Background Knowledge
Energy in Food
Food is crucial because it provides the compounds necessary for life and stores chemical energy that allows our body to function. Metabolism is the term that encompasses all of the chemical reactions that occur in the body, including growth and development, movement, and releasing the energy in our food. The amount of energy contained in foods is called calories. This term is ubiquitous, but the term has a very specific meaning in science. A calorie is the amount of energy it takes to raise 1 gram of water 1°C. One thousand calories is equal to one Calorie, as defined by nutritionists. For example, a candy bar that has 200 calories according to the nutritional label actually contains 200,000 calories as defined by scientists. Calorimetry is a method used to determine the number of calories in food (see Strategies).
Macronutrients
Three of the numerous biological molecules in food, protein, carbohydrates, and fats, are considered the macronutrients. These carbon-containing (organic) compounds are large polymers and found in abundance in foods. Each of the polymers is made of several monomers that are bonded together by a process called condensation or dehydration synthesis. The polymers are broken down by a process called hydrolysis. These processes are occurring continually in the body. Consider just two examples. Hydrolysis occurs during the digestive process as the large molecules are broken down into smaller monomers, so that they can be absorbed into the body. Condensation occurs in liver, fat and muscle cells when excess glucose is converted into the polysaccharide, glycogen.
Each of the macronutrients is discussed below in depth.
Protein
Proteins are a group of organic molecules with very diverse functions; proteins are polymers that are composed of a group of twenty related but different molecules called amino acids. The order of amino acids in a protein is coded by DNA in the cell's nucleus, though the protein is assembled in the cytoplasm of the cell. The twenty amino acids are combined in various arrangement and length to make the thousands of proteins that are present in the body. Amino acids are composed of a central carbon, which is surrounded by an amino group, a carboxyl group, an atom of hydrogen, and one or more atoms called an R group. 2 The bonds between each amino acid are called peptide bonds, and a complete amino acids chain is called a polypeptide.
Proteins function in cell structure, chemical reactions, cell movement, chemical messages (some hormones), and in immune response. 3 The proper functioning of a protein is dependent on its shape. After an amino acid chain is formed, some interactions (hydrogen bonding) among the amino acids occur which can give the polypeptide a coiled or sheet-like structure, which is known as the secondary structure. Further interactions between the R-groups of the amino acids result in tertiary structure, and finally, more than one polypeptide can interact leading to the protein's quaternary structure. 4
Proteins are found in all foods, but they are found in large quantities in meats, dairy products, eggs and legumes. Of the twenty amino acids, all but nine can be synthesized by the body. The nine essential (or indispensable) amino acids not made by the body must be ingested in foods, in order to maintain health. The essential amino acids are: leucine, isoleucine, valine, lysine, threonine, tryptophan, methionine, phenylalanine and histidine. 5 Generally, animal sources of protein are considered complete protein, although some studies have shown that beef is deficient in cystine and methionine. 6 Most vegetarian sources of protein are known as incomplete protein, because they don't contain all of the essential amino acids. Yet, millions of people from varied cultures acquire their protein from these sources. By combining plant proteins—for example, by eating combinations of beans and rice—vegetarians can acquire all the essential amino acids needed for protein synthesis. 7
Fats
Fats are another class of organic compounds, which are also called lipids. Fats primarily function in energy storage, insulation and cell membrane composition. Fats are composed of carbon, hydrogen and oxygen atoms in various arrangements. The basic unit of fat structure is a chain, called a fatty acid. A fatty acid is a nonpolar molecule that is hydrophobic, meaning it does not dissolve in water.
