The chemical family name for fats and related compounds is lipids, from the Greek word lipos, which means fat. Liquid fats are called oil, solid fat is fat and fat in food is called dietary fat.
Fats are high-energy nutrients. Gram for gram, fats have more than twice the amount of energy than protein and carbohydrates (check calories). One gram of fat equals nine calories, one of protein is four.
Usage of Fat
The human body needs fat to build body tissues and manufacture chemicals, such as hormones.
Some of the adipose (fat) tissue can easily be seen; even though our skin covers the fat, we can see it on for instance hips and bellies. This visible fat has a few functions:
- Source of stored energy.
- Cushions the skin.
- Acts as insulation, i.e. reduces heat loss.
- Provides shape to our body.
Non-visible fat in and around organs is:
- Part of every cell membrane.
- An element of hormones and other biochemicals, such as vitamin D.
- A shock absorber protecting our organs in case of falls and other injuries.
Energy from Fat
It is more difficult to extract energy from fat than from proteins and carbohydrates – fat is digested more slowly. Fat is broken down into glycerol and fatty acids (see more below), which may be stored in fat cells in adipose tissue or absorbed into cells in the intestinal wall, combining with oxygen to produce heat/energy, water and a waste product, carbon oxide.
The body extracts energy from stored fat when there is no carbohydrates/glucose available. The first step being for an enzyme in the fat cells to break up stored triglycerides, the type of fat in adipose tissue. Glycerol and fatty acids are released by the enzyme action and they are transported by the blood to body cells, where they integrate with oxygen to produce heat/energy.
Fat in Food
Food contains three kinds of fats:
- Sterols. Fat and alcohol compounds with no calories, e.g. vitamin D or cholesterol.
- Triglycerides. For adipose tissue which later could be used for energy.
- Phospholipids. Hybrids that transport hormones and vitamins A, D, E and K through the blood.
Nutritionists characterize fatty acids as saturated fatty acids, monounsaturated fatty acids, or polyunsaturated fatty acids, depending on how many hydrogen atoms are attached to the carbon atoms in the chain. The more hydrogen atoms, the more saturated the fatty acid. Depending on which fatty acids are in majority, a food fat is also characterized as saturated, monounsaturated, or polyunsaturated.
- A saturated fat, such as cheese, has mainly saturated fatty acids. Saturated fats are firm at room temperature and get harder when chilled.
- A monounsaturated fat, e.g. sunflower oil, has mostly monounsaturated fatty acids. Monounsaturated fats are liquid at room temperature – getting thicker when chilled.
- A polyunsaturated fat, such as soybean oil, has mainly polyunsaturated fatty acids. Liquid at room temperature and they stay liquid when chilled.
There are also trans fatty acids (more below) that are used in the manufacture of food to help it stay fresh longer.
By adding hydrogen atoms to unsaturated fatty acids, an oil, e.g corn oil, turns into solid fat, such as margarine. A fatty acid with extra hydrogen atoms is called a hydrogenated fatty acid. Trans fatty acids are hydrogenated fatty acids. Because of those extra hydrogen atoms, hydrogenated fatty acids behave like saturated fats, clogging arteries and raising the levels of cholesterol in the blood.
One answer to the problem of hydrogenated fatty acids is plant sterols and stanols. Plant sterols are natural compounds in the oils in fruits, vegetables and grains. Stanols are compounds created by adding hydrogen atoms to sterols from wood pulp and other plant sources. Sterols and stanols work like little sponges, sopping up cholesterol in our intestines before it can make its way into the bloodstream. As a result, the total cholesterol levels and levels of low—density lipoproteins go down.
Food and Fat
- Fruits and vegetables have merely traces of fat, typically unsaturated fatty acids.
- Grains have small amounts of fat, up to three percent of the total weight.
- Dairy products vary. Cream is a high-fat food, regular milks and cheeses lower but still high.
- Meat is moderately high in fat, mostly saturated fatty acids.
- Poultry, turkey and chicken, without the skin is low in fat.
- Fish can be high or low in fat, typically unsaturated fatty acids.
- Vegetable oils and butter are high-fat foods. Unsaturated fatty acids in oils and saturated in butter.
Our body needs cholesterol:
- Facilitate nerves sending messages.
- It protects the integrity of cell membranes.
- Is a building block for vitamin D.
- Being a base for building of hormones, e.g. estrogen.
There are two sources of cholesterol: food and our body. Our body, specifically the liver, produces naturally some of the cholesterol that is needed. We also get cholesterol directly from animal products, such as meat, eggs, and dairy products.
As with dietary fat, there are good and bad types of cholesterol. HDL cholesterol is the “good” kind of cholesterol found in our blood. LDL cholesterol is the “bad” kind. HDL helps keep the LDL from getting lodged into our artery walls.
Cholesterol and Cardiovascular Disease
Too much cholesterol in the blood could lead to cholesterol crossing into arteries where it could stick to the walls, form deposits that eventually block the blood flow and increase the risk of heart attack or stroke. LDL, together with other substances, can form plaque, a thick, hard deposit that can narrow the arteries and make them less flexible, a condition known as atherosclerosis.
National Heart, Lung and Blood Institute.