Originally Posted by AnyOldIron

you should always have a good amount of fat in your diet to optimise hormone production and provide essential fatty acids. 20-30% of overall calories is good, however keep it largely unsaturated if you are eating carbs.

the most important fats are the omega3 EPA/DHA.. you need fish oil to get these, or oily fish (mackerel is a good source). 10g fish oil a day or 3g EPA/DHA is a minimum. you can't get these directly from seeds or flax, the body must convert these from the omega3's in these foods (mostly ALA - a different omega3 acid) and the process is inefficient and inconsistent depending on your genetics. there are also omega6 and omega9 fatty acids but these are covered by most people's diets.

the rest of your fats should be mono's from things like olive oil, avocado, animal fat etc (animal fat is 50/50 sat and mono fat). aim for 1-2 tablespoons of olive oil a day for its benefits (which are many). mono fats are a great fuel source for the body as they burn easily.

saturated fat should be kept low in a high carb environment, so just get this from your usual meat/eggs and don't add any (butter, coconut oil, lard..). saturated fat is a testosterone regulator; my belief is this day's metro egg white/lean chicken brigade is rife with impotency for this very reason.. best natural test booster you will ever get is to eat high fat with about 50% sat fat (but you can't do this if you eat high carbs).

so, in simple terms: you need fat in your diet. poly's for omega3's (fish oil), mono's and sat fats for fuel/hormone production. (simplified model)

finally - do not use rapeseed, peanut, corn, sunflower, safflower or canola oils for fat. they disrupt the omega3:omega6 ratio which is skewed bigtime in favour of omega3's because of the popularity of vegetable oils when the world freaked out over sat fats and stopped using butter to cook with.. this imbalance of omega3's:omega6's messes with your insulin management and can cause you to age quicker.