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Theory Notes

The amount of energy required by an individual varies directly with the degree of activity and environmental conditions, but the rate of energy consumption in an individual by its overall cellular metabolism is more or less constant under some basal conditions i.e. standard conditions. This cellular metabolism under basal conditions is  Basal metabolic rate. The rate of energy consumption under such basal conditions per unit time (one hour) per square meter of surface area is known as “Basal metabolic rate”. A constant ratio of carbohydrates, lipids and proteins are metabolized under such basal conditions.

To simplify BMR

  • The energy required by an individual during physical, emotional and digestive rest.
  • It is the minimum energy required to sustain vital functions like working of heart, brain, circulation, respiration, ion transport and maintenance of cellular integrity

Basal Conditions

1) Person should be awake but at complete physical and mental rest

2) Person should be without food for 12-18 hours- Post absorptive state. This is to avoid the effect of digestion and absorption, the SDA of food stuffs and to prevent any chances of starvation.

3) He should be in recumbent or reclining position in bed.

4) There should be normal conditions of environment- temperature, pressure and humidity. The temperature should be between 200– 250 C. Under above conditions, energy expended by the individual is to maintain respiration, circulations, muscle tone, functions of viscera like kidney, heart, liver and brain and for the maintenance of body temperature.

Energy Expenditure in basal metabolism–   Usually 50- 70% of the daily expenditure in sedentary individuals, BMR can be responsible for burning 70% of the total calories but the figure varies due to different factors. The factors that affect BMR are as follows-

Factors affecting BMR

1) Age- BMR decreases with advancing age. In children it is high due to more surface area.

2) Sex-Males have more muscle mass and lower body weight. Thus they have high BMR. Females have lower muscle mass more body fat thus have lower BMR.

3) Hereditary factors– Some people are born with faster metabolism and some with slower metabolism

4) Body Surface area- Surface area depends on weight and height. Greater the surface area, higher is the BMR and vice versa. Tall, thin people have a higher BMR than short persons. Since much of the basal metabolism is for the maintenance of body temperature and the heat loss is proportional to body surface area. Thus BMR is proportional to surface area.

5) Environmental conditions- Temperature outside body also affects BMR. Exposure to cold causes an increase in BMR so as to create the extra need for heat for the maintenance of body temperature. A short exposure to heat has little or no effect on BMR but upon prolonged exposure to high temperature there is compensatory heat loss, this results in increase in BMR. In colder regions of the world, BMR is higher and in tropical regions the BMR is lower.

6) Body Temperature-For every 0.5 degree rise in body temperature there is 7% increase in BMR. With the rise in temperature, there is increase in the rate of chemical reactions causing increased BMR.

7) Exercise- Physical exercise not only influences body weight by burning calories. It also helps to raise BMR by building extra lean tissues. Lean tissue is metabolically more demanding than fat tissue. The increase in BMR due to exercise is also due to increased cardiac output.

8) Drugs- Caffeine, Benzedine, alcohol, epinephrine and nicotine increase BMR. Reverse is seen with certain anesthetics.

9) Pregnancy-  The BMR of pregnant mother rises after 6 months of gestation. BMR of mother is a sum total of her own BMR as in the non pregnant state and of that of fetal metabolism.

10) Racial variations- BMR varies with different racial groups. Higher values of > 33 % above normal have been reported in Eskimos.

11) Barometric pressure- Moderate decrease in pressure causes no effect on BMR, but a fall of pressure to half an atmosphere as occurs in mountain climbing increases BMR.

12) State of nutrition- BMR is lowered in states of starvation, malnutrition and wasting diseases.

13) Hormones-Thyroid hormone increases BMR. In thyrotoxicosis BMR rises 50-100% above normal, while in Myxoedema, BMR falls 35-45% below normal. Adrenaline, catecholamines, growth hormone, all of them increase BMR, Male sex hormone increases BMR to 10% or more. Anterior pituitary through its effect on TSH also affects BMR.

Short term factors affecting BMR

Illnesses such as fever, high level of stress hormones in the body and either an increase or decrease in environmental temperature result in an increase in BMR. Fasting, starvation or malnutrition all result in a lowering of BMR.

Measurement of BMR

BMR can be determined by the following methods-

1) Open Circuit system- In this system both O2 consumption and CO2 output are measured. Although it is very accurate but due to high degree of skills involved, it is less commonly used.

2) Closed circuit method-In clinical practice, the BMR is estimated by measuring O2 consumption of the patient for 2-6 minutes period under basal conditions. The O2 consumption is measured under closed circuit system. The apparatus commonly used is Benedict’s Roth metabolism apparatus. The test is usually run for 6 minutes and the volume of O2 consumed in that period is measured and corrected to standard conditions of temperature and pressure.

