Low-fat diets do not prevent dementia deaths in older women

Adopting a diet low in fat and high in fruits, vegetables, and grains does not lower the risk of dying from dementia for most postmenopausal women. While earlier research hinted at potential cognitive benefits, a secondary look at a large clinical trial revealed that these dietary modifications did not prevent dementia related deaths over a twenty year period. The findings were published in the journal Menopause.

Dementia and cognitive impairment represent major health risks associated with aging. Women make up more than two thirds of people over the age of sixty five living with Alzheimer’s disease in the United States. Researchers want to understand whether modifying everyday habits like eating patterns can protect brain health and delay the onset of severe cognitive decline.

In the past, observational studies have linked diets lower in fat to better cognitive health. An observational study is one where researchers simply track people’s behaviors over time without asking them to change their habits. However, these studies can only show associations, rather than proving that a specific diet directly prevents cognitive decline. To find stronger evidence, scientists rely on randomized clinical trials, where participants are randomly assigned to follow a specific intervention or act as a control group.

A major project called the Women’s Health Initiative previously conducted one such trial. This large study tracked the health of tens of thousands of postmenopausal women, meaning women who had permanently stopped having menstrual periods. In previous reports from a subset of this trial, researchers noticed that women assigned to a low fat diet had lower rates of possible cognitive impairment, based on scores from a test of global cognitive functioning, compared to those eating their usual diet.

Based on those initial results, researchers wanted to test if this dietary pattern also translated to a lower risk of eventually dying from dementia. Rowan T. Chlebowski, a researcher at The Lundquist Institute in Torrance, California, led a team of investigators to examine this question. They looked at the long term survival data of the original trial participants. The goal was to see if the dietary intervention provided lasting protection against fatal neurodegenerative conditions.

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The researchers analyzed data from a large randomized clinical trial that originally enrolled 48,835 postmenopausal women in the United States. The participants ranged in age from fifty to seventy nine years old. When they joined the study between 1993 and 1998, all of the women consumed diets where at least thirty two percent of their daily calories came from fat. Cognitive function was not a strict requirement for joining the study, but all participants had to be functionally independent.

The research team randomly assigned the women into two groups. Forty percent of the participants joined a dietary intervention group. Nutritionists guided this group to reduce their fat intake to twenty percent of their daily calories while increasing their consumption of fruits, vegetables, and whole grains. The remaining sixty percent of the women served as a comparison group.

The women in the comparison group received standard health materials but did not receive ongoing counseling from study nutritionists. The intensive dietary intervention lasted for a median of eight and a half years. During this time, women in the intervention group successfully reduced their fat intake and increased their consumption of plant based foods. The differences in fat intake between the two groups remained measurable for up to thirteen years.

To determine how this dietary shift affected dementia mortality, the researchers tracked the participants for a median of twenty years. They verified causes of death using medical records, death certificates, and the National Death Index, which is a centralized database of death record information in the United States. They looked specifically at deaths caused by Alzheimer’s disease and non-Alzheimer’s forms of dementia.

The results showed that the dietary intervention did not prevent deaths from dementia. Out of the original study population, 1,386 women eventually died from dementia related causes. The rates of dementia mortality were almost identical between the group that ate a low fat diet and the group that consumed their usual diet. The difference between the two groups was not statistically significant.

The researchers also looked at specific types of dementia to see if the diet had a targeted effect. They found that the low fat eating pattern did not change the rate of death from Alzheimer’s disease. It also did not change the rate of death from other types of dementia, such as vascular dementia or Lewy body dementia. The overall impact on severe cognitive decline resulting in death was effectively zero.

The research team did perform analyses on specific subgroups of women based on their age at the start of the study. In one of these subgroup analyses, they noticed a trend toward lower dementia mortality for younger women, particularly those in their fifties and sixties when the trial began. The researchers noted a gradual decrease in dementia deaths for these younger age brackets compared to older participants. However, this trend did not extend to women who were seventy or older when they altered their diets.

The researchers noted that the low fat diet did produce other measurable health benefits during the trial. The intervention positively influenced body weight, physical activity levels, and the risk of developing diabetes and high blood pressure. Medical professionals widely consider these factors to be potential contributors to dementia risk. Because the diet improved these health markers but failed to reduce dementia mortality, the study suggests that improving cardiovascular health in later life may not be enough to reverse existing neurodegenerative processes.

While the study tracked thousands of participants over two decades, readers should consider a few important caveats. The researchers evaluated dementia mortality, which means they only counted cases where dementia was recorded as a cause of death. They did not measure dementia incidence, which is the number of people who developed the condition but perhaps died of something else. This distinction means the diet might still influence the onset of mild cognitive issues without necessarily changing the ultimate cause of death.

Determining the exact cause of death in older adults can also be imprecise. As people age, they often develop multiple overlapping health conditions, making it difficult for doctors to pinpoint a single fatal disease on a death certificate. This inherent ambiguity in medical records could obscure subtle benefits of the dietary intervention. The reliance on death certificates is a standard practice in long term studies, but it is an imperfect measure of cognitive health.

The positive trend observed in women under the age of seventy also requires cautious interpretation. Because the researchers analyzed many different subgroups, this single positive finding could simply be a result of statistical chance. Alternatively, it might suggest that lifestyle changes need to start much earlier in life to effectively protect the brain. Future clinical trials will need to test dietary interventions specifically in younger age groups to confirm if early changes offer true protection against dementia.

The study, “Low-fat dietary pattern and dementia mortality: a secondary analysis of the Women’s Health Initiative dietary modification randomized clinical trial with long-term follow-up,” was authored by Rowan T. Chlebowski, Stephen Rapp, Aaron K. Aragaki, JoAnn E. Manson, Kathy Pan, Marian L. Neuhouser, Karen C. Johnson, Linda G. Snetselaar, Victor W. Henderson, Lihong Qi, Kathleen M. Hayden, Laura D. Baker, Orena Garcia, Margaret D. Pichardo, Jean Wactawski-Wende, and Ross L. Prentice.