I think we'd all agree that for us guys the most important part of the body is not actually our penis, but rather our brain. Right? Well, even if you would put your brain second on the list, you have to admit that without it, life would never be the same.
So, with that thought in mind, we want to give you a list of things that can erase, shrink and disconnect your grey matter. You would be suprised how many things can injure your precious neurons, dendrites and synapses. It is easy to build the brain at any age and it is easy to tear it down as well.
Here are some of the biggees lurking in your diet, lifestyle and environment that you may not have thought of:
2) Excitotoxins are legalized brain poisons put everyday into packaged goods and all manner of edible products, including that Diet Coke you may have had this morning trying to be healthy. Please, please read this incredibly important link on Excitotoxins for the benefit of yourself and your loved ones.
3) Blood Pressure and Metabolic Disorder. One large scale  study of 999 men - why not add one more guy, right? - showed that "cross-sectional measurements at age 70 showed that high 24-hour BP [blood pressure], nondipping, insulin resistance, and diabetes all were related to low cognitive function ". A 2009 study found that the lower blood pressure number is ultracritical: for every 10 point increase you get about a 7 percent increase in likelihood for "cognitivie impairment", which is polite way of saying memory and thinking problems.  Blood pressure is also a very strong predictor of stroke which often does nasty things to your grey and white matter.
4) Insulin and Blood Sugar. Diabetes is VERY hard on the brain: the ups and downs in diabetic blood sugar levels literally causes neurons to disconnect themselves. Researchers have even found that pre-diabetes Metabolic Syndrome leads to the same slow destruction of the brain.  High blood sugar has been associated with both memory problems AND even shrinkage of the hippocampus, the part of the brain that handles working memory. Read this link on how to avoid Metabolic Syndrome, or Syndrome X as it is sometimes called, which is the body's pre-diabetic state where insulin and blood sugar start to go out of control. (This is the health curse of all people on a Western Diet.)
5) Advanced Glycation End Products (AGEs). Advanced Glycation End Products, or AGE's, occur as a normal byproduct of glucose metabolism. However, they are produced at greatly accelerated rates when blood sugar is too high, etc. Unfortunatly, these Advanced Glycation End Products are associated with a host of physiological ills, including dementia, Alzheimer's Disease  and skin damage. Please read this link for how to combat this neuroligical and metabolic nemesis.
6) Second Hand Smoke. Second hand smoke has been linked to cancer, heart disease and stroke. But the good news is that you probably won't even remember what went wrong as scientists have just discovered that it's also correlated with dementia.  So avoid second hand smoke like the plague or you might as well sign up for work in an an asbestos factory...
7) Saturated and High Fat Diets. Several studies (in older populations) show that saturated fat (and cholesterol and total fat) is associated with decreased cognition, i.e. a decreased ability to think, learn and process and information. In fact, one recent large study of Latin American and Asian populations showed that the more meat consumption, the more the dementia.  In addition, a 2009 study on rats showed that a high fat diet (55%) impaired both cognition and exercise capacity. The animals essentially became "lazy and stupid". And many people eating fast food and/or Atkins-esque or Lower Carb Diets approach levels of 55% fat.  We know that in Western societies a high fat diet goes hand in hand with saturated fat and so, basically, these have the potential to "make you stupid". NOTE: One of the positive things that the neurotransmitter glutamate does is activate NMDA receptors. Scientists recently discovered that one of the saturated fats, palmitate, also plays an important role in activating and maintaining these same receptors which are critical for memory and learning.  (Palmitic acid is one of the most common saturated fats in plants and animals.)
8) Overweight. Researchers found a very significant increase in dementia in those whose waist was too big in midlife.  However, just being overweight, regardless of the location on the body, is likely to cause issues: one recent UCLA study found that "overweight people had 4% less brain tissue than normal-weight adults".  This is likely due to clogged arteries and vessels which lowers blood flow and leads to cell shrinkage. Another study based out of Sweden found that being overweight at midlife increased your likelihood of dementia by about 60%. 
