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Plant compared with marine n–3 fatty acid effects on cardiovascular risk factors and outcomes: what is the verdict?

Abstract

Plants provide α-linolenic acid [ALA; 18:3n–3 (18:3ω-3)], which can be converted via eicosapentaenoic acid (EPA; 20:5n–3) to docosahexaenoic acid (DHA; 22:6n–3), which is required for normal visual and cognitive function. Dietary ALA is provided mainly by vegetable oils, especially soybean and rapeseed oils, but is destroyed by partial hydrogenation; it is also present in high amounts in walnuts and flaxseed. Dietary EPA and DHA are provided mainly by fish and so are absent from vegan diets and only present in trace amounts in vegetarian diets. Vegetarians and vegans have lower proportions of DHA in blood and tissue lipids compared with omnivores. High intakes of EPA and DHA (typically in the range of 3–5 g/d) but not ALA have favorable effects on several cardiovascular disease (CVD) risk factors and have been postulated to delay arterial aging and cardiovascular mortality, but these intakes are beyond the range of normal dietary intake. Arterial stiffness, which is a measure of arterial aging, appears to be lower in vegans than in omnivores; and risk of CVD is approximately one-third lower in vegetarians and vegans compared with omnivores. Prospective cohort studies showed higher intakes of EPA+DHA, and less consistently ALA, to be associated with a lower risk of CVD, especially fatal coronary heart disease, but meta-analyses of randomized controlled trials of supplementation of EPA+DHA or ALA in secondary prevention of CVD showed no clear benefit. Current evidence is insufficient to warrant advising vegan and vegetarians to supplement their diets with EPA or DHA for CVD prevention.

Full-text

Alex’s Notes: Humans (with a few exceptions) eat a very land-based diet. This is problematic because low intakes of fish are associated with a reduced risk of cardiovascular disease (CVD), with as little as one serving per week showing a 16% reduction in risk of death from CVD. The notable difference between land and sea creatures is their fat; fish and seafood are rich in the long-chained omega-3 fatty acids EPA & DHA. Conversely, the land-based food chain (and typical Western diet) is saturated with the short-chained omega-6 fatty acid linoleic acid (LA) from seed oils and cereal grains. This pro-inflammatory fat moves its way up the food chain with most long-chained polyunsaturated fatty acids (PUFAs) in eggs, milk, and meat being from the omega-6 family.

The other short-chained PUFA is alpha linolenic acid (ALA), which exists in various nuts, seeds, and dark green leafy vegetables. The problem is that ALA is not converted into EPA or DHA in any significant amount in humans, and it has been demonstrated that the proportions of DHA in the blood lipids of vegans and vegetarians were lower than those in omnivores. This raises important questions with regard to the health benefits of omega-3 fatty acids. With a focus on cardiovascular health, this review helps illuminate the differences between ALA and EPA/DHA.

Right off the bat EPA & DHA smack down ALA, with intakes of more than 1g of EPA/DHA shown to reduce plasma triglycerides in a dose-dependent manner while ALA doesn’t have this effect, even at high doses. Of course, this benefit may be offset if you have the wrong genes. Some people may respond to fish oil with an increased LDL-cholesterol amount. But neither of these effects matter if you are already healthy because low intakes of fish oil or fish do not affect total cholesterol or LDL in normolipidemic subjects.

Blood cholesterol levels are a useful marker for cardiovascular risk, but it’s only one piece of the puzzle. Elevated blood pressure means the heart needs to work harder to pump blood, and this can put excessive strain on it if the elevation is chronic. EPA/DHA supplementation of greater than 3g daily, but not ALA, has been shown to lower blood pressure. As an added bonus, EPA/DHA are anti-inflammatory.

Many observational studies support the protective effect of fish consumption, but it is difficult to gauge the specific contribution of DHA/EPA from other nutrients or compounds present in the fish. The Alpha-Omega Trial comparing EPA/DHA supplementation to ALA supplementation showed no difference in effect on adverse cardiovascular events between the treatments. Conversely, the DART study showed that eating two portions of oily fish per week decreased total mortality over the following two years by 29% in men who previously suffered from a heart attack, but a second study by the same group found no such benefit with fish oil.

Overall, it appears a hierarchy exists in the realm of omega-3s. Fish is the absolute best and it is prudent to consume several servings per week of fatty fish (salmon, sardines, and mackerel). If that is not an option, for whatever reason, then supplementing with fish oil in modest amounts is the next best thing. Finally, don’t waste your time with ALA because it doesn’t even have the same effects as EPA/DHA, at least with regard to cardiovascular health.

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