Energy Expenditure

Tools and Techniques in Nutrition and Metabolism

Faidon Magkos, PhD, Professor in Obesity & Metabolism, Department of Nutrition, Exercise, and Sports Energy Expenditure. Date: 01.09.2025

Energy Balance

Energy balance is a fundamental concept expressed by the equation:

$\text{Energy Balance} = \text{Energy Intake} - \text{Energy Expenditure}$

Energy Intake and Energy Expenditure

• Energy Intake: Refers to the total amount of calories ingested through food.

• Energy Expenditure: The calories expended via various metabolic processes and physical activities.

• The foundational reference for this equation can be traced back to Danforth (1985) published in American Journal of Clinical Nutrition, 41:5S1132.

Caloric Values of Macronutrients

Understanding the energy provided by dietary sources is crucial:

• Proteins: 4 kcal/g

• Fats: 9 kcal/g

• Carbohydrates (including fiber): 4 kcal/g

• Alcohol: 7 kcal/g

For comparative analysis, the caloric content of fuels is highlighted:

• Coal: 4–7 kcal/g

• Gasoline: 10 kcal/g

• Natural Gas: 13 kcal/g

Basal/Resting Metabolic Rate (BMR/RMR)

BMR, or Resting Metabolic Rate (RMR), indicates the energy required for essential physiological functions at rest, including heart activity, breathing, organ functioning, and cognitive processes. It is a critical measure of energy expenditure while the body is inactive. Essentially, it comprises:

• Resting Energy Expenditure (REE) or Basal Metabolic Rate (BMR) represents this baseline energy utilization.

Contribution of Body Tissues to BMR

Energy consumption is relevant to body composition, notably skeletal muscle and adipose tissue contributions:

• Reference values for a male (70 kg, 1680 kcal/d) and female (58 kg, 1340 kcal/d) highlight the percentage contribution of various body tissues to BMR and body weight:

  • Adipose Tissue: 16% in men and 21% in women

  • Skeletal Muscle: 39% in men and 33% in women

  • Other Tissues: Include the liver, brain, heart, and kidneys with varying percentages contributing to total BMR.

This data is sourced from Elia (1992) regarding energy metabolism's tissue determinants and cellular corollaries.

Resting Metabolic Rate Studies

Research by Owen (1988) involving 104 subjects aged 18 to 82 years found that the resting metabolic rate correlates with fat-free mass (FFM) across genders.

RMR Measurement vs. Fat-Free Mass

• The plot illustrates the relationship between resting metabolic rate and fat-free mass ranging from 0 to 100 kg.

Formula for Resting Metabolic Rate:

• $\text{RMR} = 186 + 23.6 \times \text{FFM}$

• It can vary based on gender and obesity classification, with lean females showing significantly different RMR values compared to lean males.

Diet-Induced Thermogenesis (DIT)

Diet-induced thermogenesis, commonly referred to as the thermic effect of food (TEF), involves the energy expended during the digestion, absorption, transportation, and storage of nutrients.

• DIT Composition:

  • Obligatory Component: Necessary energy to process and store nutrients.

  • Facultative Component: Associated with heat production.

DIT Response Over Time

The increase in energy expenditure due to food intake peaks within a certain timeframe post-meal, following a characteristic response curve observed in studies measuring DIT.

Macronutrient Effects on DIT

The variation in DIT depends on the energy content and macronutrient composition of meals:

• For instance, for every 100 kcal ingested as protein, about 25 kcal are expended to process the nutrients. Conversely, only about 3 kcal are expended for processing 100 kcal ingested from fat.

Physical Activity Energy Expenditure (PAEE)

Physical Activity Energy Expenditure refers to energy used during exercise, occupational tasks, commuting, and non-exercise activities. It constitutes 15-30% of total energy expenditure (TEE).

Exercise Energy Expenditure (EEE)

EEE covers calories burned due to structured exercise, while Non-Exercise Activity Thermogenesis (NEAT) encapsulates the energy consumed through unstructured, conscious activities.

Exercise and Energy Expenditure

The intensity of exercise significantly affects energy expenditure, as demonstrated in cycling studies by Cheneviere (2010), where energy expenditure rose proportionally with exercise intensity compared to resting measures:

• $\text{Energy expenditure} \approx 0.94 \text{kcal/min} \times 18.5 \times 3.75$

• Graphical Depiction: A displayed graph quantifies energy expenditure as a function of exercise intensity (% VO2max).

Body Weight Effects

A study involving a range of subjects highlighted the direct correlation between body weight and energy expenditure during standardized physical activities like walking and stepping. Higher body weight resulted in increased energy expenditure rates, validated by a linear regression model:

• $y = 0.0628x + 0.2507$

Total Energy Expenditure (TEE) Calculation

Total energy expenditure is composite, calculated as the sum of resting metabolic rate, diet-induced thermogenesis, and physical activity energy expenditure. Typical proportions are:

• BMR/RMR: 60-75%

• DIT/TEF: 5-15%

• PAEE: 15-30%

Variability of Energy Intake and Expenditure

The variance in daily energy intake and expenditure can yield significant intra-individual variations noted in studies reporting a ~25% intra-individual coefficient of variation for intake compared to an 8% variation for energy expenditure.

• Data references: Bray (2008), Am J Clin Nutr 88:1504; Black (2000), Eur J Clin Nutr 54:386.

Measurement Techniques of Energy Expenditure

Direct Calorimetry

This method assesses energy through heat production measurement, leveraging the principles of thermodynamics in a bomb calorimeter setting. Combustion reactions yield energy offline monitored calorimetrically to calculate caloric content:

• Formula: $\Delta E = \Delta U + Q - \text{Work}$ for calorimetric analysis.

Indirect Calorimetry

Utilizing stoichiometric models, this technique calculates energy expenditure based on oxygen consumption and carbon dioxide production, relying on the premise that oxygen oxidizes fuel molecules.

• Key equations delineate substrate oxidation based on measured gas volumes and energy yield:

  • Example for glucose: $C<em>6H</em>{12}O<em>6 + 6 O</em>2 \rightarrow 6 CO<em>2 + 6 H</em>2O$

• An average respiration quotient ( ext{RQ}) facilitates calculating the contribution of specific energy sources during metabolism.

Prediction Equations for RMR and TEE

Estimation formulas for resting metabolic rate vary by gender, age, and body composition:

• Several established formulas present. For men:
  $\text{REE} = 66.5 + 13.75W + 5H - 6.76A$ (Harris-Benedict, 1919)

• For women:
  $\text{REE} = 655.1 + 9.56W + 1.85H - 4.68A$ (Harris-Benedict, 1919)

Furthermore, equations based on fat-free mass also offer predictive accuracy for estimating resting and total energy expenditure across diverse populations.

Practical Application of Indirect Calorimetry

Setting varies across methodologies:

• Canopy Mode: Effective for REE and DIT measurements in a referenced state.

• Chamber Mode: Allows long-term measurements over a multitude of days, capturing complete energy expenditure metrics in realistic living conditions.

• Doubly Labeled Water Method: A tracer method that estimates energy expenditure through urinary analysis, presenting high precision in free-living conditions.

Conclusion and Discussion

The comprehensive exploration of energy balance encompasses the intricate details surrounding caloric intake and expenditure, underpinned by various measurement methodologies crucial to understanding energy metabolism in nutritional science. Integrating these insights offers a foundational framework for further exploration of energy dynamics in health and disease states.

Acknowledgments

Faidon Magkos | Email: fma@nexs.ku.dk
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