From Mouth to Gut: Understanding the Intricacies of Digestion
The digestion: Have you ever wondered how the food you eat is transformed into energy for your body? The answer lies in the intricate process of digestion. The process of digestion is necessary for our body to use the nutrients in food, The Latin term for digestion, "digerere," captures this process well. Digestion involves breaking down food into smaller molecules that can be transported and utilized by our cells. (eg. Proteins are broken down into amino acids, carbohydrates into glucose, and fats into fatty acids, allowing them to be transported and used by cells throughout the body). It's crucial to understand this process and the composition of food to maintain a healthy diet and provide our bodies with the necessary nutrients for optimal function. In this article, we will delve into the intricacies of digestion, starting from where it all begins - in our brain.
Food Composition: Food is composed of macronutrients like proteins, carbohydrates, and fats, as well as micronutrients like vitamins and minerals, phytonutrients and zoonutrients, and water. Additionally, food contains other substances like fiber that are not used directly by the body but are important for the health of the bacteria living in our intestines. Because food was once living organisms like plants and animals, it is chemically complex and must be broken down into smaller pieces that can be safely transported and used by the body. Understanding the composition of food is important for maintaining a healthy diet and ensuring that the body has the necessary nutrients for optimal function. How digestion works: Where digestion begins:
1- The brain and nervous system: The brain and nervous system play a crucial role in the process of digestion. Hunger activates systems in our brain that stimulate movement, curiosity, and the desire to find food. Our brain also prepares the rest of the body for eating by telling our mouth to start salivating and our stomach to produce digestive enzymes. The autonomic nervous system (ANS), which is responsible for regulating our internal organs, controls our digestion. The ANS has two branches: the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). The SNS shuts down digestion and appetite during intense activity, stimulation, or stress, while the PNS regulates digestion and movement through the GI tract during rest and relaxation. Digestion can also be influenced by internal and external stressors, including our thoughts, feelings, and environment. Understanding this relationship is important for you to maintain healthy digestion and eating habits. 2- Nose and Mouth: a) Smelling Food:
When we smell food, it tells our brain that food is coming, and it helps us decide whether or not we want to eat it. Smelling food also activates our digestive system, which is why it's important to take time to smell and appreciate our food before we eat it.
As we chew and swallow, odors from the food travel up the back of our throat into our nasal passages in a process called retronasal olfaction. This process is important for satiation or fullness because it lets us smell and taste the food better. Slow, mindful eating can help us feel more satisfied with less food.
b) Chewing and Salivating:
Once the food is in our mouth, our teeth and powerful jaw muscles grind it into smaller pieces while our saliva moistens and breaks it down with enzymes. Our tongue tastes the food and eventually pushes it back into our throat to swallow.
Teeth are uniquely adapted to each organism's diet. We have omnivore teeth that allow us to chew many types of foods. Our jaw muscles are among the strongest in our body, and chewing stimulates pleasurable neurotransmitters, which is why we may enjoy eating. The palates on the roof of our mouth separate the nasal passages and the mouth, protect the teeth, and help us create sounds.
Our tongue is covered with papillae, tiny bumps that increase the surface area of the tongue and help move food around the mouth. Most types of papillae contain taste buds, and our mouth is lined with a mucous membrane that helps us absorb many substances through it.
We have salivary glands under our tongue, along our bottom jaw, and in our cheeks, along the backs of our jaws, along with up to 1000 tiny minor salivary glands scattered through our mouth. Saliva is mostly water, but it also contains mucus, enzymes, glycoproteins, and antimicrobial chemicals that help prevent pathogens from getting into our system. Saliva moistens our mouths, keeps our mucous membranes healthy, and washes away harmful bacteria.
c) Swallowing:
Once the food mass is chewed, lubricated, and formed into a lump, it's known as a bolus. The bolus is passed to the pharynx, and the process of swallowing itself is actually involuntary. When swallowing, the epiglottis involuntarily closes to stop food from entering our trachea, and the uvula also closes off the nasopharynx, preventing food from entering the nose. These actions stop food from getting into our airway.
Overall, our nose and mouth play important roles in the digestion process. Smelling our food, chewing and salivating, and swallowing all work together to help us enjoy and digest our food properly.
