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FACTORS IN CARDIOVASCULAR DISEASE
ESSENTIAL FATTY ACIDS vs. CHOLESTEROL
Summary:
Data in the research literature, while not exhibiting uniform and homogeneous results, generally indicate that consumption of fish and vegetables rich in essential fatty acids (particularly omega-3 fatty acids) reduces blood levels of cholesterol, LDL-cholesterol and triglycerides, while mildly elevating HDL-cholesterol levels. The consequence is reduced incidence of cardiovascular disease and associated coronary artery disease.
Essential Fatty Acids
Western diets generally are deficient in essential fatty acids (EFA). Fatty acids in general are molecules with straight chains of carbon atoms. One end has a methyl group (a carbon atom with three attached hydrogen atoms). The other end has a carboxyl group (a carbon atom with one attached hydrogen atom and one attached oxygen atom - this oxygen atom gives the molecule its weak acidity).
A saturated fatty acid (SFA) has single bonds between all the carbon atoms. (C-C-C-C etc.). Since each carbon atom has a valence of four (meaning that each atom seeks to form four bonds), each interior carbon atom in the chain has two available bonds to form, beyond the two bonds forming the chain with adjacent carbon atoms, one on each side. In a saturated fatty acid, the two available bonds are "saturated" with hydrogen atoms.
An unsaturated fatty acid, in contrast, has at least one double bond between two adjacent carbons, rather than having all single bonds between adjacent carbons. An example is linoleic acid.
The EFAs known to be beneficial are unsaturated fatty acids. Linoleic acid (LA) is an omega-6 fatty acid, designated C18:2n6. This formula designates a molecule with 18 carbon atoms containing 2 double bonds, with the first double bond located between carbons numbered 6 and 7. The presence of the double bonds renders the molecule an unsaturated fatty acid, in this case doubly unsaturated. Linoleic acid is one of the essential fatty acids.1
In addition to omega-6 fatty acids, several omega-3 fatty acids have been found essential for health. They include eicosapentaenoic acid, C20:5n3 (EPA); another is docosahexaenoic acid, C22:6n3 (DHA). A third example is linolenic acid (LNA), designated C18:3n3. The body can make EPA and DHA from LNA if sufficient LNA is available in the diet.
Dietary Supply of EFA
Western diets are generally deficient in omega-3 fatty acids, and too rich in omega-6 fatty acids. Both types of fatty acids are necessary (leading to the name essential fatty acids or EFA). EFAs are required for proper function of the nervous system and contribute to mental health. However, the ratio of omega-3 and omega-6 fatty acids must be in balance (in roughly 1:1 ratio), and the Western diet is not in balance in this regard (for many people, the ratio is 1:10). Therefore, as advocated by Weil, good nutrition requires selectivity in food choices, so as to obtain more omega-3 fatty acids, and less omega-6 fatty acids, than occurs with the typical Western diet.2
Humans and animals cannot make omega-3 and omega-6 fatty acids (but they can make fatty acids with the double bonds further along the molecule). Therefore, to get the omega-3 and omega-6 fatty acids, humans and animals must eat plants that have them, or eat animals that have stored them from ingested plants. In the case of salmon, the fish eat algae containing the omega-3 fatty acids.
A table listing the sources of omega-3 and omega-6 fatty acids is provided below.
Conflicting Diets -- EFAs and Cholesterol Levels
Farmed salmon generally eat less algae than open-ocean salmon, and therefore have less omega-3 fatty acid content. Also, farmed fish are more subject to disease and to ingestion of toxins from nearshore sources. Therefore, open-ocean salmon are preferable. Some obstetricians advise pregnant women to eat salmon and sardines, in order to get the needed omega-3 fatty acids.3 Others warn pregnant women to be very modest in fish consumption because of the possibility of mercury contamination.
