The Science of Pet Nutrition: Understanding Indirect Calorimetry

The Science of Pet Nutrition: Understanding Indirect Calorimetry

In this post, we delve into a method that is used by researchers at the University of Guelph to study the impact of environmental factors, such as nutrition, on companion animal metabolism—Indirect Calorimetry.

Indirect calorimetry allows researchers to measure energy expenditure – calories burned – and substrate utilization – the main source of energy being used – by an animal.

By understanding how pets use nutrients and burn energy, we gain invaluable insights into the metabolic processes that are taking place in our pets under different circumstances. Our research findings can then be used to develop equations to calculate the energetic and nutrient needs of our pets.  

In fact, previous research with indirect calorimetry has contributed to the development of energy requirement calculations in animals. The best examples of this are the feeding guidelines on your pets’ food label. Those guidelines are based on equations for the daily energy (or number of calories) required by the animal, which were developed using results from studies that incorporated indirect calorimetry methods.

In this article, we will explore the ‘what’, ‘how’ and ‘why’ of indirect calorimetry, its significance in pet nutrition, and how our OVC Nutrition Team has employed this methodology to advance our understanding of the dietary needs of dogs and cats.

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What Is Indirect Calorimetry?

Indirect calorimetry is a scientific method that measures the amount of energy an animal uses without directly measuring the heat they release. Historically, measuring the energy expended by an animal was done by measuring the heat that animal produced. The “indirect” part of this technique allows us to measure energy used, not by the heat given off, but by measuring concentrations of oxygen breathed in and carbon dioxide given off.

Measuring Energy Expenditure and Respiratory Quotients

Indirect calorimetry operates on the principle that, as animals break down nutrients for energy, they consume oxygen and produce carbon dioxide in the process. By measuring the amount of oxygen consumed and carbon dioxide produced, energy expenditure (or output) and Respiratory Quotients (RQ) can be calculated.

Energy expenditure refers to calories, usually represented as kcal or kJ, that an animal uses in a given time frame. When measuring energy expenditure for cats and dogs through the process of indirect calorimetry, we are estimating their energy needs in a resting state under normal environmental conditions. That said, there is a myriad of applications and measures that can be extrapolated through indirect calorimetry, which will not be discussed here.

Typically, we will measure over 24 hours, to encompass the post-meal response of the animal. This can also allow for measurements of basic functions such as eating, drinking, urinating and defecating; and in some circumstances, activities like playing. Trying to capture the energy expenditure for these types of activities will depend on the duration (Iength of time) and the environment the animal is in during these measurements.

The RQ is a ratio of carbon dioxide produced to oxygen consumed. An RQ measurement helps researchers understand how an animal is using the key macronutrients from any diet (proteins, fats, or carbohydrates) to burn calories or produce energy. An RQ of 1.0 indicates the animal is predominantly relying on carbohydrates, while an RQ of 0.7 suggests more of a preference for burning fat.

There are a variety of fundamental equations that can be used to estimate the number of calories an animal needs throughout the day. This, in turn, informs the development of various pet foods and feeding guides! Figure 1 shows a simple schematic of a typical indirect calorimetry setup to measure energy expenditure and RQ.

Figure 1. Diagram of a typical indirect calorimetry system to measure energy expenditure (EE) and respiratory quotient (RQ).

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Other Methods for Measuring Energy Expenditure

Besides indirect calorimetry, there are other ways to measure energy expenditure:

1) Direct Calorimetry: Direct calorimetry measures the amount of energy used/burned by assessing the heat produced by an animal in a specially designed chamber. This provides an accurate, but resource-intensive, method for determining metabolic rates in dogs or cats.

2) Doubly Labelled Water: Doubly labelled water involves administering isotopically labeled water (H₂O) and measuring the enrichment in urine. This method is considered non-invasive and can provide a fairly accurate assessment of total energy expenditure over an extended period in dogs or cats. However, it is not without limitations as it is not possible to tease apart the various contributions.

3) 13C Bicarbonate Isotope: This method utilizes the administration of 13C-labeled bicarbonate. The amount of exhaled 13CO₂ is then measured and used to estimate energy expenditure. This method also requires a mask to capture the breath, or an enclosed environment similar to calorimetry.

