UNIT V: Integration of Metabolism Nutrition. 1. Overview Nutrients are the constituents of food necessary to sustain the normal functions of the body

UNIT V:Integration of Metabolism

Nutrition

1. OverviewNutrients are the constituents of food

necessary to sustain the normal functions of the body.

All energy is provided by three classes of nutrients: fats, carbohydrates, protein (Figure 27.1).

The intake of these energy-rich molecules is larger than that of the other dietary nutrients Therefore, they are called the

macronutrients. This chapter focuses on the kinds and

amounts of macronutrients that are needed to maintain optimal health and prevent chronic disease in adults.

Those nutrients needed in lesser amounts, vitamins and minerals, are called the micronutrients.

2

Figure 27.1 Essential nutrients obtained from the diet.

[Note: Ethanol is not considered an essential component of the diet but may provide a significant contribution to the daily caloric intake of some individuals.]

3

2. Dietary Reference Intakes

Dietary Reference Intakes (DRI)- estimate of the amounts of nutrients required to prevent deficiencies and maintain optimal health and growth.

The DRI also establish upper limits on the consumption of some nutrients, and incorporate the role of nutrients in lifelong health.

4

The DRI consist of four dietary reference standards for the intake of nutrients designated for specific age groups, physiologic states, and gender.

A. Definition of the DRI

Figure 27.2 Components of the Dietary Reference Intakes (DRIs).

5

1. Estimated Average Requirement (EAR): The EAR is the average daily nutrient intake

level estimated to meet the requirement of one half of the healthy individuals in a particular life stage and gender group.

Requirements for half the healthy individuals


6

2. Recommended Dietary Allowance (RDA): The RDA is the average daily dietary intake

level that is sufficient to meet the nutrient requirements of nearly all (97–98%) the individuals in a life stage and gender group.

The RDA is not the minimal requirement for healthy individuals; rather, it is intentionally set to provide a margin of safety for most individuals.

The EAR serves as the foundation for setting the RDA.

Requirement for 97–98% of all healthy individuals


7

3. Adequate Intake (AI): The AI is set instead of an RDA if sufficient

scientific evidence is not available to calculate an EAR or RDA.

The AI is based on estimates of nutrient intake by a group (or groups) of apparently healthy people that are assumed to be adequate.

For example, the AI for young infants, for whom human milk is the recommended sole source of food for the first four to six months, is based on the estimated daily mean nutrient intake supplied by human milk for healthy, full-term infants who are exclusively breast-fed.

Observed or experimentally determined Used when RDA is not available


8

4. Tolerable Upper Intake Level (UL): The UL is the highest average daily nutrient

intake level that is likely to pose no risk of adverse health effects to almost all individuals in the general population.

As intake increases above the UL, the potential risk of adverse effects may increase.

The UL is not intended to be a recommended level of intake.

The UL are useful because of the increased availability of fortified foods and the increased use of dietary supplements.

The UL applies to chronic daily use. For some nutrients, there may be insufficient data on which to develop a UL.

Level at which risk of harm begins to increase


9

Most nutrients have a set of DRI (Figure 27.3).

Usually a nutrient has an EAR and a corresponding RDA.

Most are set by age and gender, and may be influenced by special factors, such as pregnancy and lactation in women.

When the data are not sufficient to estimate an EAR (or an RDA), then an AI is designated.

The AI is judged by experts to meet the needs of all individuals in a group, but is based on less data than an EAR and RDA.

B. Using the DRI

10

Figure 27.3 Dietary Reference Intakes for vitamins and minerals in individuals one year and older. EAR = Estimated Average Requirement; RDA = Recommended Dietary Allowance; AI = Adequate Intake; UL = Tolerable Upper Intake Level; — = no value established.


11

DRIs for selected nutrients

Institute of Medicine (http://www.iom.edu)

*Reference woman = 57 kg; †Reference man = 70 kg;

‡Dietary Guidelines for Americans, 2010

Nutrient Age, condition DRI Women Men

Protein (g/day)

> 19 years EAR RDA

38 g46 g* (0.8 g/kg)

46 g56 g† (0.8 g/kg)

Phosphorus (mg/day)

> 19 years

> 19–70 years> 70 years

EARRDAUL

580700

4,0003,000

580 7004,0003,000

Potassium(mg/day)

> 19 years AI 4,700 4,700

http://www.iom.edu/

Intakes below the EAR need to be improved because the probability of adequacy is fifty percent or less (Figure 27.4).

