“Eat your fruits and vegetables!” A common refrain from frustrated parents as they watch their kids disdainfully play with the peas and carrots on their plate. The emphasis on fruits and veggies is sound, based on a legion of studies demonstrating that populations with mostly plant-based diets are less prone to cardiovascular disease, diabetes, obesity and cancer.
Why this should be so is a matter of debate. Is it a matter of what is eaten or what is not eaten? Are the benefits due to some naturally occurring substances in plants, with antioxidants, vitamins and fibre usually being placed on the pedestal, or the negative effects of refined grains, sugar or meat? Or is it a combination of the two?
Dietary supplements containing the supposed beneficial substances in plants have been extensively hyped by marketers, but have in general failed to deliver the goods. However, in the last few years, another candidate purporting to solve some of the mysteries of the relationship between diet and health has appeared, stimulating vigorous research and prompting both reliable and overzealous media accounts. We are talking about our “microbiome.”
Our skin, mouth and, mostly, our digestive tract are hosts to somewhere between 30 and 50 trillion bacteria of hundreds of species, collectively referred to as the “microbiome.” Actually, there are somewhat more bacterial cells in our body than human cells, but luckily, our appearance is dictated by the human cells. Prevailing opinion had been that we all have roughly the same composition of bacterial species, and that aside from helping to break down dietary fibre during digestion, the bacteria do not interact with the body. Recent research indicates that this is far from being the case and that the specific variety of microbes we harbour may well be instrumental in shaping our risk for disease.
For example, microbiologists have found a significantly greater intestinal microbe diversity in young villagers in the African country of Burkina Faso than in Italian children. The Africans’ diet is based mostly on whole grains like millet and sorghum, while the Italians eat a western diet with refined grains and simple sugars. It turns out that the more diverse African microbiota produce twice the amount of short-chain fatty acids, with butyric being a prime example. These products of fibre digestion in the colon are small enough to diffuse through the mucus layer lining the intestine into the circulation and can calm low-grade inflammation, a condition that many researchers believe drives many of the chronic “western” diseases.
On the other hand, microbes can also stimulate inflammation if their hunger for fibre isn’t satisfied. They will then start feeding on the mucus lining of the gut, eroding it, and allowing bacterial breakdown products called endotoxins to enter the bloodstream. These are perceived as a threat by the immune system, triggering an inflammatory response aimed at eliminating them.
What is the take-away message here? That increased fibre intake leads to greater diversity of bacteria in our gut, which enhances short chain fatty acid production, and at the same time helps maintain the mucus lining of the gut, with the result being a reduction in what may be described as dangerous simmering inflammation.
The exact nature of the bacteria in our gut may even be important in determining the risk of other conditions, such as Parkinson’s disease. Parkinson’s is characterized by a lack of dopamine stimulation of nerve cells because the action of the neurotransmitter dopamine is impeded by the buildup of clumps of a protein known as alpha-synuclein, possibly initiated by a signal from gut bacteria. Mice bred to have germ-free intestines develop Parkinson’s-like symptoms upon transfer of gut microbes from Parkinson’s patients, but not with microbes introduced from healthy people. Further research may reveal which bacteria specifically are linked with Parkinson’s and whether they can be crowded out by the introduction of other bacteria, so-called “probiotics.” But yogurt is not going to do it. Neither will it prevent depression, despite the eye-catching headlines like “Feeling depressed? Eat yogurt rich in lactobacillus.”
The study that generated this seductive headline certainly did not demonstrate that eating yogurt can alleviate depression. What it showed was that stressed mice will struggle more vigorously in a tank of water if they are supplemented with Lactobacillus reuteri, bacteria that are used to make yogurt.
When a mouse is placed into a tank of water without any chance of escape it will at first vigorously swim around trying to find a way out, but will within a few minutes realize its hopeless state and sort of float, moving its legs just enough to maintain balance and keep its head above the water. The time it takes for “behavioural despair” to set in is said to be a measure of the animal’s mood. This is essentially based on the observation that treatment with antidepressants increases the time the animal struggles; in other words, it stays hopeful for a longer period.
In the study in question, researchers stressed mice by exposing them to noise, strobe lights, crowded housing and restraint in conical tubes. As if that weren’t enough, the cages were frequently tilted and the mice’s bedding wetted. These were stressed animals! And when they were subjected to the forced swimming test, they quickly gave up the struggle. This is interpreted as the mice being depressed. The gut bacteria of the “depressed” mice were then analyzed and found to have fewer lactobacilli than unstressed mice. Treating the stressed mice with these bacteria increased the time they actively struggled in the water, supposedly being more hopeful of escape, less “depressed.”
Never mind that humans are not giant mice. “Behavioural despair” in trying to escape from a water tank hardly equates to depression in people. Furthermore, the mice were treated with 2 billion colony-forming units (CFU) per day, which taking body weight into account, would require gallons of yogurt for people. You can see why I get depressed when I see headlines about treating depression with yogurt.
Joe Schwarcz is director of McGill University’s Office for Science & Society (mcgill.ca/oss). He hosts The Dr. Joe Show on CJAD Radio 800 AM every Sunday from 3 to 4 p.m.