Yogurt's Effects on Intestinal Conditions

Summarized by Robert W. Griffith, MD
October 1, 2004

Introduction

The human intestines are far from sterile - they are, in fact, loaded with bacteria. Such bacteria may have good or harmful effects. Fortunately, harmful bacterial infections of the intestines, such as typhoid fever or bacillary dysentery, are rare. One of the 'good' actions that gut bacteria may carry out is to heighten resistance to colonization by 'bad' bacteria; we can call these actions anti-pathogenic and anti-inflammatory.

Some nutritionists recommend influencing the balance between good and bad gut bacteria by having people consume cultures of beneficial live organisms, which they call 'probiotics'. Yogurt is the best known food that contains probiotics, and its effect on intestinal function has now been reviewed in the American Journal of Clinical Nutrition. This is a summary of that review.

What's in yogurt?

Yogurt is a coagulated milk product resulting from the fermentation of lactic acid in milk by Lactobacillus bulgaricus and Streptococcus thermophilus. Other lactic acid bacteria (LAB) may be used to produce different characteristics of the final product, often bacteria that are normally found in the gut. The finished product must contain live LAB at a sufficient concentration so that the cultures remain active throughout the product's shelf life. In addition, of course, there are flavoring materials, carbohydrates, and other inert constituents.

Nutritional value of yogurt

The nutritional constituents of yogurt are derived from the milk used in making it, those that are synthesized by the LABs, and those that are added by the manufacturers. The nutritional value of the milk protein is well-preserved during the fermentation process. Some LABs synthesize folic acid; other LABs synthesize lactase, an enzyme that reduces the lactose content of the yogurt. Yogurt has a high content of conjugated linolenic acid, which has been reported to have immuno-stimulatory and anti-carcinogenic properties. And finally, yogurt is an excellent source of calcium and phosphorus; the acidic nature of yogurt 'ionizes' calcium thereby improving calcium uptake into the body.

Changes in the gut microflora

Bacteria of the Lactobacilli family bind to the inner surface of the intestines, preventing harmful bacteria getting into the mucosal cell layers. However, for the LABs in yogurt to exert this useful effect, they have to survive passage through the stomach, where an acid environment is likely to kill them. The amount of LABs that reach the upper gastrointestinal tract is therefore quite limited.

Immunity of the gut

While experiments in mice show that yogurt feeding increases the number of cells secreting IgA (a marker of immunity), and similar findings, along with an increase in cytokine production, have been reported in a few human studies, there is little evidence supporting the clinical relevance of such effects (see below).

A laxative effect?

Certain strains of LABs may decrease the colon transit time, but yogurt, in general, does not have a laxative effect.

Lactose intolerance

More than half the world's adult population, depending on ethnicity, is lactose-intolerant, due to a deficiency of lactase, the enzyme that breaks down lactose. When undigested lactose reaches the colon it's fermented by colonic bacteria, which causes excess gas and diarrhea. Impaired digestion of lactose can also occur with inflammatory diseases of the small intestine, such as Crohn's disease, celiac sprue, short bowel syndrome (after surgery), and bacterial or parasitic infections.

Lactose intolerance causes abdominal cramps, diarrhea, and wind after consumption of milk. People with this problem tolerate fermented milk products, like yogurt, better than unfermented milk products. This is most likely due to the lactase activity of LABs. Whatever the mechanism, it's generally accepted that yogurt has a beneficial effect in lactose intolerance.

Diarrhea

Treatment with Lactobacillus strains is a safe and effective way to treat acute infectious diarrhea in children caused by viruses and, possibly, by bacteria. LABs are also beneficial in antibiotic-induced diarrhea. Several possible mechanisms are suggested, but the real one is unknown.

Colon cancer

While some studies have suggested that colon cancer is less common in people who eat fermented dairy products, and animal experiments provide a number of encouraging results, there is no good clinical evidence that yogurt has a protective or curative effect against colon cancer, at this time.

Inflammatory bowel disease

These include Crohn's disease and ulcerative colitis, and in both diseases the microflora of the intestine plays a crucial role. It's thought that the proportions of different microflora are altered in patients with these diseases, leading to a weakened mucosal barrier to pathogenic bacteria. Human and animal experiments indicate that LABS can improve the outcome in models of inflammatory bowel disease, but actual clinical benefit in people with these diseases is lacking, to date.

H. pylori stomach infection

Infection with the bacterium H. pylori is now known to be linked with gastric (peptic) ulcer, chronic gastritis, and gastro-esophageal reflux disorder (GERD). LABs may accelerate the eradication of H. pylori, but there is no good clinical evidence available to date that shows they actually cure such an infection.

Gut allergies

LABs have a protective effect on allergic reactions in the gut, especially those due to cow milk protein. The way(s) in which this protection is provided is unclear; a number of mechanisms have been suggested. Good clinical trials have not been reported.

The safety of yogurt?

Very rarely, in someone with a severe underlying disease that has been treated with immunosupressant drugs and antibiotics, Lactobacillus can be found in the bloodstream. However, there are no reports to date that show a connection between LAB from fermented milk (i.e. yogurt) and infection in humans. LABs have a long history of safe use in foods, going back for centuries.

Obviously, the benefits of yogurt outweigh any risks they carry. Go ahead, enjoy!

Source

• Yogurt and gut function. O. Adolfsson, SN. Meydani, RM. Russell, Am J Clin Nutr, 2004, vol. 80, pp. 245--256

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