP induced platelet aggregability. The increased MPV in vegans suggests the presence of larger, activated platelets. When platelets become activated, they change from their normal resting disc-like structure to assume a spherical shape and their volume increases substantially. This, in conjunction with the increased platelet aggregability, suggests what should be an increased thrombosis tendency in vegans, and in the case of the platelet aggregation may be related to low dietary intake of n-3 PUFA.
Plasma Hcy concentration was significantly negatively correlated with plasma phospholipid concentration of PUFA 20:5n-3 (r = - 0.226, P=0.009), 22:5n-3 (r = - 0.182, P=0.036), 22:6n-3 (r = - 0.286, P=0.001), total n-3 (r = - 0.270, P=0.002) and ratio n-3/n-6 PUFA (r = - 0.265, P=0.002), and significantly positively correlated with 20:4n-6 (r = 0.180, P=0.037). In the partial correlation analysis, after controlling for serum vitamin B12 and folate concentration, plasma Hcy was significantly negatively correlated with plasma phospholipid concentration of 22:6n-3 (r = - 0.205, P=0.019), total n-3 (r = - 0.182, P=0.038) and ratio n-3/n-6 PUFA (r = - 0.174, P=0.048) (Li et al 2006). We have also found a similar result in in middle-aged and geriatric hyperlipaemia patients (50 males, 31 females) and 65 healthy subjects (43 males, 22 females) in Hangzhou, China. Plasma Hcy demonstrated significant positive correlation with adrenic acid (22:4n-6) (r = 0.188, P = 0.018) and negative correlation with 22:6n-3 (r = - 0.277, P = 0.001) and the ratio of n-3/n-6 (r = -0. 231, P = 0.003) in sex-, age- and BMI-controlled partial correlation analysis (Li et al 2007).
Evidence from case control studies has indicated that an increased MPV is an independent risk factor for acute myocardial infarction (MI) (Martin et al 1991) and for acute and/or non-acute cerebral ischemia (O’Malley et al 1995). Large platelets, in such cases, have been shown to have increased reactivity, leading to the potential for thrombus formation. In a multiple linear regression analysis, after controlling for potential confounding factors such as dietary group, age, exercise, body mass index, and dietary PUFA and saturated fat, cholesterol, carbohydrate, and fiber intake, the MPV was still strongly negatively correlated with platelet PL 20:3n-6 (P = 0.003) and 22:5n-3 (P=0.001). The present data suggest that 22:5n-3 and 20:3n-6 may play a role in the structural function of the platelet membrane (Li et al 2002).
All above may be associated with an increased thrombotic and atherosclerotic risk. However, both the high- and moderate-meat-eater groups have a cluster of thrombotic and atherosclerotic risk factors higher than both ovolacto vegetarian and vegan groups. These factors include BMI, waist/hip ratio, blood pressure, coagulation factor VII activity, plasma TC, LDL-C and TAG concentration, ratio of TC/HDL-C, LDL-C/HDL-C and TAG/HDL-C, and serum ferritin levels.
Essential fatty acid status, lipoproteinp(a) and lipoprotein lipids in healthy female vegetarians
It has been claimed that increased serum lipoprotein(a) levels and decreased n-3/n-6 polyunsaturated fatty acid (PUFA) status are associated with increased risk of cardiovascular disease (Berner 1993, Hajjar & Nachman 1996). The aim of this study was to investigate the serum lipoproteinp(a) levels, PUFA status and correlates of serum lipoproteinp(a) in healthy female vegetarians and omnivores. We have been conducted a cross-sectional study in free-living healthy female vegetarians (n=50) and omnivores (n=24) to assess differences which may have implications for cardiovascular risk (Li et al 1999b). Subjects were asked to complete a 12-day weighed diet record within a month including three to four weekend days, and were given detailed instructions on how to complete this. A set of scales was provided for each subject. Where it was not possible to weigh food, subjects were told to describe the quantities consumed using either household measures or using the photographs of representative portion sizes of common foods provided in the back of the diet record.
Mean serum lipoprotein(a) concentration was lower in the vegetarians (171 mg/L) than in the omnivores (247 mg/L). Serum lipoprotein(a) concentration was significantly negatively correlated with carbohydrate intake (as % of energy), and positively correlated with plasma total cholesterol. Compared with the omnivores, the vegetarians had a significantly lower concentration of 20:3n-6, 20:4n-6, 22:5n-6, 20:5n-3, 22:6n-3, total n-6 and n-3 PUFA, and ratio of n-3/n-6 PUFA in serum phospholipids. Lower concentrations of plasma TC, serum phospholipids total fatty acid, total saturated fatty acid and AA, and a tendency to lower serum lipoprotein(a) in vegetarians may provide beneficial effects on cardiovascular disease risk. However, decreased concentration of serum PL n-3 PUFA may potentially promote thrombotic risk.
Conclusion
Based on the present data, we are suggesting vegetarians should perhaps increase their dietary n-3 PUFA intake, and thus improve the balance ratio of n-3 to n-6 PUFA, to reduce any thrombotic tendency that might increase their generally low risk of cardiovascular disease.
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李鐸， Prof. Dr. Duo Li 1958
Professor of Nutrition and Food Safety, Zhejiang University, China. Director, Nutrition and Food Safety Center, Asia Pacific Clinical Nutrition Society. Professor of Nutrition and Food Safety, Zhejiang University, China.
Professor of Nutrition and Food Safety, Zhejiang Gongshan University, China. Senior Research Fellow, Dept. of Food Science, RMIT University, Australia. Research Fellow, School of Health Science, Deakin University, Australia.
Australian (as delegated speaker from China)
remdl@126.com, duoli@zju.edu.cn
(a) Department of Food Science and Nutrition, Zhejiang University, China
(b) Department of Food Technology and Nutrition, Mahasarakham University, Thailand
(c) School of Biomedical and Clinical Sciences, Victoria University, Australia
(d) School of Exercise and Nutrition Sciences, Deakin University
