Brits have been warned that those who consume junk food - such as crisps, biscuits and fizzy drinks - are at risk of developing Alzheimer's disease in later life.
Shocking new research warned that every 10% increase in daily intake raises the risk of dementia by 25%.
Heavily-processed products are especially high in added sugar, fat and salt - and low in protein and fibre.
Study lead author Dr Huiping Li, of Tianjin Medical University in China, said: "Ultra-processed foods are meant to be convenient and tasty - but diminish the quality of a person's diet."
It is important to know that these foods range from burgers, chicken nuggets, sausages, pizzas and chips - to yoghurt, baked beans, ice cream, cakes, biscuits and fizzy drinks.
But they also include tomato ketchup, mayonnaise, ready meals, flavoured cereals and packaged guacamole, hummus and breads.
Dr Li added: "These foods may also contain food additives or molecules from packaging or produced during heating - all of which have been shown in other studies to have negative effects on thinking and memory skills.
"Our research not only found ultra-processed foods are associated with an increased risk of dementia, but also replacing them with healthy options may decrease dementia risk."
The study was conducted on more than 73,000 over-55s for an average of 10 years, and the participants at UK Biobank became part of an ongoing study tracking the health of half a million Brits.
They were also used to estimate what would happen if a person substituted 10% of ultra-processed foods with unprocessed or minimally processed alternatives.
This meant that swapping processed foods for fresh fruit, vegetables, legumes like peas or lentils, milk or meat reduced dementia risk by around a fifth (19%).
Participants were divided up into four equal groups - ranging from lowest to highest daily dietary percentages of ultra-processed food, with age, gender, family history of dementia, heart disease and other factors that could affect risk taken into consideration.
On average, ultra-processed foods made up nine per cent of consumption in the former (8oz on average) - compared to 28% for the highest (1lb 13oz) for example one serving of pizza or fish fingers was equivalent to 5.3oz.
The main food group contributing to high ultra-processed food intake was fizzy drinks - followed by sugary foods and ultra-processed dairy.
By the end of the study, 518 people were diagnosed with dementia - including 105 of the 18,021 in the lowest group compared to 150 of the 18,021 in the highest.
Dr Li said: "Our results also show increasing unprocessed or minimally processed foods by only 50 grams (1.8oz) a day, which is equivalent to half an apple, a serving of corn, or a bowl of bran cereal, and simultaneously decreasing ultra-processed foods by 50 grams a day, equivalent to a chocolate bar or a serving of fish sticks, is associated with a three per cent decreased risk of dementia.
"It's encouraging to know that small and manageable changes in diet may make a difference in a person’s risk of dementia."
The scientist added that junk food increases the risk of chronic diseases by ridding the body of 'good bacteria'.
Meanwhile, obese children have been found to be more prone to dementia - but decades later.
The number of dementia cases worldwide is set to triple to more than 150 million by 2050.
However, with no cure in sight, there's an increasing focus on behaviours that could protect a person from getting it, such as eating well.
Dementia affects 920,000 people in the UK - a figure forecast to reach two million within three decades.
