This may seem complicated but let’s try to simplify.
All rancid oils are bad.  If it smells bad , it probably is bad.
Some people are more talented than others at detecting rancidity
but smell is not a guarantee.  Chemicals can be added that deodorize the oil.
Heat makes oils go rancid.  Some oils are more sensitive.

Healthy oils
Flax(unrefined)--high omega 3, quickly goes rancid so keep cold
Hemp--contains GLA(only anti-inflammatory omega 6)
Olive--very low omega 6, high flavanoids, often counterfeit (be careful of source)
Butter(grass fed)--conjugated linoleic acid (good), low omega 6,
                             if clarified it has smoke-point of 500o F
Lard (non-hydrogenated) and Duck fat--moderate mono-unsaturated fat, low omega 6
Avocado--high in omega3 and omega 9
Macadamia--high omega 3 and omega 9 plus unique anti-oxidants
Coconut(unrefined)--high in good saturated fat (lauric acid), low in omega 6
Almond, Peanut, Pecan, Hazelnut (refined), Apricot kernel(refined)--tolerate moderate temperatures, contain mono-unsaturated fats, eat in moderation

Unhealthy oils
Butter--grain fed--high in “bad” saturated fat and omega 6
Canola--may have pesticide traces, high heat processing damages omega 3”s
soybean--high omega 6
corn--high omega 6
Pumpkin seed--high omega 6, goes rancid easily
Walnut--high temperature processing damages omega 3’s
             high in omega 6, goes rancid easily.  eat the whole walnut
Sesame--high omega 6
Grapeseed--high omega 6
palm and palm kernel--high omega 6
Cottonseed--high omega 6, very low omega 3
Margarine, Hydrogenated Coconut, and Shortening--contain trans fats with are very bad

Dave

Photo by Dave Hutt, www.dmddigitalphoto.com

Discussion of the Mediterranean diet has been popular in the media for years.  The results of a multi-center study of the diet, the PREDIMED, were recently published in the New England Journal of Medicine.  There were 7,447 participants studied between 2003 and 2011.  Three diet variations were studied.  Mediterranean diet supplemented with olive oil,  Mediterranean diet supplemented with tree nuts (almonds, walnuts, and hazelnuts), and a low fat diet.  After five years the two Mediterranean diets showed a highly significant 30% reduction in the risk of suffering a cardiovascular death, a myocardial infarction or a stroke compared to the low fat diet.  This study runs directly contrary to the commonly held belief that low fat diets are better to protect the cardiovascular system.  It does suggest that the type of fats are important.

--Dave

Ramón Estruch, Emilio Ros, Jordi Salas-Salvadó, Maria-Isabel Covas, D.Pharm., Dolores Corella, Fernando Arós, Enrique Gómez-Gracia, Valentina Ruiz-Gutiérrez, Miquel Fiol, José Lapetra, Rosa Maria Lamuela-Raventos, Lluís Serra-Majem, Xavier Pintó, Josep Basora, Miguel Angel Muñoz, José V. Sorlí, José Alfredo Martínez, Miguel Angel Martínez-González. Primary Prevention of Cardiovascular Disease with a Mediterranean Diet. New England Journal of Medicine, 2013

Photo by Dave Hutt, www.dmddigitalphoto.com