Healthy diets seem to prevent dementia. Why is it so hard to prove?
It is still challenging to accurately track what people eat over long periods of time. It’s even tougher to link this data to brain health. A panel of experts say the answer may lie in better clinical trial design.
The number of people living with dementia is projected to more than triple by 2050, soaring to 152 million cases worldwide. Not all types of dementia are preventable, but mounting evidence points to the possibility that some cases are. With certain environmental and lifestyle interventions, that “some” could be as substantial as two in five dementia cases delayed or dodged altogether. When it comes to a specific, clear set of instructions on How to Prevent Dementia, however, the research leaves much to be desired.
For example, there was much discussion following last week’s Alzheimer’s Association International Conference about early data from a 60-year study looking at how highly processed foods may be expediting cognitive decline in older adults. On the flip side, some researchers are “certain” that good nutrition lowers one’s risk of future cognitive impairment and dementia. But despite decades of study there is still broad consensus that the links between brain function and foods or food groups are, as of yet, inconclusive. With so encouraging evidence, why is the idea of a “dementia prevention diet” so hard to prove?
Recently, a Nutrition for Dementia Prevention Working Group made up of 27 leading researchers in the Alzheimer’s and dementia field laid out recommendations to help solidify the science around the relationship between nutritional diets and lower dementia risk. The group’s guidelines were published in The Lancet Health Longevity, where the authors also highlight the limitations of existing research.
“Many trials have not found that making people eat healthy or exercise is translating into benefits in the ways that is expected from the epidemiological research,” said Dr. Hussein Yassine, associate professor of medicine and neurology at the Keck School of Medicine of USC, said in a news release. “That means either there is no causal connection, or that these studies have not been properly designed.”
Studying nutrition: It’s complicated
The researchers involved in creating these guidelines spent two years looking at the vast body of evidence linking nutrition and dementia risk.The results of nutritional research, they found, are not always straightforward. For example, it is known that a Mediterranean-style diet is healthy and that study participants who adhere to this diet are less likely to develop Alzheimer’s or other forms of dementia. But is the diet itself the protector? Or are there other factors at work behind the scenes?
What if, for example, the consumption of a Mediterranean diet were linked to some other influential but unmeasured factor, such as socioeconomic status, which is also linked to brain health?
In addition to unmeasured variables, Yassine notes that studies may often be too short to spot any meaningful effects of a dietary intervention on cognition. “If it takes five to 10 years,” he said, “then studies that lasted for two years or less are not accurately reflecting the effect of the diet on cognition.”
Studies that have hung in there for an entire decade have still found a positive correlation between a healthy diet and a lower risk of Alzheimer’s and dementia. But the working group’s recommendations go beyond simply making studies longer.
Using brain biomarkers to analyze impact of a healthy diet
Countless studies explore the relationship between the consumption of nutrients (like folate, flavonoids, vitamin D, and certain lipids) or food groups (like seafood, vegetables, and fruits) and cognitive health, yielding unsteady results. There is no consensus on, for example, the optimal levels of certain vitamins or nutrients when it comes to brain health.
The authors argue that leveraging biomarkers from brain imaging, blood tests and can provide valuable information about how food and nutrients are processed by the body and how they may impact the brain. Neglecting these biomarkers in participants is where some studies fall short, according to Yassine. “You cannot fully study how the diet is working without studying the microbiome,” he said.
The researchers also suggested, in addition to the current gold-standard of large randomized controlled trials, using other types of trials too. Rather than only looking at broadly at a participant group of 100,000 people, the authors say, smaller personalized studies could help shed light on more nuanced factors, such as an individual’s genetic risk, diet and microbiome, and their relationship to brain health. The working group also argues that this preventative research concerned with diet and dementia risk should start in participants’ midlife years, before the symptoms of Alzheimer’s or dementia set in.
By measuring these biomarkers, collecting information about other risk factors, and carefully tracking diet and nutritional intake, it might be possible to finally answer how and why diet is protective for dementia. There might finally be a clear, conclusive answer as to which foods are most protective for individuals carrying a specific risk factor, or whether a person is getting enough of a specific vitamin or nutrient to maintain brain health.
The more trial design can improve, says Dr. Lon Schneider, professor of psychiatry and the behavioral sciences at the Keck School of Medicine, the better chance people have to prevent Alzheimer’s and dementia.
“It is important that future trials yield accurate results,” he said, “that can be translated into better clinical care for patients.