Energy and Water

Chapter 1. Energy and Water

Like all living animals, dogs and cats require a balanced diet to grow normally and maintain health once they are mature. Nutrients are components in the diet that have specific functions within the body and contribute to growth, tissue maintenance, and optimal health. Essential nutrients are those components that cannot be synthesized by the body at a rate that is adequate to meet the body’s needs. Therefore essential nutrients must be supplied in the diet. Nonessential nutrients can be synthesized by the body and obtained either through de novo synthesis or from the diet. Along with a requirement for energy, all animals have a metabolic requirement for six major categories of nutrients. These are water, carbohydrates, proteins, fats, minerals, and vitamins. Energy, although not a nutrient per se, is required by the body for normal growth, maintenance, reproductive performance, and physical work. Approximately 50% to 80% of the dry matter (DM) of a dog’s or cat’s diet is used for energy.

Nutrition is the study of food, its nutrients, and other components, including an examination of the actions of specific nutrients, their interactions with each other, and their balance within a diet. The six categories of nutrients—water, carbohydrates, proteins, fats, minerals, and vitamins—have specific functions and contribute to growth, body tissue maintenance, and optimal health.

With the exception of water, energy is the most critical component that must be considered in a diet. Like all animals, companion animals require a constant source of dietary energy to survive. Plants obtain energy from solar radiation and convert it to energy-containing nutrients. Animals consume plants and use them either directly for energy or to convert plant nutrients into other energy-containing molecules. The primary form of stored energy in plants is carbohydrate; the main form of stored energy in animals is fat. Energy is necessary for the performance of the body’s metabolic work, which includes maintaining and synthesizing body tissues, engaging in physical work, and regulating normal body temperature. Given its importance, it is not surprising that energy is always the first requirement to be met by an animal’s diet. Regardless of a dog’s or cat’s needs for essential amino acids from dietary protein or essential fatty acids (EFAs) from dietary fat, the energy-yielding nutrients of the diet are first used to satisfy energy needs. Once energy needs are met, nutrients become available for other metabolic functions.

Energy is needed by the body to perform metabolic work, which includes maintaining and synthesizing body tissues, engaging in physical work, and regulating normal body temperature. Energy is always the first requirement met by an animal’s diet.

Animals are capable of regulating their energy intake to accurately meet their daily caloric requirements. When allowed free access to a balanced, moderately palatable diet, most dogs and cats will consume enough food to meet, but not exceed, their daily energy needs. 1.2. and 3.Energy density or caloric density refers to the concentration of energy in a given quantity of food (see p. 6). When the energy density of a diet is decreased, animals respond by increasing the quantity of food they consume, which results in a relatively constant energy intake. 3. and 4. If an animal’s food intake is regulated by total energy intake, the composition of all other nutrients in the diet must be balanced with respect to the diet’s energy density. This balance should be calculated to ensure that, when a dog or cat consumes a quantity of food adequate to meet his or her caloric needs, all other nutrient requirements will be met in the same volume of food.

Although all dogs and cats have the ability to properly regulate their energy intake, this natural tendency can be overridden by environmental factors. Providing unrestricted access to foods that are both highly palatable and energy-dense can lead to chronic overconsumption in some companion animals. Today’s competitive pet food market includes many foods that are high in both palatability and caloric density. Coupled with this fact is a decline in physical activity among many pets in today’s society. Many companion animals now lead happy but relatively sedentary lives exclusively as house pets. Cats have moved from the barnyard into the house, where their former working roles as mousers and pest-controllers have been effectively eliminated. Likewise, dogs have evolved from working companions to unemployed house dogs that may lack adequate daily exercise. These two changes have led to an epidemic of obesity among dogs and cats; although reported incidence rates vary, surveys have shown that obesity is a common nutritional problem observed by practicing veterinarians and reported by owners. 5. and 6. These changes indicate that it may no longer be wise to rely on the inherent abilities of dogs and cats to regulate energy intake. Although companion animals certainly have this ability, many do not self-regulate because of the nature of the food they eat and the type of lifestyle they lead. In most cases, portion-controlled feeding is the best way to control a pet’s energy balance, growth rate, and weight status (see Section 4, pp. 194-197).


