Fats, also called lipids, are required for many important functions in your body. Fats are a main component of the membranes of all the cells in your body: without fats, your cells would have no covering or boundary. By providing the membrane around all your cells, fats are vital for insulating your body from the outside world. Fats also can be used to provide energy and are involved in supporting the immune system, brain health, and cardiovascular function.

There are many different types of fats, but only a few are essential, which means your body cannot create them internally, so you must take them in through your diet. These essential fats include an omega-6 fatty acid (linoleic acid), and an omega-3 fatty acid (linolenic acid), and are found in the highest amount in nuts, seeds, and fish. Meat contains high levels of fats that are not considered essential, called the saturated fatty acids, and it also contains cholesterol, which is also not essential and is digested in the same way as fats. High amounts of the non-essential saturated fats, and too little of the essential fats can result in problems with the immune system, hardened arteries, and scaling skin, among other symptoms.

As well as being a necessary part of your diet, during digestion, fats also act as carriers of the fat-soluble vitamins (A, D, E, and K) and the carotenoids, thus enabling their absorption. (Carotenoids, such as beta-carotene, are a group of highly colored fat-soluble compounds in plants with a wide range of health protective effects.) Without fats in your diet, you would also not be able to absorb these important vitamins, and would show deficiency symptoms such as problems with blood clotting (vitamin K), weak bones (vitamin D), or vision disturbances (vitamin A).

What happens when I eat a food containing fat?

Fats are present in food primarily as three fat molecules attached to a backbone molecule called glycerol, but your body can't absorb this molecule directly. Like protein, your body must first break down this larger molecule into smaller ones. For example, after you eat a piece of salmon, which contains essential fats, your body must first remove, or strip-off the fat molecules from the glycerol backbone to which they are attached. This process is called hydrolysis, and the types of enzymes that hydrolyze fats from glycerol are called lipases. Lipases are secreted under the tongue, in the stomach, and from the pancreas; therefore, fat hydrolysis begins the minute fats enter your mouth and continues in your stomach, where the majority of fat hydrolysis occurs.

After hydrolysis, the absorption of fats is complicated by the fact that, like any oil, they are insoluble in water, and therefore the body has a system in place to provide a solubilized fat aggregate. The body uses bile acids, which act as detergents, to make fat globules, or aggregates. After aggregation with bile, the fat aggregates, also called miscelles are transported to the small intestine, where they can be taken up directly by the intestinal cells and absorbed into the body.

Absorption of the fat from the miscelles begins in the first part of the small intestine, the duodenum, with the majority of absorption occurring in the mid-section of the intestine, the jejunum. The bile acids generally stay behind in the intestinal tract, acting more as a shuttle.

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