Scientists have known for years that unhealthy diets—particularly those 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 a person’s control, such as genetics and socioeconomic factors. But ongoing research increasingly indicates that a poor diet is a risk factor for memory impairment during normal aging and increases the risk of developing Alzheimer’s disease.
But in evaluating how some diets can affect brain health as we age, there’s been little research to date on the effects of consuming minimally processed versus ultra-processed foods.
Two recent large-scale studies suggest that eating ultra-processed foods may exacerbate age-related cognitive decline and increase the risk of developing dementia. In contrast, another recent study reported that consumption of ultra-processed foods was not associated with poorer cognition in those over 60.
While more research is needed, as a neuroscientist exploring how nutrition can influence cognition later in life, I think these early studies add another layer to how fundamental nutrition is to brain health.
Lots of ingredients, minimal nutrition
Ultra-processed foods are generally lower in nutrients and fiber and higher in sugar, fat and salt compared to 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 foods. Even packaged breads, including those high in nutritious whole grains, qualify as ultra-processed in many cases because of the additives and preservatives they contain.
Another way to look at it: the ingredients that make up most of these foods you probably won’t find in your own kitchen.
But don’t confuse ultra-processed foods with processed foods, which still retain most of their natural characteristics even though they have undergone some form of processing, such as canned vegetables, dried pasta, or frozen fruit.
Dissect the research
In a December 2022 study, researchers compared the rate of cognitive decline over about eight years between groups of people who consumed different amounts of ultra-processed foods.
At the start of the study, more than 10,000 participants from Brazil reported their dietary habits over the past 12 months. Then, over subsequent years, the researchers evaluated the participants’ cognitive performance with standard tests of memory and executive function.
Those who ate a diet high in ultra-processed foods at the start of the study showed slightly more cognitive decline compared to those who ate little to no ultra-processed foods. This was a relatively modest difference in the rate of cognitive decline between experimental groups.
It is not yet clear whether the small difference in cognitive decline associated with higher consumption of ultra-processed foods will have a meaningful effect on an individual person’s level.
The second study, involving around 72,000 participants in the UK, measured the link between eating ultra-processed foods and dementia. For the group that ate the most ultra-processed foods, about 1 in 120 people were diagnosed with dementia over a 10-year period. For the group that consumed little to no ultra-processed foods, this was 1 in 170.
Research on the relationship between health and ultra-processed foods uses the NOVA classification, a categorization system based on the type and size of industrial food processing.
Some nutritionists have criticized the NOVA classification for lacking clear definitions of food processing, which could lead to misclassification. They also argue that the potential health risks of consuming ultra-processed foods may be explained by low fiber and nutrient content and high levels of fat, sugar and salt in the diet rather than the amount of processing.
Many ultra-processed foods are high in additives, preservatives or dyes, while also having other characteristics of an unhealthy diet, such as low in fiber and nutrients. Thus, it is unclear whether eating foods that have undergone more processing has an additional negative effect on health beyond low nutritional quality.
For example, you could eat a burger and fries from a fast food chain, which is high in fat, sugar, and salt and is also ultra-processed. You could make that same meal at home, which can also be high in fat, sugar, and salt, but not ultra-processed. More research is needed to determine if one is worse than the other.
Even when the processes that lead to dementia are not taking place, the aging brain undergoes biochemical and structural changes associated with deteriorating cognition.
But for adults over age 55, a healthier diet could increase the chances of maintaining better brain function. In particular, the Mediterranean diet and the ketogenic diet are associated with better cognition later in life.
The Mediterranean diet emphasizes the consumption of plant foods and healthy fats, such as olive oil, seeds and nuts. The ketogenic diet is high in fat and low in carbohydrates, with the primary source of fiber being vegetables. Both diets minimize or eliminate sugar consumption.
Our research and the work of others show that both diets can reverse some of these changes and improve cognitive function — possibly by reducing harmful inflammation.
While 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 contribute to chronic inflammation, and ultra-processed foods can also exacerbate harmful inflammation.
Another way diet and ultra-processed foods can affect brain health is through the gut-brain axis, the communication that takes place between the brain and the gut microbiome, or the community of microorganisms that live in the digestive tract.
The gut microbiome not only aids in digestion, but also influences the immune system, producing hormones and neurotransmitters critical to brain function.
Studies have shown that the ketogenic and Mediterranean diets change the composition of microorganisms in the gut in a way that benefits the person. Ultra-processed food consumption is also associated with changes in the type and amount of gut microorganisms that have more harmful effects.
Disentangling the specific effects of individual foods on the human body is difficult, in part because it is difficult to maintain strict control over people’s diets to study them over long periods of time. In addition, randomized controlled trials, the most reliable type of study to establish causality, are expensive to conduct.
So far, most nutrition studies, including these two, have only shown correlations between ultra-processed food consumption and health. But they cannot rule out other lifestyle factors such as exercise, education, socioeconomic status, social connections, stress and many more variables that can affect cognitive function.
This is where lab studies with animals are incredibly helpful. Rats show cognitive decline in old age that parallels humans. It’s easy to monitor rodent diets and activity levels in a lab. And rats go from middle to old age in a matter of months, shortening study time.
Lab-based studies in animals will make it possible to determine whether ultra-processed foods play a key role in the development of cognitive impairment and dementia in humans. Now that the world’s population is aging and the number of elderly people with dementia is increasing, 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 article.