Yogurt is the stereotypical probiotic food. It’s mentioned in virtually all newspaper articles and blog posts that talk about probiotics and gut health, and is available to consumers all over the world. When the average Joe feels that his gut isn’t working properly, he goes to the store to buy some of this milk product – or another similar fermented dairy food that contains live bacteria. This isn’t surprising, as the media, many health bloggers, and the dairy industry have promoted yogurt as a healthy food that has the ability to heal a gut that isn’t working properly.
What these yogurt-lovers fail to recognize is that yogurt doesn’t provide the broad range of microorganisms needed to rebuild a damaged gut microbiota; it only contains a limited number of lactic acid bacteria, most of which are unable to colonize the human gut. Actually, some “dairy probiotics” may do more harm than good, in the sense that they could hinder the growth and survival of beneficial bacteria in the gut.
You can’t fix a damaged gut microbiota with a couple of strains of lactic acid bacteria
The human colon is home to a complex ecosystem of microorganisms. Just like all other organisms, these bugs need energy to survive and reproduce. Without energy, they wither and die.
They get this energy from us, the human hosts…
Most of the food you eat during a typical day are digested and absorbed in your small intestine. The rest passes into the large intestine, AKA the colon, where it either gets digested by bacteria or excreted.
The quantity of nutrients that escape digestion in your small intestine and is subjected to colonic fermentation depends on the composition of your diet. If you eat a lot of fibrous plants, the colonic microbiota gets to have a huge dinner, whereas if you eat an animal-based diet, low in fruits and vegetables, or a western-style, processed diet, the bugs deep in your gut don’t get to fully satisfy their appetite.
It’s important to remember these basic facts in the discussion of probiotics and gut microbiome repair, because they help guide our understanding of how the bacteria in the human gut operate.
There are several inherent problems with using fermented dairy foods (e.g., yogurt) and supplements containing “dairy probiotics” to repair an unhealthy gut microbiome.
First of all, these products only contain a handful of strains of lactobacillus and/or bifidobacterium. So, not surprisingly, they are incapable of installing a healthy ecosystem of hundreds of species of microorganisms in your gut. Second, most of the bacteria found in these products are incapable of colonizing the human large intestine. Rather than getting a green card and becoming permanent residents of the gut, these critters tend to be transient bypassers. Some probiotics may positively modulate the microbiota in various ways, but the effects tend to be temporary, rather than permanent.
Yogurt bacteria may be useful for treating lactose intolerance
To understand why yogurt bacteria come up short, we have to consider the issue from the perspective of an evolutionary biologist. The bacteria that live in yogurt are adapted to live in yogurt. They evolved to live in this fermented milk food; they didn’t evolve to live in the human colon. Yogurt bacteria possess the genetic and enzymatic machinery needed to utilize lactose, the primary sugar found in milk, as their primary energy source. They don’t produce the enzymes that are needed to break down the many complex polysaccharides and oligosaccharides that pass into the human large intestine.
In the guts of lactose tolerant individuals, lactose is digested by the brush-border enzyme lactase, produced in the small intestine. Lactose never reaches the large intestine, which is where the main microbial hub in the human body is found. In other words, the bacteria found in yogurt won’t have access to their regular staple food –lactose – in the colon of a lactose tolerant individual.
In the case of a lactose intolerant individual, however, the situation is somewhat different. Since the human host in this case is unable to break down lactose through the use of its own enzymes, the lactose that is consumed through the diet may become available to the colonic microbiota. What this means is that the bacteria found in yogurt and other fermented dairy foods can help the human host break down lactose, and hence, help alleviate symptoms of lactose intolerance (1, 2). Here, lactose acts as a prebiotic that fuels the growth of bacteria that grow on lactose.
Some probiotic supplements and industrially produced probiotic foods may do more harm than good
Microorganisms evolve at a rapid pace and are able to swap genetic material between themselves. The problem with the bacteria found in yogurt is that they don’t possess the genes/traits that people with a dysbiotic microbiota are lacking from their gut. They can contribute some genes that make the host better able to digest lactose, but they don’t do much in terms of repairing the ecosystem.
The fact that some studies indicate that “dairy probiotics” may be useful in the treatment of certain gastrointestinal diseases, depression, and other health disorders probably has more to do with the ability of these bugs to modulate the human immune system, carry out some of the same functions as a healthy gut microbiota as they pass through the system, and protect the human host against pathogens, than it has to do with their ability to repair the microbiota.
As I’ve discussed before here on the blog, there’s even evidence to suggest that some probiotic supplements (in particular high-potency, medical grade probiotics) and industrially produced fermented foods could undermine the health of the microbiota, in the sense that these products contain high numbers of just one or a couple of microbes, some of which produce bacteriocins (microbe-produced toxins) that inhibit the survival and growth of other microbes that are already present in the gut. Homemade, traditionally fermented foods can certainly be beneficial, but when it comes to most of the mass-produced probiotic foods and supplements you can find at the grocery store, I’m a lot more skeptical.
- Yogurt doesn’t provide the range of microorganisms needed to repair a damaged gut microbiota.
- The bacteria found in yogurt are adapted to utilize lactose and its constituent monomers, glucose and galactose, as their primary fuel source. They don’t produce the enzymes needed to facilitate the breakdown of the wide diversity of polysaccharides and oligosaccharides that pass into the large bowel of a human being.
- Some probiotic supplements and industrially produced, fermented dairy foods may do more harm than good, in the sense that they could block the development of a healthy gut microbiota.
- The bacteria found in yogurt can help speed up the recovery from acute gastrointestinal problems (e.g., diarrhea) and may act as a band-aid that covers up some of the symptoms of a damaged gut microbiota.