10% Human: How Your Body's Microbes Hold the Key to Health and Happiness

Our own cells, though far larger in volume and weight, are outnumbered ten to one by the cells of the microbes that live in and on us. These 100 trillion microbes – known as the microbiota – are mostly bacteria: microscopic beings made of just a single cell each. Alongside the bacteria are other microbes – viruses, fungi and archaea.

A story in which our lives are intertwined with those of our hitchhikers, where our microbes run our bodies, and becoming a healthy human is impossible without them.

Stool, far from being the remains of our food, is mostly bacteria, some dead, some alive. Around 75 per cent of the wet weight of faeces is bacteria; plant fibre makes up about 17 per cent.

Each of us contains communities of microbes as unique as our fingerprints.

Acquiring the genes to do this is quick and easy for microbes, as their generation times, and therefore opportunities for mutations and evolution, are often less than a day.

Metabolic rates do vary from person to person, but it is actually overweight people who have the faster metabolisms, not lean people. It simply takes more energy to run a big body than a small one.

It’s easy to think of the skin as the barrier between us and the outside world, but for every square centimetre of skin, you have two square metres of gut. Though it’s on the ‘inside’, the gut has just a single layer of cells between what’s essentially the outside world, and the blood.

After the small intestine has digested and absorbed as much as it can from what we’ve eaten, the leftovers move into the large intestine, where most of our microbes live. Here, they function like factory workers, each breaking down its own preferred molecules and absorbing what it can. The rest is left in a simple enough form for us to absorb throu

disruptions to the body’s microbes were behind gastrointestinal disorders, allergies, autoimmune diseases, and even obesity. And it wasn’t just physical health that could be affected, but mental health as well, from anxiety and depression to obsessive–compulsive disorder (OCD) and autism. Many of the illnesses we accept as part of life were not, it