Factors that Impact Hunger, Thirst, & Satiety
Hunger & Satiety
Humans have a complicated relationship with food. Many of us know what foods health experts say we should be eating, but even with that knowledge, we still choose to eat foods that are detrimental our long term health. Understanding hunger and satiety is important for improving both your relationship with food and long term health. Appetite, also known as hunger, is influenced by many different factors including: brain chemistry, psychology, metabolism, texture of food, smell of food, taste of food, gut chemistry, and sight of food (1). In this post I want to introduce you to the different factors that impact hunger and satiety so you can not only listen to your body, but understand why it is sending you certain messages.
Metabolism, Brain Chemistry, and Gut Chemistry
Reviewing all of the chemical reactions that are happening in the body to regulate hunger and satiety in this blog post is not realistic, or a good use of your time. I will take the time, however, to mention a few of the key chemicals that regulate hunger and satiety in the body.
Ghrelin: Ghrelin is the only hormone produced in the gut that does not promote satiety (2). The main function of ghrelin is to drive appetite by creating the sensation of emptiness that we associate with hunger (1). It does this by stimulating 3 different areas of the brain: the hindbrain, hypothalamus, and mesolimbic centers. These areas of the brain stimulate unconscious processes, metabolism, and feelings of pleasure and satisfaction respectively (1).
Leptin: Leptin is a hormone that is produced by body fat and interferes with appetite stimulation. Leptin circulates the body in levels that are proportional to the amount of body fat an individual has, therefore an individual with more body fat should have less of an appetite than someone with lower body fat (3). In times of chronic energy surplus however, the body can become resistant to leptins signals (2).
Insulin: Insulin is a hormone secreted by the pancreas during digestion that stimulates satiety.
Cholecystokinin (CKK): CKK is a hormone produced in the upper small intestine during digestion. It delays gastric emptying which induces satiety and reduces food intake (4).
If there are all of these chemicals inducing satiety, then why do we overeat? Well, this is where smell, taste, sight, texture and psychology start to impact hunger. Food wasn’t always as easy to acquire as it is today. Therefore there was selective pressure to preserve traits that allowed humans to consume and metabolize essential nutrients needed for survival (5). As a result of this, there is an anticipatory response to food, known as the cephalic phase response, that starts a digestive and endocrine cascade that improves the efficiency of digestion and metabolism (5). Essentially, the cephalic phase is preparing your body to digest food so that you can consume and absorb more calories and nutrients in a shorter time frame. Cephalic phase responses can be learned, so culture, emotion, and social factors can be involved in their expression (5). The cephalic phase response is why your mouth waters when you think about Thanksgiving dinner, it is also why your mouth waters when you walk past a bakery and can smell freshly baked bread. Pleasant taste and texture when consuming food continue to create an even larger cephalic response than sight and smell alone (5). Taste is our last safety mechanism before swallowing a bite of food, if the food tastes safe, bitter tastes used to be associated with poisonous food, our body wants to consume as much of that food as possible. We evolved to be opportunist, not grazing animals, because food sources were often few and far between. So safe food high in energy, like fat and sugar, drive us to eat more even if we are starting to feel full (5). These foods often have a high glycemic score, meaning they quickly raise blood sugar in the body. Alternatively, foods with lower glycemic scores, often foods that are high in fiber and low in sugar, improve satiety after consumption.
Research has show that foods with more complex texture and flavor profiles enhances satiety and satisfaction after a meal. The way we chew, move our tongues, and the frequency that we chew all influence how we perceive the texture of the food (6) A recent study found that participants in the study who were given foods with more complex texture compared to the other group reported feeling full longer than the group given a less complex food (6). The same study found that complex flavor profiles also improved satiety over time (6). These studies are where mindfulness becomes an important factor in improving satiety after a meal. If you pay more attention to the textures and flavors of the food as you are chewing it, you are more likely to feel full after eating.
While this is only a brief overview of how hunger and satiety is signaled in the body, it is helpful to understand so you can be more in tune to what your body is telling you. For example, are you eating because a food triggers a specific memory, or because your body needs nutrients? Can slowing down and taking the time to chew and taste your food help you feel more full for longer? Being more in tune with your body and the hunger ques it is sending you can greatly improve your relationship with food.
Thirst
Thirst is a much more complex mechanism than just experiencing a dry mouth. Adult humans bodies are over 60% water, and consuming water is essential for our survival. So no wonder thirst is regulated in a complicated way!
The reason we feel thirsty is to maintain fluid homeostasis within our body (7). There are many different chemical reactions that happen within our body that drives us to take a sip of water. However, just like with hunger, there are also social, habitual, cultural, and environmental factors that impact thirst (7). When we are not drinking to maintain fluid homeostasis, it is called non-homeostatic regulation. Often we do this because we anticipate that we will need fluid later in the day. For example, we often drink during a meal even though we aren’t thirsty. We do this partially due to cultural reasons, but also because we know that after the meal we will need additional fluid to maintain homeostasis in the body.
I encourage you to start to pay attention to your thirst ques. Are you drinking because you are thirsty, or because you feel like you need to drink water to be healthy? Just like with hunger, it is important to listen to what your body is signalling to you, because often it knows more than you do!
Sources:
1. The Science of Appetite -The Science of Appetite- Printout - TIME. http://content.time.com/time/specials/2007/printout/0,29239,1626795_1627112_1626670,00.html#. Accessed February 11, 2019.
2. MacLean PS, Blundell JE, Mennella JA, Batterham RL. Biological control of appetite: A daunting complexity. Obesity. 2017;25(S1):S8-S16. doi:10.1002/oby.21771
3. Schwartz MW, Woods SC, Porte D Jr, Seeley RJ, Baskin DG. Central nervous system control of food intake. Nature. 2000;404(6778):661-671.
4. Physiology of cholecystokinin - UpToDate. https://www-uptodate-com.nunm.idm.oclc.org/contents/physiology-of-cholecystokinin#H6. Accessed February 12, 2019.
5. Anticipatory physiological regulation in feeding biology. https://www-ncbi-nlm-nih-gov.nunm.idm.oclc.org/pmc/articles/PMC2297467/. Accessed February 12, 2019.
6. Larsen DS, Tang J, Ferguson LR, James BJ. Increased textural complexity in food enhances satiation. Appetite. 2016;105:189-194. doi:10.1016/j.appet.2016.05.029
7. McKinley MJ, Johnson AK. The Physiological Regulation of Thirst and Fluid Intake. Physiology. 2004;19(1):1-6. doi:10.1152/nips.01470.2003