Intakes between the EAR and RDA likely need to be improved because the probability of adequacy is less than 98 percent, and intakes at or above the RDA can be considered adequate.

Intake above the AI can be considered adequate. Intakes between the UL and the RDA can be considered to have no risk for adverse effects.

B. Using the DRI

13

Figure 27.4 Comparison of the components of the Dietary Reference Intakes. EAR = Estimated Average Requirement; RDA = Recommended Dietary Allowance; AI = Adequate Intake; UL = Tolerable Upper Intake Level.

B. Using the DRI

14

The Estimated Energy Requirement is the average dietary energy intake predicted to maintain an energy balance (that is, when the calories consumed are equal to the energy expended) in a healthy adult of a defined age, gender, and height whose weight and level of physical activity are consistent with good health.

Differences in the genetics, body composition, metabolism, and behavior of individuals make it difficult to accurately predict a person's caloric requirements.

However, some simple approximations can provide useful estimates: For example, sedentary adults require about 30 kcal/kg/day to maintain body weight; moderately active adults require 35 kcal/kg/day; and very active adults require 40 kcal/kg/day.

3. Energy Requirement in Humans

15

The energy content of food is calculated from the heat released by the total combustion of food in a calorimeterIt is expressed in kilocalories (kcal, or Cal)

The standard conversion factors for determining the metabolic caloric value of fat, protein, and carbohydrate are shown in Figure 27.5.

A. Energy content of food

Figure 27.5 Average energy available from the major food components.16

The energy generated by metabolism of the macronutrients is used for three energy-requiring processes that occur in the body: Resting metabolic rate, Thermic effect of food, and Physical activity

B. How energy is used in the body

17

Figure 27.6 Estimated total energy expenditure in a typical 20-year-old woman, 165 cm (5 feet, 4 inches) tall, weighing 50 kg (110 lb), and engaged in light activity.

18

1. Resting metabolic rate: The energy expended by an individual in a

resting, post-absorptive state is called the resting (formerly, basal) metabolic rate (RMR).

It represents the energy required to carry out the normal body functions, such as respiration, blood flow, ion transport, and maintenance of cellular integrity.

In an adult, the RMR is about: 1,800 kcal for men (70 kg) and 1300 kcal for women (50 kg)

From 50–70% of the daily energy expenditure in sedentary individuals is attributable to the RMR (Figure 27.6).


19

2. Thermic effect of food: The production of heat by the body increases as

much as 30% above the resting level during the digestion and absorption of food.

This effect is called the thermic effect of food or diet-induced thermogenesis.

Over a 24–hour period, the thermic response to food intake may amount to 5–10% of the total energy expenditure.


20

3. Physical activity: Muscular activity provides the greatest variation

in energy expenditure. The amount of energy consumed depends on

the duration and intensity of the exercise. The daily expenditure of energy can be

estimated by carefully recording the type and duration of all activities.

In general, a sedentary person requires about 30–50% more than the resting caloric requirement for energy balance (see Figure 27.6), whereas a highly active individual may require 100% or more calories above the RMR.


21

Acceptable Macronutrient Distribution Ranges (AMDR) are defined as: A range of intakes for a particular macronutrient

that is associated with reduced risk of chronic disease while providing adequate amounts of essential nutrients.

The AMDR for adults is 45–65% of their total calories from carbohydrates, 20–35% from fat, and 10–35% from protein (Figure 27.7).

4. Acceptable Macronutrient Distribution Ranges

22

Figure 27.7 Acceptable macronutrient distribution ranges in adults.

23

The incidence of a number of chronic diseases is significantly influenced by the kinds and amounts of nutrients consumed (Figure 27.8)

Dietary fats most strongly influence the incidence of coronary heart disease (CHD); evidence linking dietary fat and the risk for cancer or obesity is much weaker.