3- Esophagus: The esophagus is a muscular tube that moves food and water from the mouth to the stomach, aided by mucous membranes and peristalsis, a series of muscular contractions. Its name comes from the Greek words for "to carry" and "to eat". The lower esophageal sphincter, or LES, is a ring-like muscle at the end of the esophagus that regulates the flow of food into the stomach and prevents the backflow of stomach acid, which can cause gastroesophageal reflux disease (GERD). While diet does not directly cause GERD, it can aggravate the condition. Symptoms of GERD include heartburn, chest pain, coughing, hoarseness, nausea, vomiting, bloating, burping, and tooth erosion. Clients diagnosed with GERD can benefit from a simple dietary action plan will write soon. 4- Stomach: The stomach is a muscular organ that plays an important role in digestion. It is a J-shaped pouch that is located just below the breastbone, on the left side of the body. The capacity of the stomach varies from person to person, but it can hold anywhere from 0.25 to 1.7 liters of food and liquid. When the stomach starts to fill up, stretch receptors send signals to the brain to indicate that it's time to stop eating.
The innermost layer of the stomach is called the gastric mucosa, which secretes a variety of substances collectively known as gastric juice. This juice contains enzymes such as pepsin and gastric lipase, which break down proteins and fats respectively. Parietal cells, another type of cell in the stomach, secrete hydrochloric acid, which is the main acid in the stomach. The acid is necessary for the breakdown of food and also helps to kill pathogens.
Intrinsic factor is another substance secreted by parietal cells. This glycoprotein is needed to absorb vitamin B12. As we age or develop certain gastrointestinal diseases, it becomes harder for us to produce intrinsic factors, which means that we may not get enough vitamin B12 from food or supplements. In such cases, we may need to take B12 supplements via injection or sublingually.
The gastric mucosa also secretes other substances such as histamine, which increases the production of gastric acid, and glucagon, a hormone that helps to release glucose into the bloodstream. Serotonin, a neurotransmitter, is also secreted by the gastric mucosa. The gastric mucosa is also lined with mucus, which helps protect it from the corrosive effects of hydrochloric acid.
After the food has been mixed with gastric juice and broken down into a liquid called chyme, it begins to empty slowly into the small intestine. The rate at which the stomach empties depends on the composition of the food. Carbohydrates empty first, followed by proteins. Fats and fiber take longer to empty. Liquids empty more quickly than solids, and small particles empty more quickly than large ones. 5- The Small Intestine:
The small intestine is a narrow, winding tube that is part of our digestive system. Once the chyme has made its way to the small intestine, it is moved along slowly by peristalsis. This means that it can take anywhere from 4 to 8 hours, or even longer, for the chyme to pass through the small intestine.
But this sluggish pace is actually beneficial for us as it allows our small intestine to absorb most of the nutrients that we've eaten. These nutrients include electrolytes like potassium and sodium, minerals such as iron, calcium, magnesium, and zinc, as well as carbohydrates, amino acids, fats, water-soluble vitamins (like vitamin C or the B vitamins), fat-soluble vitamins (like vitamin A or E), cholesterol, bile salts, and even some nutrients that we have yet to identify.
Apart from the nutrients, the small intestine also absorbs most oral drugs such as alcohol and other medications, which are dissolved and broken down into smaller molecules for easy absorption. This is an important process as it ensures that our body can make the most of what we've consumed. 6- Organs involved in this process:
As we continue on our journey of digestion, we can’t ignore the importance of other organs involved in this process, starting with the liver. The liver is the largest gland in the body, weighing between 3-4 lb (1.4-1.8 kg), and is roughly the size of an American football.
The liver plays a crucial role in metabolism. It helps in breaking down or building up molecules as needed. Additionally, it produces about one pint of bile per day which is a mixture of bile salts, bile pigments, and cholesterol. As we learned earlier, bile is essential in the emulsification of fats.
Furthermore, the liver screens most of the nutrients that come along with the food we consume. The liver plays a critical role in the metabolism of fats. It converts fatty acids to triglycerides, which are transported in the blood by lipoproteins such as HDL and VLDL. In the absence of carbohydrates, the liver can form ketone bodies as another fuel source.
When it comes to carbohydrates, the liver helps maintain glucose homeostasis. It stores glucose as glycogen in its own cells if the blood glucose is high. When blood glucose is low, the liver provides glucose to the blood from its previously stored glycogen. It can also convert other sugars, like fructose and galactose, into glucose.
