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What is Energy?

==What is Energy?==

Energy is not a nutrient specifically, but is derived from macronutrients found in foods (protein, fat, and carbohydrate). Energy in food is usually considered at 3 different levels. Gross energy (GE): this is the total (thermic) energy in the foods released by complete oxidation. Although a food may have a high GE content, it may be indigestible and therefore unavailable to the animal. Digestible energy (DE) is the amount of energy which is digested and absorbed by the animal, and this is equivalent to GE minus faecal losses. Not all of the DE is available to the animal; some is lost in the urine as energy is metabolised by tissues and cells. Food energy that is utilised by the tissues is referred to as metabolisable energy (ME) and it is calculated from DE minus urinary losses of energy. This is the most meaningful measure of food energy as it represents energy that is truly available to the animal to use. The designated SI unit of energy is the joule (J), with kiloJoules (kJ), or kilocalories (kcal) in the United Stated, used in animal nutrition.  1 calorie=4.184 joule

Energy is not a nutrient specifically, but is derived from macronutrients found in foods (protein, fat, and carbohydrate). Energy in food is usually considered at 3 different levels.  

Why is it important?

#Gross energy (GE): this is the total (thermic) energy in the foods released by complete oxidation. Although a food may have a high GE content, it may be indigestible and therefore unavailable to the animal.  

#Digestible energy (DE) is the amount of energy which is digested and absorbed by the animal, and this is equivalent to GE minus faecal losses. Not all of the DE is available to the animal; some is lost in the urine as energy is metabolised by tissues and cells.  

#Food energy that is utilised by the tissues is referred to as metabolisable energy (ME) and it is calculated from DE minus urinary losses of energy. This is the most meaningful measure of food energy as it represents energy that is truly available to the animal to use.  

The designated SI unit of energy is the joule (J), with kiloJoules (kJ), or kilocalories (kcal) in the United Stated, used in animal nutrition.  1 calorie=4.184 joule

 

==Why is it Important?==

Energy is required for growth, gestation, lactation, and physical activity. Energy provided in the form of protein, fat, or carbohydrate will ultimately be used in production of adenosine triphosphate (ATP) in the tricarboxylic acid (TCA) cycle. ATP is then used as the metabolic “fuel” to support the normal actions of the cell.  

Energy is required for growth, gestation, lactation, and physical activity. Energy provided in the form of protein, fat, or carbohydrate will ultimately be used in production of adenosine triphosphate (ATP) in the tricarboxylic acid (TCA) cycle. ATP is then used as the metabolic “fuel” to support the normal actions of the cell.  

Roles in the body

 

==Roles in the Body==

Energy expenditure and requirement for intake is dependent on the Basal Metabolic Rate (BMR, the energy required for normal physiologic functions in a fasted, thermoneutral environment); activity level (normal physical activity and exercise); dietary thermogenesis (energy used during digestion and assimilation of food); and adaptive thermogenesis (energy needed to maintain body temperature in cold environments). In a thermoneutral environment BMR accounts for approximately 60% of the animal’s total daily energy expenditure, while normal activity is 30%, and dietary thermogenesis accounts for 10% energy utilization.1 Energy expenditure for adaptive thermogenesis will vary with temperature, humidity and coat thickness.2

Energy expenditure and requirement for intake is dependent on the Basal Metabolic Rate (BMR, the energy required for normal physiologic functions in a fasted, thermoneutral environment); activity level (normal physical activity and exercise); dietary thermogenesis (energy used during digestion and assimilation of food); and adaptive thermogenesis (energy needed to maintain body temperature in cold environments). In a thermoneutral environment BMR accounts for approximately 60% of the animal’s total daily energy expenditure, while normal activity is 30%, and dietary thermogenesis accounts for 10% energy utilization.1 Energy expenditure for adaptive thermogenesis will vary with temperature, humidity and coat thickness.2

Resting energy requirement (RER) accounts for both BMR and dietary thermogenegisis. RER is determined by lean body mass, but may vary with age, breed, gender, neuter status, and the presence of disease.  

Resting energy requirement (RER) accounts for both BMR and dietary thermogenegisis. RER is determined by lean body mass, but may vary with age, breed, gender, neuter status, and the presence of disease.  