In the past dietary recommendations emphasized decreasing the total amount of fat in the diet

Research now indicates that the type of fat is more important than the total amount of fat consumed.

5. Dietary Fats

24

Figure 27.8 Influence of nutrition on some common causes of death in the United States in the year 2000. Red indicates causes of death in which the diet plays a significant role. Blue indicates causes of death in which excessive alcohol consumption plays a part. (*Diet plays a role in only some forms of cancer.)

25

Plasma cholesterol may arise from the diet or from endogenous biosynthesis.

In either case, cholesterol is transported between the tissues in combination with protein and phospholipids as lipoproteins.

1.Low-density lipoprotein (LDL) and high-density protein (HDL):

The level of plasma cholesterol is not precisely regulated, but rather varies in response to the diet.

Elevated levels result in an increased risk for cardiovascular disease (Figure 27.9).

A. Plasma lipids and CHD

26

Figure 27.9 Correlation of the death rate from coronary heart disease with the concentration of plasma cholesterol. [Note: The data were obtained from a six year study of men with the death rate adjusted for age.]

27

The risk increases progressively with higher values for serum total cholesterol.

A much stronger correlation exists between the levels of blood LDL cholesterol and heart disease

In contrast, high levels of HDL cholesterol have been associated with a decreased risk for heart disease.

Abnormal levels of plasma lipids (dyslipidemias) act in combination with smoking, obesity, sedentary lifestyle, and other risk factors to increase the risk of CHD.

Elevated plasma triacylglycerols are also a risk factor for CHD, but the association is weaker than that of LDL cholesterol with CHD.

A. Plasma lipids and CHD

28

Triacylglycerols are quantitatively the most important class of dietary fats.

The influence of triacylglycerols on blood lipids is determined by the chemical nature of their constituent fatty acids.

The presence or absence of double bonds (saturated vs. mono- and polyunsaturated), the number and location of the double bonds (n-6 vs n-3), and the cis vs. trans configuration of the unsaturated fatty acids are the most important structural features that influence blood lipids.

B. Dietary fats and plasma lipids

29

1. Saturated fat: Triacylglycerols composed primarily of fatty

acids whose side chains do not contain any double bonds are referred to as saturated fats.

Consumption of saturated fats is strongly associated with high levels of total plasma cholesterol and LDL cholesterol, and an increased risk of CHD.

The main sources of saturated fatty acids are dairy and meat products and some vegetable oils, such as coconut and palm oils (Figure 27.10).

Most experts strongly advise limiting intake of saturated fats.


30

2. Monounsaturated fats: Triacylglycerols containing primarily fatty acids

with one double bond are referred to as monounsaturated fat.

Unsaturated fatty acids are generally derived from vegetables and fish.

When substituted for saturated fatty acids in the diet, monounsaturated fats lower both total plasma cholesterol and LDL cholesterol, but maintain or increase HDL cholesterol.

This ability of monounsaturated fats to favorably modify lipoprotein levels may explain, in part, the observation that Mediterranean cultures, with diets rich in olive oil (high in monounsaturated oleic acid), show a low incidence of CHD.


31

Figure 27.10 Compositions of commonly encountered dietary lipids.

.

32

3. Polyunsaturated fats: Triacylglycerols containing primarily fatty

acids with more than one double bond are referred to as polyunsaturated fats.

The effects of polyunsaturated fatty acids (PUFAs) on cardiovascular disease is influenced by the location of the double bonds within the molecule.

a. ω-6 Fatty acids: a. These are long-chain, polyunsaturated fatty

acids, with the first double bond beginning at the sixth carbon atom (when counting from the methyl end of the fatty acid molecule [Note: They are also called n-6 fatty acids.]


33

34

Consumption of fats containing ω-6 polyunsaturated fatty acids, lowers plasma cholesterol when substituted for saturated fats

Plasma LDL are lowered, but HDL, which protect against CHD, are also lowered.

