Many years ago, when I first started teaching about the chemistry of food, yogurt was only granted a few minutes of lecture time. It appeared as one of the first processed foods, albeit accidentally processed, dating back some 7000 years to when bacteria happened to drift into a pot of milk in Mesopotamia, thickening it and giving it a tart taste. I explained that the bacteria were likely of the Lactobacillus genus that produce lactate dehydrogenase, an enzyme that converts the milk sugar lactose into lactic acid. The acid then causes the proteins in the milk to precipitate causing the milk to thicken.
I then added an anecdote about how King Francis 1 of France, the king who dispatched Jacques Cartier to New France (Canada), suffered from chronic diarrhea and was supposedly cured by a doctor sent by his ally Suleiman the Magnificent who treated him with yogurt. I followed this with the story of how Nobel laureate Elie Metchnikoff had proposed that the long life of Bulgarian peasants was due to their regular consumption of yogurt. I capped off the discussion on yogurt with a critical comment that while Metchnikoff was undoubtedly deserving of the 1908 award for his discovery of phagocytosis, the process by which certain cells engulf and destroy microorganisms, he had no evidence for yogurt extending life.
Today, I devote considerably more lecture time to yogurt because this fermented milk product has been elevated to superfood status by vigorous advertising that promotes it as a source of 鈥減robiotics.鈥澛� By definition, probiotics are live microorganisms, usually bacteria or yeast, that provide health benefits when consumed. Some multiply and crowd out disease causing bacteria, others produce short-chain fatty acids such butyrate that reduce inappropriate immune response and prevent colon cancer cells from proliferating. Generally, the idea behind consuming probiotics is to ensure a healthy microbiome, the term for the thousands of bacterial species that populate our colon.
Disruption of this population, or 鈥渄ysbiosis,鈥� can occur due to the use of antibiotics, a poor diet, alcohol consumption, gastrointestinal infections by 鈥渂ad bacteria鈥�, exposure to environmental toxins, and the use of medications such as NSAIDS or laxatives. Symptoms of dysbiosis can manifest as inflammation, bloating, allergies, weight gain and a weakening of the gut barrier that results in toxins and bacteria entering the blood stream.
A healthy microbiome is obviously desirable, but the problem is that we do not really know what a healthy microbiome is. There are some 30 trillion bacterial cells in our gut made up of a couple of hundred genera, each of which is made up of hundreds of species and each species has hundreds of strains. Two people may have only 30-40% of species in common and within these the strains may differ. The microbiome is extremely complex!
Treating an infection with an antibiotic can result in diarrhea because beside killing the 鈥渂ad鈥� bacteria that are causing the disease, 鈥済ood鈥� bacteria are also dispatched. Diarrhea is due to the elimination of the dead bacteria. One way to try to restore the microbial population is by introducing probiotics. Yogurt is a common vehicle to attempt to do this. But yogurt contains only a few species of bacteria and only a few strains of each. Lactobacillus acidophilus is the most common species in yogurt and is 鈥済ood鈥� because it produces lactic acid that inhibits the multiplication of disease-causing bacteria and also has an anti-inflammatory effect. However, Lactobacillus isn鈥檛 well adapted to the human colon. It lives well in cold milk where there is lots of lactose, quite different from the warm conditions it encounters in the gut.
Trials of yogurt on post-antibiotic diarrhea have not produced compelling results. Some studies show a small reduction in diarrhea, others show no efficacy. This is not surprising because the trials have used different bacteria and different doses. Given the poor quality of evidence, no yogurt is allowed to claim that it can prevent antibiotic-related diarrhea. Vague claims such as 鈥渟upports digestive health鈥� and 鈥渃ontains billions of live probiotics鈥� are allowed. Previously, some products claimed that probiotics in yogurt can relieve irregularity and improve immune function, but these were removed after a successful class action lawsuit that alleged insufficient clinical evidence for such claims. A better bet seems to be to increase the intake of 鈥減rebiotics,鈥� substances that stimulate the growth or activity of beneficial gut bacteria. Chicory root, legumes, green bananas, oats, apples and whole grain wheat are all effective prebiotics.
There is certainly no harm in consuming yogurt regularly and there may be some good. At this point, though, it is not possible to come to a conclusion about which particular product is most likely to offer benefits. They contain different species of bacteria and we have no idea which strain of each species. One can peruse the scientific literature for any species mentioned on the label, be it Streptococcus thermophilus, Lactobacillus delbrueckii subsp., Lactobacillus bulgaricus, Lactobacillus acidophilus, Lacticaseibacillus rhamnosus, Enterococcus faecium or Bidifobacterium animalis, and find a clinical trial that has shown some benefit for osteoporosis, irritable bowel syndrome, inflammatory bowel disease, bloating, constipation, ulcer prevention, improved immune function, obesity, lactose intolerance, diabetes, cholesterol reduction and even enhanced cognitive function.
The fly in the ointment here is that the trials used different bacteria for different conditions and in doses generally larger than what is found in yogurt. Producers will sometimes point at a clinical trial that has shown a benefit for a probiotic in their yogurt, but when one checks the actual trial, it turns out that the bacteria were delivered in a much higher dose and yogurt was not the vehicle. Of commercial products, kefir generally contains the greatest variety of bacteria in significant amounts, so it is most likely to have some benefit if consumed regularly.
The number of bacteria in a product obviously matters, but there is also the question of whether they can actually make it through the stomach and small intestine to end up in the colon where they can become part of the microbiome. Numbers are usually expressed in terms of colony forming units (CFU) and should be in the billions per serving. However, that number is only 鈥済uaranteed at the time of manufacture鈥� and may be considerably less after time spent on the shelf. Then there is also the question of how the bacteria are prevented from being destroyed in the acid environment of the stomach. 鈥淓nteric coating鈥� means that the capsules that contain the bacteria have a coating that resists acids and then dissolves in the alkaline environment of the small intestine. 鈥淢icroencapsulation鈥� is superior because the bacterial cells themselves are coated with a protective polymer like gelatin or chitosan that ensures passage into the colon.
Research in probiotics is exploding so it is possible that one day a yogurt will be able to claim that it contains a specific blend of bacteria in the appropriate dose to treat some condition. Until that day, I鈥檒l have my daily dose of kefir.
