NUTR 3210 Lecture Notes - Lecture 3: Citric Acid Cycle, Energy Economics, Negative Energy
27 Jun 2018
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Learning outcomes:
Describe the contribution of macronutrients to the gross energy of food items
Describe the losses which occur during digestion, absorption and metabolism, leading to the
concepts of digestible energy, metabolizable energy and net energy
Describe physiological fuel values and the chemical basis for differences among macronutrients
in terms of energy density
Identify and explain the components of energy expenditure
Describe the use of calorimetry in the measurement of energy expenditure
Interpret Nutrition Facts labels
Apply the principles of respiratory quotient to quantify energy expenditure
Critically evaluate experimental data regarding energy
Defining energy metabolism:
Energy metabolism is defined as the sum of all chemical reactions involved in obtaining and
utilizing food energy
This is a very numerical discipline of Nutritional Sciences, requiring the quantification of energy
in and energy out of living systems:
oTo quantify energy in (energy intake) we need to understand caloric values of the
different macronutrients found in our diet, and the obligatory losses that occur as we move
those substrates into metabolism
oTo quantify energy out (energy expenditure), we need to understand the physiology of
heat production, and how it can be measured directly or estimated by measuring oxygen
consumption and carbon dioxide release
It is important to start with some definitions before describing energy metabolism. The main
ideas are:
oEnergy is the capacity to do metabolic or physical work
oAt the cellular level, the predominant source of energy is ATP
Cellular ATP pools are supported by dietary macronutrient intake
oThe energy value of foods in measured by calories
When foods are consumed and the macronutrients are broken down into useable units, they
serve various functions, but will eventually be oxidized to carbon dioxide and produce energy for
bodily functions
ATP will be utilized when the organism does physical or biochemical work - hydrolysis of the ATP
molecules release the energy captured in the chemical bonds, driving forward metabolic reactions
and physical movement, and releasing heat
Works which require ATP are:
oPhysical activity
oAnabolic pathway
oActive transport systems which move molecules through cellular membranes
oCell division, growth, reproduction
A calorie is a measure of heat, more specifically it is the amount of energy required to raise the
temperature of 1g of water by 1 C - also know as small calorie which is different from Cal -> 1000
cal or 1kcal which is the energy required to raise 1 kg (or 1 L) of water by 1C
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oThe Cal or kcal is the usual unit of energy in nutritional sciences
1 kcal = 1 Cal or Calorie = 1000 calories
1 kcal - 4.18 kJ
Energy balance and nutrition facts food label:
On nutrition facts food labels, the energy is stated in Calories (kcal)
This energy is originally stored in the chemical bonds of fat, carbohydrates and proteins, and can
be ultimately released as heat
Energy balance:
Energy balance in a system is defined as the difference between energy intake and energy
expenditure:
oEnergy balance = energy in - energy out
If a person does not consume as much energy as he/she is expending, then total body energy
will decrease
When dietary energy intake is greater than energy expenditure, the body is in positive energy
balance
Positive energy balance is seen in growth and development, which is very important, but also
occurs with weight gain and obesity in adults
Negative energy balance occurs during weight loss and infection, for example when fever is
present, and as the body degrades protein faster as it tries to support immune functions
Energy balance also depends on brain function and desire for food intake and the ability to
sense satiety
The conditions of employment and the designs of homes and neighborhoods may contribute to
an obesogenic environment
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There are scientists in many different disciplines that are trying to figure out what is causing
increased obesity in recent decades, and how it might be reversed
Energy intake:
Humans consume macronutrients that are made up of proteins/ amino acids, fats, and
carbohydrates and use them for essential roles in building cells and tissues and to perform work
Sooner or later, these molecules that have been extracted from foods will be oxidized via
glycolysis and the TCA cycle, to produce metabolizable energy
The body captures some of the energy stored in the chemical bonds of the macronutrients, in
the form of ATP - that energy is ultimately released in the form of heat which eventually emanates
from the body
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Describe the contribution of macronutrients to the gross energy of food items. Describe the losses which occur during digestion, absorption and metabolism, leading to the concepts of digestible energy, metabolizable energy and net energy. Describe physiological fuel values and the chemical basis for differences among macronutrients in terms of energy density. Identify and explain the components of energy expenditure. Describe the use of calorimetry in the measurement of energy expenditure. Apply the principles of respiratory quotient to quantify energy expenditure. Energy metabolism is defined as the sum of all chemical reactions involved in obtaining and utilizing food energy. This is a very numerical discipline of nutritional sciences, requiring the quantification of energy in and energy out of living systems: o. To quantify energy in (energy intake) we need to understand caloric values of the different macronutrients found in our diet, and the obligatory losses that occur as we move those substrates into metabolism o.
