How Is REE Measured? Exploring Human Metabolism and Rare Earth Elements

The term REE can refer to two very different scientific measurements, each requiring distinct methodologies. While your resting metabolic rate accounts for up to 70% of your daily energy expenditure, the techniques for measuring this human-centric REE are vastly different from the high-tech, lab-based processes used to analyze the concentration of rare earth elements (REE) in geological samples. This article will demystify how each type of REE is measured.

Quick Summary

This article explains how REE is measured in two contexts: Resting Energy Expenditure in humans and Rare Earth Elements in materials. It details diagnostic and estimation methods for metabolism and explores analytical techniques used in geochemistry and manufacturing for element detection and quantification.

Key Points

• Dual Meaning of REE: The acronym REE can refer to either Resting Energy Expenditure (human metabolism) or Rare Earth Elements (geochemical analysis), each with distinct measurement methods.

• Indirect Calorimetry for Human REE: The most accurate method for measuring human Resting Energy Expenditure involves indirect calorimetry, which analyzes oxygen consumption and carbon dioxide production to determine calories burned at rest.

• Predictive Equations for Human REE: For less precise estimation, mathematical formulas like the Mifflin St-Jeor and Harris-Benedict equations can approximate Resting Energy Expenditure based on factors like age, gender, and weight.

• ICP-MS for Elemental REE: In geochemistry, Rare Earth Element concentrations are most commonly measured using highly sensitive lab techniques like Inductively Coupled Plasma Mass Spectrometry (ICP-MS).

• XRF and LIBS for Elemental REE: Rapid field-deployable techniques for Rare Earth Element analysis include X-ray Fluorescence (XRF) for solid samples and Laser Induced Breakdown Spectroscopy (LIBS), which requires minimal sample preparation.

• Normalization for Geochemical REE Interpretation: Geologists normalize Rare Earth Element data against standard rock references like chondrites to better interpret elemental patterns related to geological processes.

Understanding the Different Meanings of REE

Before diving into the measurement methods, it's essential to clarify the dual meaning of the acronym REE. In the health and fitness world, REE typically stands for Resting Energy Expenditure, a measure of the calories your body burns at rest to perform basic functions. In a scientific, chemical, or geological context, REE refers to Rare Earth Elements, a group of 17 metallic elements with similar properties, vital for modern technology. The measurement techniques for these two distinct fields are completely different, ranging from sophisticated metabolic testing in a clinical setting to powerful spectrometry in a lab.

Measuring Resting Energy Expenditure (REE) in Humans

For most individuals concerned with health, weight management, or nutritional planning, measuring REE is about determining their metabolic rate. This is most accurately done through indirect calorimetry, but it can also be estimated using predictive equations.

Indirect Calorimetry: The Gold Standard

Indirect calorimetry is the most precise and reliable method for measuring a person's REE. It is based on the principle that the body's heat production can be calculated by measuring its oxygen consumption (VO2) and carbon dioxide production (VCO2).

The Process of Indirect Calorimetry:

1. Preparation: The individual must be in a fasted state (typically 12+ hours) and refrain from intense physical activity beforehand.
2. Procedure: A metabolic cart is used to collect and analyze expired gases. The person lies down or sits comfortably under a clear hood or wears a face mask for a period of 15 to 30 minutes.
3. Measurement: The machine measures the minute-by-minute exchange of oxygen and carbon dioxide. A steady state is achieved when VO2 and VCO2 readings are stable for a minimum of five minutes.
4. Calculation: The respiratory gases are used in an equation, such as the Weir equation, to calculate the 24-hour REE in kilocalories (kcal).

Predictive Equations for Estimation

When a metabolic cart is unavailable, REE can be estimated using several mathematical equations. While convenient, these are not as accurate as indirect calorimetry, particularly for individuals with metabolic disorders or those at weight extremes.

Commonly used equations include:

• Harris-Benedict Equation: Developed in 1919, this is one of the oldest and most widely used formulas. It uses height, weight, age, and gender.
• Mifflin St-Jeor Equation: A more modern formula often considered more accurate for a wider population than the Harris-Benedict equation.
• Katch-McArdle Formula: Specifically calculates Basal Metabolic Rate (BMR) based on lean body mass, making it potentially more accurate for very lean or muscular individuals.

Other Advanced Measurement Methods

• Doubly Labeled Water (DLW): This method is considered the 'gold standard' for measuring total energy expenditure (TEE) in free-living individuals over a period of 1 to 3 weeks. Water containing non-radioactive hydrogen and oxygen isotopes is ingested, and their elimination rates from the body are used to calculate CO2 production and, subsequently, TEE.
• Wearable Devices: Many fitness trackers and smartwatches estimate RMR, but their accuracy can vary significantly. They rely on algorithms that incorporate heart rate, movement, and personal data to provide an estimate, not a true metabolic measurement.

Measuring Rare Earth Elements (REE) in Geology and Chemistry

In the context of chemistry and geology, measuring REE involves advanced analytical techniques used to determine the concentration and distribution of rare earth elements in geological samples, industrial products, and environmental materials.

Inductively Coupled Plasma (ICP) Techniques

These are among the most powerful and widely used analytical methods for determining REE concentrations.

