Role of Nutrition and Major Nutrients for Human beings
Overview
Burtis, C.A., and E.R Ashwood, eds. Tietz Textbook of Clinical Chemistry,
2nd ed. Philadelphia:W. B. Saunders Co., 1994
Role of Nutrition and Major Nutrients for Human Beings
Abstract: Major nutrients, which are carbohydrates, proteins, and lipids (fats), are used as energy sources or as building blocks for larger biochemical compounds. Minor nutrients, which include all vitamins and minerals, assist the chemical reactions that occur with major nutrients. A balanced diet includes all of the necessary major and minor nutrients. If the diet is not balanced, some energy sources or building blocks will be missing and the body will not function properly.Nutrients serve as building blocks for larger chemicals and the energy that fuels all of the body’s processes, from cellular repair to the use of the muscles.
Keywords: Nutrients, carbohydrates, proteins and lipids(Fats), ATP , cholesterol
Introduction
People need to eat because they need energy. Food provides that energy. The body needs energy to make and break chemical bonds that exist in complex biochemical compounds, to hold these compounds together, and to change them. The digestive system and its accessory organs have evolved to supply individuals with the energy they need to work with these chemical bonds. There are three types of chemical bonds. An ionic bond is made between charged atoms where positive and negative charges attract each other. These bonds are fairly strong, but not so strong that energy is needed to alter them. A hydrogen bond is a weak chemical bond that is used to gently hold onto substances during chemical reactions or to fine-tune the structure of strands of proteins so that they can function properly. These bonds also exist between water molecules and anything mixed in water. Hydrogen bonds allow the water molecules to support the compounds that are dissolved in the solution, but are weak enough to allow the compounds to diffuse through the water. Hydrogen bonds are so weak that the chemicals held with them can separate just by drifting off into the surrounding water. The third type of chemical bond, a covalent bond, requires energy to make or break it.
This bond is made when electrons from two or more atoms begin to rotate around all of the atoms, forming a tight bond, almost like a wall around the core of the atoms. Covalent bonds hold biochemical compounds together until the body’s cells force them apart or the bonds wear out from repeated use of the compounds in the body.
Fig(1): ATP is the form of energy that cells use to complete their functions, from replication and division to making proteins and extracting nutrients from food. A molecule of ATP, illustrated here, contains three phosphate groups
Energy that has been extracted from the breakdown of these chemical bonds must be put into a form that cells can use. Cells use a chemical form of energy called adenosine triphosphate (ATP), which is an RNA nucleotide. The three phosphates are attached to the adenosine in series so that the molecule looks like this: A-P~P~P (Figure 1). The phosphates are negatively charged and repel each other. Attaching the second and third phosphate requires energy to force the phosphates onto the molecule. The energy stored in ATP is the energy that holds the repelling phosphates together When the energy is used, the third phosphate is removed, and the energy that was holding the phosphate onto the ATP molecule is usedto make or break a covalent bond. The resulting adenosinediphosphate (ADP) can become ATP by extracting energyfrom a nutrient and using it to attach another phosphate.These energy transport molecules function like rechargeable batteries, with the difference being that the energy is completely discharged each time the ATP is used.
TYPES OF NUTRIENTS
Nutrients are divided into major and minor nutrients. Major nutrients, which are carbohydrates, proteins, and lipids (fats), are used as energy sources or as building blocks for larger biochemical compounds. Minor nutrients, which include all vitamins and minerals, assist the chemical reactions that occur with major nutrients. A balanced diet includes all of the necessary major and minor nutrients. If the diet is not balanced, some energy sources or building blocks will be missing and the body will not function properly.
Carbohydrates
Carbohydrates, a group of molecules that include sugars and starches, provide energy to the body when the molecules are broken down.All carbohydrates contain carbon, hydrogen, and oxygen. They are categorized by size: monosaccharides, disaccharides, and polysaccharides.