The powerful benefits of lowering LDL are only partially offset because of the decreased HDL

b. ω-3 Fatty acids: These are long-chain, polyunsaturated fatty

acids, with the first double bond beginning at the third carbon atom (when counting from the methyl end of the fatty acid molecule).

Dietary ω-3 polyunsaturated fats (Fish oil) suppress cardiac arrhythmias, reduce serum triacylglycerols, decrease the tendency for thrombosis, lower blood pressure, and substantially reduce risk of cardiovascular mortality (Figure 27.12), but they have little effect on LDL or HDL cholesterol levels.


35

Figure 27.12 Dose responses of physiologic effects of fish oil intake.

.

36

4. Trans fatty acids: Trans fatty acids (Figure 27.13) are chemically

classified as unsaturated fatty acids, but behave more like saturated fatty acids in the body, that is, they elevate serum LDL (but not HDL), and they increase the risk of CHD.

Trans fatty acids do not occur naturally in plants, but occur in small amounts in animals.

However, trans fatty acids are formed during the hydrogenation of liquid vegetable oils, for example, in the manufacture of margarine and partially hydrogenated vegetable oil.

Trans fatty acids are a major component of many commercial baked goods, such as cookies and cakes, and most deep-fried foods.

A clue to the presence of trans fatty acids in a food is the words ‘partially hydrogenated’ on the list of package ingredients.


37

Figure 27.13 Structure of cis and trans fatty acids

.

38

5. Dietary cholesterol: Cholesterol is found only in animal products. The effect of dietary cholesterol on plasma

cholesterol (Figure 27.14) is less important than the amount and types of fatty acids consumed.


Figure 27.14 Response of plasma LDL concentrations to an increase in dietary cholesterol intake.

39

Figure 27.15 Effects of dietary fats.40

The primary role of dietary carbohydrate is to provide energy.

Carbohydrate consumption has significantly increased, leading some observers to link obesity with carbohydrate consumption.

However, obesity has also been related to increasingly inactive lifestyles, and to calorie-dense foods served in expanded portion size.

Carbohydrates are not inherently fattening.

6. Dietary Carbohydrates

41

Figure 27.16 Total caloric consumption and distribution of calories between the macronutrients in adults.

42

Carbohydrates in the diet are classified as either monosaccharides and disaccharides (simple sugars), polysaccharides (complex sugars), or fiber.

1. Monosaccharides: Glucose and fructose are the principal

monosaccharides found in food. Glucose is abundant in fruits, sweet corn, corn

syrup, and honey. Free fructose is found together with free

glucose and sucrose in honey and fruits.

A. Classification of carbohydrates

43

2. Disaccharides: The most abundant disaccharides are:

Sucrose (glucose + fructose), is ordinary “table sugar,” and is abundant in molasses and maple syrup.

Lactose (glucose + galactose) is the principal sugar found in milk.

Maltose (glucose + glucose) is a product of enzymatic digestion of polysaccharides. It is also found in significant quantities in beer and

malt liquors.

The term “sugar” refers to monosaccharides and disaccharides.


44

3. Polysaccharides: Complex carbohydrates are polysaccharides

(most often polymers of glucose), which do not have a sweet taste.

Starch is an example of a complex carbohydrate that is found in abundance in plants.

Common sources include wheat and other grains, potatoes, dried peas and beans, and vegetables.

4. Fiber: Dietary fiber is defined as the non-digestible

carbohydrates and lignin present in plants. Several different terms are used to described

this complex group of compounds. For example, functional fiber is the isolated,

extracted, or synthetic fiber that has proven health benefits.


45

Total fiber is the sum of dietary fiber and functional fiber.

Soluble fiber refers to fibers that form a viscous gel when mixed with a liquid.

Insoluble fiber passes through the digestive track largely intact.

Dietary fiber provides little energy but has several beneficial effects.


46

Figure 27.17 Actions of dietary fiber.

47

Some carbohydrate-containing foods produce a rapid rise followed by a steep fall in blood glucose concentration, whereas others result in a gradual rise followed by a slow decline.

The glycemic index has been proposed to quantitate these differences in the time course of postprandial glucose concentrations (Figure 27.18).