Types of ICP techniques:

• ICP-Mass Spectrometry (ICP-MS): Known for its high sensitivity, this technique measures the mass-to-charge ratio of ions produced from a sample plasma. It is effective for multi-element REE detection, even at extremely low concentrations.
• ICP-Optical Emission Spectrometry (ICP-OES): This method analyzes the light emitted by plasma-excited atoms and ions. While very capable, it requires exceptional plasma robustness, especially for samples with high matrix content like geological materials.

Other Advanced Analytical Methods

• X-ray Fluorescence (XRF): This technique uses X-rays to excite atoms in a sample, which then emit fluorescent X-rays of characteristic energies. XRF is a rapid and advantageous method for analyzing solid samples directly, without extensive preparation, though it is less sensitive for lighter elements.
• Instrumental Neutron Activation Analysis (INAA): A highly sensitive method that measures gamma radiation emitted from elements after they are exposed to a neutron source in a nuclear reactor. It is very precise but requires access to a reactor.
• Laser Induced Breakdown Spectroscopy (LIBS): A rapid, in-situ spectroscopic technique that uses a high-energy laser pulse to create a plasma on a sample's surface. The plasma's light emission is then analyzed for elemental composition, requiring little to no sample preparation.

Normalizing REE Data for Interpretation

In geochemistry, simply measuring REE concentrations isn't enough. Scientists normalize the data to a standard reference material (such as chondrite or shale) to remove natural variations and the "Oddo-Harkins effect," where even-numbered elements are more abundant than odd-numbered ones. This process makes it easier to spot relative enrichments or depletions of certain REEs, which can provide clues about the sample's geological history.

Human vs. Geological REE Measurement: A Comparison

Feature	Resting Energy Expenditure (REE)	Rare Earth Elements (REE)
Purpose	Determine caloric requirements for basic bodily functions.	Determine elemental concentration and distribution in materials.
Sample Type	Human breath (expired gases).	Geological rocks, soils, sediments; industrial products.
"Gold Standard" Method	Indirect Calorimetry with a metabolic cart.	ICP-MS (Inductively Coupled Plasma Mass Spectrometry).
Alternative/Estimation	Predictive equations (e.g., Mifflin St-Jeor, Harris-Benedict).	XRF, NAA, LIBS for solid samples.
Accuracy Factors	Fasting state, physical rest, minimal stress, body composition.	Sample preparation, spectral interferences, detection limits, normalization.
Output Units	Kilocalories per day (kcal/day).	Concentration (e.g., parts per million, ppm) and normalized ratios.

Conclusion

While the abbreviation REE refers to two distinct scientific fields—human metabolism and elemental analysis—the methods for measuring each are highly specialized. For human health, indirect calorimetry provides the most accurate measure of Resting Energy Expenditure, though practical estimates can be made with predictive equations. In geology and industry, the precise quantification of Rare Earth Elements relies on a suite of advanced analytical tools like ICP-MS and XRF. By understanding the context and the specific techniques involved, one can accurately interpret what is being measured when discussing REE in either capacity. The choice of method, whether for calculating a personalized diet plan or analyzing a new mineral deposit, is always determined by the desired level of accuracy and the nature of the sample being studied.

Frequently Asked Questions

What is the difference between Resting Energy Expenditure and Basal Metabolic Rate?

Resting Energy Expenditure (REE) is slightly less restrictive than Basal Metabolic Rate (BMR). BMR measures the minimum energy needed for vital functions under highly controlled conditions (e.g., after a 12-14 hour fast, in a comfortable thermal environment). REE is a measure of energy expenditure at rest but under less strict conditions, and it is slightly higher than BMR due to the body's recent physical activity and feeding.

How accurate are REE predictive equations compared to indirect calorimetry?

Predictive equations are less accurate than indirect calorimetry, particularly for individuals who are ill or at the extremes of weight. Indirect calorimetry is considered the gold standard and can provide an individualized assessment, whereas equations provide only an estimate based on population data.

What kind of sample is needed to measure Rare Earth Elements?

The sample type depends on the analytical technique. For liquid samples, ICP-MS is often used, requiring the sample to be dissolved in acid. For solid samples like rocks or industrial products, techniques like XRF can analyze the sample directly, while others like LIBS ablate a small surface area.

What is geochemical normalization for REE and why is it necessary?

Geochemical normalization is the process of dividing the measured Rare Earth Element concentration in a sample by the concentration in a standard reference material (like chondrite meteorite or shale rock). This removes the natural fluctuation in elemental abundance (the Oddo-Harkins effect), allowing geologists to see distinct patterns related to geological processes more clearly.

What is the respiratory quotient (RQ) and how does it relate to human REE measurement?

The respiratory quotient (RQ) is the ratio of carbon dioxide produced to oxygen consumed during indirect calorimetry (VCO2/VO2). This value can indicate which type of fuel (carbohydrates or fats) the body is predominantly using for energy at rest. An RQ closer to 1.0 suggests carbohydrate burning, while an RQ closer to 0.70 suggests fat burning.

Is measuring Rare Earth Elements expensive?

Many of the high-precision analytical techniques used to measure Rare Earth Elements are quite expensive, particularly instruments like ICP-MS. Some techniques, like handheld EDXRF, are more portable and cheaper, but may offer lower sensitivity or resolution.

What factors can influence a person's Resting Energy Expenditure?

A person's REE is influenced by several factors, including body composition (especially lean body mass), age, gender, genetics, and hormonal status. Environmental temperature, illness, and physical fitness level also play a role.