hen eggs are common food widely acceptable by people around the world , either rich or poor .
not only treated as a main dish and used as an ingredient in cooking and bakery , hen eggs can be made into yolk oil ( generically known as  dn yu ,  lun yu ,  lun jng ,  dn hung yu or  j z yu )
. yolk oil is popular in japan and korea , and among chinese people ( including chinese people from the mainland china , taiwan , hong kong , malaysia , etc ) . in japan ,
yolk oil products with trademark can be purchased over the counter from pharmacies or cosmeceutical stores . in taiwan and china ,
most of these products can be purchased from homemade market or traditional chinese herbal shops , mostly without a brand .
yolk oil is described in the well known chinese medical encyclopedia flora sinensis (  bn co gng m ) as a remedy for every diseases , with specific indications in skin sore and ulcer on head and mild fever with diarrhea in children ( boym , 1956 ) .
nowadays , yolk oil is widely used as a complimentary medicine for treating or preventing heart diseases , especially in japan
current methods for obtaining yolk oil include heating and chemical extraction , or a combination of both .
it is generally believed among yolk oil users and chinese medicine practitioners that yolk oil made from heating ( yoheat ) method , mostly by using the traditionally wok , is more effective than yolk oil made from laboratory based chemical extraction ( yoext ) .
the yolk is dried in oven or on the wok , and then continuously heated and fried without cooking oil , until yellowish or slightly charred yolk oil effuses . the oil is then collected and stored for further use .
also , it is generally thought that charred yolk oil ( cyo ) is more effective than the yellowish yolk oil as a medicine .
however , whether yolk oil should be extracted at yellowish or charred state , remains an argument .
the opponents argue that cyo may contain carcinogens and thus the threat of cancers can not be ignored , while the supporters claim that the toxicity of carcinogens is equalized by other 
good  components of the oil , or the black materials in the oil are harmless something else .
indeed , kato et al ( 1990 ) detected at least 7 heterocyclic amine mutagens in cyo branded samples .
the worrying part of cyo is that heating method can simply be done in kitchen , and drug safety issues may not be taken into account in the heating process .
an improvement of the heating method for avoiding the formation of heterocyclic amines is warranted .
lipids are the main components in egg yolk , including triglycerides ( 65% ) , phospholipids ( 29% , out of which 86% are phosphatidylcholine and 14% phosphatidylethanolamine ) , cholesterol ( 5% ) , and free fatty acids ( < 1% ) .
there have been very few literatures attempting to analyze the compositions of yoheat , and thus it is unclear which components are potentially effective to human health and diseases .
only a very small number of publically available modern scientific literatures have reported the effects of yoheat in human health and diseases . among these , lin et al reported the effects of yoheat in reducing lipid levels using rodent models ( lin et al , 2004a ; lin et al , 2004b ) .
the three experimental groups in the hamster model were fed with 2% acidic fraction oil , 4% of acidic faction oil and 2% of neutral fraction oil in their basal diets for 6 weeks , respectively , while the two experimental groups in the rat model were fed with 2% and 4% of yolk oil in their basal diets for 7 weeks , respectively .
the authors showed that yoheat reduced liver total cholesterol in rats ( lin et al , 2004a ) and triglycerides in hamsters ( lin et al , 2004b ) .
a yoext , al721 , has been reported to reverse the brain membrane hyperviscosity and , concomitantly , markedly reduce , or even completely abolish the withdrawal symptoms precipitated by naloxone in morphine - addicted mice ( heron et al , 1982 ) .
the compound has been shown to reduce htlv - iii infectivity in human peripheral - blood lymphocytes ( salin et al , 1985 ) .
a study design comparing the effects of yoheat and yoext in cellular and animal models may reveal essential information on the usefulness of the yolk oil .
because heart diseases have become common diseases around the world , it is expected that an easy - to - make complimentary medicine like yolk oil may become even more common in the next few decades .
although generic information on yoheat can be obtained from commercialized companies ( e. g. , http://www.tezukuri-ranyu.jp/ ) or books published by user groups ( nomoto , 1989 ) , there is an obvious need to verify the treatment effects , production methods and guidelines and safety of yoheat by evidence based studies .
 yolk oil is common in asia . according to the flora sinensis , 
 yolk oil is a multipurpose medicine , with specific dermatological and fever indications . 
 nowadays , it is generally used as a complimentary medicine for heart diseases . 
 yolk oil can be made from heating or chemical extraction method . 
 it is generally believed that yolk oil made from heating ( yoheat ) method is more effective as a medicine than that from extraction ( yoext ) . 
 the technical details of the heating method remain an issue of argument , including the degree of char and the threat of carcinogens formed during the heating process . 
 most yolk oil related studies used yoext as research material . 
 nevertheless , animal studies have showed that yoheat reduced triglycerides and total cholesterol in rodent liver . 
 it is expected an easy - to - make complimentary medicine like yoheat may become even more common and thus evidence based studies should be conducted to verify its pharmacological effects and safety . 
Introduction
Manufacture, Effects and Risks
Evidences for Applications
Future Perspectives and Conclusion