There are a few reasons.
One reason to be selective with your fats is because of Omega 6 to Omega 3 ratios. People of this era consume way too many Omega 6's and are suffering a lot more inflammatory diseases. Long, long ago people had an Omega 6 to Omega 3 ratio of about 1:1... now we are closer to 10:1. Too many Omega 6's make us sick.
Another reason is that some fats are just not built to withstand heating. Fats that are not made for heat can go rancid, oxidize, and form free radicals if exposed to heat.
Fats to Cook With:When choosing which fats to cook with, you want to look to saturated fats. Why? Because saturated fats are heat stable. Oils which are not heat stable can oxidize and that is not a good thing. An easy way to tell if a fat is a saturated fat is to see if it's a solid form at room temperature. If it's solid at room temp then it's a saturated fat. Saturated fats are the ideal choice for cooking with but of course, consuming them cold is great too.
Coconut Oil - cold pressed or expeller pressed
Ghee - preferably from grass-fed cows
Palm Kernel Oil
Fats to Cook Gently With:There are other fats that while having a lot of saturated fat, they also have a bit of monounsaturated fat. The fact that there are saturated fats means you can cook with it but since there are some monounsaturated fat; it's not as heat stable and therefore should be cooked gently or at lower temperatures.
Lard - preferably from pastured hogs
Butter - preferably from grass-fed cows
Fats to Use ColdPolyunsaturated fats are also important fats to consume. However, they are more delicate and easily go rancid, oxidize, and form free radicals when heated because of this, these fats are best consumed raw or cold. There are many polyunsaturated fats but many of them are very high in Omega 6's. We consume too many Omega 6's as it is so when choosing oils it's best to choose ones on the lower end of the Omega 6 scale.
Extra Virgin Olive Oil - cold pressed.
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