For both dogs and cats, RER can be calculated using exponential equations based on body weight using (70*BWkg0.75).3 A number of factors can influence daily energy requirements, such as breed, reproductive or neuter status, activity level (e.g., sedentary vs. working dog), and environment (e.g., indoor vs. outdoor, kennel/cattery vs. a home) and relying on published maintenance energy requirement (MER) equations can be problematic if these variants are not account for. Normal MER variation in cats4 can range from 29-85.5 kcal/BWkg0.75 and in dogs5 can range from 54.5-441.1 kcal/BWkg0.75. It is important to note that in both dogs and cats daily MER values can actually fall below calculated RER based solely on body weight. Adipose tissue is less metabolically active than muscle and obese dogs and cats will have lower than expected RER based on body weight measurements alone. Larger cats (>5.5 kg) have lower metabolic energy requirements on a per kg basis than lean or “normal” weight cats.4 In a meta-analysis study on energy requirements of adult cats, the MER was best represented by the equation 77.7 * BWkg 0.711. Activity level has the most significant impact on canine energy requirements with inactive dogs having lower metabolic energy requirements on a per kg basis than sporting or working dogs.5 In one cross-sectional survey of pet owners in Australia and the US, only 60% of dog owners reported walking their dogs on a regular basis, with 40% receiving no walks at all.6 The average activity level for those receiving regular walks was four 40 minute walks per week. In a recent meta-analysis5 study pet dogs with the lowest activity (resting) level had an energy requirement of 95*BWkg0.75. Energy requirements for different life-stages:

For both dogs and cats, RER can be calculated using exponential equations based on body weight using (70*BWkg0.75).3 A number of factors can influence daily energy requirements, such as breed, reproductive or neuter status, activity level (e.g., sedentary vs. working dog), and environment (e.g., indoor vs. outdoor, kennel/cattery vs. a home) and relying on published maintenance energy requirement (MER) equations can be problematic if these variants are not account for. Normal MER variation in cats4 can range from 29-85.5 kcal/BWkg0.75 and in dogs5 can range from 54.5-441.1 kcal/BWkg0.75. It is important to note that in both dogs and cats daily MER values can actually fall below calculated RER based solely on body weight. Adipose tissue is less metabolically active than muscle and obese dogs and cats will have lower than expected RER based on body weight measurements alone. Larger cats (>5.5 kg) have lower metabolic energy requirements on a per kg basis than lean or “normal” weight cats.4 In a meta-analysis study on energy requirements of adult cats, the MER was best represented by the equation 77.7 * BWkg 0.711. Activity level has the most significant impact on canine energy requirements with inactive dogs having lower metabolic energy requirements on a per kg basis than sporting or working dogs.5 In one cross-sectional survey of pet owners in Australia and the US, only 60% of dog owners reported walking their dogs on a regular basis, with 40% receiving no walks at all.6 The average activity level for those receiving regular walks was four 40 minute walks per week. In a recent meta-analysis5 study pet dogs with the lowest activity (resting) level had an energy requirement of 95*BWkg0.75.  

a. Growth: Energy requirements for newborn puppies and kittens are estimated at 25 kcal/100g BW and 20-25 kcal/100g BW, respectively, until weaning.7 After weaning puppies and kittens should be fed approximately 2*MER until they reach 40-50% of expected adult weight, this should be decreased to  1.6*MER until 80% of their expected adult weight is reached, and then further decreased to 1.2*MER until they are fully grown. At maturity food intake should be adjusted to maintain an optimal body condition. Rate of growth and time to reach each change will vary with breed and individual requirements.

Energy requirements for different life-stages:

 

*'''Growth''': Energy requirements for newborn puppies and kittens are estimated at 25 kcal/100g BW and 20-25 kcal/100g BW, respectively, until weaning.7 After weaning puppies and kittens should be fed approximately 2*MER until they reach 40-50% of expected adult weight, this should be decreased to  1.6*MER until 80% of their expected adult weight is reached, and then further decreased to 1.2*MER until they are fully grown. At maturity food intake should be adjusted to maintain an optimal body condition. Rate of growth and time to reach each change will vary with breed and individual requirements.

b. Gestation:  

*'''Gestation''':  

i. Dogs: Most foetal weight gain occurs after day 40 of gestation. Until that time, maternal energy requirements do not change significantly. After day 40, energy demand increases and bitches should be allowed free access to food.

