Bad diet causes cognitive decline: Fact or myth?

More than 400-500 calories per day of ultra-processed foods such as burgers, chips and packaged cookies is enough to increase the risk of cognitive decline, a new study suggests.

That's roughly equivalent to two donuts or half a frozen pizza, depending on which brand you choose.

The study, involving 10,775 men and women in Brazil, found people who ate more ultra-processed foods had a 28% faster rate of global cognitive decline compared to people who ate the least amount of ultra-processed foods over an 8-year period.

With people in countries like the US and the UK getting over 50% of their dietary intake from ultra-processed foods, the study, published in JAMA Neurology, indicates concerning health consequences.

Diet only one factor in cognitive health

However, experts have been quick to point out the evidence isn't so clear cut.

"This study only provides an association between intake of ultra-processed foods and cognitive decline," said Duane Mellor, a dietitian at Aston University in the UK not involved in the study.

"The problem is that it is observational data, so there is only evidence of association and not causality."

Mellor said that researchers already know a lot about the science behind the negative impact of ultra-processed foods like donuts on people's health. However, he said, it is hard to tell whether these foods are any worse than diets high in added fat, salt and sugar.

Experts also say the study fails to consider other factors contributing to cognitive health decline.

"It may be more the case they [people with diets high in ultra-processed food] were eating less [healthy] foods such as vegetables, fruit, nuts, seeds and pulses," said Mellor.

Cognitive health is a complex issue, and its decline can be due to many factors other than diet, such as exercise, smoking, alcohol, cardiovascular and metabolic disease. The study didn't include these factors in their analysis.

"This makes it virtually impossible to draw any conclusions [from the data]," said Gunter Kuhnle, a professor of nutrition and food science at the University of Reading in the UK.

Unhealthy foods are causing major global health problems

Quibbles about individual studies aside, medical and nutrition experts like Mellor agree that diet and nutrition are two of the biggest risk factors for people's health across the globe today. Evidence clearly shows obesity is now a bigger problem than world hunger.

Foods that are high in fat, sugar and salt — ultra-processed or not — are linked to everything from obesity, cardiovascular disease, cancer, and increased all-cause mortality (deaths in a given population). 

A recent report from the Global Panel on Agriculture and Food Systems Nutrition, an independent group of experts on nutrition and health, suggests developing countries, where the sale of processed foods is growing at the fastest rates, are particularly at risk in the coming years.

According to the report, over 3 billion people do not have access to a healthy diet, leading to poor nutrition.

What can you do to eat more healthily?

It's never too late to start eating healthy foods. Studies show that healthy dietary patterns such as that of the Mediterranean region help to reduce cognitive decline and reduce the risk of cardiovascular disease.

The Mediterranean-style diet emphasizes the intake of fresh foods and reduces the intake of foods that are high in processed fats, sugars and salt.

"Try to enjoy (ideally with others) a simple and varied diet based on vegetables, nuts, pulses (legumes), seeds and fruit with wholegrain and if you eat them moderate amounts of dairy products and unprocessed meats," said Mellor.

Edited by: Clare Roth