A fatty acid chain can have as many as 36 carbons, a carboxyl group, and hydrogen atoms filling all remaining bonding sites. 8 Fatty acids can be saturated or unsaturated based on the type of bonding between carbon atoms. Fatty acids that have only single bonds are called saturated fats, because each carbon is saturated with hydrogen. Saturated fats, like animal fats, have straight fatty acids chains which allow the molecules to pack tightly, resulting in a solid state at room temperature. Fatty acids that have at least one double bond between carbon atoms are called unsaturated. The double bond causes a kink in the tail which keeps the molecules from packing tightly together. Therefore, unsaturated fats, like vegetable oils, are liquid at room temperature. Monounsaturated and polyunsaturated fats refer to fatty acids with one or more than one double bonds, respectively.
Trans fats are synthetic lipids that are produced by the industrial process of hydrogenation: unsaturated fats are partially hydrogenated to make them more stable and solid at room temperature. These fats replaced saturated fats in many packaged foods because they help prolong the shelf life of the products. These trans fats have been linked to higher cholesterol, increased low-density lipoprotein (see below), and increased incidence of heart disease. 9 In 2006, the Federal Drug Administration required that trans fats be labeled, which has led many companies to eliminate the fats. 10 The city of New York banned trans fats in all restaurants, bakeries and food carts as of July 2008. 11
The two most common types of dietary lipids are triglycerides (90%) and phospholipids (6%). Triglycerides are composed of three fatty acid chains bonded to a glycerol backbone. 12 Triglycerides are the most common type of dietary fat, and they contain twice the energy of carbohydrates (9kcal/gram compared to 4kcal/gram). The higher density of energy is due to the large numbers of removable electrons in fats, which release energy as they are removed. 13 Unlike the triglyceride, the phospholipid has only two hydrophobic fatty acids chains; the third chain is replaced by a hydrophilic 'head' composed of a phosphate group and a polar molecule. The structure of this molecule is directly related to its function as the primary component of the cell membrane. The phospholipids form a double layered cell membrane; the water-loving heads of the phospholipids face the watery inside and outside of the cell, while the water-hating tails of the phospholipids attract together, forming an fairly impermeable oily layer around the cell. Other well known lipids are the sterols which include cholesterol, vitamin D, bile salts, estrogen and testosterone. 14
Lipoproteins
Lipoproteins are proteins that bond with cholesterol, triglycerides, and phospholipids that are derived from food (how they get into the body is described below). Lipoproteins are essential for the movement of fats within the body. Two types of lipoprotein are low-density lipoprotein (LDL) and high-density lipoprotein (HDL). Low density lipoprotein takes up triglycerides and cholesterol and binds to receptors on body cells. LDL can lead to atherosclerosis, the thickening of the arteries. High density lipoproteins also bind cholesterol, but these molecules move to the liver where the cholesterol is broken down, mixed with bile and excreted. This lipoprotein is beneficial because it is an essential part of the pathway that removes cholesterol from the body.
The lipoproteins, LDL and HDL, are used as a measurement for health in blood tests. The American Heart Association recommends that everyone 20 years of age and older have their LDL, HDL and cholesterol levels checked every five years. 15 However, teenagers that have family history or high risk factors can benefit by lipid screenings as well. 16 The recommended levels of LDL are 100mg/dL or less, HDL levels should be above 60mg/DL and total cholesterol should be less than 200mg/dL. See Appendix for complete table.
Carbohydrates
Carbohydrates are the class of organic compounds that primarily function as an energy source. The three classes of carbohydrates are monosaccharides, oligosaccharides, and polysaccharides. Glucose is a monosaccharide that is used directly in the production of cellular energy, ATP or adenosine triphosphate. ATP is an organic compound that stores chemical energy that is used to carry out various activities throughout cells. Many of the foods that we eat are broken down into glucose by the digestive system: fruits, vegetables, bread, and pasta. Fructose, fruit sugar, is also a common monosaccharide that we eat. Glucose can combine with one or a few other monosaccharides to form oligosaccharides, which include the disaccharide, sucrose, which is glucose bonded to fructose. Polysaccharides—such as starch, glycogen and cellulose—are large carbohydrate molecules that contain hundreds to thousands of glucose molecules joined together. Starch is the molecule that plants use to store glucose, and glycogen is the molecule that animals use to store glucose. Cellulose is an indigestible carbohydrate which contributes fiber to our diet.