#'''Dogs''': Most foetal weight gain occurs after day 40 of gestation. Until that time, maternal energy requirements do not change significantly. After day 40, energy demand increases and bitches should be allowed free access to food.

ii. Cats: Energy requirements for queens do not change significantly during gestation, but they will lose 40-50% of their body weight during lactation. During the last half of gestation queens should be fed 140*BWkg0.67 in anticipation of this extreme weight loss.8

#'''Cats''': Energy requirements for queens do not change significantly during gestation, but they will lose 40-50% of their body weight during lactation. During the last half of gestation queens should be fed 140*BWkg0.67 in anticipation of this extreme weight loss.8

==Consequences of Energy Deficiency==

 

 

 

 

Consequences of Energy Deficiency

Inadequate energy intake results in poor growth (puppies and kittens), lethargy, weakness, compromised immune function, and poor performance (whether reproductive or athletic). Complete starvation results in loss of adipose tissue as well as loss of lean body mass and atrophy of internal organs.  

Inadequate energy intake results in poor growth (puppies and kittens), lethargy, weakness, compromised immune function, and poor performance (whether reproductive or athletic). Complete starvation results in loss of adipose tissue as well as loss of lean body mass and atrophy of internal organs.  

Commercial dog and cat foods are designed to provide complete and balanced nutrition when fed to maintain optimal body weight. Underfeeding of energy from a commercial dog or cat food may also result in inadequate intake of all other essential nutrients.

Commercial dog and cat foods are designed to provide complete and balanced nutrition when fed to maintain optimal body weight. Underfeeding of energy from a commercial dog or cat food may also result in inadequate intake of all other essential nutrients.

Toxicity

 

==Toxicity==

Excess energy intake is not toxic, though long-term excess intake can result in obesity and its associated health risks. Obesity is associated with an increased risk of Diabetes Mellitus in cats,13 in growing puppies can result in development skeletal abnormalities,14,15 and can worsen clinical sign of orthopaedic disease and decrease longevity in adult dogs.16  

Excess energy intake is not toxic, though long-term excess intake can result in obesity and its associated health risks. Obesity is associated with an increased risk of Diabetes Mellitus in cats,13 in growing puppies can result in development skeletal abnormalities,14,15 and can worsen clinical sign of orthopaedic disease and decrease longevity in adult dogs.16  

Dietary Sources

 

==Dietary Sources==

Foods differ in the amount of energy, and this is a primarily a function of the amount of moisture, digestibility, and the amount and proportions of macronutrients. Digestibility (i.e., feeding) studies are the most accurate way of determining the ‘available’ energy content of a food, but these studies are expensive and require the use of laboratory animals. Many pet food companies do not have the resources to conduct digestibility studies and use predictive equations instead. There are different predictive equations for pet foods and human foods, which in part reflects differences in the digestibility of these foods. Typically for highly digestible human foods such as chicken breast, egg, rice or oils, the ‘Atwater’ factors [protein (4 kcal per gram), fat (9 kcal per gram), and carbohydrate (4 kcal per gram)] can be used to calculate energy content.  Two different approaches are commonly used for estimating the energy content of manufactured pet foods. One uses pet foods the modified Atwater factors of 3.5 kcal per gram of protein, 8.5 kcal per gram of fat, and 3.5 kcal per gram of carbohydrates. Whilst this equation is mathematically simple it has limitations, because it can over and underestimate foods with a digestibility that is lower or higher than ‘average’. An alternative but more complex equation which does account for differences in digestibility has been developed and this does appear to provide a better estimate of the ‘available’ energy content of the food.17  

Foods differ in the amount of energy, and this is a primarily a function of the amount of moisture, digestibility, and the amount and proportions of macronutrients. Digestibility (i.e., feeding) studies are the most accurate way of determining the ‘available’ energy content of a food, but these studies are expensive and require the use of laboratory animals. Many pet food companies do not have the resources to conduct digestibility studies and use predictive equations instead. There are different predictive equations for pet foods and human foods, which in part reflects differences in the digestibility of these foods. Typically for highly digestible human foods such as chicken breast, egg, rice or oils, the ‘Atwater’ factors [protein (4 kcal per gram), fat (9 kcal per gram), and carbohydrate (4 kcal per gram)] can be used to calculate energy content.  Two different approaches are commonly used for estimating the energy content of manufactured pet foods. One uses pet foods the modified Atwater factors of 3.5 kcal per gram of protein, 8.5 kcal per gram of fat, and 3.5 kcal per gram of carbohydrates. Whilst this equation is mathematically simple it has limitations, because it can over and underestimate foods with a digestibility that is lower or higher than ‘average’. An alternative but more complex equation which does account for differences in digestibility has been developed and this does appear to provide a better estimate of the ‘available’ energy content of the food.17  

Step 1 calculate carbohydrate (NFE) content:  Carbohydrate (NFE; g/100g)) = 100 - (Moisture + Protein + Fat + Ash + Crude Fibre)

