The body uses fats as a source of energy. Gram for gram, fat contains a little more than twice as much energy as carbohydrate and protein. Foods that are rich in fat include meat, milk, cheese, vegetable oils, and nuts. Muscle is often surrounded by stored fat—some animals store fat throughout muscle, leading to the marbled appearance of some red meat. Others, chickens for example, store it on the surface of the muscle where it can be more easily removed for cooking. Humans store fat just below the skin to help insulate the body from cold weather, and to store energy in case of famine. Some scientists believe that prehistoric humans often faced times of famine and may have evolved to crave fat. Another benefit of fat is its ability to help cushion and protect vital organs. Fat, like carbohydrate and protein, contains carbon, hydrogen, and oxygen, but in different proportions.
Structure of Fats
The structure of a fat is that of a three-carbon glycerol molecule with up to three long chains of hydrogen and carbons, called hydrocarbons, attached to it. The long hydrocarbon chains are called fatty acid tails of the fat molecule. The fatty acid tails are unable to dissolve in water because the hydrogen and carbons are too busy interacting with each other to form bonds with water molecules. This leads to one of the characteristic properties of fat, its inability to dissolve in water. The carbon atoms in fats—and in all molecules—are able to make chemical bonds with up to four other atoms. Each carbon in the fatty acid tail is bound to two other carbons, one in front of it and one behind it in the hydrocarbon chain. When a particular carbon is double-bonded to one of these carbons, it can only make one other bond to a hydrogen. When the carbon is single-bonded to the carbon on either side of it, it can make bonds with two hydrogen atoms.
Chemistry of Fats
When carbons are bound to as many hydrogen as possible, it is said to be a saturated fat(saturated in hydrogen . When there are carbon–carbon double bonds, the fat is not saturated in hydrogen and it is therefore an unsaturated fat. When there are many unsaturated carbons, the fat is a polyunsaturated fat. The double bonds in unsaturated fats make the structures kink instead of lying flat. This prevents the adjacent fat molecules from packing tightly together, so unsaturated fat tends to be liquid at room temperature. Cooking oil is an example of an unsaturated fat. Unsaturated fats are more likely to come from plant sources. Saturated fats, with their absence of carbon–carbon double bonds, do pack tightly together to make a solid structure. This is why saturated fats, such as butter, are solid at room temperature. Commercial food manufacturers sometimes add hydrogen bonds to unsaturated fats by adding hydrogen gas to vegetable oils under pressure. This process retards spoilage and solidifies liquid oils, thereby making food seem less greasy. Margarine is vegetable oil that has undergone this hydrogenation process.
When hydrogen atoms are added to the same side of the carbon–carbon double bond, they are said to be in the cis configuration—naturally occurring unsaturated fats have their hydrogen atoms in cis configuration. When hydrogen atoms are added on opposite sides of the double bond, they are said to be in the trans configuration. During hydrogenation, there is no way to control whether hydrogen atoms are added in the cis or trans configuration. The long term health risks of consuming foods rich in trans-fatty acids, common in fast foods, are unknown. Since fat contains more Calories per gram than carbohydrate and protein, and because excess fat intake is associated with several diseases, nutritionists recommend that you limit the number of Calories obtained from fat to less than 25% of your daily intake.
Along with fat, sweets (highly processed, sugar-rich foods) occupy a very small portion of the food pyramid since both foods provide no real nutrition, just Calories. Sweets are often referred to as “empty”Calories because consuming them provides Calories but no vitamins and minerals.