What are lipids?
The word lipid probably makes you think of butter, olive oil, margarine or lard. We usually refer to lipids simply as fats and oils. However, the lipid family includes more than that; it includes triglycerides, phospholipids, and sterols. Although their structures and functions differ, they all contain carbon, hydrogen, and oxygen and none of them dissolve in water. However, they all dissolve in organic solvents, such as chloroform, benzene, and ether.
Triglycerides in food and body fat cells are a rich source or energy, with each gram providing about 9 calories. They are the main fuel source for all body cells, except the nervous system and red blood cells. When you are resting or engaging in light physical activity, triglycerides provide 30 to 70% of the energy you burn, depending on how well fed you are, how physically active you are, and the intensity and duration of the exercise.
Triglycerides are the most common type of lipid (fat) found in foods and in the body. Around 95% of the fats we eat and 95% of the fat stored in our body is in the form of triglycerides.
Fatty acids can be saturated, monounsaturated, or polyunsaturated. To understand saturation, it is important to note that, at a maximum, a carbon atom can form 4 chemical bonds, and oxygen atom can form 2 bonds, and a hydrogen atom can form only 1 bond.
A saturated fatty acid (SFA) is called so because all the bonds between the carbons are single connections and the other carbon bonds are filled with hydrogens. To understand this concept, imagine a school bus with a child in every seat. The school bus is “saturated” with children and there are no empty seats on the bus.
Monounsaturated fatty acids (MUFA) have 1 (mono) location in the carbon chain that is not saturated with hydrogen. Using the school bus example, a monounsaturated fatty acid is like having 1 empty seat.
A polyunsaturated fatty acid (PUFA) has at least 2 double bonds in its carbon chain. If the school bus was a polyunsaturated fatty acid, it would have 2 or more empty seats.
Essential Fatty Acids
Humans can synthesize a wide variety of fatty acids, but we cannot make 2 polyunsaturated fatty acids: alpha-linolenic acid and linoleic acid. Alpha linolenic acid is the major omega-3 fatty acid in food and linoleic acid is the major omega-6 fatty acid in food. Alpha-linolenic acid and linoleic acid are essential fatty acids (EFSs). We need to obtain EFAs from food because our bodies are unable to synthesize essential fatty acids with a double bond before the 9th carbon in the chain.
Food Sources of Triglycerides
Most foods provide at least some triglycerides. Certain foods, such as animal fat and vegetable oils, are primarily triglycerides. Baked goods, snack foods, and dairy desserts also contain significant quantities of fat. Fat-free milk and milk, as well as many breakfast and cereals and yeast breads, contain little or no fat. Other than coconut milk and avocados, fruits and vegetables are low in fat.
Sources of Saturated Fatty Acids: beef, lard, pork, lamb (long chain) and milk fat, butter, coconut oil, palm oil, palm kernel oil (medium or short chain).
Sources of Monounsaturated Fatty Acids: olive oil, canola oil, peanut oil.
Sources of Polyunsaturated Fatty Acids: sunflower oil, corn oil, safflower oil, fish oil.
Sources of Omega 3: cold-water fish (salmon, tuna, sardines, mackerel), walnuts, flaxseed, hemp oil, canola oil, soybean oil.
Sources of Omega 6: beef, poultry, safflower oil, sunflower oil, corn oil
Sources of Trans Fatty Acids: margarine, shortening
Functions of Triglycerides
- Providing energy
- Providing compact energy storage
- Insulating and protecting the body
- Aid fat-soluble vitamin absorption
- Essential fatty acid functions
Eicosanoids, which are made from essential fatty acids, have over 100 different actions, such as:
- Regulating blood pressure, blood clotting, sleep cycles, body temperature, inflammation and hypersensitivity reactions
- Transporting oxygen from red blood cells to body tissue
- Maintaining normal kidney function and fluid balance
- Directing hormones to their target cells
- Regulating hormone synthesis, immune system function, and ovulation
- Regulating cell division rates, which may help prevent certain cancers or slow the growth of existing tumors
The structure of phospholipids is very similar to that of triglycerides, except a fatty acid is replaced with a compound that contains the mineral phosphorus and often has nitrogen attached. In the body, phospholipids have 2 major roles: cell membrane component and emulsifier.
The phospholipids in food often are used as an emulsifier in food preparation and manufacturing. Their ability to emulsify fats works the same way in foods as it does in the body. For example, eggs are used in many muffin recipes. The lecithins in yolks emulsify fat in muffin batter and keep it suspended in the other ingredients. Mayonnaise is thick because phospholipids in egg yolks and mustard emulsify the oil and vinegar used in the recipe. Emulsifying fats in food such as cakes, muffins, and dressings gives them body and a smooth texture.
Phospholipids can be synthesized by the body or supplied by the diet. For example, lecithins can be found in foods such as egg yolk, wheat germ, and peanuts. Although lecithin supplements are available, they are not needed because the liver can produce sufficient amounts of phospholipids.
Sterols are the last class of lipids, having a very different structure, with carbons mostly arranged in many rings.
Cholesterol is the most well-known sterol. Functions of cholesterol include: producing steroid hormones such as testosterone, estrogen, the active form of vitamin D hormone, and cortisone, and making bile which is used to emulsify fats, so they are digested normally.
Cholesterol is found in foods of animal origin, such as meat, fish, poultry, eggs, and dairy products. Plant origin foods don’t contain cholesterol. Although plants do not contain or produce cholesterol, they do make other sterols, such as ergosterol (a form of vitamin D) and sitostanol. Most people get about one-third of their cholesterol from foods they eat and the rest produced in their bodies.
Fat Digestion and Absorption
Fat digestion occurs mostly in the small intestine. The presence of fat in the small intestine triggers the release of cholecystokinin from intestinal cells. Cholecystokinin stimulates the release of bile and pancreatic enzymes. Bile emulsifies fats amd allows enzymes to efficiently break triglycerides into monoglycerides and free fatty acids.
Phospholipids and cholesterol are digested mostly in the small intestine. After absorption, short and medium chain fatty acids mostly enter the circulatory system. Long chain fatty acids enter the lymphatic system.
Recommended Fat Intakes
The Institute of Medicine (IOM) Recommended Daily Fat Intakes advise for the following fat intakes:
Total Dietary Fat – 20 to 30% of calories
Saturated Fat – As low as possible
Trans Fat – As low as possible
Unsaturated Fat – Most of fat intake
Omega 6: linoleic acid – 5% of calories
Omega 3: alpha-linolenic acid – 0.6 to 1.2 % of calories
Cholesterol – as low as possible (limited to 200-300 mg depending on region)
Very High Intakes of Fat
Diets high in total fat can increase the risk of obesity, the risk of colon, prostate, and breast cancer, and the risk of cardiovascular disease.
Atherosclerotic plaque is probably first deposited to repair injuries in the lining in any artery. As atherosclerosis progresses, plaque thickens over time, causing arteries to harden, narrow, and become less elastic.