The term scientists use for fats in foods, and the human body is Lipids. The lipids in foods and the body fall into one of three main classes. About 95 percent of all lipids are classified as Triglycerides. A triglyceride is a large molecule consisting of numerous individual fatty acids. Similar to the way glycogen consists of long chains of individual glucose molecules, a triglyceride is a storage device for fat. The name Tri- is self-explanatory, it means the compound usually contains three fatty acids, combined with a glycerol backbone. The triglyceride compound also contains some protein, but it's mostly fat. The ratio of fat to protein in a triglyceride is what makes it a Low Density Lipoprotein, LDL, or a High Density Lipoprotein, HDL. The compound also contains cholesterol, which is why the terms LDL and HDL are so important to blood pressure and cardiovascular disease. HDL compounds contain a larger amount of protein in relation to the fat and cholesterol (which is a lipid itself). This is why HDL is considered the "good" cholesterol, and LDL is known as the bad one. The other classes of fats are Phospholipids (Lecithin is an example), and Sterols (Cholesterol is the best known sterol). One other well-known sterol is Vitamin D. You shouldn't let that stop you from taking Vitamin D though, it is an essential vitamin we need in tiny amounts, and we can't metabolize calcium and phosphorous properly without it.

Fat is the body's chief storage form for the energy from food eaten in excess of need. The storage of fat is a valuable survival mechanism for people who must live in a feast-or-famine environment. Fat stored during plentiful times is what enables them to stay alive during times of famine. In addition, fats provide much of the energy needed to perform much of the body's work, especially muscular work.

Most body cells can store only limited fat, but some cells are specialized for storing fat (fat cells). These cells have the unique ability to grow in size almost indefinitely. You may think the carbohydrate glucose is not the body's major form of stored energy, but glucose is stored in the form of glycogen. A characteristic of glycogen is that it holds a great deal of water and is quite bulky and heavy. The body cannot store enough glycogen to provide energy for long periods of time. Athletes can train their body to store increased amounts of glycogen, but for the rest of us, glycogen only works until fat takes over. Usually between 10-20 minutes after the start of activity. As opposed to glycogen's heavy, bulky storage form, fats pack tightly together without water, and can store much more energy in a small space. Remember that one gram of fat contains over twice the amount of calories as a gram of either glucose or protein.

Some essential nutrients are only soluble in fat, and therefore are found mainly in foods that contain fat. These nutrients are the fat soluble vitamins: A, D, E, and K. Other essential nutrients, the essential fatty acids, or the Omega fatty acids as they are known serve as raw materials from which the body makes molecules it needs. Fats also form an important part of the cell membrane for many of the body's cells.

Satiety (feeling full)

Fat provides feelings of satiety, the lasting satisfaction of feeling full after a meal. The fat in foods also slows digestion and helps sustain satiety until the next mealtime. Research has shown that carbohydrate and fat also help to regulate the appetite, but their timing is different. During a meal, fat consumption sends no, or only weak signals of fullness, so fat-rich foods can easily be over-eaten. Carbohydrate and protein act faster and send stronger signals of satiety during the meal to stop the diner from continuing to eat. Everything about lipids is designed to add inches and pounds to our bodies. It's nature's fail-safe way to ensure that we get enough to eat. From fat's ability to make food taste and smell better, to it's aroma in food, to it's weak signals of satiety, encouraging us to eat more of it, everything about it wants to be consumed, and in large quantities! This was part of the design from our earliest ancestors who needed these extra fat stores to survive. Unfortunately they didn't have very many fast-food restaurants to choose from so their body's had to adapt to the need for greater stored energy.

Saturated and Unsaturated Fats

Fatty acid chains contain long strings of mostly carbon and hydrogen atoms arranged in a particular structure. The same atoms, arranged in a different manner can create an entirely different molecule, such as carbohydrate. Saturation refers to the number of hydrogen atoms a fatty acid chain is holding. if every available bond (attachment point), from the carbon molecules is holding a hydrogen, then that chain is known as a saturated fatty acid. It is completely filled to capacity with hydrogen. Saturated fatty acids can do the most harm in terms of poor health effects and weight gain. Examples are hard butters, margarines, and generally most things that you have to cut with a butter knife, or spread over your food.

