DIETS, NUTRIENTS AND NUTRITION

Chapter 27: Nutrition

Nutrients are the constituents of food necessary to sustain the normal functions of the body.

All energy is provided by three classes of nutrients (macronutrients): fats, carbohydrates, protein, and in some diets, ethanol.

Other nutrients like vitamins and minerals needed in lesser amounts are called micronutrients.

ENERGY REQUIREMENT IN HUMANS

Standard daily intake requirements for each of these nutrients, micro or macro, depends on age group, gender and physiologic states (i.e. either in health or disease; active or inactive) Dietary Reference intake (DRI) is the estimate of the amounts of nutrients required to prevent deficiencies and maintain optimal health.

•

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

Hence energy requirement is dependent on these factors;

Sedentary adults require about 30 Kcal/kg/day to maintain body weight;

Moderately active adults require 35 kcal/kg/day

Very active adults require 40 kcal/kg/day. The daily average requirement for energy is 2000Kcal/day.

The energy content of food is calculated in form of the total heat energy released from the total combustion of food using the calorimeter, and the unit is Cal or Kcal ; Joules (J) or KJ) are used in some countries like the USA. 1 Calorie ==4.128 Joules

The energy content of fat is more than twice that of carbohydrate or protein, whereas the energy content of ethanol is intermediate between fat and carbohydrate

How energy is used in the body

The energy generated by metabolism of the macronutrients is used for three energy-requiring processes that occur in the body:

Resting metabolic rate (RMR)

Thermic effect of food (formerly termed specific dynamic action)

Physical activity.

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 1800 kcal for men (70 kg) and 1300 kcal for women (50 kg). From fifty to seventy percent of the daily energy expenditure in sedentary individuals is attributable to the RMR

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

much as thirty percent 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 five to ten percent of the total energy expenditure.

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 thirty to fifty percent more than the resting caloric requirement for energy balance whereas a highly active individual may require 100 percent or more calories above the RMR.

Acceptable macronutrients distribution ranges in adults.

1. Dietary Fats And Chronic diseases

The incidences of a number of chronic diseases are significantly influenced by the kinds and amounts of nutrients consumed

Plasma cholesterol and coronary heart disease

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.

LDL and 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.

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.

Beneficial effect of lowering plasma cholesterol

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

The diet-induced changes of plasma lipoprotein concentrations are modest, typically ten to twenty percent, whereas treatment with "statin" drugs decreases plasma cholesterol by thirty to forty percent.

Dietary fats and plasma lipids

Triacylglycerols are quantitatively the most important class of dietary fats.

Their biologic properties are determined by the chemical nature of the constituent fatty acids.

The presence or absence of double bonds

The number and location of the double bonds

Configuration of the unsaturated fatty acids (the cis/trans ).

Saturated fat  Triacylglycerols containing primarily 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 coronary heart disease.

The main sources of saturated fatty acids are dairy and meat products and some vegetable oils, such as coconut and palm oils (a major source of fat in Latin American and Asia, although not in the US).

Experts strongly advise limiting intake of saturated fats.

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 increase HDLs.

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 coronary heart disease.

The Mediterranean diet

The Mediterranean diet is an example of a diet rich in monounsaturated fatty acids (from olive oil) and n-3 fatty acids (from fish oils and some nuts), but low in saturated fat.

The Mediterranean diet contains seasonally fresh food, with an abundance of plant material, low amounts of red meat, and olive oil as the principal source of fat.

The Mediterranean diet is associated with decreased serum total cholesterol

Polyunsaturated fatty acids

Triacylglycerols containing primarily fatty acids with more than one double-bond are referred to as polyunsaturated fats.

The effects of polyunsaturated fatty acids on cardiovascular diseases are influenced by the location of the double bonds within the molecule.

n-6 Fatty acids 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).

They are also called ω-6 (omega 6) fatty acids.

Consumption of fats containing n-6 polyunsaturated fatty acids, principally linoleic acid (18:2) ∆9,12 obtained from vegetable oils, lowers plasma cholesterol when substituted for saturated fats.

Plasma LDLs are lowered, but HDLs, which protect against coronary heart disease, are also lowered.

• The powerful benefits of lowering LDLs are only partially offset because of the decreased HDLs.

• Nuts, avocados, olives, soybeans, and various oils, including cottonseed, and corn oil, are common sources of these fatty acids.

• Linoleic acid, along with linolenic acid( an n-3 fatty acid ), are essential fatty acids required for fluidity of membrane structure and synthesis of eicosanoids.

• A deficiency of essential fatty acids is characterized by scaly dermatitis, hairloss, and poor wound healing.

n-3 Fatty acids 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 n-3 polyunsaturated fats suppress cardiac arrhythmias, reduce serum triacylglycerols, decrease the tendency to thrombosis, and substantially reduce risk of cardiovascular mortality, but they have little effect on LDL or HDL cholesterol levels.

The n-3 polyunsaturated fats are found in plants (mainly α-linolenic essential fatty acid), and in fish oil containing docosahexaenoic acid.

Antithrombotic effects of n-3 fatty acids

The reduced blood platelet reactivity observed with increased consumption of EPA (eicosapentaenoic acid ) and DHA (docosahexaenoic acid ) n-3 fatty acids results from inhibition of the conversion of arachidonic acid to thromboxane A2 by platelets.

Trans fatty acids Trans fatty acids 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 and only 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.

Dietary cholesterol

Cholesterol is found only in animal products.

The effect of dietary cholesterol on plasma cholesterol is less important than the amount and types of fatty acids consumed.

Plant sterols

Commercially available margarines containing hydrogenated plant sterols and sterol esters (predominantly sitostanol esters), when used in place of regular margarine, can reduce LDL plasma cholesterol concentrations.

