DIGESTIVE SYSTEM

What we eat affects our whole body but the digestive system is the first point of contact and thus reacts very strongly to diet.

One of the clearest effects of diet on the digestive system is the composition of gut bacteria (gut microbiome). There are many different species of bacteria that can live in the intestines and diet strongly influences what species thrive and which are suppressed. High fat and high sugar diets are suspected of contributing to growing epidemics of chronic illness, including obesity and inflammatory bowel disease.

David et al. (2014) investigated the effect of two distinctly different diets on human gut microbiome. They assigned one group of study participants to a plant-based diet based on grains, pulses, fruits and vegetables; and the second group to an animal-based diet rich in meats, eggs and cheeses. The effects of the diet change after just five days were astonishing. The animal-based diet increased the abundance of bile-tolerant microorganisms that produce many harmful by-products and decreased the levels of good bacteria that feed on plant starches and fibre. The increase in the abundance and activity of the species Bilophila wadsworthia on the animal-based diet confirms the link between dietary fat, bile acids and changes in gut microbiome capable of triggering inflammatory bowel diseases.

Another recent study focused on reviewing available data on vegan, vegetarian and omnivore gut health and, in particular, the type of gut bacteria that these diets promote (Glick-Bauer and Yeh, 2014). The study discovered that the relationship between diet and gut bacteria follows a continuum, with vegan bacterial populations being the most distinct from those of omnivores. The vegan gut microbiome has the highest proportions of health beneficial and protective bacteria. This results in reduced levels of inflammation and may be the key feature linking the vegan diet to its multiple health benefits.

In a small study, six meat-eating obese people with type 2 diabetes and/or high blood pressure were assigned to a strict vegetarian, high-fibre, low-fat diet for one month (Kim et al., 2013). At the end of the month, the diet had not only achieved weight loss but just about every marker of previous bad health – cholesterol, fats and blood sugar - was improved. Two people’s results showed they could no longer be diagnosed as diabetics. The diet also positively altered the ratio of gut bacteria – it encouraged the beneficial bacteria and decreased the numbers of pathogenic bacteria, thus significantly decreasing the level of gut inflammation in the patients. 

Chronic gut inflammation is linked to many health issues such as inflammatory bowel disease, metabolic syndrome, immune system disorders and rheumatoid arthritis. It’s now clear that a high-fat, high-sugar Western style diet significantly contributes to this scenario and encourages bacteria that produce harmful substances that not only increase inflammation but some of them are also carcinogenic (Huang et al., 2013).

Inflammatory bowel diseases (IBD) are a general term for a group of diseases that affect the intestine. Two common ones are ulcerative colitis (UC) and Crohn’s Disease (CD). UC affects mostly the colon whilst CD can occur anywhere along the gastrointestinal tract. Both cause high levels of inflammation, inability to digest or tolerate many types of food and increased permeability of the gut wall compromising the immune system. Huang et al. (2013) reviewed the evidence and suggest that a Western diet high in fat and sugars promotes pathogenic bacteria to a degree that causes high inflammation followed by disruption of gut wall integrity in genetically susceptible people. On the other hand, fibre-rich food encourages the growth of bacteria that produce compounds that help lower cholesterol, decrease inflammation and may act as anticarcinogenic agents to protect against colon cancer.

Chiba et al. (2015) used a plant-based diet to treat patients with CD. Although their diet did allow some animal products, their research points at fibre and other plant compounds as the nutrients that promote gut health, supress inflammation and act as a substrate for ‘good’ bacteria. They recommend a plant-based diet for CD for all patients as an essential part of the treatment.

Vegetarian diets have also proved very useful in prevention of diverticular disease (Crowe et al., 2011). In diverticular disease, small bulges or pockets (diverticula) develop in the lining of the intestine. This together with muscle spasms along the diverticula can cause abdominal pain, bloating and other digestive problems. Diverticula can become inflamed or infected causing further issues. In this study, the participants’ diets and health were followed for an average of 11.6 years and the observed diet effect was considerable - vegetarians had a 31 per cent lower risk of diverticular disease compared with meat eaters. The association was very strong between high fibre intake and low risk of the disease - people with the highest fibre intakes (more than 26 grams a day) had a 41 per cent lower risk.

Research clearly points at plant-based foods as very beneficial for digestive health whilst it consistently shows that animal-based diets are detrimental. A vegan diet can significantly reduce intestinal inflammation in the general population which can help prevent or treat many conditions. IBD patients may need medication but the right diet can help them heal and stay in remission. 

Case study: Mark Gibson, UK

Late teens is an important time in social development when dating is the done thing but in my case irritable bowel syndrome (IBS) made me feel really uncomfortable and embarrassed in social settings. When I started work, I had to ask to be excused more than is reasonable. On a night out to the pub I'd constantly be running to the toilet for no reason but IBS. That was my life day in day out.

I became vegetarian at around the age of twenty for moral reasons. The situation remained the same with regards to IBS. I did not receive any medical advice to rectify the problem. My diet was varied - anything as long as it was vegetarian suitable. The constant discomfort made me ask myself what could be the cause? Was it the drink? No, IBS persisted even when I avoided alcohol. Could it be due to stress? I was prone to anxiety, but when I was completely contented in life IBS still persisted. Resignedly, it was something I just had to live with...

 ...until, at about age 28, I took the logical step for a vegetarian and became vegan. I hadn't reckoned on there being a bonus health benefit in store. However, when I became vegan the irritable bowel syndrome was cured, it stopped completely! There was no regime before as nothing seemed to improve the situation. I still don't have a regime except that I am vegan. I can only conclude therefore, that eliminating dairy from my diet solved the problem of my IBS. I am 37 now and since the day I became vegan approximately ten years ago I have not experienced an irritable bowel, which looking back is quite remarkable.


For references and more information, see The Incredible Vegan Health report

See our Nutrition News for latest studies on diet and health and Going Vegan section for help with the transition.