Sometimes, especially in the fatty acids of plants and fish, the carbon chain has a place where hydrogens are missing. This empty spot is called a "point of unsaturation". A fatty acid chain that has one or more of these empty spots is known as an unsaturated fat. If there is one point of unsaturation, then the molecule is a monounsaturated fat, if there are two or more points of unsaturation, then it is a polyunsaturated fat. The unsaturated fats tend to be more liquid, as opposed to the hard saturated fats. There are exceptions to this rule, but for the most part hards are saturated, oils or liquids are unsaturated. Another general rule is that many animal fats are saturated, while plant and fish sources are usually unsaturated.

How fat is digested in the body

Food fat can eventually end up in fat stores of the body, but first it has to be digested, absorbed, and transported to it's fat storage cells. When you take a bite of food containing fat, the fat in your mouth first encounters the enzymes of saliva. One enzyme, produced by the toungue, acts on long-chain fatty acids, especially the ones in milk. This enzyme plays a major role in milk fat digestion in infants, but is not very important to fat digestion in adults. Once the food has been chewed and swallowed, it travels to the stomach, where the fat separates from other food components and floats as a layer on the top. Since fat does not mix with the stomach fluids, little fat digestion takes place in the stomach. By the time fat enters the small intestine, the gallbladder, which stores the liver's output of bile, has contracted and sent its bile into the intestine. Bile mixes fat particles with watery fluid by emulsifying them, keeping them suspended in the fluid until the fat-digesting enzymes of the pancreas can split them into smaller particles for absorption through the intestinal wall. If bile didn't mix and suspend the fat with the water, the pancreatic enzymes wouldn't be able to get to it, and the fat wouldn't be absorbed. A bile molecule, which is made by drawing cholesterol from the bloodstream works because it is attracted to both fat and water. One end of the bile attracts and holds the fat, the other end attracts and holds water. This is how the emulsion takes place and allows the pancreatic enzymes access to the fat. You may wonder how a person without a gallbladder can digest food. Remember that the gallbladder is just a storage organ for bile, the liver is the organ that actually produced the bile. Without a gallbladder the liver still produced bile and just continuously delivers it to the small intestine. People with gallbladders do have to make adjustments to their diet, but eventually their body will adapt to the new system and they will be able to absorb nearly as many nutrients as they did before.

Once the contents of the small intestine are emulsified, the pancreatic enzymes break down the long chains of fatty acids into single fatty acids, or small groups. Similar to the way a complex carbohydrate is broken down into single monosaccharides before they can be absorbed. Once broken down, the fatty acids can cross the intestinal wall barrier. On the other side, the fatty acids are incorporated into HDLs, or LDLs, which allows them to be transported throughout the body. They move first through the lymph system, as opposed to the bloodstream, but they eventually end up passing through the liver like most everything else. They are then sent out to be used for fuel, used for other activities involving fats, or stored on our waistline! The digestive system of a healthy person will absorb about 98% of all lipids that pass through it. One of the highest absorption rates for any other nutrient our body uses. Refer to the comments about fat intake and storage above. Compare this to the mineral Chromium, which is often only absorbed at a rate of about 5%!

