By Rebecca Fleeman
As the sweet potato casserole gets passed down the table and you scoot your green beans over to make room, this Thanksgiving you may think “will I be able to finish all the food on my plate?”. By the end of the meal, you are absolutely stuffed, eating more than twice what you would normally eat. However, you continue conversing, thirty minutes go by, and suddenly, you start picking up another bread roll. How could you possibly have room? What is even more inexplicable is that the instant that someone mentions pumpkin pie, you are grabbing a clean plate and some whipped cream to inhale even more. Why are you still hungry and how do we make room for more food?
Cows have four stomachs; we humans have one. Yet, once a year at Thanksgiving, we convince ourselves that we have grown an extra three stomachs and can handle more food than the year before. This year, with Thanksgiving coming, we will go over:
1) How big is your stomach and can it stretch for the holidays?
2) Why do we sometimes feel hungry, even after a huge meal?
3) How long is a Thanksgiving meal in your stomach?
Stomach size and stretch
The stomach was one of the first organs to be studied with scientific stomach observations dating back to the 1500s1. The volume of the average empty human stomach is 45-75 mL, yet the stomach can expand during feeding to anywhere from 1.5 liters in lean adults, to 4 liters in those with binge eating disorders2–4. To visualize this, think of a flattened balloon sitting in your abdomen, then consider how big that balloon is when fully blown up! Your stomach can stretch to over five times its size because it is a smooth muscle tube5. Consider Joey Chestnut, who holds over 46 food eating records, including his record of eating 74 Nathan’s Famous hot dogs in 10 minutes. Figure 1 shows how much the human stomach can distend for competitive eaters who practice this stretching6.
Each time someone eats, the stomach returns to its original size after digestion4. It is chronic overeating that will expand the stomach’s maximum capacity. People who are overweight or obese, determined by a body mass index (BMI) of over 25, have a higher stomach capacity7. Likewise, food eating competitors like Joey Chestnut have larger stomachs due to their repeated stomach stretching practices. Until recently, there was no reliable way to noninvasively measure stomach capacity. However, in 2019, researchers began using computed tomography (CT) scans of the stomach with computational modeling to determine stomach capacity7. This research allows us to better understand the size and stretch capabilities of the stomach.
Hunger and satiety
Your hunger and satiety cues are controlled by the hormones ghrelin, leptin, and insulin (Figure 2). When your stomach is empty, it releases ghrelin into your circulation. Ghrelin finds its way to the brain and stimulates neurons in the hypothalamus that tell your brain “I’m hungry!”1,8. Subsequently, as soon as your stomach begins to fill with food, ghrelin stops being secreted and thus ends the appetite stimulus1. Leptin, a hormone secreted from fat cells, aids in ending the appetite stimulus by telling the brain “stop eating, I’m full!”1,8. A third key hormone important in hunger regulation is insulin, which is secreted by the pancreas. When we eat carbohydrates, insulin is released to take the glucose in the blood into the cells. Thus, high insulin levels tell your brain that you have enough food8, while low insulin levels tell your brain “Can I have some more carbs, please?”.
Why dessert changes the equation
While these hormones and their effects on hunger seem to make a simple story for when to eat and when to stop, other factors affect hunger including stress, conditioned behaviors, and the sight or smell of food. This last variable, seeing and smelling delicious thanksgiving dessert, is what allows you to override those messages from leptin and insulin. Seeing and smelling warm home-baked apple pie actually activates salivary glands in your mouth and acid secretion in your stomach1. These responses make your body think you are hungry again when that couldn’t be further from the truth!
Speed of digestion
Digestion is a complicated process, involving both mechanical and biochemical breakdown of food9. After you chew and swallow your turkey (or Tofurky!), food travels down the esophagus to the stomach. Here, that muscular tube of an organ contracts and relaxes to guide food through the five regions of the stomach, all while acids help break down the food1. On average, food journeys from the esophagus through the stomach in about 2-5 hours2,10. It then enters the small intestine, before traversing the colon and leaves the body after a full transit time of 18 hours to 5 days10.
Cutting-edge research has developed in the past decade to study digestion. Because measuring digestion in humans can be invasive and uncomfortable, scientists have designed human-sized in vitro digestive systems (Figure 3) that mimic the shape, contractions, and acid secretions of human digestion2,9,11! These bioengineering works of genius allow us to better understand the rate of digestion and test the effects of therapeutics2,9,11. Importantly, these systems avoid the effects of age, sex, disease state, and genetic makeup that can impact normal human digestion9. However, in vitro digestion models can help us test food factors that alter digestion including food volume, texture, density, and composition9. Essentially, the combination of all the foods you choose to consume for Thanksgiving will ultimately determine how long the meal(s) stay in your digestive tract. Foods that will leave your body quicker include water, juice, and raw fruits and vegetables, while foods that linger in your system include nuts, meats, and cheeses9.
Overall, your stomach is an amazing little organ, or big organ, depending on how much you eat! Human stomachs change in size more than most organs, likely only outcompeted by the uterus that grows over 500 times its normal size during pregnancy; so maybe there is a science to calling that post-meal bump your food baby! This Thanksgiving, when you dish the stuffing onto your plate, make sure to pass it to your neighbor with a few fun facts about how their stomach is adapting to their meal and how they will make room for dessert no matter how much they stuff in the stuffing!
TL:DR
Hunger and satiety are controlled by hormones signaling from your body to your brain. The sight or smell of food can override fullness cues, enticing you to eat even when you’re full!
References
1. Soybel, D. I. Anatomy and Physiology of the Stomach. Surg. Clin. North Am. 85, 875–894 (2005).
2. Wang, J. et al. An advanced near real dynamic in vitro human stomach system to study gastric digestion and emptying of beef stew and cooked rice. Food Funct. 10, 2914–2925 (2019).
3. Stomach Capacity and Intestinal Length. NutrientsReview (2016). Available at: https://www.nutrientsreview.com/articles/stomach-capacity-intestinal-length.html.
4. Geliebter, A. Gastric distension and gastric capacity in relation to food intake in humans. Physiol. Behav. 44, 665–668 (1988).
5. Hur, M.-S. Muscular Architecture of the Abdominal Part of the Esophagus and the Stomach. Clin. Anat. 33, 530–537 (2020).
6. Levine, M. S., Spencer, G., Alavi, A. & Metz, D. C. Competitive Speed Eating: Truth and Consequences. http://dx.doi.org/10.2214/AJR.07.2342 189, 681–686 (2012).
7. SH, K. et al. Stomach Volume Assessment Using Three-dimensional Computed Tomography Gastrography for Bariatric Treatment. Obes. Surg. 30, 401–406 (2020).
8. Zanchi, D. et al. The impact of gut hormones on the neural circuit of appetite and satiety: A systematic review. Neurosci. Biobehav. Rev. 80, 457–475 (2017).
9. Peng, Z. et al. Achieving realistic gastric emptying curve in an advanced dynamic in vitro human digestion system: experiences with cheese—a difficult to empty material. Food Funct. 12, 3965–3977 (2021).
10. Lee, Y. Y., Erdogan, A. & Rao, S. S. C. How to Assess Regional and Whole Gut Transit Time With Wireless Motility Capsule. J. Neurogastroenterol. Motil. 20, 265 (2014).
11. D, D. et al. Can dynamic in vitro digestion systems mimic the physiological reality? Crit. Rev. Food Sci. Nutr. 59, 1546–1562 (2019).
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