Chapter 9. Energy Balance
ENERGY EXPENDITURE
The body’s energy expenditure can be partitioned into three major components: basal metabolic rate, voluntary muscular activity, and dietary thermogenesis. 1 A fourth component, called adaptive or nonshivering thermogenesis, represents energy that is expended in response to environmental conditions and yields heat but no useful work. Nonshivering thermogenesis was first demonstrated in small, warm-blooded animals and is essential for cold adaptation in many species, including dogs. 2. and 3.
There are three major components of energy expenditure: (1) the energy expended during rest (resting metabolic rate), (2) the energy expended during voluntary muscle activity, and (3) the energy/heat produced by thermogenesis.
Basal Metabolic Rate and Resting Fed Metabolic Rate
Basal metabolic rate (BMR) contributes the greatest portion of an animal’s total energy expenditure. It is defined as the amount of energy expended while an animal is resting in a thermoneutral environment and in a postabsorptive state (i.e., after an overnight fast). BMR represents the energy cost of maintaining homeostasis in all of the integrated systems of the body during periods of rest, when the body is not digesting food. Homeostasis refers to a state of internal stability within the body. A related value is the resting fed metabolic rate (RFMR), which is measured when the animal is not in a postabsorptive state and so includes the heat produced when food is consumed (dietary thermogenesis). The RFMR accounts for approximately 60% to 75% of an animal’s total daily energy expenditure. Factors influencing RFMR include sex and reproductive status, thyroid gland and autonomic nervous system function, body composition, body surface area, and nutritional state. 1
Research has shown that BMR and RFMR are positively correlated with the total amount of respiring cell mass present in the body. Fat-free mass or lean body mass is the closest approximation available of the total respiring cell mass. The amount of fat-free mass or lean body tissue is the strongest predictor of an animal’s metabolic rate, followed by body surface area and body weight (BW). 4. and 5. As a pet’s lean body mass and body surface area increase, BMR and RFMR increase proportionately. Similarly, when an animal becomes overweight and experiences an increase in body fat and a decrease in the proportion of lean tissue to total BW, energy expenditure per unit BW decreases. 6
Voluntary Muscular Activity
Voluntary muscular activity is the most variable component of energy expenditure. Muscular activity contributes approximately 30% of the body’s total energy expenditure in moderately active individuals. The metabolic efficiency of performing physical work is invariable, but the total amount of energy expended is affected by both the duration and the intensity of the activity. In addition, the energy cost of any type of weight-bearing activity, such as walking or running, rises as BW increases. This effect is a direct result of the added energy necessary to move a greater body mass. Therefore the energy expenditure of a pet with a high activity level depends on the duration and intensity of the exercise and the size and weight of the animal.
Dietary Thermogenesis
Dietary thermogenesis, also called the specific dynamic effect of food or meal-induced thermogenesis, refers to the heat produced in response to and following the consumption of a meal. The ingestion of nutrients causes an obligatory increase in heat production by the body as a result of the metabolic costs of digestion, absorption, metabolism, and storage of nutrients. This heat is not useful to an animal that is living in a thermal neutral environment, but will contribute to the maintenance of body temperature when an animal is exposed to a cold environment. A series of studies showed that dietary thermogenesis occurs in two phases in dogs. The first is a rise in metabolic rate that occurs in response to the presence of food, called the cephalic phase; the second, postprandial phase, occurs for up to six hours after the consumption of a meal. 7. and 8. Together, the two phases of dietary thermogenesis represent approximately 10% of daily energy expenditure for dogs. However, the magnitude of this heat production is influenced by the caloric and nutrient composition of the diet and by the nutritional state of the animal. The number of meals fed each day also affects dietary thermogenesis, with an increase in the number of meals causing an increase in the total amount of heat produced each day (see p. 65). Because cats generally consume diets that are higher in protein than dogs and tend to consume multiple meals per day, dietary thermogenesis may account for slightly more than 10% of metabolizable energy (ME) in the cat. 9
Another type of heat production is called adaptive thermogenesis. This is an additional energy expenditure that is not accounted for by the obligatory and short-term thermogenesis of meal ingestion. Adaptive thermogenesis is manifested primarily as a change in the BMR in response to environmental stresses. These stresses include changes in ambient temperature, alterations in food intake, and emotional stress. For example, cold adaptation in small mammals has been shown to rely on increased heat production that is disassociated from any productive work and is separate from shivering thermogenesis. 10 This heat loss, referred to as nonshivering thermogenesis, also occurs in dogs that are exposed to cold environments. 11
Overconsumption affects thermogenesis in some animals. When energy intake increases above daily needs in rats, dietary thermogenesis increases above the normal levels necessary for the metabolism of food and maintenance of body temperature. 12 This increased energy loss is a result of less efficient use of food calories. In the long term, the amount of weight gained during the period of overeating is less than that normally expected from the increased caloric intake. This process may represent the body’s tendency to protect the status quo of energy balance during periods of overconsumption. However, although this process has been shown to occur in laboratory animals and some human subjects, dogs do not appear to show a similar increase in dietary thermogenesis in response to overeating. 13 Dietary thermogenesis during periods of overconsumption has not been studied in cats.