4) Infrared/Thermography: Infrared or thermographic techniques capture heat radiation from an animal’s body, allowing for non-contact measurement of surface temperature and providing insights into thermoregulation and energy expenditure patterns in companion animals. However, this method is less accurate than others and is rarely used.

5) Heart Rate: Monitoring heart rate can be used to predict energy expenditure. The relationship between heart rate and energy expenditure is influenced by factors such as activity level and metabolic rate. However, this method has its limitations, as factors like stress, excitement, and health conditions can affect heart rate, making it less precise compared to some other direct and indirect calorimetry methods. Despite its limitations, it provides a real-time and continuous assessment of energy expenditure during different activities and can be a useful tool for general energy expenditure monitoring in companion animals.

6) Energy Intake: We can use the amount of food an animal has to consume to maintain a stable body weight to estimate that animal’s energy requirements. This method is commonly used in clinical practice. The amount of food is used to calculate the energy intake (calories).

7) Predictive Equations: Predictive equations use factors such as body weight, age, and activity level to estimate energy output, offering a practical and convenient method for determining energy needs in companion animals. However, there are multiple proposed equations available and individual variations, health conditions, and needs can affect the accuracy.

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Why Energy Expenditure Matters for Your Pets

Indirect calorimetry is a research tool and cannot be performed for all pets. But by extrapolating data from published research that investigated energy utilization of animals, we know that animal nutritionists and veterinarians are able to make calculations to estimate energy requirements for dogs and cats. Using this information, recommendations for daily food allotments (feeding guidelines) for your pets can be made!

Additionally, by utilizing methods such as indirect calorimetry in research, we can investigate unique dietary interventions such as testing different diets, supplements, or specific ingredients to determine whether they may be beneficial for companion animals.

Investigators can also use indirect calorimetry to study the effects of exercise routines, or medical conditions on a pet’s metabolism. Research employing indirect calorimetry for companion animals provides important information for the development of more accurate energy calculations, new ingredients for use in pet foods, treatment plans for various disease states (i.e. obesity, sarcopenia), and other advancements.

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Indirect Calorimetry Research Generated by the OVC Pet Nutrition Group

The OVC Pet Nutrition Research Team has published numerous scientific studies utilizing indirect calorimetry in collaboration with Dr. Anna Kate Shoveller. Our and the Shoveller lab’s most recent publications are listed below for those who would like to learn more:

• Rankovic, A et al. (2023) Dietary choline, but not L-carnitine, increases circulating lipid and lipoprotein concentrations, without affecting body composition, energy expenditure or respiratory quotient in lean and obese male cats during weight maintenance. Frontiers in Veterinary Science. doi: 10.3389/fvets.2023.119817  
• Rankovic, A et al. (2022) Dose-response relationship between dietary choline and serum lipid profile, energy expenditure, and respiratory quotient in overweight adult cats fed at maintenance energy requirements. Journal of Animal Nutrition. 100(7).
• Hogan, K et al. (2022) Introduction of adult cats to indirect calorimetry respiration chambers causes increased energy expenditure and respiratory quotient that decrease following acclimation. American Journal of Veterinary Research. 83(3): 264-269.
• Godfrey, H et al. (2022) Dietary choline in gonadectomized kittens improved food intake and body composition but not satiety, serum lipids, or energy expenditure. PLOS ONE. 17(3): e0264321 
• Camara, et al. (2020) The daytime feeding frequency affects appetite-regulating hormones, amino acids, physical activity, and respiratory quotient, but not energy expenditure, in adult cats fed regimens for 21 days. PLOS ONE. 15(9):e0238522
• Asaro, N et al. (2019) Modelling net energy of commercial cat diets. PLOS ONE. 14(6). e0218173.
• Asaro, N et al. (2018) Carbohydrate level and source have minimal effects on feline energy and macronutrient metabolism. Journal of Animal Science. 96(12): 5052-5063.
• Gooding, MA et al. (2015) Dietary fat and carbohydrate have different effects on body weight, energy expenditure, glucose homeostasis and behaviour in adult cats fed to energy requirement. Journal of Nutritional Science. 4: e2.

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Written by: Hannah Godfrey, BSc.H., MSc., PhD Candidate