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Sunday, March 4, 2012

DẦU CÁ (FISH OIL)


BACKGROUND
         
Fish oil can be obtained from eating fish or by taking supplements. Fish that are especially rich in the beneficial oils known as omega-3 fatty acids include mackerel, tuna, salmon, sturgeon, mullet, bluefish, anchovy, sardines, herring, trout, and menhaden. They provide about 1 gram of omega-3 fatty acids in about 3.5 ounces of fish.
 

Fish oil supplements are usually made from mackerel, herring, tuna, halibut, salmon, cod liver, whale blubber, or seal blubber. Fish oil supplements often contain small amounts of vitamin E to prevent spoilage. They might also be combined with calcium, iron, or vitamins A, B1, B2, B3, C, or D.

A further source of omega-3 FAs is from foods such as eggs, bread, and milk that have been fortified with omega-3 FAs, derived either from oily fish or from micro-algae. These can provide an alternative source in place of fish, but are not generally recommended as a substitute for fish consumption. It is not possible at this time to obtain the high level of EPA useful in cardiology from fortified foods.

There is concern over the potentially harmful effects of mercury, dioxins, and polychlorinated biphenyls present in some fish species, especially larger fish. Those containing particularly high concentrations of mercury include shark, swordfish, king mackerel, and tilefish. In a 2006 meta-analysis, Mozaffarian and Rimm concluded that the benefits of fish intake far exceed any potential risks; however, those species particularly high in contaminants should be avoided by pregnant women and nursing mothers. Omega-3 supplements provide a safe way to obtain omega-3, as the mercury remains bound to the protein of the fish and is not concentrated into the oil. Fish oil can be distilled to high levels of purity and concentration of particular omega-3s (EPA or DHA).

Fish oil is used for a wide range of conditions. It is most often used for conditions related to the heart and blood system. Some people use fish oil to lower blood pressure or triglyceride levels (fats related to cholesterol). Fish oil has also been tried for preventing heart disease or stroke. The scientific evidence suggests that fish oil really does lower high triglycerides, and it also seems to help prevent heart disease and stroke when taken in the recommended amounts. Ironically, taking too much fish oil can actually increase the risk of stroke.

Fish may have earned its reputation as “brain food” because some people eat fish to help with depression, psychosis, attention deficit-hyperactivity disorder (ADHD), Alzheimer’s disease, and other thinking disorders.

Some people use fish oil for dry eyes, glaucoma, and age-related macular degeneration (AMD), a very common condition in older people that can lead to serious sight problems.

Women sometimes take fish oil to prevent painful periods; breast pain; and complications associated with pregnancy such as miscarriage, high blood pressure late in pregnancy, and early delivery.

Fish oil is also used for diabetes, asthma, developmental coordination disorders, movement disorders, dyslexia, obesity, kidney disease, weak bones (osteoporosis), certain diseases related to pain and swelling such as psoriasis, and preventing weight loss caused by some cancer drugs.

Fish oil is sometimes used after heart transplant surgery to prevent high blood pressure and kidney damage that can be caused by the surgery itself or by drugs used to reduce the chances that the body will reject the new heart. Fish oil is sometimes used after coronary artery bypass surgery. It seems to help keep the blood vessel that has been rerouted from closing up.

When fish oil is obtained by eating fish, the way the fish is prepared seems to make a difference. Eating broiled or baked fish appears to reduce the risk of heart disease, but eating fried fish or fish sandwiches not only cancels out the benefits of fish oil, but may actually increase heart disease risk.

Omega-3 FAs, like omega-6 FAs, are a type of polyunsaturated FA (PUFA) that the body is unable to synthesize and that must, therefore, be obtained through the diet. The omega-3 FAs are distinguished from other classes of FAs by the structure of their carbon chains. FAs are chains of carbon atoms that are labeled with the ‘alpha’ carbon proximal to the carboxyl group and end with the distal ‘omega’ carbon. The omega-3 FAs contain a double bond between the third and fourth carbons from the omega carbon. All of the omega-6 FAs contain their first double bond between the sixth and seventh positions from the omega carbon. Humans lack the enzymes required to place a cis double bond at the omega-3 or omega-6 positions. Thus, omega-6 and omega-3 FA are essential FAs and must be supplied in our diet. Omega-6 FAs are present in grains and nuts. Omega-3 alphalinolenic acid (ALA) is an 18-carbon FA that is found in plants but cannot be easily elongated to the more important 20-carbon form by humans. The omega-3 FAs EPA and docosahexaenoic acid (DHA) are synthesized by phytoplankton and then consumed and concentrated by fish. They may be acquired in the diet by fish consumption, by foods fortified in omega-3, or by further concentration and specification in high-concentrate nutritional omega-3 supplements.



omega-6 linoleic acid (LA)



 omega-3 alphalinoleic acid (ALA)


omega-3 eicosapentaenoic acid (EPA)


omega-3 docosahexaenoic acid (DHA)