CSIRO Human Nutrition, PO Box 10041 BC Adelaide, South Australia
Rapeseed is a distinctive oil containing a significant amount of the essential n-3 fatty acid alpha linolenic acid plus the cholesterol-lowering oleic acid with a low level of cholesterol elevating saturated fatty acids. The only other common oil with a significant amount of alpha linolenic acid is soybean oil but this also contains a large amount of linoleic acid which reduces the effect of the n-3 fatty acid and shortens shelf life. Rapeseed oil was part of a diet which significantly reduced deaths in the three years following a myocardial infarction in the Lyon Diet Heart Study
KEYWORDS alpha linolenic acid, heart disease, cholesterol, n3, n6 fats
Rapeseed differs from other monounsaturated oils by containing, at the 10% level, the essential n-3 fatty acid alpha linolenic acid (18:3). This can be converted in vivo to a longer, more unsaturated form, EPA (20:5) which competes with the n-6 equivalent arachidonic acid (20:4). Arachidonic acid is converted to thromboxane and prostacyclin plus a whole series of 20 carbon prostaglandins and leukotrienes which have inflammatory actions. The form of thromboxane produced from EPA is far less powerful at aggregating platelets than that derived from arachidonic acid while the prostacyclin is very similar in activity. Thus an individual with a large intake of rapeseed oil can significantly modify their risk of developing a thrombosis. This is particularly important in individuals with coronary artery disease in whom the clinical event of myocardial infarction is precipitated by a thrombosis over a fissure or rupture in the atherosclerotic plaque in the coronary artery. The Cretan cohort in the Seven Country study, which had a very low rate of coronary artery disease despite obesity (Menotti et al, 1993) had three fold more alpha linolenic acid (ALA) in their plasma compared with the Dutch cohort (Sandker et al 1993). About 50g/day of rapeseed oil has the same effect on levels of EPA as a weekly portion (50-100g) of fatty fish.(Valsta et al 1996)
LYON DIET HEART STUDY
This theory was tested in the Lyon Diet Heart Study (de Lorgeril et al 1994) which randomised 605 patients after a myocardial infarction to standard care and an experimental diet. The experimental diet was aimed to be more Mediterranean in character: more bread, more root vegetables and green vegetables, more fish, less beef more poultry and more fruit while butter and cream were replaced by a margarine containing 4.8% alpha linolenic acid and 48% oleic acid. Plasma alpha linolenic acid increased by 70%, EPA by 40% and linoleic and arachidonic acid (AA) fell by 7-8%. Blood cholesterol did not change. After a mean follow up of 27 months, there were 16 cardiac deaths in the control and 3 in the experimental group; 17 non-fatal myocardial infarction in the control and 5 in the experimental groups: a risk ratio for these two main endpoints combined of 0.27 (95% CI 0.12-0.59, p = 0.001) after adjustment for prognostic variables. Overall mortality was 20 in the control, 8 in the experimental group, an adjusted risk ratio of 0.30 (95% CI 0.11-0.82, p = 0.02). Although this result cannot be directly attributable to the increase in alpha linolenic acid it is suggestive and supports the results seen in the DART study in which the same effect was seen with EPA itself .
THE DART STUDY (DIET AND REINFARCTION STUDY)
In this study 2033 men who had recovered from MI were allocated to receive or not to receive advice on each of three dietary factors: a reduction in fat intake and an increase in the ratio of polyunsaturated to saturated fat, an increase in fatty fish intake, and an increase in cereal fibre intake (Burr et al.1989) The advice on fat was not associated with any difference in mortality, perhaps because it produced only a small reduction (3-4%) in serum cholesterol. The subjects advised to eat 2-3 serves a week of fatty fish (or fish oil if fish was disliked) had a 29% reduction in 2 year all-cause mortality compared with those not so advised. This effect, which was significant was not
altered by adjusting for ten potential confounding factors.
Thus if EPA is increased directly via fish or fish oil or indirectly via alpha linolenic acid there appears to be protection from death in the 2-5 years after a myocardial infarction.
USE IN PRIMARY PREVENTION OF HEART DISEASE:LOWERING BLOOD CHOLESTEROL.
Diets low in saturated fat lower blood cholesterol. Does rapeseed oil have any advantages over sunflower seed oil or olive oil?
One of the first studies that compared sunflower and rapeseed in 1992 by Valsta et al showed that rapeseed lowered the harmful LDL cholesterol by 23% versus 17% for sunflower. The ratio of the protective apoA1 to the harmful apoB was 3.0 for rapeseed and 2.4 for sunflower. Surprisingly both these oils enhanced platelet aggregation to collagen and ADP compared with milk fat (Mutanen et al 1992). A later study from Nydahl et al (1994) in 100 young healthy subjects demonstrated no differences between sunflower -enriched diets and rapeseed oil enriched diets which lowered their LDL cholesterol levels by 5-7% . In 95 subjects with moderately high cholesterol levels both diets lowered LDL cholesterol by 14-16% but triglyceride was lowered more by the sunflower oil diet (Gustafsson et al 1994). Rapeseed oil lowers LDL cholesterol (17%) more than olive oil (13%) in hyperlipidemic subjects (Nydahl et al 1995). In another tightly controlled study rapeseed and corn oil had similar effects on LDL cholesterol (a 12 and 13%fall respectively) while olive oil was again less effective (a 7% fall) in a diet containing less than 30% fat (Lichtenstein et al 1993). Olive oil contains squalene (a cholesterol precursor) which elevates blood cholesterol (Miettinen and Vanhanen 1994).
Thus solely in terms of blood cholesterol rapeseed oil is superior to olive oil and equivalent to sunflower. Although several papers have demonstrated enhanced platelet aggregation in vitro with rapeseed and sunflower oil it does not appear to be a problem in vivo as demonstrated by the Lyon Diet Heart Study.
THE N6/N3 RATIO
There is some evidence that n3 fats are both elongated more and incorporated more into membrane phospholipids when the n6 fat linoleic acid is restricted. These two families of fats compete for the same enzymes for elongation and desaturation and their end products EPA and arachidonic acid (AA) compete for a position in the membrane. In rats a ratio of dietary ALA to LA of 1:3 and 1.3:1 produces more EPA and a lower arachidonic acid than ratios of 1:7 and 1: 4. The EPA/AA ratio increased as the ALA/LA ratio increased.(Leece and Allman 1996). In humans there is a direct linear relationship between dietary ALA and plasma and tissue phospholipid EPA levels (Mantzioros et al 1995). Rapeseed oil has an ALA/LA ratio of 1:2 thus producing near maximal incorpoartion of EPA in tissue phospholipid. In addition a low linoleic acid diet leads to lower levels of linoleic acid in membrane phospholipids and allows a 30% greater incorporation of EPA from fish oil into the membrane compared with a high linoleic acid diet (Cleland et al 1992). However membrane AA levels and leukotriene B4 production rates from AA were not changed
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