Nutrition
Nutrition is the process of consuming, absorbing, and using nutrients needed by the body for growth, development, and maintenance of life. To receive adequate, appropriate nutrition, people need to consume a healthy diet, which consists of a variety of nutrients (the natural chemical substances in foods that nourish the body). A healthy diet enables people to maintain a desirable body weight and composition (the percentage of fat and muscle in the body) and to perform their daily physical and mental activities.
If a person does not consume sufficient amounts of nutrients, a nutritional deficiency disorder may result. To determine whether a person is consuming enough nutrients, a doctor asks about eating habits and diet and performs a physical examination to assess the composition and functioning of the body. Body composition, including the proportion of body fat, can be determined accurately by weighing the person under water (hydrostatic weighing) and can be approximated by measuring skin fold thickness or performing bioelectrical impedance analysis. Laboratory tests to measure the levels of nutrients in blood and tissues can be performed. For example, the level of albumin, the main protein in blood, can be measured. Nutrient levels decrease when nutrition is inadequate.
Generally, nutrients are divided into two classes: macronutrients and micronutrients. Macronutrients are required daily in large quantities. They include proteins, fats, carbohydrates, some minerals, and water. Water is required in amounts of 1 milliliter for each calorie of energy expended or about 2.6 quarts (2,500 milliliters) a day. Micronutrients are required daily in small quantities – in milligrams (one thousandth of a gram) to micrograms (one millionth of a gram). They include vitamins and trace minerals that enable the body to use macronutrients.
Foods consumed in the daily diet contain as many as 100,000 substances. But only 300 are classified as nutrients, and only 45 are classified as essential nutrients. However, food contains many other useful components, including some fibers, such as cellulose, pectins, and gums. Food contains additives (such as preservatives, emulsifiers, antioxidants, and stabilizers), which improve the production, processing, storage, and packaging of foods. Spices, flavors, substances that add odor or color, phyto chemicals (substances in plants that have biologic activity in animals), and many other natural products improve the appearance, taste, and stability of food.
| Height-Weight Reference Chart for Adults* | ||
| Height | Weight - Women (lbs) | Weight - Men (lbs) |
| 4' 10" | 92 – 121 | — |
| 4' 11" | 95 – 124 | — |
| 5' 0" | 98 – 127 | — |
| 5' 1" | 101 – 130 | 105 – 134 |
| 5' 2" | 104 – 134 | 108 – 137 |
| 5' 3" | 107 – 138 | 111 – 141 |
| 5' 4" | 110 – 142 | 114 – 145 |
| 5' 5" | 114 – 146 | 117 – 149 |
| 5' 6" | 119 – 150 | 121 – 154 |
| 5' 7" | 122 – 154 | 125 – 159 |
| 5' 8" | 126 – 159 | 129 – 163 |
| 5' 9" | 130 – 164 | 133 – 167 |
| 5' 10" | 134 – 169 | 137 – 172 |
| 5' 11" | — | 141 – 177 |
| 6' 0" | — | 145 – 182 |
| 6' 1" | — | 149 – 187 |
| 6' 2" | — | 153 – 192 |
| 6' 3" | — | 157 – 197 |
| *Height is without shoes; weight is without clothes. | ||
Fat Versus Lean: Body Composition
Maintaining an appropriate weight is important for physical and psychologic health. A standardized height-weight table can be used as a guide. But body mass index is more reliable. It is calculated by dividing the weight (in kilograms) by the height (in meters squared).(see Determining Body Mass Index 
) A body mass index between 18 and 25 is usually considered normal for men and women. A less obvious but important consideration is how much of the body is fat and how much is muscle (body composition). There are several ways to determine body composition. Underwater (hydrostatic) weighing in an immersion pool can determine body composition. Bone and muscle are denser than water, so a person with a high percentage of lean tissue weighs more in water and a person with a high percentage of fat weighs less. Although this method is considered the most accurate, it requires special equipment, considerable time, and expertise to perform.
Body composition can be estimated by measuring the amount of fat under the skin (skinfold thickness) or by bioelectric impedance analysis. For skinfold thickness, a fold of skin on the back of the left upper arm (triceps skinfold) is pulled away from the arm and measured with a caliper. A skinfold measurement of about 1/2 inch in men and about 1 inch in women is considered normal. This measurement plus the circumference of the left upper arm can be used to estimate the amount of skeletal muscle in the body (lean body mass).