Contrary to the above recommendations by Weil are the 1990 recommendations by Ornish. He advocated an extremely low fat diet based on his view that "Fat is not 'bad' either; we just eat too much of it," and replacing saturated fats with oils will not help, because "All oils are 100 percent fat ... Adding any oil to your food will raise your cholesterol level."4 He also noted that salmon and other fish are high not only in omega-3 fatty acids, which are desirable, but also are high in saturated fat and cholesterol, and therefore advised against eating fish. He added that their omega-3 fatty acid content may be too high and "may increase the risk of hemorrhagic stroke...." and present some insulin and glucose risks for diabetics.5 He did however advocate intake of appropriate amounts vegetables containing omega-3 fatty acids and other nutrients that might be missing due to the Ornish diet's exclusion of fats in general, and allowed moderate consumption of fish by people free of heart disease.6
But the view by Ornish is not sustained by other research published in the literature, which reports beneficial effects of increased intake of certain oils. For example, daily ingestion of 2 tablespoons of virgin olive oil, for one week, reduces oxidation of LDL (oxidation of low density lipoprotein - a component of cholesterol considered injurious at high levels - worsens its effects) and raises blood levels of antioxidant compounds, particularly phenols. Virgin olive oil appears to protect LDL from oxidation.7
This type of report linking "good oils" with reduced LDL-cholesterol is sustained in a lengthy review of literature concerning fish consumption:As reviewed by Stone(1 [references not included here]) three prospective epidemiological studies within populations reported that men who ate at least some fish weekly had a lower coronary heart disease (CHD) mortality rate than that of men who ate none.(3-6) More recent evidence that fish consumption favorably affects CHD mortality, especially nonsudden death from myocardial infarction (MI), has been reported in a 30-year follow-up of the Chicago Western Electric Study.(7) Men who consumed 35 g or more of fish daily compared with those who consumed none had a relative risk of death from CHD of 0.62 and a relative risk of nonsudden death from MI of 0.33.8
After a lengthy review of literature, covering studies that show similar benefits of fish consumption, along with some studies that do not, the authors of this study concluded:
Summary: Omega-3 fatty acids have been shown in epidemiological and clinical trials to reduce the incidence of CVD. Large-scale epidemiological studies suggest that individuals at risk for CHD benefit from the consumption of plant- and marine-derived omega-3 fatty acids, although the ideal intakes presently are unclear. Evidence from prospective secondary prevention studies suggests that EPA+DHA supplementation ranging from 0.5 to 1.8 g/d (either as fatty fish or supplements) significantly reduces subsequent cardiac and all-cause mortality. For -linolenic acid, total intakes of 1.5 to 3 g/d seem to be beneficial.
Collectively, these data are supportive of the recommendation made by the AHA Dietary Guidelines to include at least two servings of fish per week (particularly fatty fish). In addition, the data support inclusion of vegetable oils (eg, soybean, canola, walnut, flaxseed) and food sources (e.g., walnuts, flaxseeds) high in -linolenic acid in a healthy diet for the general population (Table 5 [omitted here). The fish recommendation must be balanced with concerns about environmental pollutants, in particular PCB and methylmercury, described in state and federal advisories.9
Another famous diet advanced for the past 3 decades is the Atkins diet. It is diametrically opposed to the Ornish "reversing heart disease" diet. The Atkins diet advocates a low-carbohydrate diet, rich in proteins and fats. This diet has provoked numerous criticisms over the years. The American Heart Association (AHA), through its Nutrition Committee of the Council on Nutrition, Physical Activity and Metabolism, warned consumers as recently as 2001 to avoid high protein diets, including the Atkins, Zone, Protein Power, Sugar Busters and Stillman diets. The AHA view is that "[b]ecause high-protein diets rely on eating protein from animal sources, they are high in total fat, saturated fat and cholesterol, which may increase the risk of coronary heart disease."10
However, after many years of disrepute among most cardiologists and dieticians, because of its high fat content, recently the Atkins diet has been favorably reported because of new research showing that at least for some individuals, the diet causes cholesterol levels to decline. In two studies published in the New England Journal of Medicine, the Atkins diet led to higher levels of HDL cholesterol and reduced triglycerides. There was no difference in LDL cholesterol.11 Critics of the rosy interpretations of these new studies have pointed out that weight loss alone (which did occur in the two studies) can by itself lead to raised HDL and lowered triglycerides.
Other Factors Besides Cholesterol in Heart Disease
Adding further confusion are the publications that dispute the standard view altogether (that high cholesterol and saturated fat are injurious to cardiovascular health and are the cause of coronary heart disease). These publications reference the scientific and medical literature that fails to show correlations between high fat or cholesterol and coronary heart disease.12 Those who search the literature will find that there are many studies that dispute the standard view.
A wild card in the debate over cholesterol is the growing recognition of the negative impact of elevated homocysteine levels.13 Cholesterol is only one factor in athersclerosis/arteriosclerosis and cardiovascular disease. This is obvious when some people have very high cholesterol for decades, but still have clean arteries. Perhaps there is an interaction between homocysteine and cholesterol with respect to the overall effect on cardiovascular disease.
Elevated total plasma homocysteine (tHcy) level is a risk factor for occlusive cardiovascular disease (CVD). In a study in western Norway including 17,361 individuals aged 40 to 42 or 65 to 67 years at baseline, participants with preexisting CVD had higher mean tHcy values than individuals without CVD. Elevated plasma total homocysteine (tHcy) concentration has previously been associated with cardiovascular disease (CVD). During the past few years, it has become widely accepted as a risk factor based on many prospective and retrospective studies of more than 20,000 subjects.
