Biochemistry of Nutrition

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Biochemistry of Nutrition

Some people couldn't care less about the biochemical processes of dieting and nutrition. As long as they are losing weight, they're happy -- and that's fine. None of this is required reading.

However, for those who would like to know more about the basic chemistry involved in nutrition and dieting, this page provides some very basic information about just that. If you already have a degree in biochemistry, you probably won't learn much additional information, but feel free to check this information for accuracy and report errors or omissions to Jerry .

The Nutrients

Basically, food is composed of water, micronutrients, (vitamins, minerals), and three different kinds of macronutrients (or simply, nutrients). The nutrients are energy producers, and include carbohydrates, proteins, and lipids (fats).

Carbohydrates

Carbohydrates are substances whose molecules contain the elements carbon, hydrogen, and oxygen in which the ratio of hydrogen to oxygen is 2 to 1. The general chemical formula for a carbohydrate is thus CnH2nOn. However, carbohydrates are not simply hydrated carbon atoms. The ratio of H to O might be the same as water, but otherwise, water is not a component of a carbohydrate molecule.

Carbohydrate molecules can vary widely in their complexity, and in the speed with which they are digested and metabolized. Sugars are a class of carbohydrates. Sugars consist of single units, or two units bound together with covalent bonds. A "unit" is a molecule of sugar containing anywhere from 4 to 9 carbon atoms, typically 5 or 6, arranged as a ring. A simple sugar contains a single unit. For that reason, they are often called monosaccharides (meaning "one sugar"). Examples of monosaccharides include glucose, fructose, and galactose.

Two monosaccharides may even contain the exact same chemical formula. For example, glucose and fructose both have the formula C6H12O6. The difference is in the way the atoms are arranged in the molecule. Substances that have the same chemical formula, but differ in structure are called isomers.

Disaccharides are sugar molecules composed of two monosaccharide units. Sucrose (table sugar) is the most common example. It is composed of a molecule of glucose bonded with a molecule of fructose. The formation of sucrose requires that one atom of oxygen and two atoms of hydrogen be released in order to provide bonding sites for the two molecules. This is common in synthesis reactions in chemistry and is called dehydration synthesis, because it results in the formation of a water molecule:

C6H12O6 + C6H12O6 ---> C12H22O11 + H2O

Sometimes, carbohydrates can occur in very large and complex arrangements of bonded monosaccharides. These large molecules are called polysaccharides. Starch is a polysaccharide containing about 20 or more units. There are different forms of starch, too. These polysaccharides form what are generally referred to as complex carbohydrates. The general chemical formula for starch is (C6H10O5)n, where n represents the number of monosaccharide molecules in the starch molecule.

Technically speaking, a carbohydrate that contains from 3 to 6 monosaccharide units is called an oligosaccharide. These carbohydrates tend to be totally indigestible, and rather rare.

Fiber is a carbohydrate consisting of many monosaccharide units. Most fiber is derived from the cell walls of plants and is indigestible. Polydextrose is a 12-unit polysaccharide that is similar to fiber in terms of its resistance to digestion. It is often used as a bulking agent in certain processed foods.

All carbohydrates except fiber, oligosaccharides, and polydextrose are eventually converted into glucose by the body. It was once thought that simple sugars were digested and metabolized quickly, resulting in a corresponding rapid rise in blood glucose levels. It was similarly assumed that complex carbohydrates were slower to digest and, therefore, had a mitigating effect on blood glucose increases. These assumption have been proven false by work done on the glycemic index, which is a measure of how quickly carbohydrates are converted into glucose.

For example, glucose is arbitrarily assigned a glycemic index of 100. Other foods are compared to this standard. Using 100 as the reference, table sugar has a glycemic index of 65. White bread is 72 and baked potatoes have a glycemic index of 85. Corn flakes have a glycemic index of 84, while ice cream has a glycemic index of 50.

What does all this mean? When it comes to choosing foods with the idea of minimizing blood glucose spikes, pure table sugar is better than a slice of white bread, and much better than a baked potato. Ice cream is a better choice than corn flakes. Keep in mind that this says nothing about the actual nutritional value of any food. It is only a measure of how quickly foods tend to raise blood glucose levels.

Only foods containing carbohydrates have a glycemic index. Fats and proteins have little or no effect on blood sugar. Some foods with low glycemic index values include dairy products, green vegetables, beans, and surprisingly, pure fructose, which has a glycemic index of 20. Those on a low carbohydrate diet and those with diabetes should eat only low-glycemic carbohydrates (less than 50), and they must still restrict their intake of even those. The carbohydrates in foods with low glycemic indexes are absorbed and converted to glucose more slowly than higher glycemic carbohydrates, but they are still absorbed eventually and must be taken into account when dieting.

