The idea was hatched on a family holiday in Brazil, two Christmases ago. My father was researching microbes in the gut, which make up as much as three pounds of our body weight and play a big role in our health, from diabetes to obesity.

He was wondering about testing all the family. It was perfect timing. I was thinking what project I was taking on in my final-year dissertation for my degree in genetics at Aberystwyth, and there wasn't a lot of research into how much a specific diet could change gut microbiodiversity. So I suggested that I did a study, with me as a guinea pig, which my father, Professor Tim Spector, could then draw on for his research for the British Gut Project.

Our family is very medical. My father started the Twin Research Unit at King's College London and my mother is a dermatologist with an interest in genetics. My sister is the one who got away. She is a paralegal in London. But genetics have always been the staple conversation around our kitchen table, even when I was young.

When the initial findings of the project were published last weekend, a lot of people asked: "Did you do the study for your father?" But it really wasn't a John Gummer moment, a dad forcing his kid to eat burgers. It helped both of us, in fact. I was really excited to do it. And he was paying for the burgers.

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We chose McDonald's because it is a classic reference to fast food. We'd both seen the film Super Size Me, in which director Morgan Spurlock eats nothing but McDonald's food for a month. I am glad I just had to do 10 days.

My regular diet is not particularly healthy. It's a poor student diet. Toast for breakfast, sandwiches for lunch, then supper is often pizzas. Students are big fans of Iceland.

Before the start back in January 2014, I gave myself strict rules. I could have a Big Mac meal or a chicken nugget meal, both with fries and a full sugar Coke. I didn't have anything for breakfast, but I allowed myself beer and crisps in the evening. No fruit juices, though. That would be cheating as they'd affect the gut microbes.

At first it was easy. I always went to the same branch of McDonald's. I got quite friendly with them at the drive-through. "Oh, it's you again," they would say. I had to explain to them that it was part of my dissertation, I didn't just have a twice-a-day McDonald's habit.

By about day three or four it started to get harder, with the tedium of the same food. But around the sixth to the seventh day I started to have some real problems. I was feeling tired and lethargic, and I had trouble sleeping. I like to think I have a good metabolism, but I felt my body was having a hard time processing the sugar and fat.

By the sixth day I was having a food hangover for two hours after each meal and I would have to sit down as I felt quite sick. I really hit the depths. I didn't have a girlfriend at the time, but if I had, I wouldn't have been in the nicest of spirits. But I wasn't bad-tempered. Luckily, I'm not a moody chap.

During the last few days, I really started to lose my appetite. On the eighth day I had a McDonald's for lunch and it was horrible. Come supper time I was hungry, but I was aware how bad this meal was going to make me feel, so I just didn't eat.

At first, my friends were quite supportive. They thought the idea was interesting and far-fetched, a mini Super Size Me diet. But by the eight or ninth day they started saying things like: "Are your liver functions are intact? You look jaundiced. You should really consider stopping."

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Still, by then there were only a couple of days to go, so I felt I couldn't give up, even though I could almost hear my poor pancreas going: "No more! I don't have enough insulin to deal with all this sugar."

Then there was a disaster. I suddenly got very constipated for two days. No big deal, you might think, but I needed to provide a stool sample for the study. That added a whole extra layer of stress. Fortunately, it all worked out - literally - in the end.

Straight after the experiment, I drove to the supermarket and got two big bags of salad. I ate them all. I was over the moon. And the test results were fascinating. I'd lost 1,400 bacterial species in my gut in just 10 days, which was extraordinary. After a week back on my normal student diet I'd recovered a bit but not completely. I still don't know if I've restored the diversity of species to my gut.

I was the first person to look at the gut microbiome change on a McDonald's diet, but I'm just one person. In terms of scientific reliability, that doesn't score well. But we sent samples to three different laboratories, which were tested in different ways, and my results did show really interesting trends across all three.

The experiment has definitely made me think about the bad food that I am eating. I used to work as a commis chef, so I can cook. I'm eating a lot more fruit and vegetables. Plums are my favourites at the moment. It's just hard work eating healthily, although I did lose the 4lb I put on over the 10-day study. But my diet still involves a lot of pizza.

Amazingly, I have had a McDonald's since then, but not a Big Mac. Recently I was filmed for a documentary called It Takes Guts, and they brought a load of Big Macs around for some of the shots. Let's just say it did not bring back fond memories.

Interview by Xanthe Clay

A weighty problem

According to Professor Tim Spector of King's College London, highly processed junk food may kill good gut bacteria that help to keep us slim.

Spector believes that the microbes that live in our intestine affect everything from our mood to our weight control, and that this may explain why some people seemingly never put on weight, while others can't keep it off.

For the study conducted by his son Tom, samples were taken before and after he completed 10 days eating exclusively junk food.

He started with 3,500 different species, but after 10 days that had fallen by more than a third, leaving him with an imbalance dominated by one type, bacteriodetes.

Prof Spector says that while we need to cut down on wheat, soya, meat, maize and sugar, the key is to exclude nothing, eat everything. "Treat your gut like a garden, that you nourish, fertilise and sprinkle with seeds," he says.

Even a burger once or twice a week is okay, as long as you have very healthy food in between. "Our microbes are quite flexible," he adds.