Moreover, the liver can deaminate amino acids, converting them into glucose derivatives and nitrogen, which is excreted through the urine. Lastly, the liver filters out toxins, drugs, or hormones before allowing them into the bloodstream. This process is known as first-pass metabolism. That is why the liver is often damaged by oral drug overdoses or excessive intake of supplements, even if they are marketed as "healthy."
Let's take a closer look at the gallbladder and pancreas, two important organs involved in the digestive process.
The gallbladder is a small, pear-shaped sac located next to the small intestine. It plays a vital role in digestion by storing the bile that the liver produces. When food enters the small intestine, the gallbladder releases the stored bile through the common bile duct, which helps to emulsify fats and make them easier to digest.
Moving on to the pancreas is about the length of your hand and sits next to the small intestine. The pancreas releases pancreatic juices through the pancreatic duct into the duodenum, where it mixes with the bile. This mixture helps to neutralize stomach acid, making the environment more hospitable for enzymes to break down the food.
The pancreas is primarily composed of exocrine acinar cells, which secrete pancreatic juice containing digestive enzymes. These enzymes are released in an alkaline bicarbonate-based fluid and include trypsin and chymotrypsin, which help to digest proteins, amylase, which helps to digest polysaccharides, and lipase, which helps to digest fats.
Additionally, the pancreas contains about 2% endocrine cells, which produce several crucial hormones that help to regulate blood sugar levels. These hormones include insulin, glucagon, amylin, and somatostatin. The pancreas plays an essential role in the digestive process, and any dysfunction or damage to this organ can lead to serious health problems. 7- The Large Intestine (The Colon): As food enters the large intestine, also known as the colon, it moves slowly along at a pace of about 2 inches (5 cm) per hour and remains there for around 12-25 hours. Although most nutrients have been absorbed by the small intestine, the large intestine can still extract some valuable substances such as water, salts like sodium and potassium, vitamin K, acids such as acetate, butyrate, and propionate, and gases like hydrogen and carbon dioxide. In fact, the production of hydrogen in the large intestine can be a sign of a digestive disorder. The excess hydrogen can enter the bloodstream, travel to the lungs, and be exhaled, leading to a hydrogen breath test. Finally, what's left after all the absorption is feces, which is semi-solid waste.
Bacteria and their Role The large intestine is the most metabolically active organ in the body, thanks to the diverse species of bacteria that reside within it. Just as bacteria ferment sugars and starches in food to produce products such as sauerkraut or yogurt, intestinal bacteria ferment dietary carbohydrates that our small intestine cannot digest or absorb, such as fiber, oligosaccharides, lactose, and sugar alcohols. This fermentation process produces short-chain fatty acids and strong-scented gases that are beneficial to the body. Bacteria can also ferment other substances, including enzymes, dead cells, bacterial cells, and collagen and elastin found in food. Maintaining a healthy balance of bacteria can prevent harmful bacteria and yeast from colonizing the gut, remove carcinogens and boost the immune system, prevent allergy, regulate inflammation, and regulate our moods, nervous system function, hormonal responses, body fat levels, and prevent inflammatory bowel disease.
Transit and Excretion As food passes through the large intestine, it compacts into feces, which moves through the rectum and is excreted through the anus. Feces consist of dead bacteria, inorganic materials such as calcium and iron phosphate, fats, cholesterol, protein, sloughed-off cells, cellulose, digestive juices, and bile pigments that give feces their color. How fast food moves through the body depends on various factors, including stress, the amount and type of food consumed, and physical activity. A high-fiber diet and physical activity can accelerate this process, while stress typically slows it down. However, some individuals may experience the opposite effect due to stress, causing symptoms such as nausea, vomiting, or diarrhea. Supplementation with prebiotics and probiotics may be necessary for some individuals to maintain a healthy balance of gut bacteria, especially after taking antibiotics. Prebiotics provide the bacteria with dietary fiber, while probiotics are fermented foods like yogurt or sauerkraut, or supplements containing bacteria and friendly yeasts. In this article, we have discussed the process of digestion, the composition of food, and how digestion works, starting from the brain and nervous system to the nose and mouth, and down to the esophagus. The process of digestion is essential for our body to use the nutrients in food, and understanding the composition of food is important for maintaining a healthy diet. We have also learned how the brain and nervous system play a crucial role in digestion, and how the nose and mouth work together to smell, chew, salivate, and swallow our food. In the next article, we will discuss the next stages of digestion, including energy transformation in Powering the Body: Understanding the Three Energy Transfer Systems.