Monosaccharides
Monosaccharides, such as glucose, fructose, and galactose, are simple sugars.Usually, the ratio of each of carbon to hydrogen and oxygen is 1:2:1 such that there is one carbon to two hydrogens to one oxygen. Most of the sugars used in the body are six-carbon sugars, so their formula is written as: C6H12O6. The body’s sugar biochemistry is based on the breakdown of glucose. Fructose and galactose feed into the pathway of these chemical reactions.
Disaccharides
Two monosaccharides make a disaccharide. There are three types of disaccharides: sucrose, lactose, and maltose. Each one has glucose as at least one of its sugar units. Sucrose, which is made of glucose and fructose, is common table sugar. Lactose, made of glucose and galactose, is the sugar found in dairy products.Maltose, made of two glucose molecules, is found in anything “malted” and is also the sugar primarily used to make beer. Because disaccharides are too large to pass through the cell membranes, they must be broken down into monosaccharides first.
Polysaccharides
Polysaccharides are several monosaccharides linked in a chain. There are two types of polysaccharides of importance to the body: starches and glycogen. These are made up of only glucose and have slightly different forms, depending on their source and the types of chemical bonds holding them together. Both plants and animals use polysaccharides as a form of short-term energy storage. Starches are the storage carbohydrate form found in plants. There are two types of starch, depending on the complexity of the structure: amylose and amylopectin. Amylose is easily digestible and has a simple structure resembling a bunch of strings made up of glucose molecules linked together in a straight line. Amylopectin has a more complex structure, including a large number of cross-linkages between the strings, and is more difficult for the body to digest. Glycogen is the storage carbohydrate form found in animals. Glycogen is similar to amylopectin, but less complex.
Polysaccharides must be digested to their individual glucose units for the body to be able to use the energy. Monoand disaccharides are found in fruits, sugarcane, sugar beets, honey, molasses, and milk. Starches are found in grains, legumes, and root types of vegetables. Glycogen is present in all animals, although the primary source is beef.
As mentioned earlier, carbohydrates are used for energy. When glucose is broken down, some of the energy released from the chemical bonds is used in ATP molecules. If carbohydrates are not immediately needed, they are converted to glycogen or fat and stored. If not enough glucose is available, the liver breaks down glycogen to release glucose. The liver can convert amino acids into glucose, a process called gluconeogenesis. If sugar is not adequately available in the diet, amino acid supplies will be used to make glucose and not proteins.
Cellulose, another type of polysaccharide, is a major component of wood. It cannot be broken down into smaller units, so it is not digestible. When we ingest cellulose, it is considered roughage or fiber. Although we get no nutritional value from cellulose, it binds cholesterol in the intestine and helps us eliminate this chemical. Fiber also helps to regulate the digestive tract and keep people “regular.”
Proteins
Proteins have many functions in the body. They can be used for energy, structure of different parts of the body, hormones, enzymes, and muscles. Proteins are made of long chains of amino acids, of which there are 20 different types. The structure of proteins starts out simple, and then becomes more complex, depending on the protein. The function of the protein depends on its structure. The chain of amino acids will bend and twist to a three-dimensional form, depending on the sequence of the amino acids. In general, the structure and appearance of proteins can be classified as fibrous or globular.
Fibrous proteins are strand-like in appearance. Fibrous proteins, which are the main building material of the body, are called structural proteins. They include collagen, keratin, and contractile proteins of muscles. Collagen provides strength to the tendons and ligaments that hold bones and muscle together. Keratin is found in skin and “seals” the skin surface, preventing evaporation of water from underlying tissues and keeping invading microorganisms out. Contractile proteins of muscles allow muscles to contract or shorten.
Globular proteins, which are compact, spherical proteins, have a wide variety of functions. Some proteins are found in hormones, such as human growth hormone, which helps regulate growth in the body. Other types of globular proteins are called enzymes and they increase the rate of chemical reactions in the body.
