A. The science of human nutrition is less than 75 years old, and in fact, even today it would be better named "the study of nutrition." One of the most serious complications, voiced by Lucretius centuries ago, is the fact of biochemical individuality: "What is food to one may be fierce poison to others." Until this bedrock concept is incorporated into the thinking of everyone in the field of nutrition, passionate disagreements will continue.
To understand whether fat is helpful or harmful, it is necessary to understand the various classifications of fat and how each can be beneficial to some individuals and detrimental to others.
The most abundant type of fat in the American diet is "long chain saturated fat." Although much maligned, it is a concentrated source of calories needed, for instance, during pregnancy, when energy requirements are high. This sort of fat usually comes from animal products, and for sedentary Americans it is easy to get too much.
Another type of saturated fat is known as "short and medium chain triglycerides." This fat is not stored in adipose tissue, and in the body it behaves more like carbohydrates than fat. It can help cancer patients maintain their weight in a situation when starvation is often the actual cause of death. Coconut oil is a good source of this type of fat.
The simplest unsaturated fat is "monounsaturated" fat, like the fat in olive oil and avocados, and it is credited as one of the benefits of the "Mediterranean diet." Replacing solid fats from animal products with more liquid oils from vegetables appears to be "heart healthy." Because fat is an important component of cell membranes, the more liquid fats produce more flexible membranes.
The situation with "polyunsaturated" fats is more complex. There are two families of polyunsaturated fats, omega-6 and omega-3 fats, which are not only incorporated into membranes but also provide the starting material for an important family of hormones called "prostaglandins." Both omega-6 and omega-3 fats contribute to the flexibility of membranes, but omega-6 fats are primarily made into "pro-inflammatory prostaglandins" while omega-3 fats are primarily made into "anti-inflammatory prostaglandins." We need both sorts of prostaglandins, but compared to primitive diets, our modern American diet supplies about 10 times more omega-6 than omega-3. It is generally assumed that this imbalanced intake leads to production of an excess of pro-inflammatory prostaglandins and thus more inflammatory diseases. As usual, there is an exception to the above rule: At least one omega-6 fat, gamma linolenic acid, is made into an anti-inflammatory prostaglandin.
Polyunsaturated fats are generally C-shaped, but it is possible for them to be Z-shaped. These "trans-fats," until recently, were thought to be entirely detrimental to health, but one trans-fat, conjugated linoleic acid -- found primarily in cheese -- turns out to be potentially helpful for weight loss and perhaps in the fight against cancer.
The reports about very long chain fats (Lorenzo's Oil) are quite disappointing. Apparently the molecules are simply too large to get into the brain where they might be beneficial, and while the oil may slow the progress of a particular genetic disease, the cure is still ahead of us.
Match the amount and type of fat in your diet to your individual circumstances, and you will reap rewards.