Using stored fat for energy

Excess fat carried in LDL is stored by the body's fat cells for later use. When a person's body starts to run out of fuel from food, it begins to retrieve its stored fat to use for energy. It also draws on its stored glycogen. Fat cells respond to the need for energy by dismantling stored fat molecules and releasing fat components into the blood. Upon receiving these components, the cells break them down further into small fragments. Finally, each fat fragment is combined with a fragment derived from glucose, and the energy-releasing reaction continues, liberating energy, carbon dioxide, and water, the same by-products that are released from expended glucose. The way a person's body gets energy is by breaking the chemical bonds between carbon atoms. These bonds are charged ions, and when broken, they release heat, which you remember is the measuring tool used to determine calories. "Heat" energy is what makes your body go. Remember also, that fatty acids contain mostly carbon atoms, therefore, they can be recognizable as a fuel source for some parts of the body, especially the muscles. The way to use stored fat for energy is to create a demand for it in the tissues be decreasing intake of food energy, or by increasing the body's need for energy, or both. When fat is broken down for energy it is important that at least some carbohydrate be available to assist in the process. If fat is broken down without the presence of carbs, and incomplete breakdown occurs resulting in ketosis. In ketosis, the products of incomplete fat breakdown start to appear in the blood and urine, leading to upsets of the body's chemical balance. Ketosis is also identifiable as the acid-like breath from someone who has ran out of carbohydrates almost completely and is using fat as their only fuel source. A low-carb diet is not recommended as a means to lose weight. In addition to the body's exclusive need for glucose, the absence of carbohydrates in the diet usually implies increased protein, fat, or both. Too much of either nutrient is potentially harmful, especially when they displace the needed carbohydrates. In an ideal diet recommended by health experts, complex carbohydrates (not table sugar), make up about 50% of total calories, fat should be 30% or less, and protein makes up the remaining 20% or so. Table sugar is broken down to the monosaccharide glucose just like other sugars for absorption, but it isn't recommended because large amounts of it in the diet usually imply a reduction of other nutrients. Large amounts can impair your glucose tolerance, and it also isn't recommended because of it's role in dental caries. Remember that some of the body's most vital systems must have glucose to function, but the best way to get glucose is by consuming plenty of complex carbohydrates and let your body do the rest. As useful as it is, fat can never be converted to pure glucose. Protein can, but the conversion process is so costly in terms of energy, there is scarce little energy left to perform the biological function in need. Also, most people are interested in losing fat, not protein. Protein is what makes up the lean tissue (metabolically active tissue) on our bodies. In starvation conditions, your brain and central nervous system can "learn" to use fat for fuel, but this comes at a great expense to your personal health and well-being.

Some health risks of obesity

High fat intakes are associated with serious diseases. The person who chooses a high-fat diet may be inviting the risk of heart and artery disease (CVD). Heart disease is the number-one killer of adults in the United States. the person who eats a high-fat diet also incurs a risk of developing some forms of cancer, another leading killer disease.

One of the most important things in regard to fat and disease is a medical test called the "blood lipid profile", which reveals the amounts of various fats, especially triglycerides and cholesterol in the blood. It also identifies the protein carriers with which these lipids are traveling. This test can tell you a great deal about your risk for developing CVD. The most important factor for CVD is blood cholesterol. A person's blood cholesterol concentration is considered to be a predictor of that person's liklihood of suffering a heart attack or stroke, and the higher the cholesterol level, the earlier the episode is expected to occur. Blood cholesterol is one of the three major risk factors for CVD. The other two are smoking and high blood pressure. The main dietary factor associated with elevated blood cholesterol is a high intake of saturated fat, which the body uses to make cholesterol on its own. In comparison, the cholesterol you consume directly from foods makes up only a small part of the cholesterol in your blood. Everyone's ability to handle food cholesterol is determined somewhat by genetics. About 10 percent of people exhibit little increase in their blood cholesterol even with a high dietary intake. About 10 percent more respond to the same diet with greatly increased blood cholesterol. A few people have inherited a total inability to clear from their blood the cholesterol they have eaten. This condition is rare, but it is well known because the study of the condition led to the discovery of how cholesterol is transported in the body. An effective tactic against high blood cholesterol is to trim the fat, and especially the saturated fat from foods.

There are some fats you actually need

The human body can use carbohydrate, fat, or protein synthesize nearly all of the fatty acids it needs. Two are well-known exceptions: Linoleic acid and linolenic acid, these fatty acids are more popularly known as the Omega 3 and 6 fatty acids.