Polyunsaturated Fat
- Polyunsaturated fatty acid (PUFA) is a healthy fat that does not raise blood cholesterol levels. In fact consumption of PUFA lowers LDL significantly.1-4
- There are two series of PUFA that are deemed essential. Linoleic acid (C18:2 n-6) is the predominant omega-6 or n-6 PUFA. The predominant (parent) omega-3 or n-3 PUFA is linolenic acid (18:3 n-3). See Fish.
- Linoleic acid increases the fecal excretion of steroids, and inhibits the hepatic synthesis of apo B containing lipoproteins. Replacing saturated fatty acids (SAFA) with PUFA or monounsaturated fatty acids (MUFA) reverses the suppression of LDL-receptor activity by cholesterol-raising SAFA. Substituting 20% of energy from SAFA with PUFA decreases the total cholesterol (TC) level by 40 mg/dl. Most of the reduction is in LDL and the number of apo B particles. PUFA does not raise the triglyceride (TG) level, and sometimes lowers it.1-4
- The two undesirable effects of PUFA are increased susceptibility for lipid peroxidation, and lowering of the HDL level. HDL levels are reduced by about 1% for every 2% of MUFA or SAFA energy substituted with PUFA. The substitution of PUFA for SAFA energy has played a major role in reducing TC levels and CAD in the USA.4
- The CAD (coronary artery disease) mortality rate declined by 69% in the past 3 decades in the USA. About a third of the decline in CAD rates is attributed to the decrease in blood cholesterol levels from 240 to 199 mg/dl in the population; this, in turn, was due to an increase in the consumption of PUFA from 3% to 6%, and a decrease in SAFA consumption from 16% to 12% of the energy.
- The importance of PUFA is further underscored by the marked differences in PUFA consumption, which parallel the 4-fold difference in CAD rates between France and Finland.
- Vegetable oils such as soybean, corn, safflower, sunflower, and cottonseed, are the primary sources of n-6 PUFA. The impact of consuming liberal quantities of PUFA in the form of soybean oil on blood cholesterol level and recurrent heart attacks are given in figures 076 and 077. 1, 5 The 16% reduction in recurrent heart attacks achieved with one pint (473 ml) of soybean oil per week for 60 months before the advent of the statins is nearly similar to that achieved with standard dose of first generation statins.6
- The average consumption of PUFA in the western diet is 6%–8% of energy (17 g/day for men, and 12 g/day for women).Contrary to previous fears, n-6 PUFA do not antagonize the anti-inflammatory effects of n-3 PUFA nor do they raise the risks of breast, colorectal, or prostate cancer in humans.3 Up to 10% of the energy from PUFA is recommended.
- However, a very high n-6 PUFA to n-3 PUFA ratio may increase the thrombogenicity through increased production of arachidonic acid and thromboxane A2. This is because linoleic and linolenic acids use the same set of enzymes for desaturation and chain elongation. An n-6 PUFA to n-3 PUFA ratio of 3:1 appears to be optimum. Japan, which has one of the highest rates of fish consumption, has recently changed the recommendation of this ratio from 4:1 to 2:1 which may be advisable for vegetarians.3
Sources
1. Leren P. The Oslo diet-heart study. Eleven-year report. Circulation. Nov 1970;42(5):935-942.
2. Enas EA. Cooking oils, cholesterol and CAD: facts and myths. Indian Heart J. Jul-Aug 1996;48(4):423-427.
3. Enas EA, Senthilkumar A, Chennikkara H, Bjurlin MA. Prudent diet and preventive nutrition from pediatrics to geriatrics: current knowledge and practical recommendations. Indian Heart J. Jul-Aug 2003;55(4):310-338.
4. Enas EA. How to Beat the Heart Disease Epidemic among South Asians: A Prevention and Management Guide for Asian Indians and their Doctors. Downers Grove: Advanced Heart Lipid Clinic USA; 2011.
5. Clarke R, Frost C, Collins R, Appleby P, Peto R. Dietary lipids and blood cholesterol: quantitative meta-analysis of metabolic ward studies. BMJ. Jan 11 1997;314(7074):112-117.
6. Enas E.A., Hancy Chennikkara Pazhoor MD, Arun Kuruvila MBBS, Krishnaswami Vijayaraghavan MD F. Intensive Statin Therapy for Indians:Part I Benefits. Indian Heart J 2011; 63: 211-227.