Fatty acids are a simple lipid made up of a hydrophilic carboxylate group attached to a long hydrocarbon chain. These fatty acids are categorized as either saturated or unsaturated, depending on the structure of the hydrocarbon chain. If all the carbons in the chain are fully bonded to hydrogen atoms (saturated with hydrogen) then the molecule is a saturated fatty acid. Unsaturated fatty acids, which are further divided into monounsaturated or polyunsaturated fatty acids, have some carbons in the chain that form double bonds with each other. These bonds cause a change in their structure that prevents them from packing tightly together. However, saturated fatty acids do not have these double bonds and can pack together, which raises the melting point of the molecule. This means that oils or fats with high levels of saturated fatty acids, such as butter or lard, are solid at room temperature. Saturated fatty acids are also very stable and resist spoilage and oxidation due to time or high temperatures.

Like monounsaturated fatty acids and some polyunsaturated fatty acids, saturated fatty acids can be synthesized in the body. Glucose is used to create saturated fatty acids of varying hydrocarbon chain lengths. Enzymes then can convert them into certain mono or polyunsaturated fatty acids. This process is predominantly done in the liver and adipose tissue, however one saturated fatty acid called lauric acid is instead made in the mammary gland and delivered in breast milk. Saturated fatty acids can also be found in most animal products including meat, milk, and butter. Fat from animals contains between 40 to 60% saturated fat. The fatty acids are also found in some plant products such as cocoa butter and tropical oils like coconut and palm oil.

Classification and uses of saturated fat

Saturated fatty acids can be divided into three types: short, medium and long chain fatty acids. Short-chain fatty acids have a hydrocarbon chain of between four to six carbon atoms. Two common examples are butyric acid (4 carbons) and capric acid (6 carbons), both found mainly in butterfat. Medium-chain fatty acids have a chain between eight to twelve carbons long. The most common kind is lauric acid (12 carbons) found in coconut oil. Long-chain fatty acids have between fourteen to eighteen carbons. Three examples are myristic acid (14 carbons) and palmitic acid (16 carbons) both found in palm kernel and coconut oil, and stearic acid (18 carbons) found in beef and lamb fat. The length of the hydrocarbon chain appears to dictate the properties and function of the fatty acid, as described below.

The human body has many uses for saturated fatty acids. They are predominantly used as an energy source for cells. Fragments of the fatty acids are used in the Krebs cycle to produce ATP. Deposits of fat in the body called adipose tissue are composed almost entirely of saturated fatty acids that can be converted into energy. The fatty acids also make up a high percentage of cellular membranes. This helps to stabilize the membrane. The presence of saturated fatty acids helps promote the retention of healthy omega-3 fatty acids in tissues and promotes the incorporation of calcium into bones. They also help protect the liver from poisoning by alcohol and other toxins. Long-chain fatty acids make up the fat around the heart and are an energy source for the heart when it is stressed. Short and medium chain fatty acids have antimicrobial properties and are thought to help the immune response and protect the digestive tract from harmful microorganisms.

Saturated fat and health concerns

For several decades all saturated fatty acids have been considered unhealthy in relation to unsaturated fatty acids. Initial research indicated that a diet high in saturated fat raises cholesterol levels, caused blocked arteries, and led to heart disease and stroke while unsaturated fat did not have this effect. Scientists had two theories for how saturated fatty acid increases cholesterol levels in the bloodstream. First, the fatty acids could be converted directly into cholesterol in the liver. Second, excess levels of saturated fatty acids could displace cholesterol from its normal position in the cell membrane. This would force the cholesterol to instead circulate in the bloodstream. Because saturated fat caused these problems, doctors and researchers have been promoting a diet low in saturated fatty acids for decades.

However, recent research now seems to state that certain saturated fatty acids are less harmful than others. Palmitic acid is known to increase cholesterol levels and may be the most harmful of the saturated fatty acids. Myristic acid also appears to have this effect. However, stearic acid actually does not seem to affect cholesterol levels at all. There are many possible reasons for this, one is because stearic acid is quickly converted into the healthier, monounsaturated fatty acid oleic acid in the body. More research obviously needs to be done to understand the benefits or risks of saturated fat in the diet.