“I am here to do more than eat and shit,” an irate Mao ZeDong shouted during his only meeting with Josef Stalin in Moscow, having been kept waiting for days. This was Stalin’s attempt to show him who was the real boss. Yet it transpires that he was far more interested in Mao’s inner workings than he let on.

According to recent reports, former Soviet agent Ivor Atamanenko claims Stalin had ordered Mao to be fed well during his ten days of closely supervised “hospitality”. Mao was also asked to use a special toilet, where his excrement was collected daily and sent to a secret lab for analysis.

The Russian scientists wanted to determine the state of mind of the Chinese leader by looking for chemicals in the excrement they believed were linked to certain behaviour and traits. It may sound completely crazy but the anecdote does raise a valid question. How much can you tell about someone’s mental state from the chemicals in their poo? Let’s take a look at the science.

Dubious research

The researchers, tasked with analysing stool samples from a number of foreign leaders, believed that high levels of brain chemicals such as the amino acid tryptophan were a sign of someone likely to be calm and easy to deal with. If levels were low, however, it would mean the opposite. The also believe a lack of potassium in poo was a sign that somebody was nervous and suffering from insomnia.

We don’t know Mao’s actual results or even how accurately they could measure the chemicals. Many fields of science in Russia during Stalin’s time were in many fields caught up in the dogmatic political struggles of the era.

Stalin had, for ideological reasons, banned classical genetics as it was based on what he regarded as tainted Western Darwinian views. Instead, he encouraged alternative theories – for example, his favourite scientist Trofim Lysenko used Lamarckism to argue that grain yields could be trebled simply by burying seeds in cold ground. He faked huge experiments for years until the truth became too embarrassing to tell. Millions starved and the Soviet union grain supply suffered terribly.

For that reason, it was quite unlikely that tryptophan could be measured accurately enough in 1949 to detect subtle changes. The trusted secret police fixer Lavrentiy Beria, who was in charge of the project, would probably have given Stalin the results he wanted to hear. Failure was not a good career move.

Gut microbes versus DNA

So was it all just pseudoscience? Actually, while potassium is unlikely to say much about our personalities, tryptophan is more useful. It comes from the proteins in our diet and is the source of key brain chemicals produced in our intestines. These include melatonin (responsible for regulating sleep and abnormal in many anxiety states) and serotonin (associated with a variety of mental conditions such as depression but also appetite). Having good levels of tryptophan in our stool samples is probably a sign of good health and ironically gives our waste product its nasty smell. However the interplay of these gut chemicals with the brain (our gut-brain axis) is much more complex than we imagined.

Serotonin has recently been linked to our gut contents in mysterious ways. The 100 trillion microbes in our colon produce at least a third of all our bodies’ chemicals and many vitamins. The gut microbes in our large intestine are responsible for maintaining most of our serotonin supplies, which influence our mood. Anxiety and stress in lab animals lead to changes in the numbers and types of gut microbes and alter the chemicals they produce. When pellets of poo are taken from these animals and transferred to the sterile guts of normal mice they become anxious and stressed. This means that anxiety can truly be infectious.

In humans, thousands of different microbe species can be now rapidly identified by DNA techniques just by swabbing a bit of toilet paper. Tests are showing that we all have a unique microbial fingerprint that consistently identifies us throughout life. On average we share less than 20% of our common microbes with other people compared to sharing 99.9% of our DNA.

Small human studies have shown major abnormalities in microbe populations with people with chronic pain, depression and autism compared to normal controls. While disruption to our microbes could be partly due to the stress of the disease it suggests they could also be contributing. Studies using probiotics to change microbes and the chemicals they produce to improve mental symptoms have been very successful in lab animals, and in a few human studies, like a recent pilot improving exam stress in Japanese medical students.

It may seem unlikely, but current analyses of thousands of poo samples from different populations are showing that even with our crude understanding, the ability to predict a common disease like obesity, diabetes, autoimmune disease or accelerated ageing from a stool examination is orders of magnitude better than with DNA testing.

Maybe Stalin’s experiment wasn’t so crazy after all – and if we had Mao’s poo sample today we could learn a lot more about him. World leaders should take greater care of their excrement – it could fall into the wrong hands.