The digestive system is an integrated series of body parts and organs that breaks down the complicated food you eat into the simple building blocks of protein, fats, carbohydrates, and micronutrients that maintain your energy reserves and nurture/repair/renew all the cells of your body.  

Since starch digestion starts in your mouth with the release of the enzyme ptyalin from your salivary glands, it is imperative to eat slowly and chew your food thoroughly. From the mouth, the food bolus moves through the tube-like esophagus into the stomach, and then the small intestine, where it is subjected to a variety of digestive enzymes along the way. The small intestine is the biggest part of the digestive tract and also where the absorption of most nutrients take place. The pancreas, liver and gall bladder are considered accessory organs of the digestive system since they release pancreatic digestive enzymes (amylase, lipase and trypsin), and bile (liver, gall bladder) into the small intestine to complete the digestion of carbohydrates, fat and proteins. After nutrient absorption in the small intestine, the remaining food residue moves into the large intestine where any remaining water and small electrolytes are removed/absorbed, and stool is formed, stored and eliminated from the rectum and anus. 

The human digestive tract is 25-30 feet long, from mouth to anus,  and is about 10-12 times larger than the length of the trunk of the body. This comparison is consistent with the relationship seen in herbivores, and quite different than the short digestive tracts seen in true carnivores, that are only 3-6 times larger than their trunk length. This observation is a pertinent piece of anatomical data suggesting that human primates are select herbivores/frugivores, and that a diversity of whole fresh fruits and vegetables are the most ideal human food. 

The importance of whole plant food for digestive health is also highlighted and emphasized when you consider the process of transit time: the time it takes for food to travel from your mouth to anus. Due to the length and coiled/circuitous architecture of the digestive tract, both fiber and water are two essential ingredients that will foster more effective movement of your food. A whole plant food dietary plan has the highest fiber and water content of any eating plan and promotes optimal digestive function. Standard American diets of excessive animal-derived, and ultra-processed foods are both dangerously low in water and fiber content. This interferes with smooth effective transit, and can lead to stasis, irritable bowels, inflammation, and putrefaction of ingested food components rather than efficient digestion. 

This importance of fiber becomes even more significant when you consider the vast, teeming world of microbes that live within us. This population of organisms (approximately 40-100 trillion) in the intestines is referred to as the gut microbiota (GM) and is on par, if not greater, than the entire number of body cells (approximately 30-40 trillion). Several pounds of these organisms live in our large intestine as the GM. Only whole plant food provides the insoluble fiber that is the direct and beneficial prebiotic food source for the beneficial microbes in the gut, and insoluble fiber that provides bulk and optimizes transit time and stool formation. (1)

Scientific research continues to shed light on the remarkable relationship of the GM to a wide variety of body activities including healthy digestive function. The diversity, activity, and biochemistry of the GM plays an important role in health and disease and affects overall nutrition, mood and mental health, cardiovascular health, the regulation of inflammation/function of the immune system, satiety/eating behavior, and even cancer.

Many of these effects are mediated by a network of 500 million nerve cells buried in the walls of the digestive system from mouth to anus, which controls every aspect of the digestive process from swallowing to enzymatic digestion, food transport/motility, and ultimately fecal elimination. (2) (3) This enteric nervous system (ENS) releases numerous neurotransmitters, including 50% of body dopamine, and 95% of body serotonin which also promotes gut motility, and  has been referred to as the “little brain” that communicates directly with the big brain via the vagus nerve.. The incredible communication between the GM, the ENS, and the brain has been popularized as the gut-brain connection.

Whole plant food supports a diversity of healthy bacteria in the GM, while animal foods, and processed/ultra-processed foods deficient in fiber decrease gut diversity, promote an abnormal population of microbes (dysbiosis) and promote damage and inflammation in the gut. (4) This is also an important reason why probiotic supplements are not the best approach for improving the probiotic GM that has been compromised by poor food choices, or toxic excesses of stress, alcohol, and antibiotics. These supplements only provide billions of organisms into an environment that already has trillions of organisms. It makes much more sense to maintain the food and lifestyle choices that minimize the destruction of the GM, and provide the prebiotic fiber nutrition that nurtures and supports an already vast probiotic environment. 

