Energy - Nutrition

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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.

  1. 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.
  2. Digestible energy (DE): This 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.
  3. Metabolisable energy (ME):: Is food energy that is utilised by the tissues 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.

Roles in the Body

Energy expenditure and requirement for intake is dependent on:

  • 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
  • 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 varies 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.

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 accounted for. Normal MER variation in cats[4] can range from 29-85.5 kcal/BWkg0.75 and in dogs[5] 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 * BWkg0.711.

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-analysis[5] study pet dogs with the lowest activity (resting) level had an energy requirement of 95*BWkg0.75.

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.
  • Gestation:
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.
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].
  • Lactation:
Dogs: Typically lasts 6 - 8 weeks, and energy demand will vary depending on litter size and breed. Peak lactation occurs around week 4 post-partum, when weaning typically starts. The energy requirement for milk production is estimated to be 24 kcal/BWkg of bitch per puppy for litters of for 1-4 puppies; and 12 kcal/BWkg of bitch per puppy for additional puppies i.e 5 or more. The energy requirements to support lactation are added to maternal MER[7].
Cats: Typically lasts 7-9 weeks. Queens experience a net loss of body mass during lactation and should be fed at 2*MER.
  • Athletes:
Dogs: Energy intake should be adjusted to environment and condition and will vary with the activity[4][7]. Racing sled dogs may have a daily energy requirement of 6-10*MER depending on temperature, pack weight, and distance covered; whereas a racing greyhound (sprint races) may have a daily requirement of 2*MER during training and racing.
  • Neutering:
Neutering can influence energy requirements due to changes in activity, and/or in ghrelin levels in response to changes in sex hormone concentrations[9][10][11].
  • Age:
Digestive efficiency decreases with age, and older dogs and cats may need to increase energy intake to offset changes in digestive efficiency and maintain optimal body weight[4][12].

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. 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.


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

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


GE = gross energy
DE = digestible energy
DE = digestible energy
ME = metabolisable energy
CF = crude fibre
NFE = nitrogen free extract

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) provides between 330-380+ kcal per 100g;
  • Semi-moist (>14% and <60% moisture) provides between 250-350+ kcal per 100g;
  • Wet food (>60% moisture) typically provides 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

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.


  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.
  3. Klieber M. The fire of life. 1961 New York: John Wiley & sons.
  4. 4.0 4.1 4.2 4.3 Bermingham EN, et al. Energy Requirement of adult cats. Br J Nutr 2010;103:1083-1093.
  5. 5.0 5.1 5.2 Bermingham EN, et al. Energy Requirement of adult dogs: A meta-analysis. PLOSone 2014;9:e109681.
  6. Christian NE, et al. Dog ownership and physical activity: A review of the evidence. J Phys Act Health 2013; 10:750-759.
  7. 7.0 7.1 7.2 National Research Council (NRC). Energy. In Nutrient Requirements for Dogs and Cats. 2006 Washington, DC: National Academies Press p.28-48.
  8. Loveridge GG. Body weight changes and energy intake of cats during gestation and lactation. Anim Technol 1986;38:7-15.
  9. Kienzle E and Rainbird A. The maintenance energy requirement of dogs-What is the correct figure for the calculation of the metabolic body weight in dogs? J Nutr 1991;121:39-40.
  10. Backus RC, et al. Gonadectomy and high dietary fat and not high dietary carbohydrate induced gains in body weight and fat of domestic cats. Br J Nutr 2007;98(3):641-650.
  11. Root MV. Early spay-neuter in the cat: effect on development of obesity and metabolic rate. Vet Clin Nutr 1995;2:132-134.
  12. Donoghue S, et al. Body composition and diet of relatively healthy older dogs. J Nutr 1991;121:S58-S59.
  13. Lutz TA and Rand JS. A review of the new developments in type 2 diabetes mellitus in human beings and cats. Brit Vet J 1993;149:527-536.
  14. Hedhammer A, et al. Overnutrition and skeletal disease. Cornell Vet 1974;64(suppl 5):9-150.
  15. Kealy RD, et al. Effects of limited food consumption on the incidence of hip dysplasia in growing dogs. JAVMA 1992;201:857-863.
  16. Kealy RD, et al. Effects of diet restriction on life span and age-related changes in dogs. JAVMA 2002;220:1315-1320.
  17. Kienzle E, et al. The development of an improved method of predicting the energy content in prepared dog and cat food. J Anim Physiol Anim Nutr 1998; 79:69-79.
  18. Fahey GC Jr, et al. Dietary fiber for dogs. II. Isolated total dietary fiber (TDF_ additions of divergent fiber sources to dog diets and their effects on nutrient intake, digestibility, metabolic energy and digesta mean retention time. J Anim Sci 1990;68:4229-4235.
  19. Kienzle, E, et al. Prediction of Energy Digestibility in Complete Dry Foods for Dogs and Cats by Total Dietary Fiber. J Nutr 2006;136:2041S-2044S.

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