Energy has no measurable mass or dimension, but the chemical energy contained in foods is ultimately transformed by the body into heat, which can be measured. Energy in food is expressed in units of kilocalories (kcal) or kilojoules (kJ). A calorie refers to the amount of heat energy necessary to raise the temperature of 1 gram (g) of water from 14.5° Celsius (C) to 15.5° C. Because a calorie is a very small unit, it is not of practical use in the science of animal nutrition. The kcal, which is equal to 1000 calories, is the most commonly used unit of measure. The kilojoule is a metric unit and is defined as the amount of mechanical energy required for a force of 1 newton (N) to move a weight of 1 kilogram (kg) by a distance of 1 meter (m). To convert kcal to kJ, the number of kcal is multiplied by 4.184. In the United States, kcal is the most commonly used unit for energy in human and pet foods.

The caloric value of foods can be measured using direct calorimetry. This process involves the complete combustion (oxidation) of a premeasured amount of food in a bomb calorimeter, resulting in the release and measurement of the food’s total chemical energy. This energy is called the food’s gross energy (GE). The three nutrient classes that provide energy in an animal’s diet are carbohydrates, fats, and proteins. Animals cannot use all of a food’s GE because energy losses occur during digestion and assimilation. Digestible energy (DE) signifies the amount of energy available for absorption across the intestinal mucosa. Apparent DE can be calculated by subtracting the indigestible energy excreted in the feces from the GE of the food. Additional energy losses occur as a result of the production of combustible gases and the excretion of urea in the urine. The incomplete oxidation of absorbed dietary protein by the body results in the production of urea. Because the production of combustible gases in dogs and cats is minimal, only urinary losses are typically accounted for. Metabolizable energy (ME) is the amount of energy ultimately available to the tissues of the body after losses in the feces and urine have been subtracted from the GE of the food.

ME is the value that is most often used to express the energy content of pet food ingredients and commercial diets. Similarly, the energy requirements of dogs and cats are usually expressed as kcal of ME. ME can be subdivided to calculate net energy (NE) and the energy lost to dietary thermogenesis. Dietary thermogenesis, also called the specific dynamic action of food, refers to the energy needed by the body to digest, absorb, and assimilate nutrients. NE is the energy available to an animal for the maintenance of body tissues and for production needs such as physical work, growth, gestation, and lactation (Figure 1-1). The percent of ME that becomes available as NE is called the efficiency of utilization.

The ME of a diet or food ingredient depends on both the nutrient composition of the food and the animal that is consuming it. For example, because of the length and structure of its gastrointestinal tract, a nonruminant herbivore such as a horse can derive a greater amount of energy from grass than can a dog or cat. Therefore the ME value of grass for a horse is higher than the ME value of grass for a companion animal. Several different methods are used to estimate the ME values of a food ingredient or diet for a given species, each with its own strengths and limitations. These include the direct determination of ME through feeding trials and total collection procedures; calculating an estimate of ME using analyzed levels of protein, carbohydrate, and fat in the diet; and, most recently, predicting ME using regression equations based upon a food’s fiber content. New research also suggests that some in vitro methods may provide reliable estimates of the energy content of certain types of commercial pet foods.

Direct Determination in Feeding Trials

The gold standard for determining ME is through data collected in actual feeding trials with the species in question. The diet or food ingredient is fed to a group of test animals, and feces and urine are collected throughout a predesignated time period. Determination of the GE content of the food, feces, and urine allows direct calculation of ME by subtraction (Figure 1-2). Because this approach provides the most accurate estimate of ME, many pet food manufacturers periodically use feeding trials to measure DE or ME of their foods and ingredients. Data that are collected can be used to reflect changes in ME values as new products are developed and to ensure the accuracy of predictive equations that may be used more routinely. When digestibility trials are used, ME values are determined using DE values and a correction factor to account for urinary energy losses from the metabolism of protein. Because cats digest protein calories more efficiently than dogs, species-specific correction factors have been determined; for cats 0.86 kcal/g digestible protein and for dogs 1.25 kcal/g digestible protein. 7. and 8.