Bioelectric impedance analysis measures the resistance of body tissues to the flow of an undetectable low-voltage electrical current. Typically, a person stands barefoot on metal footplates, and the electrical current is sent up one foot and down the other. Body fat and bone resist the flow much more than does muscle tissue. By measuring the resistance to the current, doctors can estimate the percentage of body fat. This test takes only about 1 minute.
Dual-energy x-ray absorptiometry (DEXA) accurately determines the amount and distribution of body fat. This imaging procedure uses a very low dose of radiation and is safe. However, it is too expensive to use routinely.
A Calorie is a Measure of Energy
Foods have calories; that is, foods supply the body with energy, which is released when foods are broken down during digestion. Energy enables cells to perform all of their functions, including building proteins and other substances needed by the body. The energy can be used immediately or stored to be used later.
When the supply of energy – that is, the number of calories consumed in foods – exceeds the body's immediate needs, the body stores the excess energy. Most excess energy is stored as fat. Some is stored as carbohydrates, usually in the liver and muscles. As a result, weight is gained. An excess of only 200 calories per day for 10 days is likely to result in a weight gain of nearly ½ pound, mostly as fat.
When the intake of energy is insufficient for the body's needs, the body begins to use carbohydrates stored in the liver and muscle. Because the body mobilizes stored carbohydrates quickly and because water is usually excreted as carbohydrates are mobilized, weight loss tends to be fast initially. However, the small amount of stored carbohydrates provides energy for only a short time. Next, the body uses stored fat. Converting fat to energy is a slower process, so weight loss is slower as the body uses fat for energy. However, the amount of fat stored is much larger and can, in most people, provide energy for a long time. Only during prolonged, severe shortages of energy, does the body break down protein. For normally nourished people who experience total starvation (who do not consume any food to supply energy), death occurs in 8 to 12 weeks.
Energy requirements vary markedly from about 1,000 to more than 4,000 calories a day depending on age, sex, and physical activity. Typically, to maintain body weight, sedentary women, young children, and older adults need about 1,600 calories a day; older children, active adult women, and sedentary men need about 2,000 calories; and active adolescent boys and young men need about 2,400 calories. However, division of caloric intake by a 24-hour period (daily intake) is arbitrary. Furthermore, the needs of the body vary depending on its activity at any particular time. Vigorous activity, especially aerobic exercise, increases needs substantially, and a lack of activity decreases needs.
How Are Calories in Foods Measured?
Food labels always contain the number of calories per serving. But how is this number determined? The answer is surprisingly simple: The food is burned. A sample of the food is placed in an insulated, oxygen-filled chamber that is surrounded by water; this chamber is called a bomb calorimeter. The sample is burned completely. The heat from the burning increases the temperature of the water, which is measured and which indicates the number of calories in the food. For example, if water temperature increases by 20 degrees, the food contains 20 calories. This method of measuring calories is called direct calorimetry.
Carbohydrates, proteins, and fats supply 90% of the dry weight of the diet and 100% of its energy. As sources of energy, carbohydrates, proteins, and fats are interchangeable in proportion to their energy content. The energy content is 4 calories in a gram of carbohydrate or protein and 9 calories in a gram of fat (1 gram equals 1/28 ounce). However, these nutrients differ in how quickly they supply energy; carbohydrates are the quickest, and fats are the slowest.
Carbohydrates, proteins, and fats are digested in the intestine, where they are broken down into their basic units: carbohydrates into sugars, proteins into amino acids, and fats into fatty acids and glycerol. The body uses these basic units to build substances it needs for growth, maintenance, and activity (including other carbohydrates, proteins, and fats).
Carbohydrates: Depending on the size of the molecule, carbohydrates may be simple or complex. Simple carbohydrates are various forms of sugar, such as glucose and fructose. Because they are small molecules, they can be broken down by the body quickly and are the quickest source of energy. Fruits, dairy products, honey, and maple syrup contain large amounts of simple carbohydrates, which provide the sweet taste in most candies and cakes.
Complex carbohydrates are composed of long strings of simple carbohydrates. Because complex carbohydrates are larger molecules than simple carbohydrates, they are slower sources of energy. However, energy can be generated from them relatively quickly. Complex carbohydrates occur in wheat products (such as breads and pastas), other grains (such as rye and corn), beans, and root vegetables (such as potatoes).
The body stores very small amounts of excess energy as carbohydrates. The liver stores some as glycogen, a complex carbohydrate that the body can easily and rapidly convert to energy. Muscles also store glycogen, which they use during periods of intense exercise. The amount of carbohydrates stored as glycogen provides almost a day's worth of calories. A few other body tissues store carbohydrates as complex carbohydrates that cannot be used to provide energy.