The mechanism by which these compounds lower LDL cholesterol concentrations is to inhibit intestinal absorption of dietary cholesterol and cholesterol secreted into the bile.

Other dietary factors affecting CHD include:

Soy protein : Consumption of 25 to 50 g/day of soy protein causes an approximately ten percent decrease in LDL cholesterol in patients with elevated plasma cholesterol.

Alcohol consumption: Moderate consumption of alcohol (for example, two drinks a day) decreases the risk of coronary heart disease, because there is a positive correlation between moderate alcohol consumption and the plasma concentration of HDLs.  

Vitamins B6, B12, and folate: An elevated plasma homocysteine level (thought to be toxic to vascular endothelia) is associated with increased cardiovascular risk.

Homocystein is converted to harmless amino acids by the actions of enzymes requiring these vitamins.

2. Dietary Carbohydrates   The primary role of dietary carbohydrate is to

provide energy.

Although caloric intake has been associated with the incidence of obesity, obesity has been more directly related to increasingly inactive lifestyles, and to calorie-dense foods served in expanded portion size.

Carbohydrates are not inherently fattening.

Classification of Carbohydrates Monosaccharides: e.g. Glucose and fructose

Disaccharides: Sucrose, lactose and maltose.

Sucrose is ordinary ‘table sugar’ abundant in maple syrup and molasses

Lactose is the principal sugar found in milk

Maltose is a product of enzymatic digestion of polysaccharides, also found in beer and malt drinks

The term “sugar” commonly refers to mono and disaccharides , while added sugars are those sugars and syrups added to food during processing or preparation.

Polysaccharides are complex carbohydrates which do not have a sweet taste. Examples – starch found in plant in abundance (wheat, potatoes, dried peas and beans, and vegetables)

Dietary fibers

  The non digestible carbohydrates commonly found in plants.

Various types are known;

-functional (the isolated, extracted, synthetic fibers)

-soluble (form a viscous gel when mixed with liquid)

-insoluble which passes through the digestive tract unchanged.

Food with a low glycemic index tends to create a sense of satiety over a longer period of time, and may be helpful in limiting caloric intake.

However, many experts feel that high nutrient and fiber content, such as occurs in whole grains, fruits, and vegetables, is a better guide for selecting dietary carbohydrates.

There is no direct evidence that the consumption of simple sugars is harmful. Diets high in sucrose do not lead to diabetes or hypoglycemia.

Also, contrary to popular belief, carbohydrates are not inherently fattening. They yield 4 Kcal/g (the same as protein and less than half that of fat), and result in fat synthesis only when consumed in excess of the body's energy needs.

However, there is an association between sucrose consumption and dental caries, particularly in the absence of fluoride treatment.

DIETARY PROTEIN

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

10 of the 20 amino acids needed for the synthesis of body proteins are essential- that is, they cannot be synthesized in humans at an adequate rate.

Proteins from animal sources

: Proteins from animal sources (meat, poultry, milk, 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.

Gelatin prepared from animal collagen is an exception; it has a low biologic value as a result of deficiencies in several essential amino acids.

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

Nitrogen Balance

Nitrogen balance occurs when the amount of nitrogen consumed equals that of the nitrogen excreted in the urine, sweat, and feces.

Most healthy adults are normally in nitrogen balance

There are two extremes:

Positive

Negative

Positive nitrogen balance

This occurs when nitrogen intake exceeds nitrogen excretion.

It is observed in situations in which tissue

growth occurs, for example, in children, pregnancy, or during recovery from an emaciating illness.

Negative nitrogen balance

This occurs when nitrogen loss is 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.

Protein requirement 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.

RDA for protein is computed for proteins of mixed biologic value at 0.8 g/kg of body weight for adults, (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.

Pregnant or lactating women require up to 30 g/day in addition to their basal requirements. To support growth, children should consume 2 g/kg/day.

There is no physiologic advantage to the consumption of more protein than the RDA. Protein consumed in excess of the body's needs is and the resulting carbon skeletons metabolized to provide energy or acetyl CoA for fatty acid synthesis.

Because excretion of excess protein is accompanied by Ca2+ excretion, excess protein may be associated with osteoporosis and hypocalcemia

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 130g/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.

Protein-calorie malnutrition

In developed countries, protein-calorie malnutrition 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 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 malnutrition are kwashiorkor and marasmus.

Kwashiorkor

Kwashiorkor occurs when protein deprivation is relatively greater than the reduction in total calories.

Unlike marasmus, significant protein deprivation is associated with severe loss of visceral protein.

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, de-pigmented 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.

A child with kwashiorkor frequently shows a deceptively plump belly as a result of edema.

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.

Diet And cancer

Diet influences the risk for certain forms of cancer, especially cancer of the esophagus, stomach, large bowel, breast, lung, and prostate.

As with most chronic diseases that are influenced by nutritional factors, the incidence of cancer is also influenced by genetic and environmental factors.

High intakes of saturated fats are associated with increased risk of certain cancers, especially cancer of the colon, prostate, and breast.

For example, Figure above shows the correlation between the relative risk for colon cancer and consumption of animal fat in women. The data show that those women whose diets were rich in animal fat have significantly increased risk for colon cancer. However, whereas these studies show association between fat and cancer, they do not establish fat as the cause of cancer.

In general, populations consuming diets rich in fruits and vegetables have lower incidence of many kinds of cancer.

High fiber diets are associated with a lower risk of colon cancer and diverticulitis.

GENERAL OVERVIEW OF METABOLISM

Well-fed Versus Fasting States

Production of Energy from metabolic fuels