Fats are a form of fuel for the body, and also serve a variety of physiological functions. They are an essential nutrient. One gram of fat contains about nine calories (with a small variance according to the type of fat). The DRIs recommend that at least 20% of calories come from fat. There is no tolerable upper limit of fat intake.
It is a common philosophy, especially in the U.S., that fat consumption promotes weight gain, and fats should not be consumed in very large amounts. In recent years this has turned out to be largely untrue. Excess calorie consumption promotes weight gain, whether it is in the form of carbohydrates or fat. Without excess calorie consumption there is not weight gain. A person can healthfully consume any amount of fat within the limits of their caloric needs, as long as they reduce their intake of calories from other sources correspondingly.
Fats come in two varieties: saturated and unsaturated. Under the traditional view, high intake of saturated fats leads to higher levels of LDL ("bad") cholesterol and an increased risk of heart disease. This view has been subject to some question recently, but it remains dominant. For this reason unsaturated fats are considered more healthful than saturated fats, and it is recommended that the majority of one's fat intake be unsaturated. This is naturally achieved, for example, by consuming vegetable oils.
There are at least two known exceptions to the rule that unsaturated fats are good and saturated fats are bad. The first exception is trans fat, which is a kind of unsaturated fat that increases levels of LDL and the risk of heart disease. Trans fat is found in small quantities in animal fat, and in much larger quantities in partially hydrogenated vegetable oils, as may be found in processed foods. The prevailing recommendation is to consume little or no trans fat.
The second exception to the "unsaturated good, saturated bad" rule is found in medium chain triglycerides (MCTs). MCTs are found in large amounts in coconut oil. MCTs are generally believed to be especially healthful fats, and in particular have been found in some studies to promote fat burning and decrease appetite.
Two types of fat are essential nutrients. These are linoleic acid (LA), and α-linolenic acid (ALA). The AI for LA is 17g per day for men, and 12g per day for women. The AI for ALA is 1.6g per day for men, and 1.1g per day for women. These AI's were derived from looking at median intakes for USA and Canada because these countries do not show symptoms of deficiencies.
LA belongs to a category of fats called ω-6 fatty acids, and ALA belongs to a category called ω-3 fatty acids. It is believed today that an optimum fat intake approaches a 1:1 ratio of ω-6 to ω-3 fatty acids, or perhaps as low as 4:1. A typical fat intake contains substantially higher levels of ω-6 than ω-3, so achieving such a ratio usually requires decreasing ω-6 intake and/or increasing ω-3 intake.
Increasing ω-3 acid intake has good and bad aspects. High intakes of ω-3 acids (particularly as EPA and DHA) were associated with decreased immune response but also with decreased inflammation. Laboratory animals fed larger amounts of ω-3 had increased oxidative damage in liver, red-blood cells, kidneys, and bone-marrow DNA. This appeared to be ameliorated by vitamin E consumption.
A particular concern is the association with decreased clotting capability. The Greenlandic Inuit eat 6.5g/day of ω-3 acids and have increased risk of hemorrhagic stroke, nosebleeds, and urinary bleeding (they are also outstanding in other ways so it is not possible to pin this solely on fatty-acid consumption). A number of studies found increased bleeding with various doses starting at 2g/day above people's normal consumption. For a United States study where median group consumption was less than 0.48 g/d, no association was found between increased consumption and increased risk of hemorrhagic stroke.
The National Academy of Sciences warns against significant supplementation of ω-3 acids for individuals with diabetes or glucose intolerance because of observed negative effects. Additionally people taking anticoagulants like warfarin, aspirin or coumadin could have excessive bleeding when taking ω-3 supplements.
All of this should serve to illustrate that the topic of fats in human nutrition is perhaps quite complicated and poorly understood.