Most authorities recommend that about 50 to 55% of the total daily calories should consist of carbohydrates.
Proteins: Proteins consist of units called amino acids, strung together in complex formations. Because proteins are complex molecules, the body takes longer to break them down. As a result, they are a much slower and longer-lasting source of energy than carbohydrates. There are 20 amino acids. The body synthesizes some of them from components within the body, but it cannot synthesize nine of the amino acids – called essential amino acids. They must be consumed in the diet. They include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
The body contains large amounts of protein. Protein, the main building block in the body, is the primary component of most cells. For example, muscle, connective tissues, and skin are all built of protein.
Adults need to eat about 60 grams of protein per day (0.8 grams per kilogram of weight or 10 to 15% of total calories). Adults who are trying to build muscle need slightly more. Children also need more. If more protein is consumed than is needed, the body does not build more protein (or muscle). Instead, the body breaks the protein down and stores its components as fat, which can be broken down and used for energy later as needed.
Fats: These complex molecules are composed of fatty acids and glycerol. Fats are the slowest source of energy but the most energy-efficient form of food. Each gram of fat supplies the body with about 9 calories, more than twice that supplied by proteins or carbohydrates. Because fats are such an efficient form of energy, the body stores any excess energy as fat. The body deposits excess fat in the abdomen (omental fat) and under the skin (subcutaneous fat) to use when it needs more energy. The body also stores excess fat in blood vessels and within organs, where it blocks blood flow and damages the organs, often with devastating results.
The body cannot synthesize some fatty acids – called essential fatty acids. They must be consumed in the diet. The essential fatty acids make up about 7% of the fat consumed in a normal diet and about 3% of total calories (about 8 grams). They include linoleic acid, linolenic acid, arachidonic acid, eicosapentaenoic acid, and docosahexaenoic acid. Linoleic acid and linolenic acid are found in vegetable oils. Eicosapentaenoic acid and docosahexaenoic acid, which are essential for brain development, are found in fish oils. In the body, arachidonic acid can be formed from linoleic acid, and eicosapentaenoic and docosahexaenoic acid can be formed from linolenic acid, although fish oil is a more efficient source.
There are different kinds of fat: monounsaturated, polyunsaturated, and saturated. In general, saturated fats are more likely to increase cholesterol levels and increase the risk of atherosclerosis. Some evidence suggests that consuming trans fatty acids, a different category of fat, may increase cholesterol levels in the body and thus may contribute to the risk of atherosclerosis.
Authorities generally recommend that fat be limited to less than 30% of daily total calories, or fewer than 90 grams per day, and that the amount of saturated fat, trans fatty acids, and cholesterol in the diet be reduced. However, people with high cholesterol levels may need to reduce their fat intake even more. When fat intake is reduced to 10% or less of daily total calories, cholesterol levels tend to decrease dramatically.
A diet is whatever a person eats, regardless of the goal – whether it is losing weight, gaining weight, reducing fat intake, avoiding carbohydrates, or having no particular goal. However, the term is often used to imply a goal of losing weight, which is an obsession for many people.
Food Guide Pyramid
Standard healthy diets for children and adults are based on the needs of average people who do not need to lose or gain weight; who do not need to restrict any component of the diet because of disorders, risk, or advanced age; and who expend average amounts of energy through exercise or other vigorous activities. Thus, for a particular person, a healthy diet may vary substantially from what is recommended in standard diets. For example, special diets are required by people who have diabetes, certain kidney disorders, certain liver disorders, coronary artery disease, high cholesterol levels, osteoporosis, diverticulitis, chronic constipation, or food sensitivities. There are special dietary recommendations for young children, but little guidance is available for other specific age groups, such as older people.
Some foods contain fiber, a tough complex carbohydrate that is only partially digested by the body. The digestible part provides some calories; the indigestible part provides bulk to the feces. This bulk helps the intestine move feces along, thereby preventing constipation, and it reduces pressure in the intestine, helping prevent diverticular disease. Eating fiber also helps increase the elimination of cancer-causing substances produced by the bacteria in the large intestine and helps moderate the changes in sugar and cholesterol levels in the blood that occur after eating a meal. Authorities generally recommend that about 30 grams of fiber be consumed daily. In the United States, the average amount of fiber consumed daily is about 25 grams. An average serving of fruit, a vegetable, or cereal contains 2 to 4 grams of fiber.
