Scientists have known for years that an unhealthy diet — especially one high in fat and sugar — can cause harmful changes in the brain and lead to cognitive impairment.
Many factors that contribute to cognitive decline are beyond an individual’s control, such as genetics and socioeconomic factors. But ongoing research increasingly shows that poor diet is a risk factor for memory impairment during normal aging and increases the risk of Alzheimer’s disease.
But when it comes to assessing how certain diets erode brain health as we age, there has been little research on the effects of eating minimally and ultra-processed foods — that is, until now.
Eating ultra-processed foods may exacerbate age-related cognitive decline and increase the risk of dementia, according to two recent large-scale studies. In contrast, another recent study reported that ultra-processed food consumption was not associated with poorer cognitive performance in people over the age of 60.
While more research is needed, as a neuroscientist who studies how diet affects cognition in later life, I find that these early studies add a new layer to considering the impact of basic nutrition on brain health.
More Ingredients, Less Nutrients
Ultra-processed foods tend to be lower in nutrients and fiber and higher in sugar, fat and salt than unprocessed or minimally processed foods.
Some examples of ultra-processed foods include soda, packaged cookies, potato chips, frozen meals, flavored nuts, flavored yogurt, distilled alcoholic beverages, and fast food. Even packaged breads, including those nutrient-dense whole grains, are in many cases ultra-processed because they contain additives and preservatives.
Another way of looking at it: You’re unlikely to find the ingredients that make up most of these foods in your home kitchen.
But don’t confuse ultra-processed foods with processed foods, which still retain most of their natural properties even though they’ve been processed in some form — like canned vegetables, dried pasta, or frozen fruit.
analytical research
In a December 2022 study, researchers compared the rate of cognitive decline over about eight years in people who ate different amounts of ultra-processed foods.
At the start of the study, more than 10,000 participants living in Brazil reported their eating habits over the past 12 months. Then, over the next few years, the researchers assessed the participants’ cognitive performance with standard tests of memory and executive function.
Those whose diets included more ultra-processed foods at the start of the study had slightly higher cognitive decline than those who ate little or no ultra-processed foods. This was a relatively modest difference in the rate of cognitive decline between the experimental groups.
It is unclear whether small differences in cognitive decline associated with increased consumption of ultra-processed foods would have meaningful effects on individuals.
The second study, with around 72,000 participants in the UK, measured the link between eating ultra-processed foods and dementia. For those who ate the most ultra-processed foods, about 1 in 120 were diagnosed with dementia within 10 years. For those who ate almost no ultra-processed foods, that number was 1 in 170.
Studies examining the relationship between health and ultra-processed foods use the NOVA classification, a classification system based on the type and degree of industrial food processing.
Some nutritionists have criticized the NOVA classification for not having a clear definition of food processing, which could lead to misclassification. They also argue that the potential health risks of eating ultra-processed foods can be explained by a diet low in fiber and nutrients and high in fat, sugar and salt, rather than the amount of processing.
Many ultra-processed foods are loaded with additives, preservatives or coloring agents, along with other hallmarks of an unhealthy diet, such as being low in fiber and nutrients. Therefore, it is unclear whether consumption of more processed foods has additional negative health effects beyond lower diet quality.
For example, you can eat burgers and fries from fast food chains that are high in fat, sugar and salt and ultra-processed. You can cook the same meal at home, and it’s likely to be high in fat, sugar, and salt, but it won’t be ultra-processed. More research is needed to determine whether one is worse than the other.
Brain Healthy Eating
Even if the processes leading to dementia do not occur, the aging brain undergoes biochemical and structural changes associated with cognitive deterioration.
But for adults over 55, a healthier diet can increase the likelihood of maintaining better brain function. In particular, Mediterranean and ketogenic diets were associated with better cognition in older adults.
The Mediterranean diet emphasizes eating plant foods and healthy fats, such as olive oil, seeds and nuts. The ketogenic diet is high in fat, low in carbohydrates, and the main source of fiber is vegetables. Both diets reduce or eliminate sugar consumption.
Our study and that of others suggest that both diets can reverse some of these changes and improve cognitive function—likely by reducing harmful inflammation.
Although inflammation is a normal immune response to injury or infection, chronic inflammation can be harmful to the brain. Studies have shown that excess sugar and fat can lead to chronic inflammation, and ultra-processed foods may also exacerbate harmful inflammation.
Another way diet and ultra-processed foods may affect brain health is through the gut-brain axis, the communication that occurs between the brain and the gut microbiome, or community of microbes that live in the digestive tract.
The gut microbiota not only aids in digestion, but also affects the immune system, while producing hormones and neurotransmitters that are critical to brain function.
Studies have shown that ketogenic and Mediterranean diets alter the composition of gut microbes in ways that benefit the body. Consumption of ultra-processed foods was also associated with changes in the type and abundance of gut microbes that had more deleterious effects.
Uncertainty
Figuring out the specific effects of individual foods on the body is difficult, in part because it’s problematic to strictly control people’s diets to study them over the long term. Furthermore, randomized controlled trials are the most reliable type of research to establish causality, but they are expensive to conduct.
Most nutrition studies so far, including these two, have only shown a correlation between ultra-processed food consumption and health. But they couldn’t rule out other lifestyle factors, such as exercise, education, socioeconomic status, social relationships, stress, and many more variables that could affect cognitive function.
This is where laboratory-based research using animals can be very useful. Mice show cognitive decline similar to humans in old age. It is easy to control the diet and activity level of rodents in the laboratory. Mice go from middle age to old age in a matter of months, which shortens the learning period.
Laboratory-based animal studies will make it possible to determine whether ultra-processed foods play a key role in the development of cognitive impairment and dementia in humans. As the world’s population ages and the number of older people living with dementia increases, this knowledge cannot come fast enough.
Sara N. Burke, Associate Professor of Neurobiology and Cognitive Aging, University of Florida
This article is republished from The Conversation under a Creative Commons license. Read the original text.
