How your pet’s brain and body regulate his appetite

A properly-functioning appetite regulation system (or “appestat”) helps prevent obesity in dogs and cats by controlling feelings of hunger and satiety. However, dietary indiscretions and lack of exercise can affect this regulation, leading to overeating and weight gain.

Obesity is a major problem in both humans and their companion animals, and one of the contributors is a dysfunction in appetite regulation. (For more about obesity, turn to page xx.) If a pet or person feels hungry all the time, he can easily overeat and gain weight. The brain and central nervous system work to regulate appetite in dogs and cats (and people!), but lifestyle and dietary factors can affect this regulation.

How your dog or cat’s appetite is regulated

Appetite is controlled by feelings of hunger and satiety (the so-called “appestat”), and is regulated centrally through the hypothalamus of the forebrain, as well as the midbrain and brain stem.

1. Hypothalamus

The hypothalamus is the main regulatory effector organ of appetite, and controls the volume of food consumed. It communicates with other central nervous system (CNS) areas of the brain stem, as well as the reward-related limbic pathways of the midbrain.

It acts as a sensor via numerous hormones (more on these below), and as a biological clock that stimulates hunger. Hunger occurs when the body’s store of nutrients is depleted: short-term reservoir stores are carbohydrates while long-term reservoir stores are fat. Glucose regulates short-term control over appetite, and the lipostat (the part of the brain that controls how much weight the body should carry) controls long-term appetite through a cumulative effect over time. The hypothalamus interprets and integrates the input of neural and humoral (circulating) factors that result in the body’s coordinated feeding and energy expenditure responses.

The hypothalamus induces the secretion of adiponectin (a hormone that regulates insulin secretion and fatty acid oxidation), as well as leptin and ghrelin (the “stop” and “go” hunger hormones, respectively). Leptin comes from white adipose tissue cells and decreases appetite. Basically, these hormones act as signals for satiety, thereby regulating food intake. Leptin and another biomarker, serotonin, function in separate systems to control appetite. Serotonin (5-hydroxytryptamine or 5-HT), a biochemical marker of mood, is associated with several behavioral and psychological factors. It is also involved in the hypothalamic regulation of energy consumption and is influenced by energy conditions.

The more fat (adipose) cells that are in the body, the more they secrete pro-inflammatory cytokines that help regulate immunity and chronic systemic tissue inflammation. This induces cellular oxidative stress, which leads not only to obesity, but also to infections and even cancers.  Adiponectin and leptin are the fat-regulating hormones. The pro-inflammatory cytokines and bioactive peptides secreted from the adipose tissue are called adipokines.

2. Brain stem

The brain stem transfers information from the peripheral nervous system to the midbrain and forebrain. It directly connects with the gut via neuronal (nervous system) pathways

and regulates mechanical processes involving appetite and food intake, such as chewing and swallowing. The brain stem can organize certain aspects of feeding behavior without input from the hypothalamus. It is involved in the regulation of energy balance.

Together with the hypothalamus, the brain stem also controls carbohydrate metabolism. In one study, leptin concentrations were higher in a group of obese dogs than in a group of lean dogs. Adiponectin and cerebrospinal fluid 5-HT concentrations were higher in the lean group than in the obese group. Analysis of the gut microbiome also revealed that the diversity of microbes was lower in the obese group (see sidebar on page xx). Microbes from the phylum Firmicutes (85%) were the predominant group in the gut microbiota of the lean dogs.

3. Midbrain

The limbic structures within and around the midbrain support a variety of functions, including emotion, behavior, motivation, long-term memory, and sense of smell (olfaction). When it comes to appetite regulation, the midbrain reward system partakes in the control of “hedonic” feeding (intake of palatable foods), which can override the satiety signal.

Dogs, cats and humans are facing escalating problems associated with weight gain and obesity. Successful appetite control can depend on effective targeted conventional drugs and supplements — but these should always be used in conjunction with dietary modification (avoiding high-glycemic carbohydrates and “bad” fats) and lifestyle changes incorporating more exercise.

Contributing causes to dysfunctional appetite regulation

The desire to eat is regulated by an interaction between the digestive tract, adipose tissue and the brain. During stress, appetite levels — and therefore calories consumed — may increase, whereas dysregulation of appetite can also lead to overeating.

1. Lifestyle and lack of exercise

Two chief hormones affected by exercise also control appetite, but in different directions. Ghrelin stimulates appetite, causing hunger, while peptide YY suppresses appetite.

The sedentary lifestyle of most urban populations, and the pets that share their homes, has no doubt contributed to the reduced utilization of metabolic calorie intake. When coupled with a lack of regular exercise, caloric utilization is further stagnated.

Calorie restriction has been shown to increase lifespan by increasing the dietary activators of sirtuins, proteins that regulate metabolism and lifespan, such as resveratrol and other polyphenols (green and black tea, and grape seed extract).

2. Dietary issues and indiscretion

A transient “sugar high” results from foods with a high glycemic index, such as sugars, flour, rice, white potatoes, breads, and fruits like bananas and watermelon. This is followed by feelings of hunger, which promote a craving for more food.  Furthermore, well-intentioned people may share unsuitable foods with their “hungry” pets to stop them begging, such as bacon, burgers or fries, which just aggravates the situation.

3. The gut microbiome

Gut bacteria can affect how different foods are digested and produce chemicals that produce a feeling of satiety. People and pets that eat food high in fiber generally have lower weight, likely due to the role of gut bacteria in digesting fiber. These bacteria also digest certain antioxidants, known as flavonoids, found in plants; this helps prevent weight gain. Lastly, gut bacteria can influence how dietary fats are absorbed in the intestines, which can affect how body fat is stored.

Gut microbiota regulate the brain-gut axis. While the hypothalamus and brain stem are the primary central sites of appetite regulation, the gut microbiome can stimulate peripheral sensory neurons (cells that transmit nerve impulses). The vagas nerve is the major nervous system pathway involved and conveys information from the gastrointestinal contents to the braim: it also modulates gastrointestinal motility and feeding behavior.

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Dr. Jean Dodds received her veterinary degree in 1964 from the Ontario Veterinary College. In 1986, she established Hemopet, the first non-profit national blood bank program for animals. Today, Hemopet also runs Hemolife, an international veterinary specialty diagnostics service. Dr. Dodds has been a member of many committees on hematology, animal models of human disease and veterinary medicine. She received the Holistic Veterinarian of the Year Award from the AHVMA in 1994, has served two terms on the AHVMA’s Board of Directors, chairs their Communications Committee, and currently serves on the Board of the AHVMF, as well as its Research Grant and Editorial Committees.