*Step 1 calculate carbohydrate (NFE) content:  Carbohydrate (NFE; g/100g)) = 100 - (Moisture + Protein + Fat + Ash + Crude Fibre)

Step 2 calculate the Gross Energy (GE) content of the food:  GE (kcal/100g) = (5.7 x protein) + (9.4 x fat) + (4.1 x [NFE + Crude Fibre])

*Step 2 calculate the Gross Energy (GE) content of the food:  GE (kcal/100g) = (5.7 x protein) + (9.4 x fat) + (4.1 x [NFE + Crude Fibre])

Step 3 calculate the percentage digestibility of the food (there are different equations for cat and dog foods)

*Step 3 calculate the percentage digestibility of the food (there are different equations for cat and dog foods)

Cat, % digestibility of energy = 87.9 – (0.88 x CF x 100/[100- % moisture])

:*Cat, % digestibility of energy = 87.9 – (0.88 x CF x 100/[100- % moisture])

Dog, % digestibility of energy = 91.2 – (1.43 x CF x 100/[100- % moisture])

:*Dog, % digestibility of energy = 91.2 – (1.43 x CF x 100/[100- % moisture])

 

*Step 4: calculate DE content: DE = GE (from step 2) x % energy digestibility (from step 3)/100

Step 4: calculate DE content

*Step 5: calculate ME content (there are different equations for cat and dog foods)

DE = GE (from step 2) x % energy digestibility (from step 3)/100

:*Cat ME (kcal/100g) = DE (from step 4) – (0.77 x protein g)

 

:*Dog, ME (kcal/100g) = DE (from step 4) – (1.04 x protein g)

Step 5: calculate ME content (there are different equations for cat and dog foods)

Cat ME (kcal/100g) = DE (from step 4) – (0.77 x protein g)

Dog, ME (kcal/100g) = DE (from step 4) – (1.04 x protein g)

Key:

Key:

The specific energy contribution of a commercial pet food on an as-fed basis will also depend on dietary moisture level: dry food (<14% moisture) provide between 330-380+ kcal per 100g; semi-moist (>14% and <60% moisture) provide between 250-350+ kcal per 100g; and wet food (>60% moisture) typically provide 80-100+ kcal per 100 g. Since fat provides a larger proportion of energy relative to protein and carbohydrates, diets with higher fat levels will provide more energy per 100g as-fed. The energy contribution of total dietary fibre is negligible for dogs and cats, yet inclusion of high levels of dietary fibre, especially insoluble, non-fermentable fibre will increase volume of food while decreasing energy content.18,19  

The specific energy contribution of a commercial pet food on an as-fed basis will also depend on dietary moisture level: dry food (<14% moisture) provide between 330-380+ kcal per 100g; semi-moist (>14% and <60% moisture) provide between 250-350+ kcal per 100g; and wet food (>60% moisture) typically provide 80-100+ kcal per 100 g. Since fat provides a larger proportion of energy relative to protein and carbohydrates, diets with higher fat levels will provide more energy per 100g as-fed. The energy contribution of total dietary fibre is negligible for dogs and cats, yet inclusion of high levels of dietary fibre, especially insoluble, non-fermentable fibre will increase volume of food while decreasing energy content.18,19  

Diagnosing Energy Deficiency

 

==Diagnosing Energy Deficiency==

Most often determined by physical examination and evaluation of body condition. Dogs and cats with inadequate energy intake may have generalized sarcopenia even in the presence of excess adiposity. The most accurate way of differentiating weight loss due to inadequate energy intake versus underlying disease is by comparing actual and expected daily energy requirements.  

Most often determined by physical examination and evaluation of body condition. Dogs and cats with inadequate energy intake may have generalized sarcopenia even in the presence of excess adiposity. The most accurate way of differentiating weight loss due to inadequate energy intake versus underlying disease is by comparing actual and expected daily energy requirements.  

References

 

==References==

1. Case LP, et al. In Canine and Feline Nutrition: A resource for Companion Animal Professionals. 2011 Third Ed. St. Louis: Mosby p.59-61.

1. Case LP, et al. In Canine and Feline Nutrition: A resource for Companion Animal Professionals. 2011 Third Ed. St. Louis: Mosby p.59-61.

2. National Research Council (NRC). Physical Activity and Environment. In Nutrient Requirements for Dogs and Cats. 2006 Washington, DC: National Academies Press p.267-273.

2. National Research Council (NRC). Physical Activity and Environment. In Nutrient Requirements for Dogs and Cats. 2006 Washington, DC: National Academies Press p.267-273.