The most complete sources of proteins are found in animal tissues. Plants can also provide amino acids. There are eight amino acids, called essential amino acids, which human beings cannot make. These are tryptophan, methionine, valine, threonine, lysine, leucine, histadine, and isoleucine. Because humans cannot make them, they must be supplied in the diet. If they are not supplied, proteins cannot be made, which results in a protein deficiency. Protein deficiency during childhood can result in developmental problems that restrict both mental and physical development. Deficiencies occurring in adults cause a number of problems, such as premature aging, problems in fighting infections, and bleeding in joints and the digestive tract.
Evaluation of the amount of proteins in the body is used to determine an individual’s nutritional status, called nitrogen balance. If the person is healthy, his production of proteins is equal to the breakdown of proteins, and he is in neutral nitrogen balance. If the person is growing or repairing tissue damage and has adequate amino acid resources for protein production, his production of protein exceeds protein breakdown, and he is in positive nitrogen balance.
If a person’s proteins are being broken down faster than the body can replace them, the person is in negative nitrogen balance, which is not good. Negative nitrogen balance means that the person needs supplementation of proteins and amino acids to achieve a neutral or positive nitrogen balance.
Fats and Lipids
Lipids are insoluble in water, and thus they are difficult to carry in the blood. They are categorized into triglycerides, phospholipids, and steroids. The principal dietary lipids in the body are cholesterol and triglycerides. Phospholipids are mostly tied up in cell membranes and
do not play a significant role in energy metabolism. Triglycerides, which are made in the liver to store excess energy from carbohydrates, make up a major portion of adipose tissue. This tissue provides the body with insulation to keep warm and cushions joints and organs for protection. Triglycerides are composed of three-carbon glycerol molecules with three fatty acids attached, one to each of the three carbons.
Fatty acids are long chains of carbon atoms, 12 to 30 carbons long. Attached to the carbons are hydrogen atoms. If all the possible hydrogen atoms are attached to the chain, the fatty acid is called a saturated fat. If any of the hydrogen atoms are missing, the fatty acid is called an unsaturated fat. These forms of fatty acids behave slightly differently in the body. Saturated fats contribute more to the buildup of plaque in arteries and are considered less healthy than unsaturated fats.Saturated fats are found in all animal tissues, and unsaturated fats are found in nearly all plants. As with proteins, two fatty acids are essential for human beings: linoleic and linolenic, and are called essential fatty acids. About 90% of the body’s dietary fat intake consists of the fatty acids
palmitic acid, stearic acid, oleic acid, and linoleic acid. Linoleic acid is found in vegetable oils, especially corn and safflower oils, and linolenic oil is found in rapeseed oil. Essential fatty acid deficiencies contribute to dermatitis, a depressed immune system, anemia, growth retardation, infertility, and cardiac, liver, and respiratory problems. Steroids are another type of lipid that have hydrocarbon rings. Cholesterol, one of the most important steroids, is made in the liver and ingested with animal tissues. Plants have a counterpart to cholesterol called phytosterol, but this cannot be absorbed by humans and does not contribute to dietary fats. Cholesterol is used as a framework for hormones called steroids. Slight changes are made to the structure of cholesterol to make these hormones. Testosterone and estrogen, which are reproductive hormones, are both steroids. Aldosterone, an adrenal cortex steroid hormone, assists in the renal conservation of sodium. Cholesterol is also incorporated into cell membranes to make them pliable. It is found in the membranes of red blood cells to allow them to enter small capillaries.
Some Facts
Fats are not soluble in water. Thus, for the body to carry lipids such as cholesterol and triglycerides in the blood, which is water-based, the lipids are mixed with proteins that can dissolve in water and act as carriers for the fats. Different proteins give different characteristics to these lipid-protein mixtures. These lipid-protein mixtures are called HDL (high-density lipoprotein) and LDL (low-density lipoprotein) and neither one of them is good or bad. All dietary fats are needed by the body, just not in excess. If the fats separate from their protein carriers, they can no longer travel in the blood or mix well in cells. This is analogous to the water and oil of salad dressing. In the blood, these floating lipids attach to fatty deposits called plaques on the walls of blood vessels (Figure 2.2). If the plaque becomes large enough, it can close off part of the blood vessel. If part of the plaque breaks off from the vessel wall, it can travel to capillaries, where it may get stuck and completely block the smaller vessel. When this blockage occurs in the blood vessels of the heart, a heart attack results. If this blockage occurs in the brain, a stroke results.
