The group name for fats, oils and cholesterol is lipids, from lipos, the Greek word for fat. Liquid lipids are called oils. Solid lipids are called fats.
Fat is important for the following reasons:
- It provides a source of stored energy
- It gives shape to the body
- It cushions the skin
- It acts as an insulating blanket to reduce heat loss
- It is a part of every cell membrane
- It is a component of myelin, the fatty substance that sheaths nerve cells
- It makes up about 60% of the brain
- It is a constituent of hormones and other biochemicals such as bile and vitamin D
- It acts as a shock absorber protecting internal organs from damage
Dietary fat contains more than twice as much energy per gram than proteins and carbohydrates but it is more difficult for the body to access this energy. The fat in a meal floats to the surface of the food being digested in the stomach, limiting the effect of the lipases – the enzymes that break fats down. Fat is therefore digested more slowly than proteins and carbohydrates, making you feel fuller for longer.
When fat moves into the small intestine, the gallbladder releases bile, which starts to break the fat down into glycerol and fatty acids. These can either be stored in fat cells in the body (adipose tissue) as triglycerides, or absorbed into the intestinal wall. There they are either combined with oxygen to produce energy, or used to make lipoproteins that transport them through the bloodstream.
Glucose, produced by digesting carbohydrates, is easy for the body to burn, so carbohydrate is always used first. Once this has been used up, an enzyme starts to work on the fat cells, breaking up the stored triglycerides to release glycerol and fatty acids, which can be used for energy.
In the same way that amino acids are the building blocks of proteins, fatty acids are the building blocks of fats. A fatty acid is a chain of carbon atoms with hydrogen atoms attached. At one end is a methyl group – this is called the omega end or n end. At the other end is the acid group. The difference between the fatty acids is a result of the number of carbon atoms in the chain, and how many double carbon bonds there are in the chain.
A saturated fatty acid will have a hydrogen atom at every available carbon link. A monounsaturated fatty acid drops two hydrogen atoms at one point in the chain and forms a double bond between the adjacent carbon atoms. In a polyunsaturated fatty acid this happens more than once along the chain.
The following diagrams illustrate the typical structure of a fatty acid chain. To simplify such diagrams, the atoms can be disregarded, just showing the carbon chain as a simple zig zag line. Then when a double bond is introduced, the effect on the chain can be seen clearly.
The prefixes cis- and trans- describe whether the hydrogen atoms are on the same or opposite side of the double bond. Naturally occurring unsaturated fats are cis, except for up to 6 percent of fat from ruminant animals that is naturally trans. Nonhydrogenated unsaturated fats are the sole source of cis-fats in the diet. The double bond in a trans fat has very little effect on the shape of the carbon chain. The double bond in a cis fat produces a definite kink. The more double bonds in a cis fat, the more kinks there will be in the chain. These kinks define the properties of the fatty acid.
Plant oils are technically fats, but are liquid at room temperature. While these are considered healthier than animal fats, their cooking properties are different from those of solid animal fat. To modify them so that they could be used in similar ways to animal fats, for example baking cakes, pastry and cookies, food chemists used – and still use – hydrogenation. In this process, hydrogen is combined with the unsaturated plant oils, converting some of the molecules into saturated fats.
Complete hydrogenation converts healthy unsaturated fats into fully saturated fats, making them entirely equivalent to regular, saturated, animal fat. This would not work as a healthier substitute for animal fat, since it essentially has the same structure. The solution seemed to be to partially hydrogenate the plant oil. This way, some unsaturated molecules would remain, and keep the product healthy. This is how we get partially hydrogenated vegetable oil.
But partial hydrogenation has a down-side. Natural unsaturated fatty acids are in a configuration that is called cis, but partial hydrogenation can flip the chemical bonds into a trans configuration. As mentioned above, the difference between cis and trans is that the two H atoms are on the same side of the double bond (cis), compared to being on opposite sides (trans). So this affects the shapes of the molecules. In a cis configuration, the double bond creates a kink in the fatty acid. In the trans configuration, there is no kink. Therefore, a trans-fatty acid is structurally similar to a saturated fatty acid (which has no kinks, either).
The kinks in the molecules determine whether the material is solid or liquid at room temperature. More importantly, the linear shape of a saturated fat and also a trans fat mean that these fats will more easily form plaque, sticking to the walls of arteries. The kinks in a fatty acid chain will prevent this happening.
1. Nutrition for Dummies, Carol Ann Rinzler
2. Fat and why it Matters, Indiana University
3. Fatty Acid, Wikipedia
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