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You are what you eat: why humans can travel the world and eat anything

If you have ever wondered why humans have conquered planet earth’s most extreme environments and survived for millennia, the answer may be at hand. It looks like we had a little help from our bacterial friends.

In the human body, our cells are outnumbered one hundred to one by bugs. We have evolved in a mutually-beneficial relationship.  The bugs have enabled us to ferment foods such as proteins (the main constituent of meat) and carbohydrates (sugar, starch and fibre found in plants) just like cows do. In this tit-for-tat relationship, fermentation frees up vital nutrients in foods that we can’t digest by ourselves, and in return provides food for the bugs.

With the diversity of human habitats in mind, a paper in the scientific journal Nature took things to extremes. It asked what happened to the diversity of bugs in our guts when people were fed vegetarian-only or meat-only diets for two days. Researchers found the diversity of bugs, which included bacteria, fungi and viruses, changed rapidly in response to diet. The vegetarian diet produced more bugs that can ferment carbohydrates and the meat diet produced more bugs that ferment proteins. This provides evidence that humans as well as the bugs that live within us, can rapidly adapt to different nutritional conditions.

In addition to telling us about how rapidly we can adapt to different diets and environments, the researchers found that an exclusively carnivorous diet feeds the ‘wrong’ type of bacteria – those that produce harmful waste products shown to promote cancer and inflammatory bowel disease. This signals that we should eat meat in moderation.   

The good news is the effects of the extreme diets lasted only as long as the diets themselves. This goes some way to explaining why temporary diets are not effective in the long term.

So where do the bugs come from in the first place? Well, some of them there are probably lurking there within us all the time, lying low, waiting for a rainy day. Others hitch a ride with our food. Examples of hitch-hiking bugs are bacteria that ferment milk-based products such as yoghurt and those that ferment meat-based products such as sausages. They even found viruses which infect plants such as spinach can hitch a ride into our insides.

Remarkably, most of these bugs remain alive in our intestines and can be grown in the lab. However, most bugs cannot be coaxed into action under the bright lights of the lab. Researchers have developed a simple way to study these shy creatures.  One useful product of the Human Genome Project is that we have learned to sequence DNA and its protein-making cousin RNA rapidly and in huge quantities. This means that given a biological sample such as poo, a gene sequencer can tell us precisely how many and what quantities of any species of bacteria, virus of fungus it contains.

So what does this paper tell us about what we should eat? Because the study dealt with extremes, the answer is not that much. The likely recommendation, though, is to follow the usual guidelines of a balanced diet with plenty of fresh fruit and vegetables and most likely a little less meat than you would normally eat. And next time you eat a sausage or yoghurt remember the mostly good bugs they contain and how they have helped us, for better or worse, take over planet earth.

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