You would practically have to be as isolated as an Australian Aborigine not to have absorbed the message that dietary fats, particularly saturated fats, are unhealthy. I am extremely confident that this message is false, but it is nearly impossible to turn the opinion tide due to its pervasive presence. Most people don't question why fats are bad; they assume that researchers must have done their homework, and they trust the result.
To say that the current situation with regard to dietary fats is confusing would be an understatement. We are repeatedly told to keep our total fat intake down to, ideally, 20% of our total calories. This is difficult to achieve, and I believe it is misguided advice. In direct contradiction to this "low-fat" goal, we are encouraged to consume as much as possible of the "good" kinds of fats. Fortunately, the message is finally becoming widely embraced that omega-3 fats are healthy and that trans fats are extremely unhealthy. DHA (docosahexaenoic acid) is an omega-3 fat that is found in large quantities in the healthy brain. In the diet, it is available mainly from cold water fish, but eggs and dairy are also good sources. Trans fats are generated by a high-heat process that hydrolyzes polyunsaturated fats into a more stable configuration, which increases their shelf life but makes them so unnatural they almost can no longer be called a food. Trans fats are extremely damaging both to heart and brain health. A high consumption of trans fats has recently been shown to increase the risk of Alzheimer's . Trans fats are especially prevalent in highly processed foods -- particularly when fats are converted to a powdered form.
We are told to avoid saturated fats, mainly because they have appeared, from empirical evidence, to be more likely to raise LDL levels than unsaturated fats. Yet these fats are less susceptible to oxidation, and this may be why they show up in LDL -- because they are of higher quality and therefore should preferentially be delivered to the tissues for functional roles rather than as fuel (i.e., free fatty acids). Coconut oil, a saturated fat, has been shown to benefit Alzheimer's patients . And high-fat dairy (also highly saturated) has been shown to be beneficial both to fertility among women  and, remarkably, to heart disease .
Despite the widespread belief that fats (particularly saturated fats) are unhealthy, an article that appeared in the American Journal of Clinical Nutrition in 2004  claims that, for a group of post-menopausal women, a high-fat, high-saturated-fat diet affords better protection from coronary artery disease than a low-fat (25% of calories from fats) diet. The subjects in the study were obese women with coronary artery disease. Most of them had high blood pressure, and many had diabetes. They fit the profile for metabolic syndrome that I have previously argued is a direct consequence of a prolonged low-fat high-carb diet. I am gratified to see that my hypothesis that an increase in fat intake would decrease their risk of heart disease has been verified by a carefully controlled study.
Another investigation where fats were shown to afford protection against heart disease has just been completed. It involved a long-term study of a large number of Swedish men . The authors looked at low- vs high-fat dairy, as well as consumption of fruits and vegetables, meats, grains, etc. The only statistically significant result that afforded protection from heart disease was a combination of high-fat dairy and lots of fruits and vegetables. Fruits and vegetables with low-fat dairy afforded no protection.
I suspect one of the critical nutrients the fruits and vegetables provide is antioxidants that help prolong the life of the fats. Other excellent sources of antioxidants include richly colored fruits like berries and tomatoes, coffee, green tea, and dark chocolate, and several spices, most especially cinnamon and turmeric (a major ingredient of curry). These should be consumed in abundance along with fats for optimal results.
Polyunsaturated fats such as corn oil and canola oil are unhealthy for the brain precisely because they are unsaturated. There are two major problems: (1) they have a low melting point, which means that, if they are used for frying they will be converted to trans fats, which are extremely unhealthy, and (2) they are much more susceptible to becoming rancid (oxidized) at room temperature than saturated fats, i.e., they have a shorter shelf life.
Researchers in Germany recently conducted an ingenious experiment designed to determine how the degree of freshness of polyunsaturated fats affects the metabolism of those fats in female lactating rats . They divided female rats into two groups, and the only difference between the test group and the controls was that the test group was given fats that had been left in a relatively warm place for 25 days, which caused considerable oxidative damage, whereas the controls were fed fresh fats instead. The rats' unusual diet was begun on the day that they gave birth to a litter. The researchers examined the mammary glands and the milk produced by the two groups for apparent differences. They found that the test group's milk was markedly reduced in the amount of fat it contained, and their mammary glands correspondingly took up less fat from the blood supply. One might surmise that the rats' metabolic mechanisms were able to detect oxidative damage to the fats, and therefore rejected them, prefering to do without rather than to risk the consequences of feeding their pups oxidized fats. Consequently, the pups of the test group gained significantly less weight than the control group's pups.
Boxed items like cookies and crackers that contain processed polyunsaturated fats are doctored with antioxidants and even antibiotics to protect them from spoiling. Once they're consumed, however, they still have to be protected from going rancid. Biochemical laws work the same way whether inside or outside the body. There are plenty of bacteria throughout the body that would be eager to take up house-keeping in rancid fats. The body has devised all kinds of strategies for protecting fats from oxidation (becoming rancid) and from attack by bacteria. But its task is rendered much easier for saturated rather than unsaturated fats, and for fresh rather than stale fats.
If we stop trying to get by on as few fats as possible in the diet, then we don't have to become so preoccupied with getting the "right" kinds of fats. If the body is supplied with an overabundance of fats, it can pick and choose to find the perfect fat to match each particular need; excess or defective fats can just be used as fuel, where it's not very important which fat it is, as long as it can be broken down to release energy.