Importantly, the GM primarily ferments the insoluble fiber from plant food into short chain fatty acids (butyrate, acetate, propionate) that play a key role in supporting the integrity of the gut and optimal digestive function. Since the colon is primarily concerned with absorbing water from the food residue passing through it, the cells in the wall of the gut are fastened tightly together by special proteins that maintain a tight barrier and protect against absorbing larger molecules, and toxic/infectious agents. Short chain fatty acids from the fermentation of plant fiber promote the tight junction proteins that link colon cells together as well as promoting the construction and stability of  the mucous layer that protects the absorptive surface of the gut. 

A healthy GM supported by plant fiber also plays a major role in the digestibility and availability of key nutrients from whole plant foods. (5) Microbes from the health-promoting genera Lactobacillus and Bifidobacteria have an enzyme (myrosinase)that converts glucosinolates in cruciferous veggies (broccoli, Brussels sprouts, cabbage, kale, Bok choy etc. ) into  isothiocyanates like sulphoraphane that reduce inflammation and cell damage. The healthy GM also bioactivates and increases the availability of polyphenols from plant food, including isoflavones in soy products, catechins in green rea, and curcuminoids in turmeric, and tannins in berries, pomegranates, walnuts, and almonds to decrease inflammation/oxidative stress, and the risk of cardiovascular disease, obesity, osteoporosis and cancer. Take a moment to think about this remarkable symbiosis. When you eat whole plant food, the fiber in these foods supports the healthiest GM, that reciprocally produce metabolites and enzymes that make the most powerful plant nutrients available for your optimal health and function!

There are also a class of cells in the wall of the intestines referred to as enteric endocrine cells (EEC) that release hormones and peptides that play an important role in the regulation of digestive function, metabolism and appetite. Short chain fatty acids produced by the GM promote the release of glucagon-like peptide-1 (GLP-1) from the the EEC in lower part of the small intestine that stabilizes blood sugar, slows down food emptying from the stomach, and signal the brain to reduce hunger and cravings. This promotes both satiation and satiety, experienced as fullness at a meal with a lower volume of food intake , and fullness for longer periods between meals. 

You are probably aware of the onslaught of weight loss drugs that have been designed to mimic natural GLP-1 produced by the body. Unfortunately, these drugs have also been associated with adverse effects of thyroid medullary tumors, inflammation of the pancreas, compromise of the gall bladder, vomiting, dehydration and potential kidney dysfunction. Since your body produces this food and weight stabilizing peptide in response to high-fiber, low calorie- dense plant food and a healthy GM, it makes sense to adopt this dietary plan for optimal weight/body fat, and avoid the unnecessary risks of drug intervention.

Digestive health has a huge impact on your quality of life. Whole plant food provides the highest quality nutrients for optimal health and performance, and the best opportunity to support all the factors that affect the optimal function and activity of your digestive system,   

References

1. Holscher H. Dietary fiber and prebiotics and the gastrointestinal microbiota. Gut Microbes 2017 Feb 6;8(2):172-184.

2. Sharkey KA, and Mawe GM. The enteric nervous system. Physiol Rev, 2023 April1;103(2):1487-1564.

3. Agrawal H, et al. Enteric nervous system as a therapeutic target in gastrointestinal disorders. World J Gastrointest Pharmacol Ther. 2025 Dec 5;16(4):110843.

4. Tomova A, et al.The effects of vegetarian and vegan diets on the gut microbiota. Frontiers in Nutrirtion 2019 April;6(47). 

5. Hullar M, and Fu B. Diet, the gut microbiome and epigenetics. Cancer J. 2014 May-June;20(3):170-175.