One of the first things to be aware of when it comes to answering which fats are healthy for you, is that there are essential fats and non-essential fats. An essential fat is one that the body needs, but which the body cannot manufacture on its own from scratch. A non-essential fat can be manufactured by the body using other nutrients as starting materials. Do not confuse essential with necessary.
Almost all fats, with the exception of trans-fats, are necessary, but if the fat is one that the body can make, then it is not essential that we get it in our diet.
All the fats can also be divide into saturated, mono-unsaturated and polyunsaturated. The essential fats are polyunsaturated, specifically the omega-3 and omega-6 fats. The non-essential fats are the saturated, mono-unsaturated and the omega-9 class of polyunsaturated. Within this context, the essential fats are healthy, since, if we do not get them in our diet, our health suffers.
Just because the essential fats are healthy does not mean that the non-essential ones aren’t. The mono-unsaturated fats, for example, can be consumed in larger amounts than the saturated fats without adverse effects. It is the saturated fats that we need to keep an eye on. Our society tends to consume too many saturated fats and, in addition, excess calories are converted by our bodies to saturated fats.
The unhealthy fats then are the trans fats (probably the worse) and the saturated fats. The healthier fats are the mono-unsaturated (mostly from vegetable oils) and the omega-3 and omega-6 polyunsaturated. I say “healthier” rather than healthy because too much of them can also cause problems. The “healthiest” is the omega-3 class, not only because it is one of the essential fat classes, but, unlike the omega-6s, our modern diet tends to provide low quantities of it.
A couple of notes of caution. The way in which our bodies use the various fats is rather complex. For example, plenty of research has shown that it isn’t so much the quantity of omega-3 and omega-6 that we get that matters, so much as the ratio of the two. Our bodies evolved to use a omega-6 to omega-3 ratio of about 1-to-1.Modern diets, that are low in omega-3 and high in omega-6(who’s main source is grain and grain fed meats), have omega-6/omega-3 ratios far above this. Research has shown a link between this high ratio for several diseases including breast cancer and type II diabetes. Another note of caution has to do with how you prepare the food you eat. For example, vegetable oils tend to be healthier than saturated fats, but not so if you tend to fry the oil. The frying process creates trans fats as well as lipid hydro-peroxides, both of which have detrimental effect on health.
A good article on fats in general can be found on the Mayo Clinic website
The following medical journal article is a good reference on the importance of the omega-3-to-omega-6 ratio
2)Biomedicine Pharmacotherapy. 2002 Oct;56(8):pages 365-79.