\u201cAny important disease whose causality is murky, and for which treatment is ineffectual, tends to be awash in significance.\u201d \u2014Susan Sontag<\/p>\n
There are many contributors to depression<\/a>, including trauma<\/a>, hormones<\/a>, genetics<\/a>, stress<\/a>, bereavement<\/a>, and more. To that lineup of usual suspects, we need to add gut microbes. The gut-brain axis<\/a> represents the amazing connection between our gut microbes and our mood. <\/p>\n The gut contains about 30 trillion microbes, roughly the same as the number of cells in our body. But there are thousands of different types of microbes, each with their own set of genes. When you add up all the genes from every microbe inside you, they outnumber your human genes by 100 to 1.<\/p>\n We are seriously outnumbered. <\/p>\n Fortunately, most gut microbes are on our side. These good microbes make a substance called butyrate that heals and nourishes our gut lining but also works in our brain to encourage the growth of new neurons, thereby improving our memory<\/a> and cognition<\/a>. <\/p>\n Microbes even have genes to make neurotransmitters like dopamine<\/a> and serotonin, two of the top targets for psychoactive drugs, helping to keep our minds calm and collected. <\/p>\n This is pretty stunning behavior for such tiny creatures, but it results from a bargain we made millions of years ago, when animals first rose out of the muck. To protect us from the omnipresent pathogens, we recruited our own set of microbes to fight them. In return, we provided them with a continuous buffet and a warm, wet environment to thrive and multiply. <\/p>\n The circles of life<\/p>\n There is a lot of circularity to our association with microbes. A happy brain dials down our natural immune response to microbes, and well-behaved microbes reassure our brain that things are okay. A healthy gut-brain axis thus creates a virtuous cycle. Showing correlations is easy. Proving causality is notoriously difficult. <\/p>\n In general, a healthy collection of microbes, called a microbiome<\/a>, is a source of wonder. It is an ecosystem that, like any other, has many working parts. Diversity is key to keeping any one creature from dominating. But sometimes, due to factors like diet<\/a> and stress, that diversity gets upended and bad characters take the throne. <\/p>\n We can directly feel a lot of things in our bodies, like a thorn prick or a stubbed toe, but gut issues are weird. They typically manifest as depression or anxiety<\/a>. Often, we don\u2019t realize the gut is even involved. Psychiatry<\/a> is just waking up to the idea that depression may be more about microbe issues than mother issues. <\/p>\n A more natural solution<\/p>\n A select few psychiatrists, like Ted Dinan of Cork, Ireland, have started treating depressed patients with probiotics. Dinan calls these psychobiotics: microbes that can improve mood. (1) But because of the circular associations between microbes and mood, it has been hard to celebrate the good guys and condemn the bad guys. Correlation is not causation, and most studies don\u2019t have the power to tease this apart. <\/p>\n It is gratifying to see a new study that shows just that: A large study of Germans with major depressive disorder (MDD) from the University of M\u00fcnster, led by Dominik Heider and Tim Hahn, has used a new statistical approach that can pull causality out of the data. <\/p>\n The work makes good use of insights from Judea Pearl on how to orient the arrows of causality, mostly by demonstrating that there is no \u201cbackdoor\u201d way to point them. This is statistical causality, not as good as experimental causality. But it is ethically dubious to feed people nasty microbes to see what happens. This may be as close as we can get to genuine causality.<\/p>\n Microbiome Essential Reads<\/p>\n The study looked at 1,269 MDD patients and controls and found two microbes that cause depression. The culprits are Eggerthella and Hungatella. This discovery doesn\u2019t mean other things can\u2019t also cause depression. In fact, they also showed that being overweight can cause MDD. It also doesn\u2019t mean these two microbes will always cause depression, but they are likely to do so if they become dominant in your gut.<\/p>\n This is big news for psychiatry. We\u2019ve seen thousands of vague associations, but this causal effect is a much stronger story for the psychobiotic principle. <\/p>\n What can we do with this knowledge?<\/p>\n Unfortunately, Eggerthella and Hungatella are not super-weird microbes with easy vulnerabilities. Like most microbes, they enjoy fiber, so simply loading up on fiber is unlikely to dislodge them. <\/p>\n However, these microbes may abandon their throne and become better citizens by encouraging the growth of friendlier microbes like Lactobacillus. These are more common gut denizens and help lower the pH, which discourages pathogens. Another friendly microbe is Bifidobacteria, which encourages butyrate-producing microbes like F. prausnitzii and C. butyricum and is also found in ferments. <\/p>\n That means consuming ferments like yogurt, kraut, kimchi, and kefir may be key. Exercise also helps to balance your microbiome. <\/p>\n