Hunger, Satiety, and Thirst: Part 1
Ever wonder what makes you hungry or makes you feel full? What makes you feel satisfied after a meal? Do you wonder what makes you feel thirsty? I know looking at or thinking about a large, beautiful burger from McMenamin's makes me hungry, especially if I haven't already eaten, but there is a reason the sensation of hunger appears. It doesn't exist in a vacuum. All the senses, metabolism, and psychology are involved.
The human body is designed to manage its food needs really well and thereby regulate body weight. Dr. Alexandra Logue, in her book The Psychology of Eating and Drinking, stated, as an example, that if someone ate 2% more calories than she needed every day, after one year that would lead to a 5 pound weight gain. That 2% calories is equivalent to a little extra pat of butter every day. Yet most people are able to maintain a stable body weight year to year. In light of the obesity epidemic, regulating body weight is clearly not as straightforward, but it is interesting to me that a little extra bit of butter could lead to an extra 5 pounds one year later (p12).
Studies have shown that a rumbling stomach is not what indicates someone is hungry. So what does signal hunger? Me thinking about a McMenamin's burger and consequently salivating over the thought of biting into a burger is similar to the reflex demonstrated by Pavlov's dogs--the ones who would salivate after hearing a bell. Not only is it a reflex for me to salivate after thinking about the burger, but it is also reflexive for my body to subsequently secrete insulin, a hormone made in the pancreas, into the blood stream. Insulin helps transport blood sugar (glucose) into cells for energy production, lowering the amount of blood sugar in the blood vessels. Blood sugar can lower over several hours too. When blood sugar is low, your body tells you it's time to eat (1 p14-15). Eating sweet foods increases the amount of insulin that is released, further increasing the amount of blood sugar lowering, increasing the amount of sweet food that is eaten to meet the deficit and stop feeling hungry (1 p17). This is why sometimes it's hard to eat just ONE cookie!
Another interesting example of the relationship between insulin and hunger is one provided in Logue's book. During Ramadan, Muslim people fast sunrise to sunset. During the day, the men (typically) are at work, and the women are at home making food for their children and food to consume after sunset. At the beginning of Ramadan, which is a month long, the study indicated that women reported more feelings of hunger than the men. It makes sense because the women have to smell and see the food they are preparing all day, so their bodies are busily secreting insulin in response. However, at the end of Ramadan, levels of hunger between men and women are the same. The relationship between being around food and insulin secretion (hunger) can be severed (1 p15).
Ambient temperature can impact hunger. If it's too hot in the home or outside, someone will feel less hungry. I know people who always lose a few pounds every summer, and it makes sense. It's hotter outside! I definitely don't feel inclined to eat big meals during the hotter months. The opposite tends to be true when it's cold. My Montana family makes jokes about putting on weight during the winter, much like the bears that roam our backyards, but that phenomenon really is based on biology. It's nature to eat more when it's cold, because our bodies like to stay a certain temperature in order to function (1 p15). I now wonder if it's good or bad that my husband and I keep our apartment cold during the winter!
Chewing food can influence the feeling of hunger. Chomping down on a piece of gum is an evidence-based way of decreasing both hunger and how much food is eaten when it is time to eat (1 p16). All the more reason to actually chew food well in between bites. Conversely, as pointed about by a naturopathic physician/professor, smoothies do not involve any chewing (2). So how do smoothies influence hunger? I would presume they do not dampen hunger. At a recent conference, there was a lot of talk from one speaker that smoothies and shakes are the best way to get extra calories in, especially for patients who are wasting away and have difficulty eating--like cancer patients (3).
Insulin isn't the only hormone in the body that impacts hunger. Ghrelin is an important hormone made by the gut that increases appetite. Interestingly enough, if someone isn't getting enough sleep, his body will release more ghrelin. This can easily lead to overeating, specifically sweeter, higher carbohydrate foods (4). As a student, I make sure to get sufficient sleep every night (no all night study sessions), so that I don't find myself turning to unhealthy foods the next day in an effort to feel more awake.
Neurotransmitters, chemicals made by the gut and the brain, impact hunger too. Dopamine and serotonin work together decrease hunger; and if they're low, then hunger will be increased (2,5). Dopamine typically inhibits the frequency of eating, while serotonin regulates how much is consumed (1 p28, 2). If one of those neurotransmitters is misregulated, then the other is likely misregulated too (5). Neuropeptide Y is another important chemical in the brain, and it can modulate the other neurotransmitters in the brain, increasing the drive to eat. Studies suggest that excessive neuropeptide Y can result in overeating and obesity (1 p28).
Lastly, eating is affected by mood and memories. If someone associates food with love, then she will eat when she needs to feel loved, not necessarily because her body needs the fuel (1 p31-32). So, I wonder if someone paused to ponder why she's eating a particular food, if not because she's hungry, then what feelings would surface that maybe driving the need to eat.
This was only an overview, but we can see that so much affects hunger! But I'm sure your next question is what makes us feel full? Check back next week for Hunger, Satiety, and Thirst: Part 2.
As usual, if you have concerns about your diet and eating habits, please consult with a licensed healthcare provider first.
1. Logue A. Chapter 2: Down the Hatch. In: The Psychology of Eating and Drinking. 4th Edition. New York, NY. Routledge; 2015: 11-32.
2. White W. Hunger, Satiety, Thirst. Psychology of Eating. January 2018.
3. MacOdrum M. Cancer Diets: Facts vs. Fads. Food as Medicine Symposium. February 2018.
4. Broussard JL, Kilkus JM, Delebecque F, et al. Elevated ghrelin predicts food intake during experimental sleep restriction. Obesity (Silver Spring). 2016;(24)1: 132-138.
5. Zwickey H. Neuroprotective Nutrition: Nutrition, Inflammation, and the Gut-Brain Axis. Food as Medicine Symposium. February 2018.