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Energy

A food calorie is a kilocalorie (1000 cal, , ). One kcal is the amount of energy (heat) required to raise

the temperature of 1 kg water by 1o Celsius from 15

to 16o Celsius.

1 kcal = 4.184 kilojoule (kJ, )

Food energy is captured when macronutrients are oxidized.

Oxidation can take place in the chamber of an apparatus or

intracellularly within an organism.

Measuring gross energy (a direct method

to capture energy released)

What chemical reaction takes place

when food is burned?

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[1-9%]

(E intake E feces)/E intake * 100

Energy value of foods (Atwater factors)

Release in metabolism /

futile cycles

[~50%]

[25-40%]

Food energy available for body functions that needs ATP

Alcohol

7 kcal/gm

(6.9 to be exact)

Atwater Factors

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Question: Do you think 1 kcal derived from protein is

same as from fat and glucose?

i.e. Is a calorie a calorie?

Remember, the Atwater factors are AVERAGE values. They are in

error to some degree for basically any food item. The error comes

from the complex structure of food that can change the gross energy as

well as availability.

For example, a food containing high proportion of unavailable dietary

carbohydrate would increase the excretion of protein and fat in the

feces. Thus, the factors overestimate the energy derived from such

foods.

Implication: we cannot determine the exact metabolizable energy of a

given mixed diet.

According to the 1st Law of Thermodynamics, energy can

be neither created nor destroyed, but only transformed.

Hence, food energy that gets inside the body will either be

used (work/heat) or stored. So, body energy balance could

be an estimate of how energy derived from the

macronutrients is utilized.

Energy balance = Output Input

On a long-term basis

Output > Input decrease energy reserve & BW

Output < Input increase energy reserve & BW

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Determinants of energy Expenditure

By the Factorial Method

1) Basal metabolism (basal metabolic rate, BMR)

2) Physical activity

3) Thermic effect of food (specific dynamic action of food,

diet-induced thermogenesis)

4) Growth

pregnancy, lactation, baby, children & teenager

30-50%

Physical activities

10%

Thermic effect

of food 50-65%

Basal metabolism

Components of Energy Expenditure

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Energy expenditure measurement:

1)Direct body heat loss

2) Indirect via O2 consumption or

CO2 production

Indirect calorimetry

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Determinant of energy need - Basal Metabolic Rate (BMR)

The minimum amount of energy

spent to maintain life.

Measured under a quiet,

inactive & non-stress post-

absorptive state.

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Respiratory quotient (RQ) and fuel usage

When our body burns fuel, the amount of

oxygen consumed and carbon dioxide produced

would depend on what is being burned

For carbohydrate, oxygen consumed & carbon

dioxide produced are on equal molar basis

But, for fat, more oxygen is consumed than

carbon dioxide produced - WHY?

RQ = moles carbon dioxide produced / moles

oxygen consumed

RQ for carbohydrate, fat and protein is ~ 1, 0.7

and 0.85, respectively

RQ data will suggest the major fuel being burned

and energy expenditure could also be estimated

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Miscellaneous factors influencing BMR Sex

Developmental stages

age, sexual maturity, pregnancy, lactation

Stress

Diurnal/seasonal cycle (torpor / hibernation)

Thyroid Hormone

Assume one person (A) is tall

and slender and the other (B) is

shorter and wider. However,

they have the same weight.

Will they have similar BMR?

Why?

A

B

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Semi-log plot of specific O2 consumption

against body mass for mammals

the mouse to elephant curve

Surface area as site of

heat loss

Tissue as site of heat

production

The smaller the animal,

the higher the

area : volume ratio

Physical Size

Mass / Size

MR= aWb

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Components of Energy Expenditure Basal Metabolism

W in kg

H in cm

A in year

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Determinant of energy need physical activity

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Effects of Meal Ingestion & Menstrual Cycle

Thermic effect of food

(Diet induced thermogenesis)

Increases in MR due to

mastication

gut movement

digestion & absorption, etc

Note: in each diagram, DIT is the

difference in area under the curves.

Li et al. 1999

Menstrual phase caused

a 16% difference (luteal > follicular)

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Understanding Nutrition, 2011

Question: Is a calorie a calorie?

This could be answered partly by studying the effects of

isocaloric diets on body weight and composition.

To be covered in tutorials