We’ve heard that trans fats are bad for us and something we should avoid. In fact, trans fats notoriety earned them a place on the Nutrition Facts food label. We are also familiar with the fact that polyunsaturated fats are good fats. We are told that we should consume them preferentially over fully saturated fats. Although we use these words in conversation, do we really know what they mean? Do we understand why food manufacturers, restaurant cooks, and home cooks use one type of fat instead of another when producing a complex food, such as a cake or French fries?
The answers to these questions are the meat and potatoes, so to speak, of food science, the science of modern day food processing and manufacturing. It is a fascinating field. We would all benefit from learning at least some of the basics of food science so that we can make informed and healthy choices when we shop.
A bit of food science
Fat is solid at room temperature and oil is a liquid. Fats and oils come from animals, including marine animals and vegetables. Examples of animal fats include lard from hogs and butterfat from milk. Fish oils include cod liver oil (does anyone use this anymore?) and the ever-popular fish oil rich in omega-3 fatty acids that come from fatty fish, such as salmon, albacore tuna, lake trout, and herring. Cocoa butter is an example of a vegetable fat. And corn oil, olive oil, canola oil, and soybean oil are examples of vegetable oils.
Fats and oils are made up of two different components: fatty acids and glycerol. Fatty acids are chains of carbon atoms which have different amounts of hydrogen atoms attached to them. These chains vary in a number of ways:
- Chain length. Thus, you will hear about short-chain fatty acids and long-chain fatty acids.
- Number of hydrogen atoms per carbon atoms. When the maximum number of hydrogen atoms are present, the fatty acid is said to be “fully saturated.” When there is room for only one more, we say the fatty acid is “monounsaturated”. A fatty acid like linoleic acid is missing four hydrogen atoms and is said to be “polyunsaturated”. A molecular structure called a double bond exists at the site where the hydrogen atoms are missing. The importance of this is explained when in the discussion of trans fats (below).
- Melting point. The more unsaturated fats are the lower their melting points. When a fat is so unsaturated that it is liquid at room temperature, we call it an oil. Adding hydrogen atoms during food processing is called hydrogenation. It is used to convert an oil into a solid shortening.
Fatty acids that are unsaturated can exist in two forms: the cis form and the trans form. Cis forms have hydrogen atoms on the same side of the double bond and trans forms have hydrogen atoms on the opposite sides. Cis fats have different properties from trans fats.
Most naturally occurring unsaturated fatty acids are in the cis form. They are liquids at room temperature. Cis fats can be converted into the trans form by heating liquid vegetable oils in the presence of metal catalysts and hydrogen. This is called partial hydrogenation. It hardens the liquid vegetable oils into solids, such as shortenings and margarine. Partial hydrogenation also destroys certain fatty acids, such as linolenic and linoleic acid, that oxidize over time causing the fat to become rancid.
Processed vegetable fats have displaced animal fats in Western diets since the early part of the last century. There were two driving factors: lower cost and a belief that vegetable fats were better for us than animal fats. Trans fats also have properties that make them desirable in the production of commercially-baked products and fast foods. They prolong shelf life, for example.
Health impact of saturated fats and trans fats
Both fully saturated fats and unsaturated, but partially hydrogenated, trans fats raise LDL cholesterol (the “bad” cholesterol). However, trans fats also lower HDL (“good”) cholesterol, whereas saturated fats do not. The result is that the ratio of LDL to HDL is more unfavorable in a diet high in trans fats than a diet high in saturated fats. Trans fats also raise triglyceride levels and levels of Lp(a), other risk factors for cardiovascular disease.
Several large studies have shown increased risk for coronary artery disease with high trans fat ingestion. It is also increased in diets high in saturated fats. Despite the failure of the much-publicized Women’s Health Initiative study that failed to find a beneficial impact of low-fat diets, we must keep in mind that the type of fat ingested (saturated vs. unsaturated, trans vs. cis) was not examined in that study. The weight of the evidence suggests that we should avoid ingestion of trans fats whenever possible and we should limit our consumption of saturated fats.
One way to do this is to read food labels. Not just the hype on the front of the package, but the FDA mandated Nutrition Facts labels on the back. You may find that the food that says “cooked in vegetable oil”, in large brightly colored letters on the front, is found to be partially hydrogenated with significant trans fat levels when the fine print of the FDA label is perused.
When it is not possible to read food labels, such as when you are eating out, try your best to choose unprocessed, fresh foods. When oils are added, such as salad dressing, opt for liquid vegetable oils, such as olive oil. Avoid fried foods (of course) and other foods known to be high in trans fats, such as doughnuts and pastries, chips, and even some microwave popcorn.
The bottom line
What’s it boil down to? You know I am going to say this. Go heavy on the fruits and vegetables and light on fat and processed foods. What you eat really does make a difference to your health.