Glycemic index is defined as: The area under the blood glucose curves seen

after ingestion of a meal with carbohydrate-rich food, compared with the area under the blood glucose curve observed after a meal consisting of the same amount of carbohydrate in the form of glucose or white bread.

B. Dietary carbohydrate and blood glucose

48

Figure 27.18 Blood glucose concentrations following ingestion of food with low or high glycemic index.

49

Carbohydrates are not essential nutrients, because the carbon skeletons of amino acids can be converted into glucose

However, the absence of dietary carbohydrate leads to ketone body production, and degradation of body protein whose constituent amino acids provide carbon skeletons for gluconeogenesis.

Adults should consume 45–65 percent of their total calories from carbohydrates.

C. Requirements for carbohydrate

50

Humans have no dietary requirement for protein, per se, but, the protein in food does provide essential amino acids.

Nine of the 20 amino acids needed for the synthesis of body proteins are essential—that is, they cannot be synthesized in humans.

7. Dietary Protein

51

1. Proteins from animal sources: Proteins from animal sources (meat,

poultry, milk, and fish) have a high quality because they contain all the essential amino acids in proportions similar to those required for synthesis of human tissue proteins

A. Quality of proteins

52

2. Proteins from plant sources: Proteins from wheat, corn, rice, and beans have

a lower quality than do animal proteins. However, proteins from different plant sources

may be combined in such a way that the result is equivalent in nutritional value to animal protein.

For example, wheat (lysine-deficient but methionine-rich) may be combined with kidney beans (methionine-poor but lysine-rich) to produce a complete protein of improved biologic value.

Thus, eating foods with different limiting amino acids at the same meal (or at least during the same day) can result in a dietary combination with a higher biologic value than either of the component proteins

A. Quality of proteins

53

The amount of dietary protein required in the diet varies with its biologic value.

The greater the proportion of animal protein included in the diet, the less protein is required.

The RDA for protein is computed for proteins of mixed biologic value at 0.8 g/kg of body weight for adults, or about 56 g of protein for a 70-kg individual.

People who exercise strenuously on a regular basis may benefit from extra protein to maintain muscle mass; a daily intake of about 1 g/kg has been recommended for athletes.

Women who are pregnant or lactating require up to 30 g/day in addition to their basal requirements.

To support growth, children should consume 2 g/kg/day.

C. Requirement for protein in humans

54

1. Consumption of excess protein: There is no physiologic advantage to the

consumption of more protein than the RDA. Protein consumed in excess of the body's

needs is deaminated, and the resulting carbon skeletons are metabolized to provide energy or acetyl coenzyme A for fatty acid synthesis.

When excess protein is eliminated from the body as urinary nitrogen, it is often accompanied by increased urinary calcium, increasing the risk of nephrolithiasis and osteoporosis.


55

2. The protein-sparing effect of carbohydrate:

The dietary protein requirement is influenced by the carbohydrate content of the diet.

When the intake of carbohydrates is low, amino acids are deaminated to provide carbon skeletons for the synthesis of glucose that is needed as a fuel by the central nervous system.

If carbohydrate intake is less than 130 g/day, substantial amounts of protein are metabolized to provide precursors for gluconeogenesis.

Therefore, carbohydrate is considered to be “protein-sparing,” because it allows amino acids to be used for repair and maintenance of tissue protein rather than for gluconeogenesis.


56

In developed countries, protein-energy (calorie) malnutrition (PEM) is seen most frequently in hospital patients with chronic illness, or in individuals who suffer from major trauma, severe infection, or the effects of major surgery.

Such highly catabolic patients frequently require intravenous (parenteral) or tube-based (enteral) administration of nutrients.

In developing countries, an inadequate intake of protein and/or energy may be observed.

Affected individuals show a variety of symptoms, including a depressed immune system with a reduced ability to resist infection. Death from secondary infection is common.

Two extreme forms of PEM are kwashiorkor and marasmus.

D. Protein-calorie malnutrition

57

1. Kwashiorkor: Kwashiorkor occurs when protein deprivation is

relatively greater than the reduction in total calories.

Kwashiorkor is frequently seen in children after weaning at about one year of age, when their diet consists predominantly of carbohydrates.

