The Neurobiology of Appetite
Metabolic set point
People alter the quantity and frequency of food consumption daily and yet the brain seems to have a regulatory process that allows people to maintain a relatively stable body weight.
Isn’t that crazy?
Anyone who has ever tried to diet knows all too well about this metabolic set point. There are staggeringly low rates of success with diet programs. A systematic review of studies published between 1931 and 1999 found that only 15% of patients achieved dietary success after 5 years. Most people who diet will slowly return to their preexisting weight within 1 year.
This metabolic set point appears to be controlled by our genetics. There is a strong correlation between the body mass of biological parents and adoptees in adoption-based studies. In the case of weight, genetics has far more influence than environmental factors.
Despite all this obesity rates in the United States as well as other developed countries continues to rise, so what gives?
Our genes have difficulty responding to the modern environment. 3000 years ago, when food sources were scarce, it was advantageous to consume and store as many calories as possible. However, in the modern world where there is no shortage of opportunity to consume calorie dense foods, our genetics are working against us. The weight issue is genetic but also influenced by availability of high-calorie delicious food.
When it comes to weight, energy in (food) must equal energy out (heat and work). The energy out is made up of the resting metabolic rate (calories burned when the body is stationary) and physical activity. The brain has a unique mechanism for managing the RMR. When more calories are consumed the RMR increases and when we diet the RMR is turned down.
To solidify the point, we can look no further than The Biggest Loser competition. Investigators assessed 14 of the 16 contestants before the competition, after completion of the 30-week program, and 6 years after the show. 13 of the 14 study participants regained weight and 4 were heavier than when they started the competition 6 years ago. The real downer was they all burned less calories at rest 6 years after the show ended. Despite exercising more and theoretically being much healthier their RMR decreased.
What are the important signals used by the body that indicate when to eat and when to stop eating?
Short-Term signels include:
Glucose: This is the primary nutrient that mediates satiety. Hypoglycemia will stimulate hunger and increase eating, while glucose infusions will decrease food intake.
Mechanoreceptors in the gut: The physical presence of food in the stomach activates these receptors due to stretching, the vagus nerve transmits signals of gastric stretch to the hindbrain to decrease eating.
Gut Hormones: The most well understood is cholecystokinin (CCK) which is released by endocrine cells in the small intestine. This will inhibit further food intake by stimulating the vagus nerve and decreasing gastric emptying. People have tried using CCK as a weight loss measure but all it does is decrease the size of meals but increases the frequency of eating thus producing a net zero effect on weight loss.
Ghrelin is the only gut hormone that stimulates hunger. Some suggest that decreased ghrelin produced by the stomach is the reason gastric bypass surgery is effective for weight loss.
It’s now known that adipose tissue releases a hormone that conveys information about energy stores. Leptin is produced by fat cells and increases or decreases based on the total amount of fat. Leptin is a hormone that tells the body to stop eating. In the case of obesity leptin levels are high and energy expenditure increases while food intake decreases. When someone goes on a diet and fat stores decrease leptin decreases resulting in decreased energy expenditure and increased food intake.
Two groups of neurons in the arcuate nucleus of the hypothalamus mediate the leptin signal, proopiomelanocortin (POMC) and neuropeptide Y (NPY). POMC stops eating and NPY increases food intake and decreases energy expenditure. In obesity there is increased leptin which inhibits NPY and activates POMC resulting in increased energy expenditure and decreased food intake. The opposite is true for the lean individual.
Eating and Pleasure
It’s well established that eating can result in pleasure, we have all had this experience after a stressful week a good meal can instantly change our mindset. The pleasure from food is likely an adaptation that enhanced survival when food sources were scarce. Increased dopamine in the nucleus accumbens and release of endogenous opioids appears to be more active when we are eating a meal we enjoy.