Energy Balance, Metabolism, and Dietary Recommendations

The human body utilizes energy that is released from the chemical bonds found within food sources in various forms.
The vast majority of the body's total energy supply is dedicated to meeting basal metabolic needs, which are the fundamental requirements for maintaining life at rest.

Energy Needs: * Involuntary body work: This represents the greatest use of energy in the human body. It includes all autonomous functions required to sustain life. * Voluntary work and exercise: This includes conscious physical movements and structured physical activity.
Measurement of Energy: * Kilocalorie ( $kcal$ ): Defined as the specific amount of heat energy necessary to raise the temperature of $1\,kg$ of water by $1^{\circ} C$ . * Joule ( $J$ ): This is the international unit of measure for energy. * Energy conversion: To convert kilocalories ( $kcal$ ) into kilojoules ( $kJ$ ), multiply the number of kilocalories by $4.184$ .
Food as Fuel for Energy: * Adenosine triphosphate ( $ATP$ ): This is the metabolic end product derived from the energy-yielding foods consumed by the body. It serves as the primary energy currency for cellular processes. * Fuel Factors: These represent the specific energy values provided by different macronutrients and substances: * Carbohydrate: $4\,kcal/g$ * Fat: $9\,kcal/g$ * Protein: $4\,kcal/g$ * Alcohol: $7\,kcal/g$

Energy Density: Refers to foods that contain a high concentration of energy (calories) relative to their weight.
Nutrient Density: Refers to foods that contain a high concentration of essential vitamins and minerals relative to their calorie content.
Empty Calorie Foods: These are the functional opposite of nutrient-dense foods; they provide energy (calories) but little to no significant nutritional value in terms of vitamins or minerals.

Maintaining life and health requires a consistent balance between the intake of food energy and the output of body work.
States of energy imbalance occur when intake does not match output, which may manifest physically as a body weight that is either underweight or overweight.

The support of life relies on two interconnected energy systems: * External Energy Cycle: This occurs in the environment where plants transform radiation from the sun into stored chemical energy through photosynthesis. * Internal Energy Cycle: This occurs within the human body. People consume plant and animal foods, breaking down the stored energy from its complex forms into simple fuels. This energy is used for body processes and is eventually returned to the external environment as heat and metabolic waste.

Main Sources of Energy: The primary intake comes from food and caloric drinks. This is supplemented by energy that has been stored within body tissues.
Estimating Dietary Energy Intake: This involves tracking all food and drink consumption and calculating the total energy value based on fuel factors.
Stored Energy Reserves: * Glycogen: Provides a short-term energy reserve lasting approximately $12$ to $48$ hours. * Adipose Tissue (Fat): This is the body's primary energy storage site; the amount varies significantly by individual. * Muscle Mass: This tissue is generally preserved but can be used as an energy source during longer periods of fasting or extreme starvation.

Definition of BEE: The Basal Energy Expenditure (BEE) is the sum of all internal activities of the body while it is at a state of total rest.
Measurement of BEE or REE: These can be measured using two primary methods: * Direct calorimetry. * Indirect calorimetry.
Predicting BEE or REE: A general rule of thumb for prediction is: * For women: Multiply $0.9\,kcal/kg$ of body weight by the $24$ hours in a day. * For men: Multiply $1\,kcal/kg$ of body weight by the $24$ hours in a day.
Factors Influencing BEE: * Influence from organs with high metabolic activity (e.g., liver, brain, heart, kidneys). * Body composition (the ratio of lean mass to fat mass). * Growth periods (infancy, childhood, adolescence). * Body temperature (fever increases metabolic rate). * Hormonal status (e.g., thyroid hormones). * Disease state.

Physical Activity: This component of energy output includes energy spent on professional work, recreation, and the necessary activities of daily living.
Thermic Effect of Food (TEF): This is the energy required specifically for the physical and chemical processes of digestion, absorption, and the transportation of nutrients to the body's cells.

The Total Energy Expenditure ( $TEE$ ) is the sum of three distinct components: 1. Basal Energy Expenditure ( $BEE$ ). 2. The physical activities of the person. 3. The thermic effect of food ( $TEF$ ).
Component Percentages of TEE: * Basal Energy Expenditure ( $BEE$ ): Comprises the largest portion, between $60\%$ and $75\%$ of total needs. * Physical Activity: Comprises between $15\%$ and $30\%$ of total needs. * Thermic Effect of Food ( $TEF$ ): Comprises approximately $10\%$ of total needs.

General Life Cycle Considerations: * Growth Periods: During stages of rapid growth, additional energy per unit of body weight is required to facilitate the building of new body tissue. * Adulthood: Energy needs generally decline as a person ages, though the specific rate of this decline varies greatly between individuals.
Dietary Reference Intakes (DRIs): * Note: DRIs for vitamins and minerals are specifically set at two standard deviations above the mean. This is designed to meet the nutritional requirements of $97.5\%$ of the total population.
Dietary Guidelines for Americans: * Recommendation to choose a healthy eating pattern while maintaining an appropriate calorie level for the individual. * Recommendation to meet the Physical Activity Guidelines for Americans.
MyPlate: This tool is used to create individualized plans for energy and nutrient balance.

Scenario: Sara is a $35$ -year-old female with a Resting Energy Expenditure ( $REE$ ) of $1500\,kcal/d$ . She works $6$ to $8$ hours a day and usually participates in a $2$ -mile walk after work.
Question: What is the difference between REE and BEE?
Question: Which organ systems use much of the body energy for cellular function?
Exercise: Calculate your own $REE$ according to the Mifflin-St. Jeor equation. What is the result?