Factors Affecting Energy Expenditure
Various factors influence a pet’s total daily energy expenditure (Table 9-1). An individual animal’s BMR is affected by body composition, age, caloric intake, and hormonal status. The BMR component of energy expenditure naturally decreases as a pet ages, primarily as a result of a gradual loss of lean body tissue. Changes in BMR can also occur as a result of food restriction. When caloric intake is decreased, an initial decrease in BMR occurs because of hormonal influences. If caloric restriction continues, the loss of lean body tissue due to weight loss causes a persistent reduction in BMR. This decrease will not be corrected until normal levels of lean body tissue have been restored. Similarly, persistent overeating can lead to an increase in total energy expenditure. Part of this increase results from the increase in lean body tissue with weight gain and increased dietary thermogenesis. A pet’s reproductive status also affects resting metabolic rate (RMR). A study with cats reported that gonadectomized (neutered) male and female cats have lower estimated RMR values than those for intact cats of the same age. 14 These differences may be a factor in overweight conditions in pets (see Section 5, pp. 316-318).
| C omponent | F actors |
|---|---|
| Basal metabolic rate | Gender, reproductive status, hormonal status, autonomic nervous system function, body composition, body surface area, nutritional stage, age |
| Voluntary muscular activity | Weight-bearing activity, duration of exercise, intensity of exercise, size and weight of animal |
| Meal-induced thermogenesis | Caloric and nutrient composition of meal, nutritional state |
| Adaptive thermogenesis | Ambient temperature, alterations in food intake, emotional stress |
Changes in voluntary activity and exercise level can significantly affect energy expenditure in dogs and cats. Just like people, companion animals tend to become more sedentary as they age. This change is usually first observed when the pet reaches maturity. In many breeds and individuals, play behaviors do not persist strongly into adulthood, and the onset of maturity is accompanied by a decline in physical activity. Later in life, voluntary activity may decline further because of chronic disease, the onset of arthritis, or a decreased tolerance for exercise. These changes will be reflected in a decline in the pet’s total energy requirement. It follows that increasing a pet’s daily exercise will increase the energy requirement. A portion of the higher energy expenditure occurs because of the direct calorie-consuming benefit of exercise. However, just as important, the long-term, cumulative effects of exercise cause changes in BW and condition. Regular exercise results in a higher proportion of lean tissue to fat tissue in the pet’s body. The amount of exercise necessary to decrease body fat and maintain or increase lean body tissue is related to the duration and intensity of the physical activity. As discussed previously, an increase in lean body tissue increases BMR. Therefore voluntary activity not only directly burns energy; it also contributes to a higher percentage of lean body tissue and a higher BMR over the long term.
As pets age, regular exercise is important to maintain adequate energy expenditure and to help to maintain the body’s lean mass. Voluntary activity burns energy, increases lean body tissue, and results in a higher basal metabolic rate.
FOOD AND ENERGY INTAKE
The other half of the energy balance equation is energy intake. Food intake is regulated in all animals by a complex system involving both internal physiological controls and external cues. The internal signals and external stimuli that affect appetite, hunger, and satiety are presented in Box 9-1. A growing number of studies have investigated the internal signals that govern food intake in dogs and cats. Although much of the scientific knowledge regarding these signals has been collected primarily in laboratory animals, it can be used to provide insight into mechanisms that may be operating in other species.
BOX 9-1
| I nternal signals | E xternal stimuli |
|---|---|
| Gastric distention | Food availability |
| Physiological response to sight, sound, and smell of food | Timing and size of meals |
| Changes in plasma concentrations of specific nutrients, hormones, and peptides | Food composition and texture |
| Diet palatability |
Internal Controls of Food Intake
In all mammals, the natural state for the body is one of hunger. This state is held in check by the presence of food in the gastrointestinal tract; the digestion, absorption, and metabolism of nutrients; and the amount of nutrients stored in the body at any one time. The systems controlling food intake are complex, and include feedback to the brain from adipose and the gastrointestinal tract via hormonal and nervous system signals.