Digestive System
The digestive system is a network of organs that takes in food, breaks the food down into its component parts for transport throughout the body, and eliminates food waste. The digestion process begins in the mouth, and food passes through the esophagus into the stomach for storage and continued digestion. Food is moved into the small intestine where nutrients are absorbed and indigestible matter moves into the large intestine. After water is removed and final breakdown of digestible material is completed, the remaining material moves into the rectum and out the anus. Many organs contribute to the digestive process even though food does not pass directly through them. The pancreas and the gall bladder release different kinds of digestive fluids into the small intestine. The blood vessels that surround the intestines—and receives all of the nutrient molecules as they are absorbed in the intestines—flows directly into the liver, so that cells in the liver can begin the process of using and storing the nutrients.
Digestive Processes
The mouth is the beginning of the digestive system, where the teeth begin to grind up food, and salivary glands release saliva and mucin to soften food. Saliva contains salivary amylase, an enzyme that breaks down on the digestion of carbohydrates. Food moves down the esophagus due to the muscular contraction of smooth muscle in a process called peristalsis. Food is pushed by peristalsis into the stomach, passing through the lower esophageal sphincter, which is a ring of muscle that opens to allow the passage of food and closes to keep the stomach acids from damaging the esophagus.
The stomach is a muscular pouch that mixes and digests food. In the stomach, the cells of the stomach lining release gastric juice into the stomach's interior, or the lumen. The gastric juice consists of mucus, hydrochloric acid, and pepsinogen, which is converted into pepsin in an acid environment. 17 The acid causes proteins to denature, or unravel, and pepsin, an enzyme, breaks the peptide bonds between the amino acids of proteins. The acid environment also kills microorganisms that may be present in food. The muscular wall of the stomach contracts, churning and grinding food into a watery fluid called chyme. The chyme is slowly moved on through the pyloric sphincter to the small intestine where digestion continues and nutrient absorption occurs.
The small intestine is 20 feet long with a diameter of 1.5 inches. It is divided into three segments: the duodenum, the ileum and the jejunum. The small intestine's job of digestion is done in conjunction with three accessory organs, the liver, the pancreas, and the gall bladder. As food enters the small intestine through the pyloric sphincter, the pancreas is triggered to release digestive enzymes that break down protein, carbohydrates, fats and nucleic acids. Among these enzymes are pancreatic amylase to break down carbohydrates, lipase to digest fats, proteases (for example, trypsin) to digest proteins, and nuclease to digest nucleic acids. The pancreas also releases bicarbonate to neutralize the acid within the liquid food coming from the stomach. This is important since the enzymes trypsin and chymotrypsin that break down protein can only function in a neutral pH. The liver has multiple functions in the body, several of which are involved in digestion. The liver produces bile, which is used to breakdown fat globules into smaller and smaller droplets. The bile salts act to surround the hydrophobic fat molecules. The gall bladder's function is to store excess bile before it is excreted into the intestine.
The small intestine is designed for absorption. Its wall is covered with millions of tiny folds, called villi that project out into the lumen of the small intestine. The villi increase the surface area for absorption, allowing the small intestine to absorb nutrients into the blood stream efficiently. As food passes to the jejunum and the ileum, the small monomers of nutrients are absorbed from the lumen, through the endothelial cells of the intestinal walls into the blood stream.
After food has passed through the small intestine, most of the digestible nutrients have been absorbed into the bloodstream. The indigestible food moves into the large intestine, which is approximately 5 feet long. However, some remaining nutrients are broken down by bacteria in the large intestine. These organisms are responsible for the production of Vitamin K in the body. Excess water in the indigestible material is absorbed and the waste is excreted through the rectum and anus.
