In order to understand how eating ages our brain, we must first ask why are our brains located in our heads? Wouldn't they be safer if they were deep in our chest, similar to the location of our hearts? Brains, regardless of how small or simple, have evolved at the best possible location to perform their principal function: survival of the individual and the species. With very few exceptions, brains are always located at the front end of an animal's feeding "tube" or mechanism, which in humans and many other organisms is the tubular system (the alimentary canal) that extends from the mouth to the anus. Your brain makes it possible for you to find food by sight, sound, and smell and then to organize your behavior so that the front end of your feeding tube can get close enough to taste the food and check it for beneficial or potentially harmful contents before you ingest it. Once the food is in your feeding tube, it is absorbed and becomes available to the cells of your body. Your entire feeding tube and associated organs, also known as the gastrointestinal system, use nearly 70% of the energy you consume just to make the remaining 30% available to the rest of your body. Your brain uses about 14% of the available consumed energy, and your other organs that allow you to reproduce and move around your environment (including your muscles and bones) utilize about 15%. As you can see, very little energy is left over for other tasks in the body. These percentages give you some idea of the priorities-sex and mobility-that billions of years of evolution have set for your body to achieve.
Thus, today we have a big brain and a gastrointestinal system that is fairly efficient at extracting energy for itself and its principal customer: the brain. The food we eat must be metabolized, a process that requires the oxygen in the air we breathe. Unfortunately, our most basic acts of survival, breathing and eating, are what age our bodies and our brains. If this sounds like the proverbial damned-if-you-do, damned-if-you-don't scenario, well, it sort of is, and yet somehow our species has managed to survive this challenge for several hundred millennia.
Like most other animals on this planet, we humans acquire energy for our biochemical machinery by breaking down the carbon bonds found in fats, sugars, and proteins and then gobbling as much energy from the process as possible. The fact that we do this so inefficiently means that much of the energy in our food is lost as heat. This process also leaves our cells with left-over carbon atoms. The problem is what to do with all of this carbon waste. More than 2 billion years ago, the solution for a small independently living single-celled organism, which might have closely resembled our own mitochondria (the furnace that handles almost all of our cells' energy production needs), was to combine these left-over carbons with a readily available gas, oxygen, and to expel the product as a gas called carbon dioxide. Thus, thanks to our current symbiotic relationship with the descendants of these ancient bacteria, our mitochondria, the way our bodies obtain energy to live is as follows: carbon bonds come into the front end of our feeding tubes in the form of fats, carbohydrates, and proteins; we then extract energy and excrete the residue as carbon dioxide and water vapor.
Because oxygen is also exceedingly toxic to cells, it must be utilized very carefully and conservatively. Indeed, scientists have recently discovered that the genes that control energy metabolism have been highly conserved across millions of years of evolution, from yeast to humans, and that these genes influence the rate of the aging process. Essentially, the better we negotiate our energy-oxygen exchange with our indwelling mitochondria, the longer and healthier we live as a single individual and as a species. Disrupt the balance in this exchange, and the impact can be harmful.
In general, the hemoglobin in our blood does a decent job of regulating the oxygen levels near the individuals cells of our bodies so that those cells have the oxygen they need for respiration but not too much to kill them outright. These cells have also evolved numerous anti-oxidant systems that would allow us live to be 115 years old, if we were lucky and ate very, very little food. But most of us are not that lucky, and most of us eat all of the time and just keep on breathing, making ourselves vulnerable to the consequences of oxygen. Thus our bodies and our brains age more rapidly.
With normal aging, because we insist on eating and breathing, tissue-damaging molecules called oxygen-free radicals are formed by our mitochondria. Free radicals are not always harmful; however, they become more prevalent with age and may slowly overwhelm our natural anti-oxidant systems, destroying our neurons and just about every other cell in our bodies. According to another recent discovery, the overproduction of these oxygen-free radicals may encourage cancer cells to metastasize and move around the body. Think about the unbelievable irony of this process: The mitochondrial power plant that resides in quite large numbers in every cell of our bodies is actively injuring those cells by the very process of trying to keep them alive. It turns out that each species' maximum lifespan may be determined by how many free radicals are produced by the hundreds of mitochondria that live in each of their cells. We are, indeed, always our own worst enemy.