Calculation

The average O2 consumption for 6 minutes is multiplied by 10 to convert into hourly basis and then multiplied by 4.825oC. The heat production is represented by each litre of O2 consumed. This gives the heat production in C/hour. Since the BMR is expressed as C/Sq.met/Hour, the energy output per hour has to be divided by the surface area of the individual.

 Calculation of surface area of an individual

 The surface area of an individual adult is about 1.8 square meter.

1) Du- Bois Surface area formula- A simple formula for calculating the surface area is as follows

BSA = (W 0.425 x H 0.725) x 0.007184

where the weight is in kilograms and the height is in centimeters and body surface area is in square meters.

2) By using normograms– Most conveniently the surface area is calculated from the normograms if the height and weight are known. Height is measured in feet/cm ; weight is measured in pounds/kg and surface area is calculated in square meters.

3) Harris–Benedict equations

BMR calculation for men (metric) 
BMR = 66.47 + (13.75 x weight in kg ) + ( 5.003 x height in cm ) – ( 6.755 x age in years )

BMR calculation for women (metric) 
BMR = 655.1 + (9.563 x weight in kg ) + ( 1.850 x height in cm ) – ( 4.676 x age in years )

4) Read’s formula

BMR = 0.75 (PR +0.74 x PP)

PR is pulse rate

PP is pulse pressure.

The result is expressed as percentage of the normal and is corrected within a range of ± 10%

Normal range of BMR

A healthy adult male has a BMR of about 40C/sq.m/hour and adult female has about 37 C/Sq.m/hour

Example of calculation of BMR

1) The normal BMR for an individual of the patient’s age and sex is obtained from the standard tables.

2) The patient’s actual rate is expressed as + or – % of the normal.

3) In a male aged 35 years with height-170 cm , weight- 70 kg and O2 consumption 1.2 liters in a 6 minutes time, the BMR can be calculated as follows-

O2 consumption/hour = 1.2 x10 = 12 liters

  • or 12 x 4.825 = 58 Kcal/hour

Surface area from normogram = 1.8 sq.m.

Therefore BMR= 58/1.8 = 32 C/Sq.m/hour

The normal BMR for this patient by reference to standard table is 39.5 C/Sq.m/hour.

Hence the patient is having surface area of    

Cal

 = – 18.98 % below normal.

Significance of BMR

1) Diagnostic Aid

It is used for the diagnosis of various pathological conditions specially assessing the thyroid functions. The pathological variations in BMR are as follows-

a) Fever- Infections and febrile diseases elevate BMR, usually in proportion to increase in body temperature.

b) Diseases- BMR is increased in-

  • Leukemias
  • Polycythemia
  • Some types of anemia
  • Cardiac failure
  • Hypertension
  • Dyspnea
  • Perforation of ear drum(False increase)
  • Hyperthyroidism
  • Cushing syndrome
  • Acromegaly

BMR is deceased in-

  • Addison’s disease
  • Hypothyroidism

2) Calculation of Caloric requirement- It is essential in the calculation of caloric requirement of an individual for prescribing a diet of adequate calorific value and planning nutrition.

3) To know the effect of food and drugs- BMR is calculated to know the effect of special food stuffs and drugs.

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Dietary fiber is indigestible part of plant foods that makes stool soft and thus enables smooth bowel movements, prevents constipation, hemorrhoids and diverticulosis. 

It consists of all of the components of the cell walls of plants that are not broken down by the body’s digestive enzymes.

Dietary Fiber and Total Fiber

“Dietary fiber” as noted on the “Nutrition Facts Label” of commercial foods, is fiber originally present in the food. “Total fiber” consists of dietary fiber and added fiber – substances that are adding to original food to increase their fiber content or to change their physical properties. Pectin, for example, is added to jam to give it a gel form.

Dietary fiber can be grouped into two main categories, those that are soluble and those that are insoluble in water.

Soluble Fiber

Soluble fiber (viscous fiber) partly dissolves in water and forms gel with it. Foods rich in soluble fiber include beans and other legumes (peas, soy, and lentils), oats, barley, citrus fruits (oranges, grapefruit), psyllium husk and flax seedSubstances found in soluble fiber are gum, pectin, some hemicelluloses, mucilage and storage polysaccharides (starch and glycogen).

Beneficial Effects of Soluble Fiber

  • Soluble fiber may prevent both diarrhea and constipation. It absorbs water from the gut and thus makes stool soft, but not bulky.
  • Soluble fiber may help in weight loss. It slows down the speed of the passage of food through the gut thus giving a feeling of fullness.
  • Soluble fiber may help to prevent and treat diabetes type 2. It slows down absorption of glucose from the intestine into the blood thus preventing high spikes of glucose in the blood after a meal.
  • Soluble fiber may lower total and LDL cholesterol and thus helps to prevent ischemic heart disease and stroke. It binds bile acids in the bowel and removes them from the body and thus reduces their absorption into the blood. Lost bile acids are replaced by synthesis from blood cholesterol. This is one theory about how soluble fiber lowers blood cholesterol levels.
  • Soluble fiber may prevent bile salt diarrhea after a gallbladder removal.   