Digestion and Absorption of Protein, Fats and Carbohydrates
In the digestive system, proteins are primarily digested in the stomach and small intestine, in the presence of protease enzymes like pepsin in the stomach and trypsin and chymotrypsin in the small intestine. Proteins are broken down into their component units, the amino acids, which are absorbed through the small intestine and enter the blood stream to be transported throughout the body. The amino acids enter cells so that cells can use the amino acids as raw material to make new proteins.
The breaking down of fat molecules begins in the stomach where the mechanical force of the stomach begins to break fats into smaller globules. Fat digestion continues in the small intestine due to the action of enzymes from the pancreas and bile from the liver. The bile salts surround the small fat droplets to keep them from reforming large globules that can resist breakdown. Bile salts are negatively charged and therefore these negatively charged droplets repel each other. Enzymes that digest fats can act more efficiently on the small droplets, forming monoglycerides and fatty acids. 18 These hydrophobic molecules easily diffuse through the lipid-containing cell membrane of the epithelial cells. Inside the epithelial cells of the small intestine, triglycerides reform from monoglycerides and fatty acids, and the triglycerides are transported into lymph vessels and then blood vessels. 19 These triglycerides are stored in adipose (fat) tissue until needed by cells as an energy source when glucose levels are depleted.
The digestion of carbohydrates begins in the mouth and continues in the small intestine. In the small intestine, digestion of carbohydrates occurs both in the lumen along the surface of the epithelial wall (brush border). Breaking starch molecules in the intestine requires multiple enzymes for the complete breakdown into glucose. Both salivary and pancreatic amylase can break the linkages between glucose molecules, except those at the terminal end or those proximal to a site of branching. The resulting molecules are disaccharides (maltose), oligosaccharides (maltotriose) and the small branched dextrins. These molecules move to the epithelium wall where an enzyme complex that consists of sucrase and isomaltase breaks the remaining molecules into monosaccharides. These monosaccharides move into the epithelial cells through specialized protein channels and enter the blood stream. 20 The glucose molecules are taken to cells that use glucose to produce cell energy, also known as adenosine triphosphate (ATP). Excess glucose is either stored as glycogen in the liver and in muscle cells or it is converted into lipid and stored in the adipose tissues in the body.
Pancreas and Insulin
The pancreas has a dual function in the body; it is a crucial part of both the digestive system and the endocrine system. As a digestive organ, the pancreas produces digestive enzymes that break down sugars, fats and proteins into their building blocks. The enzymes enter the duodenum through the pancreatic duct, and become fully functional in the duodenum. The nutrients that are broken down are absorbed through the intestinal wall and into the blood stream. These nutrients first pass through the liver for further metabolism and removal of toxins. The nutrient-rich blood then travels to the heart and is thus circulated throughout the body.
As an endocrine organ, the pancreas responds to the surge in glucose levels in the blood by producing insulin. Insulin is produced in the beta cells of the pancreas, which are found within tiny units called the islets of Langerhans. Glucose enters the beta cell, triggering a cascade of reactions that result in the release of insulin. Inside the beta cells, glucose is broken down in the mitochondria, releasing ATP. The production of ATP changes the voltage gradient of the cell, which results in an increase of calcium uptake by the cell. Insulin is then released from the beta cells through exocytosis.
The increased level of insulin in the blood stream triggers cells to take up glucose out of the blood stream. Insulin has effects throughout the body, primarily in the liver, in fat cells and in muscle cells. When glucose levels are high in the body, insulin levels increase and direct the storage of glucose as glycogen and the synthesis of lipids. When glucose levels are low, levels of insulin drop, causing lipids to be metabolized and proteins and amino acids are converted into glucose.