During a meal, food causes stomach distension and the immediate release of gastrointestinal hormones such as cholecystokinin (CCK) and glucagon-like peptide 1 (GLP-1), which signal fullness in the short term. 15 Physical distention of the stomach and the distal small intestine stimulates the vagus nerve and relays satiety information to the brain. 16 However, the presence of food in the stomach alone will not inhibit food intake until significant gastric distention occurs, so the relative importance of this mechanism in influencing meal size and meal termination, especially when consuming an energy-dense diet, is fairly minor. A second physiological control of food intake is the ileal brake. Under normal physiological conditions, undigested nutrients can reach the terminal small intestine and cause delayed gastric emptying and reduced intestinal tract motility, making the ileal brake a relatively important mechanism in food intake control. 17 Activation of the ileal brake reduces hunger and food intake in addition to influencing gastrointestinal motility and secretions.
In the stomach, gastric cells release one of the few known orexigenic hormones, ghrelin. Blood ghrelin peaks prior to meal initiation and the administration of ghrelin stimulates appetite and increases gastric emptying rate in dogs and cats. 18.19. and 20. In the proximal small intestine, I-cells in the duodenum and jejunum release CCK in response to the presence of fat and protein. CCK mediates gastric acid secretion, gastric emptying, gall bladder contraction, and pancreatic enzyme secretion in multiple species, including the dog and cat, where it also acts as a potent anorectic agent. 18. and 21. In dogs and cats, GLP-1 and peptide YY (PYY) both are released from L-cells in the ileum and colon and influence satiety in response to the presence of unabsorbed carbohydrates and fats. 22. and 23. GLP-1 increases insulin secretion and reduces pancreatic enzyme secretion. It also reduces gastric acid secretion, slows gastric emptying rate, and functions to stimulate the ileal brake, exerting an endocrine distal-to-proximal feedback in the gastrointestinal tract. 24 In contrast, PYY acts as a paracrine or neurocrine agent, as plasma levels do not reflect the local activity of PYY. 25
Other hormones influence the sensation of satiation and hunger over longer periods of time. These include leptin and insulin. Leptin is a product of the ob gene and is synthesized primarily by adipose tissue. Leptin signals the availability of energy stores to the hypothalamus and, when it is increased, reduces food intake and BW. Blood leptin concentrations do not change in response to meals, but are proportional to total body fat stores in dogs and cats. 26. and 27. Increased leptin concentrations also are associated with the diminished insulin sensitivity seen in overweight cats. 28
Insulin may be an important internal control signal for both appetite and satiety. The exogenous administration of this hormone stimulates hunger and increases food intake in human subjects. The mechanisms involved appear to be an insulin-induced decrease in the use of cellular glucose (glucoprivation) and severe hypoglycemia. Insulin may also act directly on the hypothalamus to mediate this effect. Studies with rats have shown that both insulin and the adrenal glucocorticoid corticosterone function synergistically with central neurotransmitter substances to stimulate eating. In human subjects, feelings of hunger are positively correlated with low levels of blood glucose. 29 Excess plasma glucose, however, does not depress food intake.
Insulin may also be involved in signaling satiety and the cessation of eating. It has been theorized that the size of the fat deposit in an animal’s body may be regulated by the concentration of insulin in the cerebrospinal fluid. The insulin levels in the cerebrospinal fluid increase and decrease proportionately as fat cells increase and decrease in size. These changes happen without the daily fluctuations that occur in plasma insulin levels. The insulin receptors of the cerebrospinal fluid, which are not accessible to the plasma insulin pool, appear to be involved in the regulation of food intake and total body adiposity. A study with rats demonstrated that when insulin was infused into the cerebrospinal fluid over a period of several weeks, food intake and BW decreased significantly. 30 On the other hand, when the spinal pool of insulin was experimentally decreased by the injection of insulin antibodies, food intake and BW both increased. These changes occurred independently of changes in plasma insulin concentration. Insulin levels in cerebrospinal fluid may modulate the brain’s response to other internal satiety signals, such as the release of gut peptides, and may be important in the long-term control of body fat stores.
