5.8: Summary

Except for people who consume lots of alcohol and little food, we get most of our calories (energy) from carbohydrate, fat, and protein. Fat has the most concentrated calories at 9 calories per gram. Alcohol has 7 cal/g, carbohydrate 4 cal/g, and protein 4 cal/g.

Glucose is the basic unit of the most common carbohydrates found in food. Hundreds to thousands of glucoses are linked together in starch and glycogen. The most common sugars found in food are the single sugars glucose, fructose, and galactose, and the double sugars sucrose (glucose +fructose), lactose (glucose+galactose), and maltose (glucose+glucose). Sucrose is commonly known as table sugar.

With the exception of lactose in milk, virtually all of the carbohydrate in our diet comes from plant foods. Only trivial amounts of glycogen (“animal starch”) and sugar are found in beef, fish, etc. Glycogen is the complex carbohydrate found in our bodies, two thirds in muscle, one third in liver.

Strictly speaking, some dietary fibers are carbohydrates, but in nutrition, carbohydrate refers to digestible ones, and fiber refers to the indigestible parts of plant foods. Thus, dietary fiber is found only in plant foods.

Fats include triglycerides, lecithin, and cholesterol. In common usage, fat refers to triglycerides, which make up most of the fat that we eat and the fat that we store in our bodies. A triglyceride is made of three fatty acids attached to a backbone of glycerol.

Triglycerides have a variety of fatty acids, making one triglyceride different from another. The most common fatty acids in our diet have a chain of 16 to 18 carbons. Two fatty acids are essential in the diet, linoleic acid (an omega-6 fatty acid) and linolenic acid (an omega-3 fatty acid). Omega-6 and omega-3 refer to the location of a key double bond in the carbon chain.

Hydrogen atoms are attached to these carbon chains. When the carbon chain has as many hydrogens as it can hold, it’s a saturated fatty acid. If there’s one place (a double bond) in the chain that can take on more hydrogen, it’s monounsaturated. When there are two or more double bonds in the chain, it’s polyunsaturated.

Food manufacturers can hydrogenate (add hydrogen to) some of the double bonds in vegetable oils to solidify the oils and to lengthen the shelf-life of their products. Hydrogenation creates a trans arrangement around some of the remaining double bonds; these are trans fatty acids. Trans fatty acids, though unsaturated, act like saturated fat—they’re more solid and can raise blood-cholesterol levels.

Depending on which kinds of fatty acids predominate in the triglyceride, the fat is called saturated, monounsaturated, or polyunsaturated. Saturated fatty acids predominate in the triglycerides in beef, so beef fat is called a saturated fat. In olive oil, monounsaturated fatty acids predominate. In safflower oil, polyunsaturated fatty acids predominate. Saturated fat is solid at room temperature, whereas mono- or polyunsaturated fat is liquid.

Lecithin differs from a triglyceride: One of the three fatty acids in a triglyceride is replaced by a phosphorus-containing substance. Fatty acids in lecithin can differ, just as they can in triglycerides. In food, lecithin is useful as an emulsifier. In the body, lecithin forms the basic structure of cell membranes. Lecithin isn’t required in our diet; the body makes it.

Unlike triglycerides and lecithin, cholesterol has no caloric value and is found only in animal foods. (Plants can contain trivial amounts.) Cholesterol can’t be broken down to carbon dioxide and water (unlike carbohydrate, protein, triglycerides, and lecithin). Thus, excess amounts in the diet can sometimes raise blood-cholesterol levels. Cholesterol is essential in the body as a component of cell membranes and as starting material to make sex hormones, bile acids for digestion, etc. Cholesterol isn’t required in the diet; the body makes it.

Amino acids have nitrogen and are the basic units of protein. Amino acids can be broken down and used for energy, or can be used to make protein, or other substances (e.g., neurotransmitters). Twenty different kinds of amino acids are used to make protein—the body can make 11, the other 9 are required in the diet.

Proteins have from a few to more than a thousand amino acids, linked together in a precise sequence, forming a single chain. The chain folds into a distinctive shape, according to the sequence of amino acids. A protein’s shape is crucial to its function. A protein is said to be denatured when its shape changes permanently, altering its normal function.