Lipids are organic compounds that cannot dissolve in water but are soluble in organic solvents. They provide the highest amount of energy per gram of food we eat. This amounts to 9 Calories per gram. Lipids that are solid at room temperature such as butter, are called fats, while lipids that are liquid at room temperature such as vegetable oils, are called oils. Lipids in foods generally consist of two main components; glycerol and fatty acid.
There may be one, two or three fatty acids present in the lipid. If there is only one, this is called a monoglyceride. If there are two, this is called a diglyceride. And, if there are three, this is called a triglyceride. Most of the lipids present in food is in the form of triglycerides.
The fatty acids that are attached to the glycerol unit may be saturated or unsaturated. A saturated fatty acid is one that has all its carbons fully filled or “saturated” with hydrogen. An unsaturated fatty acid does not have all the carbons fully filled with hydrogen, due to the presence of one or more double bonds. If there is only one double bond in the fatty acid, it is called a monounsaturated fatty acid. If there are more than one double bonds, it is called a polyunsaturated fatty acid.
So why is this important? Saturated fatty acids are associated with increased incidence of heart disease, while unsaturated fatty acids are associated with improved heart health. You can use the the word “sad” to remember that it is the saturated fatty acids that are bad for you. Less consumption of meat and meat products will generally reduce your intake of saturated fatty acids. On the other hand, more consumption of plant-based products will increase your consumption of unsaturated fatty acids. Two groups of unsaturated fatty acids are essential, meaning that they cannot be made by the body and therefore must be obtained from food. These are omega-3 and omega-6 fatty acids. They are so named because the first unsaturated bond is located on the omega-3 and omega-6 positions of the fatty acid respectively. Omega-3 fatty acids are precursors for important biomolecules such as ALA (alpha-linolenic acid), DHA (docosahexaenoic acid), and EPA (eicosapentaenoic acid). These molecules provide benefits such as reduction in inflammation, improved fetal and cognitive development, and healthy aging. Omega-3 fatty acids are abundant in fatty fish (e.g. tuna, herring and salmon), flax seed, walnut and chia. The most important omega-6 compound is linoleic acid which is helpful in improving heart heath. Linoleic acid is abundant in oil seeds and vegetable oils. We tend to get enough omega-6 in our diet but often lack omega-3.
The nutritional label on your food product will tell you the amount of total fats, saturated fats, and unsaturated fats (mono and polyunsaturated) contained in the product. If the saturated fat content is less than 5% daily value (DV), this is considered low. If it is 10% this is considered moderate and if it is 20% or more, this is considered to be high.
Apart from saturated and unsaturated fats, you will see the term “trans fats” declared on food labels. This is a very bad type of fat that have been found to significantly increase the risk of heart disease. So you want to see this at 0%; even though 0 % really means <0.5 g/serving according to FDA definition. Trans fats are generally made during the hydrogenation process of oil production. Hydrogenation is a process where hydrogen is added to vegetable oils to make them more saturated. This is desirable because saturated fats are generally firm, stable and have additional functional properties compared to unsaturated fats. Explore this article to learn of different functional properties of lipids.
Lipids are oxidized by the process of either autoxidation or photo-oxidation. Autoxidation is the autocatalytic generation of free radicals. It is the most common oxidation mechanism. It is catalyzed in the presence of light, oxygen, heat and metals. Autooxidation occurs in three steps – initiation, propagation, and termination.
Initiation: Hydrogen is abstracted from an unsaturated fatty acids by a reactive oxygen specie (OH. or HOO.), making it an unstable free radical.
Propagation: The fatty acid radical then reacts with molecular oxygen, producing a peroxyl radical, which in turn reacts with other lipids to produce hydroperoxides and more free radicals
Termination: Two radicals react to form a non-radical. Biological systems have natural antioxidants that help to speed up termination and reduce cellular damage .
Photo oxidation occurs when UV light interferes with normal molecular oxygen (called triplet oxygen). This causes one of its electrons in its outer shell to become excited and move to a higher orbital level. This is called the singlet state. This is a high-energy and very reactive state that is capable of reacting with unsaturated fatty acids to extract a proton leading to formation of a free radical and then initiate autoxidation. Food processors add antioxidants in foods to prevent free radical chain reactions and buildup of rancid compounds and off-flavors.
Systematic Names of Saturated and Unsaturated Fatty Acids
Classification of Lipids
So far I have talked about fats and oils. These fall under the category of simple lipids as do waxes which have not been yet mentioned. Waxes are are esters of fatty acids and alcohols other than glycerol.
Other lipids falling under the category of compound lipids contain chemical groups in addition to alcohol and fatty acids. These include:
- Phospholipids: Contain a phosphate group
- Glycolipids: Contain hexose units
- Sulpholipids: contain sulfated hexose with fatty acids and alcohol
- Lipoproteins: Contain protein subunits
A final category of lipids is Sterol. These are characterized by four fused rings, three with six carbons and one with five carbons. The most abundant of the sterols is cholesterol.
Important function of cholesterol include,
- Production of hormones
- Production of bile
- Part of cell membrane structure
Cholesterol travels through the blood in the form of lipoproteins. High density lipoproteins (HDL) are called good cholesterol and low density lipoproteins (LDL) are called bad cholesterol. LDL’s make up most of your bodies cholesterol. They raise the risk of heart disease and stroke by building up on the walls of blood vessels in the form of “plaque”. HDL cholesterol absorbs cholesterol and carries it back to the liver which is responsible for breaking it down. High HDL can therefore reduce your risk of heart disease.
Food labels declare the total amount of cholesterol present. You need to check to make sure that the amount present is low or at 0%. This is because high total cholesterol is associated with cardiovascular disease. Dietary cholesterol is not necessary since the body can make all that it needs. So how much cholesterol does your body need? Check out my article on lipid digestion to find out.
Reference: Potter, N. N. & Hotchkiss, J. H. (1998). Food Science, 5th edition. New York, NY: Springer Science+Business Media LLC.