It sounds like the stuff of some rather low-budget horror film, but scientists now say that it is possible (with a diet so high in fat that it causes obesity) to alter the immune cells in our brain to such an extent that they become sedentary and start consuming the connections between our neurons, resulting in shrinking cognitive ability.
Scary, but reversible, says Dr. Alexis M. Stranahan, neuroscientist in the Department of Neuroscience and Regenerative Medicine at the Medical College of Georgia. But, if the mouse model is to be believed, following a low-fat diet for just two months reverses this trend, as the weight normalises.
Too much fat in the body, explains Stranahan, produces chronic inflammation, which stimulates microglia (the active immune defence) to have an autoimmune response.
Where normally these microglia have the ability to ingest infectious agents in the brain and dispose of them, which in turn supports the health of neurons, in cases of obesity in mice these microglia turn to ‘overeating’. “Normally in the brain, microglia are constantly moving around. They are always moving around their little fingers and processes.
What happens in obesity is they stop moving,” Stranahan said. “They draw in all their processes; they basically just sit there and start eating synapses. When microglia start eating synapses, the mice don’t learn as effectively.” Stranahan’s study featured normal male mice. One group ate a diet in which about 10 percent of the calories came from saturated fat, and another consumed a diet of around 60 percent fat. Other factors were equal.
After four weeks, all mice were faring similarly, except for those on the higher fat diet getting fatter. By 12 weeks however, not only were this group obese but cytokine levels (which are produced by the microglia when they start getting activated and angry) were elevated, and there was a reduction in the markers for synapse number and function in the mice’s hippocampus – the centre of learning and memory.
When the mice on the high-fat diet were switched to the low-fat version, their weight returned to normal within about two months. And once back on a low-fat diet, Stranahan maintains, these cellular processes in the brain can be completely reversed, and cognition maintained. The original paper is below and the image with the supplementary table (left) gives an exact breakdown of the diets for readers. Please note, the high fat diet consisted of 54.4g of lard whereas the low fat diet consisted of only 4.4g of lard, so the researchers were essentially testing a high animal fat diet in these mice versus a low animal fat diet.