The Science of Hunger By Cristen Conger Cristen specialises in technology and the science of everyday living. Her articles have featured on Huffington Post, ABC Science and MSNBC.com. She also co-­‐hosts the popular Stuff Mom Never Told You podcast, which explores gender issues such as relationships, careers and women's history. Hunger and cravings are two different sensations. The body regulates the former, while the mind wields greater power over the latter. Hunger serves a more utilitarian purpose, signalling our brains that it's time to eat. To cross that distance between the stomach and brain, the vagus nerve is the communication highway that runs between the noggin and the abdomen. Our stomach hunger cycle, in a nutshell, begins with a hormone called ghrelin. When our bodies have burned up the food in our stomachs and our blood sugar and insulin levels begin to drop, ghrelin communicates with the hypothalamus in the brain. The hypothalamus, housed in the deep center portion of our brain cavity, regulates our basic body functions such as thirst, sleep and sex drive. Once it receives the message, delivered by ghrelin, that we need to eat something to keep our bodies running, the hypothalamus triggers the release of neuropeptide Y, which stimulates our appetites [source: Wright]. When we recognize that our body wants food and we start to refill it, another process happens to counter that hunger feeling in order to keep us from gorging ourselves. First, the fat tissues expel the hormone leptin. This chemical tells our brains that our bodies are satisfied and we can stop eating. It does so by turning down the production of neuropeptide Y and cranking up levels of proopiomelanocortin, an appetite suppressant, in our bloodstream [source: Wright]. The hypothalamus also monitors our insulin and blood sugar levels to ensure that we've eaten enough to bring those levels back up. Since this process doesn't happen instantaneously, we may feel uncomfortably stuffed after finishing a large meal. The Science of Cravings If we were to compare stomach hunger and mind hunger to different types of telephones, stomach hunger would be a one-­‐line analog phone, and mind hunger would take the form of a high-­‐end cell phone with camera, Internet and MP3-­‐player capabilities. Far more elaborate and complicated, mind hunger isn't entirely necessary for survival. In fact, scientists have largely debunked the theory that food The Science of Hunger © 2014 www.carrensmith.com 1 cravings are our bodies' ways of nagging us that we need a specific type of nutrient [source: Pressman and Clemens]. The dopamine stimulation and chemical reward that we obtain from fulfilling food cravings has been compared to drug addiction because both behaviours follow similar neural pathways. The pleasure and reward that we derive are simply milder versions of those that drug addicts experience when they get high. Neurologically speaking, that fast food joint isn't much different from a person selling drugs. Psychological factors can also influence the intensity and nature of our food hankerings. Studies on mood have found that our emotional state normally has a greater impact on cravings than hunger [source: Hill]. Diet influences our levels of the hormone serotonin, which regulates our disposition [source: Pressman and Clemens]. Hunger Hormones By Elaine Magee, MPH, RD If there were a hormone in your body whose chief job was to make you feel hungry, most of us probably wouldn't be too keen on it. (I don't know about you, but having a healthy appetite has never been a problem for me.) But if there were a hormone that decreased our appetites, we'd order buckets of it! Well, let me introduce you to some hormones that do just those things: the "hunger hormones," leptin and ghrelin. Leptin is a hormone, made by fat cells, that decreases your appetite. Ghrelin is a hormone that increases appetite, and also plays a role in body weight. Levels of leptin -­‐-­‐ the appetite suppressor -­‐-­‐ are lower when you're thin and higher when you're fat. But many obese people have built up a resistance to the appetite-­‐
suppressing effects of leptin, says obesity expert Mary Dallman, PhD, from University of California at San Francisco. Here's what we know so far about the "hunger hormones" and what we can do to help control our appetites. What We Know About Ghrelin Ghrelin, the appetite increaser, is released primarily in the stomach and is thought to signal hunger to the brain. You'd expect the body to increase ghrelin if a person is under eating and decrease it if he or she is overeating. Sure enough, ghrelin levels have been found to increase in children with anorexia nervosa and decrease in children who are obese. German researchers have suggested that ghrelin levels play a big role in The Science of Hunger © 2014 www.carrensmith.com 2 determining how quickly hunger comes back after we eat. Normally, ghrelin levels go up dramatically before you eat; this signals hunger. They then go down for about three hours after the meal. But some researchers believe that ghrelin is not as important in determining appetite as once thought. They think that its role in regulating body weight may actually be a more complex process. What We Know About Leptin Of the two hormones, leptin -­‐-­‐ the appetite suppressor -­‐-­‐ appears to be the bigger player in our bodies' energy balance. Some researchers think that leptin helps regulate ghrelin. Leptin helps signal the brain that the body has enough energy stores such as body fat. But many obese people don't respond to leptin's signals even though they have higher levels of leptin. In general, the more fat you have, the more leptin is in your blood. But the level varies depending on many factors, including when you last ate and your sleep patterns. A study showed that rats that were given doses of leptin ended up eating less, but this effect lasted only about two weeks. It seems that the rats developed a resistance to leptin's appetite-­‐cutting effects. How to Control Hunger Hormones Are there ways to control our "hunger hormones," and thus rein in our appetites? Possibly -­‐-­‐ by avoiding high-­‐fat processed foods. When we eat, messages go out to various parts of our bodies to tell us we've had enough. But when we eat fatty meals, this system doesn't work as well, says Dallman. Eating processed fat tends to lead to eating more calories, gaining weight, and storing fat, Dallman says. Researchers have seen some of these effects after only three days of a high-­‐fat diet. But researchers have shown that either a diet rich in either "good" carbohydrates (like whole grains) or a diet high in protein suppresses ghrelin more effectively than a diet high in fat. Something that might help (and certainly won't hurt) is to get enough sleep! In a study of 12 young men, sleep deprivation was associated with an increase in ghrelin levels, appetite, and hunger compared with when they slept 10 hours a night. All in all, this adds to the huge amount of evidence showing that avoiding a high-­‐fat, processed food diet is one of the keys to maintaining a healthy weight. The Science of Hunger © 2014 www.carrensmith.com 3