What is surprising about what the glycemic index shows, is that some simple sugars, like fructose, have a very slow impact on blood glucose levels, whereas complex carbohydrates contained in potatoes act very quickly to raise blood sugar. On the other hand, the complex starches found in many legumes act slowly to raise blood glucose levels. Modified corn starches, such as maltodextrin and corn syrup solids have a glycemic index equal to that of glucose.

Lipids (Fats) Lipids include the fats and oils. The basic difference between the two is that oils tend to be liquid at room temperature, and fats tend to be solid.

Lipids are typically composed of large molecules. Like carbohydrates, a lipid molecule also contains carbon, hydrogen, and oxygen, but the proportion of H to O is not 2:1.

Structurally, a fat molecule consists of one molecule of glycerin which has been bonded by dehydration synthesis to three molecules of fatty acids. Glycerin is an oily substance whose molecule consists of a 3-carbon chain containing 8 atoms of hydrogen and 3 atoms of oxygen. Fatty acids are long-chain hydrocarbon molecules which contain a carboxylic acid group (-COOH) on one end. During dehydration synthesis, three fatty acid molecules bond with the three different -OH groups of the glycerin molecule to form a molecule of fat and 3 molecules of water.

There are 3 different kinds of fats (and fatty acids). A saturated fat molecule contains as many hydrogen atoms as the carbons in the fatty acids can hold. In an unsaturated fat, one or more double bonds between carbon atoms takes the place of hydrogen atoms. If only one double bond is present, it is a monounsaturated fat. If two or more double bonds are present, it is a polyunsaturated fat.

Our bodies require that certain fatty acids be included in the food we consume. These are called essential fatty acids, because our bodies cannot manufacture them. That's why it is important to consume these fats as part of our diet. Flax seed oil, olive oil, and canola oil are good sources of mono- and polyunsaturated fatty acids. Butter contains mostly saturated fatty acids.

Fatty acids are used by the body in various ways. They are a primary component of cell walls, for example, and they are used to make hormones and hormone-like substances called eicosanoids that help to regulate various bodily functions.

Some studies link saturated fats with an increase in serum cholesterol. However, those studies did not take into account the negative effects of carbohydrates in that equation. Absent carbohydrates, saturated fats have less of an impact on cholesterol. Monounsaturated fats and polyunsaturated fats tend to lower cholesterol. Trans fats are a type of fat produced by artificially hydrogenating (adding hydrogen atoms to) vegetable oil to make it more solid. Margarine and shortening contain trans fats. Trans fatty acids are the most unhealthful of all the fats and should be avoided on any diet.

Proteins

Proteins are the most complex of the three basic nutrients. They consist of very large molecules consisting of carbon, hydrogen, oxygen, and nitrogen. The nitrogen atoms are contained in structures called amino groups. An amino group connected to a carboxylic acid is called an amino acid. Proteins consist of long chains of amino acids, folded back onto themselves in a deliberate manner and held in position with hydrogen bonds.

Protein molecules are sensitive to heat. When heated to temperatures above about 180 deg. F, they become denatured. A denatured protein is one in which the hydrogen bonds have been broken, causing the protein to coagulate. A good example of a denatured protein is an egg white that has been cooked. The amino acids are still present in a denatured protein, so cooking proteins does not reduce their nutritional value.

The amino acids in proteins are used to build structures in the body, for cell repair, and in building hormones.

As with some fatty acids, amino acids cannot be synthesized by the body and must be provided in the diet. These are called essential amino acids. It is noteworthy that fats and proteins are an essential part of the diet, but that carbohydrates are not, in that the only carbohydrate that is required by the body, glucose, can be synthesized by the body without the necessity of including it in the diet.

Other Nutrients

There are a few other minor nutrients which do not fit clearly into any of the three groups outlined above, but which are metabolized by the body and produce energy. These include ethyl alcohol, glycerin, and sugar alcohols.

Ethyl alcohol, contained in alcoholic beverages, cannot be used directly by the body as a source of energy. Instead, the alcohol that is consumed goes to the liver, where it is converted into fat and carbohydrates. Glycerin is a by-product of fat metabolism. It can also be ingested as an ingredient in some foods. Glycerin is metabolized much like carbohydrates, except that there is no corresponding increase in either insulin levels or blood glucose. Sugar alcohols are chemically alcohols, but they are derived from sugar molecules. They are much slower to be absorbed from the intestine than sugars and are metabolized differently. Therefore, they have little or no effect on insulin production. Sugar alcohols are partially metabolized by the flora of the large intestine, which may produce intestinal gas.

Nutrient Metabolism

The three major nutrients are used by the body in different ways. Carbohydrates are used solely for energy, although a limited amount can be stored by the body in the liver in the form of glycogen (animal starch). Fats are also used for energy, but they are also important as a body mass builder. Normally, healthy men have up to 20% of their total weight as fat, and women have as much as 25% fat. Of course, obese people will have a higher percentage of body fat than normal. A person weighing 300 pounds, for example, may have a body fat index of 40%.

Fat is a normal and healthy constituent of the body, helping to cushion internal organs from shock. It also helps insulate the body and keep it warm.

Primarily, however, the body uses fat for energy. Fat contains more than twice as much energy per gram as do carbohydrates. Carbohydrates (in the form of glucose) are typically used for quick energy, while fat is burned during sustained exercise. Fat is also the primary fuel of choice during slow aerobic exercise, while glucose is used during fast aerobics or anaerobic exercise.

Proteins can also be used for energy, but primarily, they are used as the building blocks of the body. Muscle tissue is composed of proteins. A protein matrix is also contained in the integument and the linings of organs. If the body runs out of glucose and fat during exercise, it will burn protein.

The amount of energy contained in food is typically measured in calories. Technically, a dietary calorie is actually a kilocalorie using the chemical definition. A calorie is the amount of heat energy that is required to increase the temperature of 1 gram of water by 1 degree Celsius. A dietary calorie (Kcal) is, therefore, the amount of heat it takes to raise the temperature of 1000 grams (1 liter) of water by 1 degree Celsius.

Carbohydrates and proteins burn at the rate of 4 Kcal per gram. Fat produces 9 Kcal of heat per gram. There are 30 grams in an ounce, and 16 ounces in a pound. Therefore, one pound of fat produces 4320 Kcal of energy, enough to raise the temperature of more than 10 gallons of water from the freezing point to the boiling point.

Relevance to Dieting

Since a calorie has but one definition, does that mean all calories are equal? A calorie is a calorie, right? Technically, that is true. But dieting has a lot to do with what type of food your body prefers and what it does with the excess.

If you eat a hypothetical diet consisting of 70% carbohydrates and 30% protein with no fat, some of the protein will be used for body building and repair, and some will be converted into glucose. All the carbohydrates will be converted to glucose initially. The result will be a rapid and sustained elevation in blood glucose levels, which will stimulate insulin production. Insulin is a hormone that does many things. Among them, it stimulates your cells to metabolize glucose for energy. A side effect of insulin, however, is an increase in appetite. That causes most people to eat again, not long after eating a high-carbohydrate meal. Importantly, however, insulin also stimulates your body to store fat. In the absence of insulin, your body cannot store fat.

It is plain to see that a high-carbohydrate diet will provide many more grams of carbohydrates than are necessary for immediate energy usage. Some carbohydrates are converted to glycogen and stored in the liver, but this represents only a small percentage of the carbohydrates consumed. The remainder become excess and most of that excess is converted into fat for storage in the body tissues. Thus, eating a high-carbohydrate diet can result in weight increase, especially in non-athletes.

Now, look at another hypothetical diet, consisting again of 30% protein but 70% fat with no carbohydrates. The proteins are used in the same way as the first diet, so even without consuming any carbohydrates, as much as 58% of the protein consumed is converted to glucose. This will not be enough to satisfy the energy needs of the body, so in the absence of carbohydrates, the body begins to burn the fat it consumes. This causes the body to "convert" to a fat-burning engine instead of primarily being a glucose-burning engine.

Fats, unlike carbohydrates, have a high satiety factor. Whereas carbohydrates make you hungry a couple of hours after eating, fats make you full, and the satiety lasts for hours. Therefore, you tend to consume fewer calories on a high-fat diet than on a high-carbohydrate diet. Also, remember, without carbs, your body does not produce much insulin. Therefore, the fat you eat cannot be stored. Thus a high-fat diet, in the absence of carbohydrates, typically results in weight loss. Yet your blood glucose does not drop too low, because your liver continues to convert some of the dietary protein into glucose. Any excess dietary fat is not stored but broken down by a process known as lipolysis (the opposite of dehydration synthesis) and excreted.

The products of fat metabolism are fatty acids and glycerin. The glycerin is used as a fuel source and the fatty acids are broken down further into ketone bodies, which become the primary fuel of the body in the absence of glucose. Any excess ketones are not stored but are excreted in the urine. The production of ketones during fat metabolism is called ketosis.

The upshot of these different metabolic processes is that a calorie of carbohydrates will tend to make you fatter than a calorie of fat that is eaten in the absence of carbohydrates. That is because excess carbohydrates turn to fat, which is stored, but excess fat is broken down and excreted.

Copyright © 1999 by Jerry

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