We are suffering a worldwide epidemic of obesity and accompanying heart disease and diabetes – incidence of obesity almost tripled between 1975 and 2016. The conventional explanation is simply over-eating carbohydrates and fats.

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But American nutritionist Kevin Hall has formulated a promising evidence-based hypothesis. Much obesity is caused by eating ultraprocessed foods that disrupt the signals between the gut and the brain that would otherwise tell us we have had enough to eat. The story is well told by Ellen Shell in a Scientific American issue of October 2019.

Unprocessed foods are the edible parts of plants and animals, eg seeds, leaves, tubers, meat, fish, eggs, etc. No salt, sugars or fats are added, and food nutrients remain in their natural proportions and combinations.

A processed food is a natural food that has been treated (processed) to enhance a particular quality, eg smoking meat to add flavour and extend storage life, adding sugar or salt to enhance flavour.

Ultraprocessed food is extensively processed food, even separating ingredients and reconstituting them into combinations/proportions far different from those in unprocessed food. For instance, in unprocessed food carbohydrate is usually packaged in fibre, but fibre is often missing in ultraprocessed food.

Ultraprocessed foods include potato crisps, sugary breakfast cereals, powdered instant soups, hotdogs, zero-calorie drinks, pizza, chicken nuggets, and so on. Shell reports that almost 60 per cent of calories consumed in America and 90 per cent of all added sugars come from ultraprocessed food – people prefer these foods.

The popular hypothesis that over-eating carbohydrates (sugars, starches and fibres) is the cause of obesity is underpinned by plausible theoretical reasons. But Kevin Hall’s carefully controlled laboratory experiments show no meaningful difference in body fat loss or calorie expenditure between subjects on very low carbohydrate diets and high carbohydrate diets.

Laboratory conditions

Again Hall carefully compared people on ultraprocessed diets (sugary cereals, egg substitutes, etc) with people on unprocessed diets (roast beef, eggs, vegetables etc) under controlled laboratory conditions. Participants on either diet were instructed to eat as much or as little as they liked. Participants ate one of these diets for two weeks and then switched to the other diet for two weeks. Those on the ultraprocessed diet ate about 500 extra calories per day than they did when eating the unprocessed diet, gaining two pounds in two weeks.

The gut sends messages to the brain via the vagus nerve, including messages about the amount of food calories entering the stomach and intestines. If these messages get scrambled the brain gets confused, and Harvard University neuroscientist Janet Small proposes that this is what happens when ultraprocessed foods are ingested.

Small explains by comparing a naturally sweet food such as honey and an ultraprocessed food sweetened with saccharin. Detection of sweetness in the gut, whether natural sugar or saccharin, signals the brain to get ready for the calories that are on the way. But in the case of saccharin the expected calories don’t arrive. The brain now thinks something is missing and encourages us to keep eating.

Also experiments with rats on junk-food diets indicated that the ultraprocessed food caused food cravings in those rats that became obese.

Throughout almost all our evolutionary history humans ate only unprocessed food, and evolution adapted people in different parts of the world to accommodate to locally available natural diets. These diets differ widely from each other in nutritive content – high-fat diets, high-carbohydrate diets, all-meat diets, all-plant diets and so on. For example the Masai of Africa subsist on meat, blood and milk, eating virtually no plant food.

Odd pizza

Of course, eating the odd pizza doesn’t cause problems, which occur only when ultraprocessed food constitutes a significant ongoing fraction of overall diet. And, as Shell points out, people differ in their reactions to ultraprocessed foods making it quite unlikely that these foods explain the full extent of modern obesity. However, the evidence cited by Shell strongly indicates ultraprocessed foods are one significant cause of the obesity epidemic.

The human body can accommodate over time to virtually any unprocessed whole food diet, but it cannot accommodate to the modern western diet, that is so high in processed foods. American food writer Michael Pollan explains this memorably in his book In Defence of Food (Penguin Books 2008). His simple mantra for a healthy diet is – “eat food [unprocessed], not too much, mostly plants”.

William Reville is an emeritus professor of biochemistry at UCC