Your Gut Bacteria Doesn't Like Junk Food—Even If You Do - Blue Zones (2024)

by Tim Spector, Professor of Genetic Epidemiology, King’s College London

When Morgan Spurlock famously spent a month eating large portions of McDonalds for the purposes of his documentarySupersize Me, he gained weight, damaged his liver, and claimed to have suffered addictive withdrawal symptoms. This was popularly attributed to the toxic mix of carbs and fat plus the added chemicals and preservatives in junk foods. But could there be another explanation?

We may have forgotten others who really don’t enjoy fast food. These are the poor creatures that live in the dark in our guts. These are the hundred trillion microbes that outnumber our total human cells ten to one and digest our food, provide many vitamins and nutrients, and keep us healthy. Until recently we have viewed them as harmful – but those (like salmonella) are a tiny minority and most are essential for us.

Studies in lab micehave shownthat when fed an intensive, high-fat diet their microbes change dramatically and for the worse. This can be partly prevented by using probiotics; but there are obvious differences between us and lab mice, as well as our natural microbes.

Arecent studytook a group of Africans who ate a traditional local diet high in beans and vegetables and swapped their diet with a group of African Americans who ate a diet high in fat and animal proteins and low dietary fibre. The Africans fared worse on American-style food: their metabolism changed to a diabetic and unhealthy profile within just two weeks. The African Americans instead had lower markers for colon cancer risk. Tests of both groups showed very different microbiomes, the populations of microbes in their guts.

Home testing

Surprisingly, no one has specifically investigated the effect of junk food on Westerners from the perspective of the microbiome.

For the sake of science and research for my bookThe Diet Myth, I have been experimenting with several unusual diets and recorded their effects on my gut microbes. These include fasting, a colonoscopy diet, and an intensive unpasteurised French cheese diet. My son, Tom, a final-year student of genetics at the University of Aberystwyth, suggested an additional crucial experiment: to track the microbes as they changed from an average Western diet to an intensive fast food diet for over a week.

I wasn’t the ideal subject since I was no longer on an average diet, but Tom, who like most students enjoyed his fast food, was. So he agreed to be the guinea pig on the basis that I paid for all his meals and he could analyze and write up his results for his dissertation. The plan was to eat all his meals at the local McDonald’s for 10 days. He was able to eat either a Big Mac or chicken nuggets, plus fries and co*ke. For extra vitamins he was allowed beer and crisps in the evening. He would collect poo samples before, during and after his diet and send them to three different labs to check consistency.

Tom started in high spirits and many of his fellow students were jealous of his unlimited junk food budget. As he put it:

“I felt good for three days, then slowly went downhill. I became more lethargic, and by a week my friends thought I had gone a strange grey colour. The last few days were a real struggle. I felt really unwell, but definitely had no addictive withdrawal symptoms and when I finally finished, I rushed (uncharacteristically) to the shops to get some salad and fruit.”

While it was clear the intensive diet had made him feel temporarily unwell, we had to wait a few months for the results to arrive back. The results came from Cornell University in the U.S. and the crowdfundedBritish Gut Project, which allows people to get their microbiome tested with the results shared on the web for anyone to analyze. They all told the same story: Tom’s community of gut microbes (called a microbiome) had been devastated.

Tom’s gut had seen massive shifts in his common microbe groups for reasons that are still unclear. Firmicutes were replaced with bacteroidetes as the dominant type, while friendly bifidobacteria that suppress inflammation halved. However the clearest marker of an unhealthy gut is losing species diversity, and after just a few days Tom had lost an estimated 1,400 species – nearly 40 percent of his total. The changes persisted and even two weeks after the diet his microbes had not recovered. Loss of diversity is a universal signal of ill health in theguts of obeseanddiabetic peopleand triggers a range ofimmunity problemsin lab mice.

That junk food is bad for you is not news, but knowing that it decimates our gut microbes to such an extent and so quickly is worrying. Many people eat fast food on a regular basis and even if they don’t get fat from the calories, the body’s metabolism and immune system are suffering via the effects on the microbes.

We rely on our bacteria to produce much of our essential nutrients and vitamins while they rely on us eating plants and fruits to provide them with energy and to produce healthy chemicals which keep our immune system working normally.

We are unlikely to stop people eating fast food, but the devastating effects on our microbes and our long-term health could possibly be mitigated if we also eat foods which our microbes love like probiotics (miso, apple cider vinegar, sourdough bread, sauerkraut, yogurts), root vegetables, nuts, olives and high-fibre foods. What they seem to crave, above all else, is food diversity and a slice of gherkin on the burger just isn’t enough.

Tim Spector is a Professor of Genetic Epidemiology at Kings College, London & Director of the Department of Twin Research and Genetic Epidemiology at St Thomas’ Hospital, London. Professor Spector graduated from St Bartholomew’s Hospital Medical School, London. After working in General Medicine, he completed a MSc in Epidemiology, and his MD thesis at the University of London. He went on to found the UK Twins Registry of 11,000 twins in 1993, which is one of the largest collections of genotype and phenotype information on twins worldwide. Its breadth of research has expanded to cover a wide range of common complex traits many of which were previously thought to be mainly due to aging and environment. Excerpted and adapted from the original article posted onThe Conversation.

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