Carbohydrates and Blood Sugar
Carbohydrates contribute between 45 to 65% of the calories in the average diet. 21 Based on evidence from countries like China, that had both a relatively high carbohydrate diet and low rates for heart disease and diabetes, carbohydrates had been deemed healthy, compared to fat. As a result, doctors and nutritionists advised a low-fat and high carbohydrate diet. How successful was the advice? Today, sixty percent of adults are overweight and obese, and diabetes rates have tripled. 22 What went wrong? Evidently, the type of carbohydrate is crucial. The terms simple and complex carbohydrates refer to the size of the carbohydrate molecule, like monosaccharide and polysaccharide. Simple carbohydrates have been portrayed as bad and complex carbohydrates have been held as the good carbohydrates. 23 In fact, polysaccharides such as starch (complex carbs) are broken down into the monosaccharide glucose (simple carbs) in the small intestine.
Today, nutritionists recommend making whole grain carbohydrates an important part of the diet, and they warn against eating refined carbohydrates. Refined grains, such as bread made with white flour, white rice and potatoes, are easily and quickly digested. This rapid digestion results in a rapid rise of glucose in the blood and a corresponding flood of insulin to shuttle all of that glucose into cells for use and storage. The sudden release of large amounts of insulin can cause glucose levels to fall so low that the brain sends signals of hunger, leading us to find more sources of glucose. In contrast, whole grains take longer to be broken down, which ultimately leads to a more prolonged release of glucose and insulin response. This slower response keeps blood sugar levels in the blood from plummeting quickly, giving us the effect of feeling fuller longer. The fiber (indigestible cellulose) in whole grains plays an important role in these benefits, slowing the absorption of glucose.
Because of the physiological difference in insulin production of refined and whole grain carbohydrates, two values have been developed: Glycemic Index and Glycemic Load. Glycemic Index is a measure of the affect on blood sugar and insulin levels. Foods are ranked on the GI Scale, the highest value being 100, which is pure glucose. A Glycemic Load factors the amount of carbohydrate as well as the Glycemic index, thus giving a more compete picture of a food's effect on the body. 24
Diabetes
Diabetes is a disease that causes high blood glucose levels in the bloodstream. There are two type of diabetes: Type 1 and Type 2. Type 1 diabetes results when the insulin secreting cells of the pancreas are destroyed and can no longer produce insulin. This attack on the beta cells in the pancreas is waged by the person's own immune system. It is not clear why this happens. Type 1 affects about 5% of those that have diabetes. Type 2 diabetes affects 95% of all diabetics. Type 2 diabetes is caused by a malfunction in the pancreas, in which insulin is either not produced in sufficient quantities or the cells have become resistant to insulin. Type 2 diabetes had been previously called adult-onset diabetes, but this type of diabetes is now common in young people as well. 25 I am focusing on Type 2 diabetes in this unit because of the prevalence of this disease, and its increasing importance among young people.
Type 2 Diabetes is characterized by blood glucose levels of 126 mg/dL and above (fasting test results). Elevated glucose levels are caused by insulin resistance, which may be caused by genetic susceptibility, but is made worse by lack of exercise, obesity and smoking. Insulin resistance, also known as Syndrome X, leads to high levels of insulin (hyperinsulinemia) in the blood because the pancreas pumps out more insulin in efforts to reduce glucose levels in the blood. 26 The high levels of glucose in the blood are removed by the kidneys, along with water, causing extreme thirst and frequent urination. Although there is plenty of glucose in the blood, cells cannot use the glucose because they do not respond to insulin properly. Therefore, cells are starved for glucose, which can cause feelings of fatigue in the individual.