The complex actions of the myriad of orexigenic and anorexic hormones are received and coordinated by the brain. Specifically, the hypothalamus is known to be involved in mediating both quantitative and qualitative changes in food intake. The arcuate nucleus in the hypothalamus plays a central role in mediating signals of energy storage and needs. The arcuate nucleus stimulates food intake through neuropeptide Y (NPY)-containing neurons and signals satiety through proopiomelanocortin (POMC) neurons. 31 Insulin and leptin inhibit NPY-containing neurons and stimulate POMC-containing neurons. PYY also inhibits NPY release in the brain. 16 In addition, several different neurotransmitter substances are believed to be involved in this process. 31 Stimulatory neurotransmitters include catecholamine, norepinephrine, and three classes of neuropeptides (opioids, pancreatic polypeptides, and galanin). Direct injections of these compounds into the hypothalamus of rats potentiate eating in both hungry and satiated animals. In addition, obesity as a result of overeating can be induced in laboratory animals by the chronic administration of norepinephrine. Although multiple sites of the brain and nervous system respond, the medial paraventricular nucleus is the area of the hypothalamus most sensitive to these neurotransmitters. Interestingly, there is evidence suggesting that these compounds affect specific nutrient selection by animals, rather than simply increasing total caloric intake. 31 Norepinephrine injection causes an increase in the consumption of carbohydrates, and the administration of opioids and galanin results in increased fat consumption.
Aberrations in any of the internal control systems for appetite, hunger, and satiety can result in pathological changes in food intake. For example, lesions involving the ventromedial center of the hypothalamus lead to overeating, but lesions of the lateral nucleus result in an inhibition of food intake. Endocrine imbalances such as insulinoma, hypopituitarism, hyperadrenocorticism, and possibly hypothyroidism may affect food intake. Any metabolic dysfunction that affects neurotransmitter substances or the gut peptides could also potentially result in changes in food intake.
Interestingly, the condition of obesity can further perturb appetite, hunger, and satiety signaling. Excess weight gain can elicit insulin resistance in cats and dogs. 27.28. and 32. Relative to normal-weight controls, obese humans have faster gastric emptying rates and lower postprandial PYY and GLP-1 responses. These abnormalities were ameliorated with weight loss and suggest weaker satiety signaling in obese individuals. 16 In addition, other physiological conditions, such as spaying and neutering, may impact internal controls of food intake. In a survey of dogs in the United Kingdom, neutered females and males were approximately twice as likely to be obese as their intact counterparts. 33 Neutering has been demonstrated to increase food intake, BW, and body fat in male and female cats, which can be mitigated almost entirely by the administration of estradiol. 34. and 35. Furthermore, intact female rats and ovariectomized female rats and male rats administered estrogen, were much more sensitive to the anorectic effects of leptin in the brain compared to male rats and ovariectomized female rats not supplemented with estrogen. 36 This suggests that sex hormones play a central role in regulating internal satiety signals.
External Controls of Food Intake
External controls of food intake include stimuli such as diet palatability, food composition and texture, and the timing and environment of meals. Exposure to highly palatable foods is considered an important environmental factor contributing to food overconsumption in humans, laboratory animals, and companion animals. 37 Studies with human subjects have demonstrated that the quantity of food consumed varies directly with its palatability, and palatability does not appear to increase with levels of food deprivation. In other words, if food is perceived to be very appealing, an individual tends to consume more of it, regardless of the initial level of hunger. Similarly, when rats are offered a highly palatable diet, they overeat and become obese. 38 This effect has been observed with high-fat diets, calorically dense diets, and “cafeteria” diets that provide a large variety of palatable food items. 39. and 40. It appears that the novelty of being presented with several different types of palatable foods can override normal satiety signals. 41 A similar practice that is not uncommon with companion animals is the feeding of a variety of table scraps and calorically dense treats. The persistent feeding of highly desirable and appealing foods to some dogs and cats may override the body’s natural tendency to balance energy intake and lead to the overconsumption of energy.
Dogs and cats have preferences for certain flavors and types of pet foods, and these preferences are influenced by a number of factors. For example, an early study reported that beef was a preferred type of meat for dogs, and that cooking the meat enhanced its attractiveness. 42 It was theorized that early experience with cooked meat, such as that present in commercial pet foods, was the cause of the development of a preference for cooked products. Dogs also have a strong preference for sucrose, while cats do not show a strong attraction to sucrose-sweetened foods or fluids. 43. and 44. Both dogs and cats prefer warm food to cold food, and palatability generally increases along with the fat content of the diet (although this increase in acceptance may be related to texture as well as taste). Many of the taste preferences of dogs and cats can be explained by the type of taste buds or “units” found on their tongues (see Chapter 7, pp. 46-47). 45 For example, both dogs and cats have a high proportion of taste buds that are sensitive to amino acid flavors. It is postulated that these provide them with the ability to distinguish among the different types of meats that may be found in a carnivorous diet.