Unwanted Effects of Soluble Fiber

Soluble fiber, if ingested in excess may cause:

  • Abdominal bloating and flatulence
  • Dehydration, if ingested without water
  • Pectin may reduce absorption of cholesterol-lowering drugs, like Lovastatin

Soluble Fiber Supplements

Examples of soluble fiber supplements:

  • Metamucil (psyllium – isphagula)
  • Citrucel (methylcellulose)
  • Benefiber (wheat dextrin)
  • FiberChoise (Inulin)
  • Pectin

The soluble fibers such as pectin and true plant gums are mucilaginous and are digestible.

Pectins are predominantly polygalacturonic acids with varying amounts of other hexose or pentose residues.

True plant gums are complex poly saccharides composed of primarily arabinose, fucose, galactose, mannose, rhamnose, and xylose. The gums are soluble in water and are digestible by the enzymes in the intestinal tract. Both pectins and gums are mucilaginous; they absorb water to form viscous gels in the stomach that decrease the rate of gastric emptying. The mucilaginous nature of the soluble fibers, pectins, and gums tends to decrease the rate at which carbohydrates are digested and absorbed, thus decreasing both the rise in blood glucose levels and the ensuing increase in insulin concentration.

Insoluble Fiber

Insoluble fiber can not be dissolved in water. Foods rich in insoluble fiber include whole wheat and other whole grains and most dark green leafy vegetables, like cabbage and cauliflower. Substances found in insoluble fiber include cellulose, hemicellulose and lignin.

Beneficial Effects of Insoluble Fiber

  • Insoluble fiber may help to prevent constipation, hemorrhoids and diverticulosis. It binds water and thus makes stool soft and bulky; it also speeds up the passage of food through the intestine.

Unwanted Effects of Insoluble Fiber

  • Ingesting foods with insoluble fiber containing sulphur (garlic, onions) may result in excessive gas.
  • Insoluble fiber eaten on an empty stomach may aggravate symptoms of irritable bowel syndrome.
  • Insoluble fiber ingested without water may result in severe constipation or even intestinal obstruction.
  • Certain types of insoluble fiber may trigger diarrhea in sensitive people.
  • Excessive ingestion of supplements containing insoluble fiber, especially in small children, may reduce absorption of calcium, magnesium, iron, copper and zinc.

1) Cellulose is a major structural component of plant cell walls. Cellulose is a long, linear polymer of glucose (β-D-glucopyranose) units that are joined by β(1→4) glycosidic bonds . Cellulose molecules have an extended, rigid structure that is stabilized by interchain hydrogen bonds.

Starch, the plant storage polysaccharide, which is also a polymer of glucose, differs in its structure in that the glucose monomer units are joined by α(1→4) glycosidic bonds .Starch is composed of two types of polymers, amylose, which has a nonbranched helical structure, and amylopectin, which is branched with α(1→6) glycosidic bonds joining the branches to the main polymer chain. Although starch is easily digested by salivary and pancreatic amylase and the disaccharidases present on the brush border of intestinal mucosal cells, cellulose cannot be hydrolyzed. The β(1→4) glycosidic bonds of the cellulose chain cannot be cleaved by the amylases present in the digestive tract.

 

 Figure -1- Showing the molecular structure of cellulose, indicating the repeating disaccharide unit, cellobiose.

 

 

Figure- 2-The molecular structure of starch, indicating the repeating disaccharide unit, maltose, as well as the α-1,6-glycosidic bond present in the branch points of amylopectin.

2) Hemicelluloses are also polysaccharides that are structural components of plant cell walls. However, unlike what their name implies, they are unrelated to cellulose. They are polymers that are made up of a variety of sugar monomers that include glucose, galactose, mannose, arabinose, and xylose, as well as acidic forms of these monosaccharides. Xylose is the monosaccharide that is most abundant. Hemicelluloses have a random, amorphous structure that is suitable for their location in the plant cell wall matrix. Depending on their molecular structure, hemicelluloses are partially digestible.

 3) Lignins are formed by the irreversible dehydration of sugars that result in aromatic structures. The remaining alcohol or phenol OH groups can react with each other and with aldehyde and ketone groups to form polymers. These polymers cannot be broken down by the digestive enzymes and, like cellulose and the indigestible portion of hemicelluloses, form the stool bulk.

 

Figure-3-  A lignin molecule in an early stage of condensation. The aromatic rings are a result of irreversible dehydration of sugar residues.

Although cellulose and hemicellulose are insoluble, they absorb water to swell and increase the stool bulk. This results in larger, softer stools. It has been shown that diets plentiful in insoluble fiber also increase the transit time of food in the digestive tract and decrease intracolonic pressure. Lignins, in addition to increasing stool bulk, also bind organic molecules such as cholesterol and many potential carcinogens. 

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