Eating processed food is nothing new. Humans have been crushing grains to make bread for thousands of years. But in recent decades, our food supply has shifted, with an increasing number of ultra-processed products made with fillers, additives, stabilizers and synthetic ingredients that our grandparents wouldn't recognize.

A recent analysis by the Access to Nutrition Initiative finds about 70% of food products sold in the U.S. are unhealthy — and much of the food can be classified as ultra-processed.

So, are we the frogs in the boiling pot, adjusting to the shift toward ultra-processed foods without realizing these foods may promote excess consumption, which may harm us?

Amid the global boom in diet-related diseases, including obesity and Type 2 diabetes, Dr. Chris van Tulleken, the author of Ultra-Processed People, made himself a test subject for a brief, one-month experiment.

Van Tulleken, an infectious disease physician in his mid-40s, swapped his normal, healthy diet full of fruits, vegetables, lean proteins and whole grains for foods that mostly came from packages, boxes and bottles.

We spoke to van Tulleken about his book and his research.

This conversation has been edited for length and clarity.

How exactly did your diet change — what did you eat?

I started eating more snack food, so I'd have midmorning snacks. And I went back to eating the kind of cereal I loved as a child, so I would eat my chocolate-covered breakfast cereal. I drank more soda. And I made little swaps. For example, instead of eating nuts as a snack I'd eat chips. And then in the evening I had a lot more convenience food, so I was still eating a few vegetables, but I'd have microwavable lasagna or I'd have takeaway fried chicken or pizza.

Overall, about 80% of my calories came from ultra-processed foods, and that's very easy to do. Most bread in the supermarket is ultra-processed, almost all of our breakfast cereals and snack foods are ultra-processed, and most of our convenience foods are ultra-processed too.

Note: A classification system used by researchers categorizes foods into one of four categories, from unprocessed and minimally processed, to ultra-processed.

In your book you describe how you gained weight, which isn't too surprising given the foods you ate. You also measured what happened to your gut hormones. What did you find and how did you feel?

I became very unwell very quickly. I felt terrible. I stopped sleeping, I developed anxiety and became very unhappy. I was the pilot patient in a study I'm running with colleagues at University College London. We found there were effects on my gut hormones. So inside all of our bodies we've got hormones that tell us when to stop eating. They're very well evolved. All animals have them and ultra-processed food interferes with those hormones. So at the end of a meal, my hunger hormones would still be sky high.

This sounds very dramatic. Why do you think you were left wanting to eat more, when you'd eaten sufficient calories?

I think a lot of this food has been engineered to drive excess consumption. This food is energy dense. It's full of fat, salt and sugar. So you can consume calories at a much higher rate than when you're eating whole foods. Also, ultra-processed food is often processed into much smaller particles. So it may be being absorbed in a different part of the gut than the part that releases the fullness signal. So I suspect you're eating this food faster than your body's ability to send a signal to the brain saying, "I'm done now."

This fits with the evidence that foods full of refined carbohydrates such as bread, crackers and chips — which is basically what a lot of the ultra-processed foods are — can drive up blood sugar and insulin which may stimulate appetite. What's new here?

There is a very reputable scientist, Kevin Hall, in the United States. He ran a clinical trial and found that when people eat an ultra-processed food diet, they eat about 500 calories more per day, compared to people on a whole food diet, eating the same amounts of fat, salt, sugar and fiber. And there's a lot of epidemiological evidence that shows it is the ultra-processed food that interferes with our body's ability to say, "you know what, I can stop eating now."

This was a small study, 20 people. Are you investigating this further?

Yes, we were gathering data to get funding for a bigger study that we're now running.

Note: Van Tulleken and colleagues have been recruiting participants to investigate the effects of an ultra-processed versus minimally processed diet in the U.K. The researchers say it will be the longest diet trial of ultra-processed food and the first to help people reduce their consumption.

If ultra-processed foods had the same effect on everyone, wouldn't we expect to see the whole population gaining weight and becoming unhealthy or do people respond differently?

I think there are two groups of adults. Many people will be able to have a relationship with ultra-processed food that is somewhat like a relationship with alcohol. A lot of people can just enjoy two glasses of wine or a bottle of beer on a Friday night, and that's fine. They can have that relationship with it. But many people will recognize that actually their relationship with these food products is much more addicting in nature.

But as a society, can we really go back to whole food — or diets made up of minimally processed foods?

There isn't an absolutely clear boundary between traditional food processing and ultra-processed foods. We chop, cook, smoke, pickle, salt, grind, pulverize — we've been doing all that for millennia and we have to do it. So processing is fine. But ultra-processing is where food is made in factories. It's wrapped in plastic. It has strange additives in it that you don't find in kitchens. And the purpose of the food is profit. So, I eat cheese, but I don't eat processed cheese. I eat butter, but I don't eat margarine. I eat traditional flour bread, but I don't eat emulsified, supermarket bread.

There are multiple causes of obesity, and some of it is tied to food insecurity. People who have the least to spend on groceries often buy the most shelf-stable, processed food because it's affordable. Do you think governments should step in to regulate?

We need governments to start treating the products a little bit like tobacco. We need to limit the marketing of these products, and we need to change the labeling on the packets. The simplest thing is to do what Chile is doing. They put a black hexagon label on the packages of ultra-processed food. So governments shouldn't ban it, or tax the food, because it's the only affordable food for many people. But governments can start to warn people that it has negative health outcomes strongly associated with it.

Edited for broadcast and web by Jane Greenhalgh.