Not surprisingly, most dogs prefer canned and semimoist pet food rather than dry; cooked rather than uncooked meat; beef over other meats; and warm food rather than cold. To a degree, palatability increases along with the fat content of the diet.
Palatability is an important diet characteristic that is heavily promoted in the marketing of commercial pet foods. In addition to the pet’s preferences, many pet owners select a pet food based on their own perceptions of the food’s appeal (see Section 3, pp. 177-180 for a complete discussion of palatability of commercial pet foods).
The timing and social setting of meals also influence eating behavior. Dogs and cats rapidly become conditioned to receiving their meals at a particular time of day. This conditioning manifests itself both behaviorally and physiologically. Pets generally become more active at mealtime, and gastric secretions and gastric motility increase in anticipation of eating. In addition, dogs tend to increase food intake when consuming food in the presence of other dogs in their social group (conspecifics). This process is called social facilitation. For example, some pet owners find that a dog fed free-choice without difficulty begins to overconsume and gain weight when another dog is added to the household. In most dogs, social facilitation causes a moderate increase in the dog’s interest in food and an increased rate of eating. However, for some, the increase in food intake that occurs in response to another animal’s presence can be extreme enough to singularly cause excessive food intake. In some situations, however, the addition of a new dog or cat to the home can inhibit food consumption in other pets. This can occur when agonistic relationships develop or when one pet is fearful of another.
There is also evidence that food choice in dogs can be influenced by the experience of conspecifics and by the behavior of their owner. For example, in a small pilot study, 12 pairs of dogs were matched according to body size and then randomly assigned to be either a demonstrator or an observer dog. 46 Demonstrator dogs were taken to another room and were offered a serving of dry dog food flavored with either dried basil or dried thyme. After the demonstrator had consumed at least 20 grams (g) of the food, the two dogs were reunited and allowed to socialize for 10 minutes. The observer dog was then removed and offered an equal amount of both of the flavored foods. Although all of the observer dogs sampled both foods, dogs showed a significant preference for the flavor that had been previously consumed by their paired demonstrator dog. Because all of the observer dogs sniffed the mouths and heads of their demonstrators, it was theorized that olfactory cues may be important for the social transmission of food preferences. Interestingly, another set of studies demonstrated that dogs were capable of performing correctly in quantity discrimination tasks and consistently selected a large quantity over a small quantity of palatable food. 47. and 48.
Recently, another form of social learning affecting food choice has also been described in dogs. An owner’s food preferences can influence the food choices that their dog makes. In one study, a group of 50 dogs was first tested for quantity discrimination and showed a significant preference for the larger quantities of food (1 piece vs. 8 pieces of kibble). 49 However, when the owner of the dog demonstrated a preference for the smaller quantity of food before allowing the dog to choose, dogs switched and began to choose the smaller quantity of food more frequently. In addition, when dogs were presented with two bowls containing identical quantities of food, and the owner showed an interest in one of the bowls, their dogs chose the preferred bowl 82% of the time. These results illustrate the importance of the dog’s social environment, and specifically the influence of an owner’s preferences, upon feeding behavior in dogs. Similar studies are needed that examine the influence of owner behavior and food choice on food preferences in cats.
A dog’s social environment can influence feeding behavior and food selection. For example, dogs often respond to the presence of other dogs by increasing their rate of eating and are also capable of learning food preferences from other dogs. Similarly, the owner’s behavior can influence a dog’s food choices!
The frequency with which meals are provided is another external factor that can affect food intake and energy needs of dogs and cats. Metabolically, increasing the number of meals per day while keeping total energy intake constant results in increased energy loss from dietary thermogenesis. In a study with adult dogs, a group that was fed four times per day increased oxygen consumption 30%, but a second group that was fed the same amount of food in just one meal daily exhibited only a 15% increase in oxygen consumption. 50 In contrast, the presence of food, particularly palatable food, is a potent external cue for meal ingestion and offering increased number of meals per day may lead to excess consumption in individuals that are highly sensitive to external cues. A study was conducted to compare the effects of free-choice feeding with portion-controlled feeding on the growth and development of growing puppies. 51 Puppies that had access to food throughout the day gained weight more rapidly and were heavier than puppies fed using the portion-controlled regimen. However, the two groups exhibited similar amounts of skeletal growth as measured by forelimb and body length. These results indicate that both groups were developing maximally, but the free-choice fed group was depositing more body fat than was the portion-controlled group. In addition to affecting growth, multiple feedings of a highly palatable food may lead to overconsumption and excess weight gain in adult dogs and cats. This tendency to overconsume may more than compensate for the increased energy loss from dietary thermogenesis.
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