“Tell me what you eat, and I will tell you what you are.”
This phrase, coined by Jean Anthelme Brillat-Savarin in the The Physiology of Taste, was over a century ahead of its time.The commonly-held aphorism is true in more ways than one. In one respect, it means that the food you eat becomes a part of your person, and this has long been known — amino acids from digested proteins are incorporated into our own proteins, and the energetic sources from our diet (such as sugars or fatty acids) are added to our own stores of energy.In another respect, the quote can mean that the food you eat influences who or what kind of person you are. This interpretation is also true – the substances you consume can alter your brain chemistry, and thus behavior.Microorganisms in your diet have a fascinating route by which they can change the brain through our microbiome – the ecosystem of bacteria, archaea, protozoa, fungi, and viruses that live on and interact with our bodies. Each adult has about 1 kg of these microbes, which are highly diverse, containing approximately 100 times as many genes as the human genome.The diversity and composition of these microbes in the gut is strongly influenced by diet. For example, mice fed a plant based low-fat diet have a microbial profile that is completely altered by exposure to a high-sugar, high-fat ("Western") diet, which increases the proportion of several classes of bacteria, including Erysipelotrichi and Baccili.
How metabolism affects the brain
Microbes participate in metabolism, in part, by producing bile acids to help digest food and by synthesizing choline and short-chain fatty acids (SCFAs). Deficiencies in choline or SCFAs can cause fatty liver disease or even cirrhosis. Additionally, butyrate derived from the microbiome is necessary to regulate energy use in the colon.Many of the metabolites produced by microbes are also active in the nervous system, and the bacterium Bifidobacteria infantis can even act as an antidepressant through its regulation of kynurenine/tryptophan metabolism, similar to the actions of some antidepressant medications such as elective serotonin reuptake inhibitors (SSRIs), which seek to increase the concentration of synaptic serotonin.