Typical symptoms include stunted growth, edema, skin lesions, depigmented hair, anorexia, enlarged fatty liver, and decreased plasma albumin concentration.

Edema results from the lack of adequate plasma proteins to maintain the distribution of water between blood and tissues.


58

59 Figure 27.22: A.Listless child with kwashiorkor. B.Child suffering with marasmus.

2. Marasmus: Marasmus occurs when calorie deprivation is

relatively greater than the reduction in protein. Marasmus usually occurs in children younger

than one year of age when the mother's breast milk is supplemented with thin watery gruels of native cereals, which are usually deficient in protein and calories.

Typical symptoms include arrested growth, extreme muscle wasting (emaciation), weakness, and anemia.

Victims of marasmus do not show the edema or changes in plasma proteins observed in kwashiorkor.


60

8. Chapter Summary

Estimated Average Requirement (EAR) is the average daily nutrient intake level estimated to meet the requirement of one half the healthy individuals in a particular life stage and gender group.

The Recommended Dietary Allowance (RDA) is the average daily dietary intake level that is sufficient to meet the nutrient requirements of nearly all (97–98%) individuals.

Adequate Intake (AI) is set instead of an RDA if sufficient scientific evidence is not available to calculate the RDA.

The Tolerable Upper Intake Level (UL) is the highest average daily nutrient intake level that is likely to pose no risk of adverse health effects to almost all individuals in the general population.

61

The energy generated by the metabolism of the macronutrients is used for three energy-requiring processes that occur in the body: resting metabolic rate, thermic effect of food, and physical activity.

Acceptable Macronutrient Distribution Ranges (AMDR) are defined as the ranges of intake for a particular macronutrient that is associated with reduced risk of chronic disease while providing adequate amounts of essential nutrients.

Adults should consume 45–65% of their total calories from carbohydrates, 20 –35% from fat, and 10–35% from protein (Figure 27.22). 62

8. Chapter Summary

Elevated levels of cholesterol or LDL cholesterol result in increased risk for cardiovascular disease.

In contrast, high levels of HDL cholesterol have been associated with a decreased risk for heart disease.

Dietary or drug treatment of hypercholesterolemia is effective in decreasing LDL, increasing HDL, and reducing the risk for cardiovascular events.

Consumption of saturated fats is strongly associated with high levels of total plasma and LDL cholesterol. When substituted for saturated fatty acids in the diet, monounsaturated fats lower both total plasma and LDL cholesterol, but increase HDL. 63

8. Chapter Summary

Consumption of fats containing n-6 polyunsaturated fatty acids lowers plasma LDL, but HDL, which protect against coronary heart disease, are also lowered.

Dietary n-3 polyunsaturated fats suppress cardiac arrhythmias and reduce serum triacylglycerols, decrease the tendency for thrombosis, and substantially reduce the risk of cardiovascular mortality.

Carbohydrates provide energy and fiber to the diet. When they are consumed as part of a diet in which caloric intake is equal to energy expenditure, they do not promote obesity.

Dietary protein provides essential amino acids. The quality of a protein is a measure of its ability to provide the essential amino acids required for tissue maintenance. 64

8. Chapter Summary

Proteins from animal sources, in general, have a higher- quality protein than that derived from plants. However, proteins from different plant sources may be combined in such a way that the result is equivalent in nutritional value to animal protein.

Positive nitrogen balance occurs when nitrogen intake exceeds nitrogen excretion. It is observed in situations in which tissue growth occurs, for example, in childhood, pregnancy, or during recovery from an emaciating illness.

Negative nitrogen balance occurs when nitrogen losses are greater than nitrogen intake. It is associated with inadequate dietary protein, lack of an essential amino acid, or during physiologic stresses such as trauma, burns, illness, or surgery.

Kwashiorkor is caused by inadequate intake of protein.

Marasmus results from chronic deficiency of calories.

65

8. Chapter Summary

66

Documents

UNIT V: Integration of Metabolism Nutrition. 1. Overview Nutrients are the constituents of food necessary to sustain the normal functions of the body