Elevated tHcy level is a strong predictor of CVD mortality, especially among patients already suffering CVD and among the elderly. From correlation studies, elevation of tHcy is caused by deficiencies of folate and B12. Other causes include impaired renal function, older age, male sex, post-menopausal status, and lifestyle factors including smoking, heavy coffee consumption, and lack of exercise. Elevated homocysteine has weaker associations with blood pressure and serum cholesterol level.14
However, as with cholesterol studies, some studies have not shown high correlations between tHcy and CVD. More research is in order to determine if additional factors not yet understood are producing the contrary results.
In addition to cholesterol and homocysteine, another substance has been linked to coronary artery disease (CAD), called c-reactive protein (CRP). According to one report published in 2001, a study of 1286 women has shown that CRP appears to be a specific marker for cardiovascular disease in women, and a predictor for future cardiovascular events such as heart attacks.15
Thus, cholesterol is not the only dietary factor for CVD, and it may not even be the most important factor. How all factors (cholesterol, homocysteine, and CRP, and possibly others) function together is a subject for further study. As Dr. Ridker, first author of the cited CRP study, has noted, "half of all heart attacks and strokes occur among people with normal cholesterol levels, so we have to move beyond [mere] cholesterol measurement."
Conclusion
In the meantime, until there is clarification from new research, which can help us understand the various studies showing somewhat conflicting results, we may rely on general dietary recommendations -- these include eating lots of fresh fruits and vegetables, reducing intake of "empty" calories composed of table sugar and white starch, and moderating the intake of animal protein, in addition to avoiding the bad kinds of fats (saturated fats and trans-fats). Adding fish rich in omega-3 fatty acids is on balance very desirable. Also, all diets should include sufficient amounts of folate (folic acid) and vitamin B12. A very specific list of recommendations for dietary fats is provided by Weil.16
Sources of Essential Fatty Acids17
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Omega-3
| Omega-6 |
Fish oil
Flaxseed oil
Flax seeds
Hemp seeds
Herring
Mackerel
Purslane
Salmon
Sardines
Soybean oil
Soybeans
Walnut oil
Walnuts
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Corn oil
Cottonseed oil
Grapeseed oil
Meat
Peanut oil
Poultry
Safflower oil
Sesame oil
Soybean oil
Sunflower oil
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REFERENCES
1. Andrew Weil, Eating Well for Optimum Health (New York: Alfred A. Knopf, 2000), 82.
2. National Institutes of Health, "Workshop on Essentiality and Recommended Dietary Intakes for Omega-6 and Omega-3 Fatty Acids," 1999, as reported by Weil, 88.
3. Weil, 88.
4. Dean Ornish, Reversing Heart Disease (New York: Random House, 1990), 264-265; italics in the original text.
5. Ornish, 279.
6. Ornish, 279-280.
7. M. C. Lopez-Sabater et al., European Journal of Clinical Nutrition 2002;56:114-120.
8. Penny M. Kris-Etherton, William S. Harris, and Lawrence J. Appel, "MPH Fish Consumption, Fish Oil, Omega-3 Fatty Acids, and Cardiovascular Disease," Circulation 2002;106:2747.
9. Kris-Etherton, Harris and Appel.
10. Robert H. Eckel et al., Circulation 2001;104:1869-1874.
11. Gary D. Foster et al., "A Randomized Trial of a Low-Carbohydrate Diet for Obesity," New England J. Medicine, May 22, 2003; vol. 348, no. 21, 2082-2090; Frederick F. Samaha et al., "A Low-Carbohydrate as Compared with a Low-Fat Diet in Severe Obesity," New England J. Medicine, May 22, 2003, vol. 348, no. 21, 2074-2081.
12. An example is Judith A. DeCava, Cholesterol - Facts and Fantasies: What do the Experts REALLY Say (Centerville, MA: a Printery, 1994).
13. One of the major books on homocysteine is by Dr. Kilmer S. McCully, The Homocysteine Revolution (New Canaan, CT: Keats Publishing, 1997).
14. Eha Nurk et al., "Plasma Total Homocysteine and Hospitalizations for Cardiovascular Disease," Arch Intern Med 2002; 162:1374-1381.
15. Paul M. Ridker et al., Ann Intern Med 2002;136:529-533.
16. Weil, 99-102.
17. Weil, 85.
2003 © John C. Munday Jr.
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