Calculation Methods

Although direct measurement in the target species is the most accurate method for estimating ME, it is also very time consuming and costly, and requires access to large numbers of representative animals. As a result, routine determinations of ME are conducted with mathematical formulas that estimate ME from analyzed carbohydrate, protein, and fat content, or from fiber content of the food. 9 Recently, although not yet commonly used in industry, experimental methods that utilize in vitro enzymatic assays, measurement of the ratio of total amino acids to non–amino acid nitrogen, or near-infrared spectroscopy have also been reported as accurate predictors of the energy value of dog foods. 10.11. and 12.

Pet food manufacturers measure a food’s metabolizable energy (ME) either by direct measurement using feeding trials or by calculation from analyzed nutrient levels or standard table values. New in vitro methods of estimating ME include enzymatic assays, measurement of the amino acids and amino acid nitrogen, and near-infrared spectroscopy.

Formulas that have been derived for dog and cat diets include constants that account for fecal and urinary losses of energy. The GE values, which represent total energy content, for mixed carbohydrate, fat, and protein are 4.15, 9.40, and 5.65 kcal/g, respectively. 8 However, as mentioned earlier, animals are incapable of using all of the energy present in food nutrients. Inefficiency in digestion, absorption, and assimilation results in energy losses. In human foods, the Atwater factors of 4-9-4 kcal/g are commonly used to estimate ME values for carbohydrate, fat, and protein, respectively. These factors are calculated using estimated digestibility coefficients of 96% for fat and carbohydrate and 91% for protein. 13 A digestibility coefficient is the proportion of the consumed nutrient that is actually available for absorption and use by the body. The ME value of protein was further reduced to account for urinary losses of urea.

The Atwater factors work quite well for estimating the ME of homemade dog and cat diets and for commercial products with very high digestibilities such as milk replacers for puppies and kittens and enteral feeding formulas. 8 However, digestibility data collected in dogs and cats fed typical commercial pet foods have shown that Atwater factors tend to overestimate the ME values of most commercial foods. This miscalculation occurs because the digestibility of many pet food ingredients is lower than the digestibility of most foods consumed by humans. Digestibility data collected in dogs from 106 samples of dry, semimoist, and canned commercial dog foods showed that the average digestibility coefficients for crude protein, acid-ether extract (a measure of fat content), and nitrogen-free extract (NFE) (a measure of soluble carbohydrate content) were 81%, 85%, and 79%, respectively. 14 The fact that some pet food ingredients are generally lower in digestibility than the foods consumed by humans causes the Atwater factors to be inaccurate for use in estimating the ME of pet foods. The National Research Council’s (NRC’s) 1985 recommendations for dogs suggested that digestibility coefficients of 80%, 90%, and 85% be used for the protein, fat, and carbohydrate in commercial dog food, respectively. When GE values were readjusted for digestibility and urinary losses, ME values of 3.5, 8.5, and 3.5 kcal/g were assigned to protein, fat, and carbohydrate, respectively (Table 1-1). These values are referred to as modified Atwater factors. Although these values provide a better estimate of ME values for pet foods than do the Atwater factors, they may still underestimate the ME values of highly digestible foods and overestimate ME values for foods containing high amounts of fiber or poor quality meat sources. 15

N utrient H uman food digestibility coefficient A twater factor P et food digestibility coefficient M odified A twater factor
Carbohydrate 96% 4 kcal/g 85% 3.5 kcal/g
Protein 91% 4 kcal/g 80% 3.5 kcal/g
Fat 96% 9 kcal/g 90% 8.5 kcal/g
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Jul 31, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on Energy and Water

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