The high glucose levels in the bloodstream that are characterized by diabetes can result is extensive damage to blood vessels, especially capillaries. If the areas of the body that are supplied by the capillaries don't get the nutrients and oxygen that they need, they eventually die. The areas most often affected are the lower legs and feet, the eyes and the kidneys. Blindness is also common in diabetics, each year 24,000 people become blind from the effects of diabetes. Amputation is even more common in this group; over 80,000 diabetics each year have a toe, foot or leg amputated due to gangrene. Heart disease is also extremely common in diabetics because of high triglyceride levels in the blood and high blood glucose levels which cause inflammation. 27
Risk Factors
Diabetes affects almost 24 million people (7.8% of the population) in the United States. Of these, it is estimated that over 5 million people have yet to be diagnosed with this serious condition. While diabetes is ranked as the sixth leading cause of death, having diabetes is linked to other serious diseases such as heart disease, stroke, high blood pressure and kidney disease. 28 Obesity and lack of exercise are the primary risk factors for diabetes, though genetics plays a role as well. Peoples of Asian, African, Hispanic and Native American decent are more likely to develop diabetes. It is hypothesized that a trait, known as the "thrifty gene" may have enabled people to survive when food was scarce. 29 Cells may have had a natural insulin-resistance, breaking down fats rather than using glucose for energy. 30
In addition to obesity, inactivity, and family history, there are other risk factors for diabetes. Individuals who are over the age of 45, have high blood pressure, and have high triglyceride and low HDL levels in the blood are at risk for diabetes. Prediabetes is a condition give to those that have one or more early indicators that may lead to diabetes. This condition affects approximately 54 million people, who may have elevated glucose levels in the blood (100 to 125 mg/dL) and urine. Early indicators for prediabetes also include a waist measuring more than 35 inches for males and more than 40 inches for females, blood pressure of 130/85 mmHg or higher, and low levels of HDL in the bloodstream. 31
Obesity is clearly a risk factor for diabetes; approximately 80% of diabetics are obese. However, not all obese individuals develop diabetes, actually the percentage if fairly low, only 5-10%. How body fat is distributed influences diabetes risk; it is more common in individuals that carry most of their body fat in the central part of their abdomen. 32 Obesity increases the likelihood of insulin resistance, and it has been shown that weight-loss can reduce insulin resistance. 33 But how does obesity cause diabetes? One theory proposes that fat interferes with the body's ability to use insulin. 34 Specifically, excess lipid molecules in the bloodstream are taken up by muscle and liver cells interrupt the signals that would allow insulin into the cells. 35
High blood pressure, or hypertension, is also a risk factor for diabetes. Again, the mechanism that links blood pressure to diabetes is not well understood. High levels of insulin that would be present in a prediabetic or diabetic have been found to affect sodium retention, which could contribute to high blood pressure. 36 Also, insulin affects the endothelium (inner wall) of the blood vessels, causing dilation of blood vessels and capillary recruitment. Insulin resistance leads to dysfunction of the endothelium, which is related to hypertension, as well as coronary artery disease, and arteriosclerosis. 37
Treating and Preventing Diabetes
Diabetes can be prevented by maintaining a healthy diet and getting proper exercise. The diet prescribed by Walter Willett, M.D. would be similar to a Mediterranean Diet, consisting predominantly of whole grain foods, unsaturated plant oils, fruits and vegetables, nuts and legumes and fish, poultry and eggs. 38 Refined carbohydrates, like white rice, white bread, white pasta, potatoes, soda and sweets, red meat, and butter should be eaten only sparingly. 39
Diabetics also need to check their blood sugar regularly and take medicines as prescribed. Not all type 2 diabetics take insulin; some take medicines to increase the uptake of glucose into cells, to increase the amount of insulin produced by the pancreas, or to delay the uptake of glucose from the small intestine. 40 There are also medications to lower glucose levels in the blood. Most importantly, some people with type 2 diabetes can stop taking medications by losing weight and increasing their physical activity. The pancreas can make its own insulin and a careful diet can control their blood glucose levels. 41
The most important routes to prevent diabetes are sustained exercise and healthy diet. The same diet that can help control diabetes is beneficial for preventing the occurrence of diabetes. That is, we all should be sure to eat plenty of fruits and vegetables and limit our intake of refined carbohydrates. As well, those over the age of 45 should have their glucose levels checked every three years, or younger if you have increased risk for getting diabetes.
Comments: