You are what you eat

P, my esteemed correspondent from New England, sent me a link to a recent review in Science: The gut microbiota at the intersection of diet and human health by Gentile and Weir of Colorado State U. It was right on message for one of my project students, who is currently writing her literature review on whether bacteria in the human GI tract are able to manufacture neurotransmitters. Reviews are handy because, if they are written well, they can be a tl;dr executive summary of a topic. But a good recent review can be a jump-station to a chunk [87 references here] of original research papers. Acronyms:

• MAC - microbiota accessible carbohydrate
• SCFA - short chain fatty acids
• NNS - non-nutritive sweeteners
• FODMAPs - fermentable oligosaccharides, disaccharides, monosaccharides & polyols

I think the key fact is that the human genome is finite: we have something like 25,000 protein coding genes - to do everything: enzymes, receptors, pores, channels, hormones. We are much more complex, genetically and biochemically than a fungus like the thrush yeast Candida albicans [6,000 genes] or a bacterium like Escherichia coli [4,000 genes] both of which can be found about our persons. But if we are lugging around thousands of different species of microbe (and we are) then their collective biochemical tool-box = ?10 million proteins? makes ours look paltry.

We do have an enzyme called sucrase which splits white-p'ison cane sugar into its constituent glucose and fructose. Every living thing we know about can get energy from glucose, some more efficiently than others. If a large fraction of the carbohydrate intake is sucrose, and it is, then a small fraction of the microbes streak ahead, over-grow and leave nothing for the slower, but biochemically more versatile, bacteria. That limits the amount of interesting compounds that we can harvest or which can encourage a diverse flora capable of withstanding the invasion of a wide variety of pathogens. It's like a metaphor for a healthy society: we all benefit if, rather than subsistence hunter-gatherers, there are pianists, blacksmiths, telephone-sanitisers, life-coaches, bankers, bakers, bus-drivers, PE teachers and rocket-scientists. If we all went out foraging like hunter-gatherers and came back with a variety of leaves, fruit and fungi for dinner then eating would be more interesting and minority bugs would be given a leg up in the struggle for existence in the dark-down-there.

We are subject to a lot of faddy nonsense recommending named diets with supposed but largely untested and aspirational benefits: Atkins, lactose-free, gluten-free, Ketogenic, Lutefisk, Veggie, Vegan, Palaeo, Mediterranean etc. One trouble is that you can make 'bread' without gluten but only with the help of food engineers who lurry in the emulsifiers, stablisers, guar-gum, anti-oxidants to make something that looks like the real thing. The emulsifier polysorbate-80 induced obesity and intestinal inflammation in experimental mice; which is a worry when you consider how widely used the stuff is in value-added packaged food.

Plant-based foods [aka 'food'] are the most likely source of MACs which will help keep your microbial flora a) diverse and b) stable. Diversity probably providing a buffer against the slings and arrows of outrageous incoming bacteria [aka 'disease']. But plants are also a rich source of their own weird metabolism. The same P who recommended this Gentile and Weir article spent a couple of years in Australia studying plant secondary compounds and their effect on fertility in kangaroos. In an attempt to deter herbivory some australian grasses produce phyto-estrogens which trick about with the fertility of female red kangaroos Macropus rufus. Actually, the 'roos have subverted this defensive chemical warfare and seem, after drought-break triggers a flush of new vegetation, to use a jolt of phyto-estrogens to start gestating their young. Move - counter-move: the dance of evolutionary imperative drives us all around the ballroom of life.

Exec summ: "Eat food, not too much, mostly plants" then you can largely forget about the detail of diet.