The Memory of Older People Can be Impaired Even After Just Three Days of a High-Fat Diet

Even a few days of a high-fat diet could cause memory problems and associated brain inflammation in older adults, a new study in rats suggests.

Adverse Inflammatory Changes in the Brain and Eating Habits

Researchers fed separate groups of young and old rats a high-fat diet for three days or three months to compare how quickly changes in the brain occur when an unhealthy diet is consumed, relative to the rest of the body. As expected from previous diabetes and obesity research, consuming fatty foods for three months caused metabolic problems, intestinal inflammation, and dramatic shifts in gut bacteria in all rats, compared to those who received normal food, while only three days of high fat did not cause any major metabolic or intestinal changes.

However, the researchers found that only older rats – regardless of whether they were on a high-fat diet for three months or just three days – performed poorly in memory tests and showed negative inflammatory changes in the brain. The findings refute the notion that diet-related inflammation in the aging brain is caused by obesity, according to senior study author Ruth Barrientos, a researcher at The Ohio State University’s Institute for Behavioral Medicine Research.

Most research on the effects of fatty and processed foods on the brain has focused on obesity, but the effects of unhealthy diets, independent of obesity, are still largely unstudied. Unhealthy eating and obesity are linked, but not inextricably linked. “We are really looking for the direct effects of diet on the brain. And we have shown that tremendous neuroinflammatory changes occur within three days, well before obesity sets in,” said Barrientos, who is also an adjunct professor of psychiatry and behavioral health and neuroscience at The Ohio State College of Medicine.

Changes in the body are slower in all animals and are actually not necessary to cause memory impairment and changes in the brain. The researchers would never have known that encephalitis was the main cause of memory impairment caused by a high-fat diet if they had not compared the two time lines. The research results were recently published in the journal Immunity & Ageing. Years of research in Barrientos’ laboratory have shown that aging leads to a long-term “preparation” of the brain’s inflammatory profile, coupled with a loss of brain cell reserves to recover, and that an unhealthy diet can make things worse for the brains of older adults. Fat makes up 60% of the calories in the high-fat diet used in the study, which could match a range of common fast food options: for example, nutrition data shows fat makes up around 60% of the calories in a McDonald’s Double Smoky BLT Quarter Pounder Burger with Cheese or a Burger King Double Whopper with Cheese.

Memory Appears to be More Resilient to the Effects of a High-Fat Diet at a Young Age

After giving the animals a high-fat diet for three days or three months, the researchers conducted tests that looked at two types of memory problems that often occur in older people with dementia and are based on separate brain regions: contextual memory, which is mediated by the hippocampus (the brain’s primary memory center), and cue-evoked fear memory, which originates in the amygdala (the brain’s fear and danger center). Compared to control animals that received normal chow and young rats that received a high-fat diet, older rats displayed behaviors suggesting that both types of memory were impaired after only three days of a high-fat diet—and the behaviors persisted as they continued the high-fat diet for three months. The researchers also found that the levels of a number of proteins called cytokines in the brains of the older rats changed after three days of a high-fat diet, indicating a disrupted inflammatory response. Three months into the high-fat diet, some of the cytokine levels had shifted but remained dysregulated, and the cognitive problems persisted in behavioral tests. A shift from baseline in the inflammatory markers is a negative response and is known to impair learning and memory functions, Barrientos said.

Compared to rats fed a normal diet, both young and old rats fed the high-fat diet for three months gained more weight and showed signs of metabolic dysfunction – poor insulin and blood sugar control, inflammatory proteins in adipose tissue, and changes in the gut microbiome. The high-fat diet did not affect memory and behavior in young rats, nor did it affect brain tissue. These diets cause obesity-related changes in young and old animals, but young animals appear to be more resilient to the effects of the high-fat diet on memory. Researchers believe this is likely due to their ability to activate compensatory anti-inflammatory responses that the older animals lack. Because glucose, insulin, and adipose tissue inflammation are elevated in both young and old animals, there is no way to distinguish what causes the memory impairment in old animals just by looking at what is happening in the body. What happens in the brain is important for the memory response.