Weight Loss Diets
Weight loss requires consuming fewer calories than the body uses. Losing ½ pound of fat by dieting requires 10 days of consuming 200 fewer calories per day than the body uses. If 400 fewer calories are consumed than needed, a dieter can hope to lose ½ pound every 5 to 7 days. One pound of body fat stores about 3,500 calories.
In most conservative weight loss diets, the number of calories consumed is usually reduced to 1,200 to 1,500 a day. Diets containing fewer than 1,200 calories often lack essential nutrients, such as protein, iron, and calcium. To be healthy, weight loss diets should provide about the same volume of food (by including more fiber and fluids) as the normal diet. They should also be low in saturated fat and sugar and include essential nutrients, including antioxidants. Reading food labels makes people aware of the composition and calorie (energy) content of food, including beverages. Counting calories helps people control calorie intake. Using sugar and fat substitutes and eating foods that contain them help some people reduce calorie intake.
Combining increased exercise with dieting greatly enhances weight loss because exercise increases the number of calories the body uses. For example, vigorous walking burns about 4 calories per minute, so that 1 hour of brisk walking per day burns about 240 calories. Running is even better, burning about 6 to 8 calories per minute.
Eating small meals frequently can help with weight loss for several reasons. Insulin levels usually increase after eating, and more insulin is produced when many calories are consumed, especially when the meal is rich in carbohydrates. High insulin levels promote the deposition of fat and increase appetite. Eating small, frequent meals prevents levels from increasing, thus discouraging fat deposition and helping suppress appetite. Eating certain types of foods at certain times of the day may also promote weight loss. For example, fast-energy foods, such as carbohydrates, are best eaten when the body needs a large supply of energy – that is, in the morning and during vigorous exercise. The body's need for energy is lowest at night, so avoiding carbohydrates in the evening may help.
High Protein-Low Carbohydrate Diets: Diets high in protein and low in simple carbohydrates have become popular as a way to lose weight. Such diets usually also restrict fat, because each gram supplies so many calories. However, some high protein-low carbohydrate diets, such as the Atkins diet, do not restrict fat.
The theory behind these diets is that slower-burning energy sources – protein and fat – provide a steady supply of energy and thus are less likely to lead to weight gain. In addition, people tend to feel full longer after eating protein than after eating carbohydrates, because carbohydrates empty from the stomach quickly and are digested quickly. Carbohydrates also strongly stimulate insulin production, which promotes fat deposition and increases appetite.
Some of these diets recommend avoiding foods with a high glycemic index. Foods that contain carbohydrates are given a glycemic index, which indicates how quickly the carbohydrates are digested and thus how a food affects the level of sugar in the blood. Foods that contain large amounts of sugar (such as maple syrup, honey, and candy) and starchy foods (such as carrots, potatoes, and some cereals) have a high glycemic index because they are quickly digested and quickly increase blood sugar levels. Foods that are high in fiber (such as whole-grain rye bread and all-bran cereals) have a low glycemic index because they are digested more slowly and do not quickly increase blood sugar levels. This index is useful for diabetics who need to control their blood sugar levels and for athletes who need to restore their blood sugar levels after an athletic performance. However, the glycemic index is not useful for dieters. The difference between the speed of carbohydrate digestion for foods with the highest and lowest glycemic indexes is so small that it makes little difference to most dieters. Avoiding foods with a high glycemic index does not promote weight loss, and it eliminates foods with valuable vitamins and minerals.
Some experts do not recommend following a high-protein diet for long periods of time. Some evidence suggests that over years, very high protein diets impair kidney function and may contribute to the decrease in kidney function that occurs in older people. People with certain kidney and liver disorders should not consume a high-protein diet.
Very low carbohydrate diets (of less than 100 grams a day) can also cause a problem – the accumulation of keto acids (ketosis). When a person does not consume enough energy for the body's needs, the body breaks down fats. As part of this process, the body produces keto acids. In small amounts, keto acids are easily excreted by the kidneys without causing symptoms. However, in large amounts, they can cause nausea, fatigue, bad breath, and even more serious symptoms, such as dizziness (due to dehydration) and abnormal heart rhythms (due to electrolyte imbalances). People following a low-carbohydrate diet (or any other weight loss diet) should drink large amounts of water to help flush keto acids from the body.
Low-carbohydrate diets tend to cause large amounts of weight to be lost during the first week or so, as the body converts stored carbohydrates (glycogen) to energy. As glycogen is broken down, the body also excretes large amounts of water, adding to the weight loss. However, once the body begins to use stored fat for energy, weight loss slows. Low carbohydrate diets may be high in fat and thus the total caloric intake may exceed what the body uses; in such cases, weight loss may stop after glycogen is used up.
