When you think of beauty, the brain is probably not the first thing that leaps to mind. But if you care about gaining and maintaining beauty, you cannot ignore the organ that controls just about everything that goes on inside of you, largely determining how you appear on the outside.When YouBeauty founder Michael Roizen, M.D., wrote his book, “RealAge: Are You as Young as You Can Be?,” he analyzed studies that revealed the effects of lifestyle factors on the aging processes of many body organs and systems. Evidence overwhelmingly suggests that our daily habits impact the rate and way in which we age. The aging brain has received particular attention in the last decade, as scientists have debunked the notion that the adult brain remains stuck on a one-way track to atrophy. In fact, lifestyle factors, including exercise, can protect against many of the negative effects of brain aging.QUIZ: Are Your Habits Aging You?The detrimental changes that occur in your brain as we get older come from a variety of factors, but most can be boiled down to a few primary causes. These include oxygen-related free radicals (unstable molecules that damage the molecules around them), compromised energy breakdown and usage in the cells (caused largely by free radical damage to the energy-producing mitochondria in the cells) and the buildup of damaged proteins and cell parts (think of it as mini junkyards in your brain). These are all associated with cognitive decline and Alzheimer’s disease.The good news? Physical exercise is one of four main lifestyle components (the others are socialization, exercising your brain’s cognitive abilities and brain-healthy eating habits) found to mitigate these processes that lead to cognitive impairment and decreased brain function.MORE: Memory-Boosting Tips from Dr. Oz and Dr. RoizenStudies link exercise to decreased stress and damage from free radicals. Regular, moderate exercise produces a mild oxidative stress that leads to an increased capacity to counter the negative effects of free radicals, no matter what their cause. In the case of the brain, studies on rats have shown decreased levels of protein carbonyl (a sign of free radical damage) and significant improvements in cognitive function after just nine weeks of moderate exercise. Numerous human studies have reproduced these beneficial effects of exercise on brain function.But that’s not the only way exercise combats mental decline: When junk collects in the factories of your cells, which it inevitably does, given all of the construction, production, degradation and transportation going on, the junk needs to be removed. Your cells have their own “Got Junk” waste disposal service. They are specialized cell structures such as lysosomes and proteasomes that get rid of damaged proteins and other cell parts. These waste disposal systems are often compromised in the aging brain—this plays a central role in decreased brain function.Moderate exercise increases the activity of both repair enzymes and waste disposal enzymes in the brain, which clean things up so your brain isn’t a junkyard.MORE: Brain Foods That Fight Alzheimer’sPhysical activity also protects the brain in other ways. Exercise activates the genes that lead to neurogenesis, which is the creation of new brain cells. As pointed out by neurologist and dementia expert Paul Bendheim, M.D., author of “The Brain Training Revolution: A Proven Workout for Healthy Brain Aging,” “Humans who engage in regular aerobic physical exercise build ‘brain reserve,’ an insurance policy against age-associated memory impairment and Alzheimer’s disease. Modest physical exercise has been shown to promote the birth of new brain cells and actually enlarge key areas of the brain involved in memory formation and critical thinking and planning skills. “Just as you would make more mistakes copying a manuscript when you are tired and have been at it for hours with no break, the number of errors your body makes transcribing genetic material increases with age. Exercise activates DNA repair enzymes—your body’s internal “spell check”—reducing the damage from transcription errors that accompany aging.Physical activity also enhances the body’s ability to alter the strength of the connection between nerve cells (known as synaptic plasticity). This phenomenon is vital to memory and learning. Lastly, exercise is associated with a reduced risk of developing Alzheimer’s disease, even in those who carry the APOE4 gene, which is most closely linked to an increased risk of late-onset Alzheimer’s.John C. Morris, M.D., author of a recent study looking at the effects of exercise on the expression of this gene said, “Carriers of the Alzheimer’s gene APOE-4 who regularly exercised over a decade were five to ten times less likely to have brain plaques linked to the disease than those with the gene who weren’t physically active.”Bottom line: Moderate exercise is one of the most effective tools for slowing the rate at which your brain ages and reducing your likelihood of developing life-altering cognitive conditions such as Alzheimer’s.So the next time you head out for a run, hop on your bike or jump into the pool, think about all of those new neurons being created, the genetic errors being corrected, the free radicals getting neutralized and the brain’s junk being cleared away. That vision may give your workouts new meaning—and keep you going back for more.MORE: Brain Games That Improve Your Health