Most of the fat in our food is in the form of triglycerides. Each molecule of triglyceride consists of a molecule of glycerol bound to three molecules of fatty acid. The reason that the fat from chocolate stays solid at room temperature but melts in your mouth, while the fat in olive oil stays liquid at room temperature, has to do with which fatty acids the fat contains.
The fatty acids are a large group of chemicals. They are all made up of a long string of carbon atoms, studded with hydrogen atoms. This hydrocarbon string is what makes them “fatty.” All fatty acids also have a carboxyl group at one end. That’s what makes them “acid.”
The smallest carboxylic acid is formic acid, which puts the sting in a fire ant’s venom. It’s a carboxyl group (-COOH) with just a hydrogen attached. If you replace that hydrogen with a methyl group (one carbon and three hydrogens), you end up with acetic acid, which puts the bite in vinegar. Propionic acid is a carboxylic acid with a three carbon chain. Butyric acid has a four-carbon chain.
The longer the carbon chain, the more the carboxylic acid behave like a fat and the less it behaves like an acid. Formic acid and acetic acid are completely soluble in water. Butyric acid can mix with water, but it is somewhat oily. The fatty acids with a longer carbon chain, such as the oleic acid you find in olive oil or the stearic acid you find in chocolate, do not mix with water.
Fats versus oils
An oil is simply a fat that is liquid at room temperature. In other words, its melting point is lower than room temperature.
The longer the carbon chain in a fatty acid, the higher its melting point will be. That’s why the stearic acid (18 carbons) in chocolate stays solid at room temperature while butyric acid (four carbons) is a liquid at room temperature.
The more hydrogen atoms a fatty acid has, the higher its melting point will be. when a fatty acid is holding as many hydrogen atoms as it possibly can, it’s said to be “saturated.” Saturated fatty acids have no double bonds between any of the carbons in its carbon chain. A fatty acid with one double bond is said to be “monounsaturated.” Fatty acids with more than one double bond in the carbon chain are said to be “polyunsaturated.”
You can see the effects of saturation when you compare four different fatty acids that each have 18 carbons: stearic acid, oleic acid, linoleic acid, and alpha-linolenic acid. Chocolate stays solid at room temperature because it contains a lot of stearic acid. Olive oil, on the other hand, stays liquid because it contains the monounsaturated fatty acid oleic acid. Linoleic acid is a major constituent of corn oil, and alpha-linoleic acid is found in flaxseed oil.
Saturated fats have desirable traits. They stay solid at room temperature and have a better shelf life. You can convert unsaturated fats to saturated fats through a process called hydrogenation.
Triglycerides and fatty acids
If you’ve ever gotten soap in your eye, you know that fatty acids are irritating. That’s because the carboxylic acid tail of the free fatty acid is caustic. So you can understand why our cells like to bind up the sharp carboxylic acid end of a fatty acid with glycerol. A glycerol molecule can bind up to three molecules of fatty acid in this way.
Are some fats “healthy”?
Fat is extremely high in calories. As you can see from these illustrations, a fat molecule contains very little oxygen. It will take a lot of oxygen, and a lot of oxidation (burning) to convert them to carbon dioxide and water. That means that they will release a lot of energy. If you are struggling to get enough calories to survive, fatty foods are a real advantage. That’s why people instinctively like fatty foods.
Unfortunately, people in Western society have too much access to fatty foods. As a result, we suffer from obesity and heart disease. When scientists first noticed that heart disease and obesity were much more common in Northern Europe and the United States than in Southern Europe or in Asia, they figured that the problem was saturated fat. So the recommendation came for people to switch from butter to margarine, which was lower in saturated fat. Then people realized that margarine, being made by hydrogenation of polyunsaturated oils, could contain a lot of trans fat. So then people were told to switch to olive oil, which contains monounsaturated fat. Some scientists noticed that the Native Greenlanders didn’t have quite as many heart attacks as you might expect, given their high-fat diet. So then people told us to eat fish and fish oil. Lately, people have been telling us to eat a “Mediterranean diet,” which is described as featuring fish and olive oil. In reality, what made the Mediterranean diet healthy is all the vegetables. People who avoid all animal foods, including fish, and all oils can be even healthier than people eating a Mediterranean diet.
All of these shifting recommendations are confusing, but the simple truth has been obvious all along, to those who read the scientific articles carefully. The people who are the healthiest and least likely to die of heart attacks are the ones who eat very little fat of any kind. The people with the lowest blood cholesterol levels are not only heart-attack-proof, they have a low risk for cancer and other “diseases of affluence.” The healthiest diet consists of low-fat unrefined plant foods and gets less than 10% of its calories from fat. People who eat a low-fat diet that includes lots of fresh vegetables will get enough fat from their food, and they’ll get a good balance of the omega-6 and omega-3 essential fatty acids. Adding a spoonful of ground flaxseed or a few walnuts can ensure a plentiful intake of omega 3 fatty acid.
Often, you will see people recommending that toddlers be fed whole cow’s milk instead of 2% or skim milk. In reality, there’s no need for children to receive any cow’s milk at all.
Can you get sick from eating too little fat?
Although your body does need small amounts of two “essential” fatty acids: linoleic acid and alpha-linolenic acid, it’s practically impossible to find people who get sick from eating a low-fat diet. The typical cases of essential fatty acid deficiency occur in people who have been fed nothing but sugar intravenously for an extended period. They are getting no fat from their diet, and the high insulin levels make it hard for them to release the essential fatty acids from their fat stores. These people’s requirement for essential fatty acids can be met by rubbing some vegetable oil on the skin.
Some people worry that they won’t be able to absorb the fat-soluble vitamins (vitamins A, D, E, and K) if they don’t get enough fat in the diet. In reality, ordinary unprocessed plant foods contain enough traces of fat to enable people to absorb vitamin A, E, and K. You don’t need to put oily salad dressing on your salad in order to absorb vitamins from the salad! Vitamin D is made in our own skin when it is exposed to sunlight and doesn’t have to come from the food.
There are, of course, some people who have trouble absorbing fat from their food. Fat malabsorption can result from many diseases, including cystic fibrosis. People who have trouble absorbing fat from their food should be getting nutritional advice from a registered dietitian, along with medical care from a doctor, nurse practitioner, or physician assistant.