Low-Fat Diets: This type of diet is the most effective for losing weight and maintaining weight loss. Fat supplies a large number of calories per gram and is more readily deposited as body fat than are proteins and carbohydrates. Reducing the amount of fat rather than the amount of protein or carbohydrate may be an easier way to reduce total caloric intake because a small reduction in fat saves so many calories. A reduction of only 10 grams of fat per day saves about 900 calories. For weight to be lost, the amounts of protein and carbohydrates consumed should not increase greatly as the amount of fat decreases. However, the main reason for reducing the amount of fat in the diet is to lower cholesterol levels in the blood – which a dieter may or may not need to do.
High-Fiber Diets: Fiber indirectly helps with weight loss in several ways. It provides bulk, which makes people feel full faster. It slows the rate at which the stomach empties so people feel full longer. Fiber also requires more chewing, forcing people to eat more slowly and perhaps less. High-fiber foods, such as fruits and vegetables, wheat bread, and beans, are filling without providing many calories. Eating more high-fiber foods may enable people to eat fewer less filling, high-calorie foods, such as high-fat foods. However, fiber supplements, such as guar gum and cellulose, are not effective for weight loss.
Liquid Diets: Many people use liquid diets to lose weight, mainly because they are convenient. However, the contents of such liquids vary, and many are unlikely to be of much help in losing weight. Some commercially available liquid diets are well balanced, with appropriate proportions of protein, carbohydrates, and fat plus supplemental vitamins and minerals. But others contain a large proportion of carbohydrates, producing a sweet and tasty drink, and are not necessarily low in calories. Such liquid diets are more useful as a supplement to other foods for people who are trying to gain weight.
Very low calorie commercial liquid diets contain all needed nutrients. Usually, a drink that contains 220 calories is consumed 4 times a day instead of meals. Such diets are effective for short-term weight loss. For long-term weight loss, two or three meals are replaced with a liquid-diet drink. The remaining one or two meals should be low-fat, low-calorie, and nutritious.
An alternative to commercial diets is the all milk diet. This diet is simple and inexpensive and may be useful for short-term weight loss.
Grapefruit Diet: One popular fad diet involves consuming large amounts of grapefruit and grapefruit juice. The theory behind this diet is that grapefruit contains an enzyme that helps burn fat, but this theory has never been proved.
Although grapefruits are a healthful food – containing no fat, little sodium, and large amounts of vitamin C, beta-carotene (at least in pink grapefruits), and fiber – a diet based primarily on one fruit is nutritionally unsound. A grapefruit diet may help some people reduce total caloric intake, but it does not supply a balance of nutrients, which is needed for good health. Furthermore, eating grapefruit alters the levels of several drugs in the blood, and eating large amounts of grapefruit often causes diarrhea.
Food-Combining and Food-Cycling Diets: These diets are based on a theory that eating certain kinds of foods at different times promotes weight loss. An example is the Beverly Hills Diet, which recommends cycling different foods, usually over a 6-week period. For part of the time, a person eats nothing but fruits. Later, the person eats only breads, then only protein, then only fats. No scientific evidence supports this approach to weight loss, and the diet is intrinsically unhealthful.
Other Fad Diets: There are many fad diets. Some require extreme reductions in the number of calories consumed; others rely on supplements alleged to help burn fat; and still others are based on eating a single type of food. The Richard Simmons diet (900 calories a day) and the Atkins Diet (2,000 calories a day) are both low in carbohydrates and may result in dehydration if not enough fluids are consumed. Following the Richard Simmons diet for a long time may lead to deficiencies of iron, calcium, and protein as well as vitamins A, B1 (thiamin), B2 (riboflavin), and B3 (niacin). The Atkins diet is particularly high in fat and cholesterol. The Beverly Hills, Rice, and Pritikin diets are low in fat and protein and high in carbohydrates. The Beverly Hills and Rice diets are deficient in protein, iron, calcium, zinc, and vitamin B12. The Pritikin diet is relatively adequate in nutrition, but its low-fat content makes it unpalatable and less likely to be followed. These diets have not been shown to lead to sustained weight loss, and many are dangerous, supplying inadequate amounts of essential nutrients and leading to serious metabolic disturbances, such as loss of bone density and strength (including osteoporosis), problems with menstruation, abnormal heart rhythms, high cholesterol levels, kidney stones, and worsening of gout.