LDL is assembled in the liver from proteins, cholesterol, and triglycerides and sent into the blood to deliver these fats to the body’s tissues. The lipids and proteins tend to separate, especially if there is an increase in blood pressure, as in hypertension. Thus, LDL has earned the name “bad” cholesterol. HDL protein is made in the liver and released into the bloodstream
without any lipids. Its job is to scavenge cholesterol from the body’s tissues and blood vessels. When the HDL proteins are full of cholesterol, they are removed from the blood by the liver and the cholesterol is made into bile, a digestive fluid. Because HDL removes cholesterol from tissues and does not significantly contribute to the buildup of plaque, it has earned the name“good” cholesterol.
The result is the number of calories a person should burn in a day to maintain body weight. If the person eats fewer than this, the person will lose weight. If the person eats more calories than this, the person will gain weight. Table shows the number of calories burned through different activities. When people increase their level of this type of basic physical movement, in conjunction with doing at least five hours of purposeful high intensity exercise per week, the magic starts to happen.
APPROXIMATE NUMBER OF CALORIES BURNED PER HOUR BY ACTIVITY
Activity
| 45 kgs | 68 kgs | 90kgs |
Bicycling 6 mph
| 160 | 240 | 312 |
Bicycling 12mph
| 270 | 410 | 534 |
Jogging 5.5 mph
| 440 | 660 | 962 |
Jogging 10 mph
| 850 | 1,280 | 1664 |
Jumping rope
| 500 | 750 | 1,000 |
Swimming 25 yds/min
| 185 | 275 | 358 |
Swimming 50 yds/min
| 325 | 500 | 650 |
Walking 4.5 mph
| 295 | 440 | 572 |
Tennis (Singles)
| 265 | 400 | 535 |
CONCLUSION
Humans need to eat to gain energy for chemical reactions involving a type of chemical bond called a covalent bond. This bond keeps complex biological chemicals together and requires energy to make it or break it for repair, growth, or development. Metabolic pathways for carbohydrates, proteins, and lipids intersect and allow the body to use nutrients to both make and burn proteins and lipids. Carbohydrates exist as monosaccharides, disaccharides, and polysaccharides, depending on the number of sugar units. Monosaccharides include glucose, fructose, and galactose. Disaccharides include sucrose, lactose, and maltose. Biologically
important polysaccharides come either from plants as starch or from animals as glycogen. Proteins are made from a mixture of 20 amino acids and fulfill a variety of functions in the body. Cholesterol and triglycerides are important dietary lipids. Triglycerides are an important form of long-term energy storage and will be made from excess carbohydrates. Both vitamins and minerals are important in metabolizing the major nutrients of carbohydrates, proteins, and lipids. Deficiencies of vitamins or minerals compromise cell metabolism.
The body takes food and breaks it down into the nutrients it can use, both major and minor. The major nutrients include carbohydrates, proteins, and lipids. Vitamins and minerals are types of minor nutrients . Nutrients serve as building blocks for larger chemicals and the energy that fuels all of the body’s processes, from cellular repair to the use of the muscles. Sometimes foods are described as having empty calories. This means that the item is made mostly of sugar, probably sucrose, and not much of anything else. When carbohydrates are ingested along with proteins, lipids, vitamins, and minerals, they form part of a balanced diet that fills our nutritional needs.
What is especially exciting about nutrition is not just the possibility of prevention, but the reversal of diseases. The body has a tremendous ability to heal itself including repair of the damage to our DNA. Certain vitamins, minerals, and special phytonutrients can promote this healing and strengthen our cells so that they are better able to protect themselves against future injury. In fact, recent evidence has shown that a common herb can rescue injured brain cells even when taken hours after the injury occurs!