It’s no secret anymore that obesity is a world-wide epidemic. It’s not something that’s easy to hide or overlook: obesity is everywhere, especially in the Western and developed worlds. Something else that is becoming glaringly obvious is that the Western diet is not something to be proud of. With deep fryers and excessive portions dominating the everyday diet, it’s no wonder there are health complications that were not even considered before.
One such complication was addressed in 2017 in a study published in The Neurobiology of Aging (Spencer, D’Angelo, Soch, Watkins, Maier, & Barrientos, 2017). These researchers addressed the growing issue of obesity in the aging population (those between 60-74 years old), and its effect on their memory. They acknowledged the shocking fact that obesity rates in the United States in this age range had doubled since 1980, and that unique consequences could be occurring due to the weakening of older brains.
To study if this was the case, these researchers decided to test aging rats on their memory compared to younger rats when both on and off a high-fat diet. More specifically, they wanted to find out if a short-term high-fat diet would produce similar inflammatory effects (swelling) as other known memory-impairing events such as surgery or infections. Such injuries had been shown in other studies to cause the brain to swell and release harmful levels of chemicals that promote further swelling, overwhelming the weakened immune cells in the aging brain and damaging memory. Knowing this, the researchers predicted similar results from short-term high-fat diets in older rats (and ultimately in older humans).
Male rats were used to perform this study, some older and some younger. Older rats were 24 months old and weighed about 500g, whereas the younger rats were 3 months old and weighed about 300g. Each group of rats was split into those that were fed a regular diet of rat chow (17% calories from fat), and those that were fed a diet of high-fat rat chow (60% calories from fat). These four groups were tested the same way to see if there were any differences in memory retention.
The memory test was for contextual fear conditioning: if the rats would remember a frightening event that occurred in a certain context. The rats were fed their assigned chow for three days before being introduced to the frightening context: exploring a new chamber, hearing a long tone after a few minutes, followed by a small shock sent through the floor. This shock was not enough to hurt the mice, but enough to startle them and make them wary of the chamber and tone. After this task the rats were all put back onto the regular chow (overall replicating a three-day binge of high-fat food before continuing with regular diet, much like a long weekend of humans binging on high-fat foods). Four days after this task the rats were all reintroduced to the context and tested on their fear of a) the chamber, and b) the tone (each stemming from a different part of the brain that forms and stores memories: the hippocampus and the amygdala respectively). Their level of fear was measured by freezing behaviour, their most obvious defensive response.
What the researchers found was that all the rats that were fed the high-fat diet gained weight, particularly the aged rats, and that they all froze significantly less than the regular-diet rats when reintroduced to the fear context (therefore a lack of short-term memory formation). The aged rats who were fed the high-fat diet performed particularly poorly on this task, showing no freezing behaviour at all when the tone was repeated.
An even more interesting discovery was made when the researchers compared the brains of all the rats after the experiment. The hippocampus and amygdala regions of the brain that were noted to be responsible for different types of memory formation had significantly different levels of the chemicals that increased inflammation (meaning that age and diet both influenced the amount of these chemicals that were released). The aged rats who were fed the high-fat chow had significantly higher levels of one of these chemicals, called IL-1B protein, in their amygdala than the other rats, indicating a strong negative consequence of the short-term high-fat binge. Furthermore, voluntary exercise (running on a wheel) was shown to prevent the memory impairment that was caused by the high-fat diet in the aged rats.
Overall what the researchers discovered in this study was that short-term high-fat binges affected both the hippocampus and the amygdala of the rats in terms of memory formation. The fear memories that were tested (hearing the tone and shocking the paws) were especially impaired for the aged rats. The inflammation-causing chemicals (specifically IL-1B protein) became elevated in the rats whose brain immune response was weakened (aged rats), resulting in memory impairment.
This study provided strong evidence to suggest that diet (and exercise!) are important for the aging population to promote healthy brain functioning, particularly memory formation. In a society where high-fat diets and obesity are on the rise, it is important to understand how they affect the brains of the most vulnerable populations, including the aging.
Spencer, S. J., D’Angelo, H., Soch, A., Watkins, L. R., Maier, S. F., & Barrientos, R. M. (2017). High-fat diet and aging interact to produce neuroinflammation and impair hippocampal-and amygdalar-dependent memory. Neurobiology of aging, 58, 88-101.