9) High Cholesterol. High cholesterol, among many other nasty things, has been associated now with Alzheimer's and dementia.  The study found that "people with total cholesterol levels between 249 and 500 milligrams were one-and-a-half times more likely to develop Alzheimer's disease than those people with cholesterol levels of less than 198 milligrams. People with total cholesterol levels of 221 to 248 milligrams were more than one-and-a-quarter times more likely to develop Alzheimer's disease". They were specifically talking about people who had cholesterol in their early 40's by the way and this backs up other previous research.  NOTE: The best way to lower high cholesterol is through the Low Fat (Ornish) Diet.
10) Stress. "Don't get stressed out" is good advice for your brain. It will age your cardiovascular system and literally rip apart your brain. And the suprising thing is that many counterintuitive aspects of life are actually stressors for your body and mind. For more information, read this link. By the way, Depression is a form of stress and very often leads to elevated Cortisol levels. High cortisol levels literally destroy your brain and some studies have shown that depressed individuals have a shrunken hippocampus, the center for memory. Read here for more details.
12) Inflammation. Researchers have long suspected that inflammation was a leading component of Alzheimer's and other dementias. For example, many researchers have noticed that those taking NSAIDs (Advil, Ibuprofen, Motrin, etc.) have significantly better outcomes for Alzheimer's. (Unfortunately, NSAID's are also associated with erectile dysfunction and a wide variety of GI disorders!) Furthermore, it was well-known that head traumas and all the associated inflammatory response was a major risk factor for developing Alzheimer's later in life. Just recently the suspicions were strongly confirmed in a September 8, 2009, print issue of Neurology study that showed that seniors with inflammation from an infection were had memory loss at twice the rate of someone without such an infection. And participants that happened to have high TNF alpha levels before the study had on average ten times the rate of memory loss. Please see this link on How to Lower Inflammation for more information.
13) Lack of Sleep. Evidence is continuing to mount that lack of sleep helps lead to dementia. Of course, this is no shock because anything that leads to increase arteriosclerosis, inflammation and high blood pressure is bound to lead to troubles for your grey matter. In addition, scientists recently found that it also directly increases beta amyloid and the associated plaques that are so characteristic of Alzheimer's.  You don't want plaque in your brain anymore than you want it in your arteries!
14) CoQ10 Megadosing. According to one animal study, megadosing on CoQ10 will damage hearing and memory. See this link on The Dangers of Supplementation for more details.
15) Heavy Alcohol Consumption. Chronic, heavy drinking is so hard on the brain that it actually shrinks it. Furthermore, alcohol is neurotoxic and the reason appears to be that it actually stimulates glutamate - can you say Excitotoxin? - activity leading to neuronal death and injury. 
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5) Amer J Clin Nutr, Received for publication February 3, 2009. Accepted for publication May 21, 2009; Emiliano Albanese, et al; "Dietary fish and meat intake and dementia in Latin America, China, and India: a 10/66 Dementia Research Group population-based study"
6) FASEB J, 2009 Aug 10. [Epub ahead of print], Murray, et. al., "Deterioration of physical performance and cognitive function in rats with short-term high-fat feeding"
7) Neurology, Received August 15, 2007, Accepted December 14, 2007, "Central obesity and increased risk of dementia more than three decades later", R. A. Whitmer PhD, et. al.
8) American Academy of Neurology (2008, April 16). High Cholesterol In Your 40s Increases Risk Of Alzheimer's Disease. ScienceDaily.
9) NEUROLOGY 2007;68:751-756, "Serum cholesterol changes after midlife and late-life cognition"
10) J of Neurology, Neurosurgery, and Psychiatry, 2008;79:864-868, Published Online First: 5 Feb 2008,"Changes in folate, vitamin B12 and homocysteine associated with incident dementia"
11) Science, Science Express Index, DOI: 10.1126/science.1180962, Published Online September 24, 2009, Submitted on August 24, 2009, Accepted on September 11, 2009, "Amyloid- Dynamics Are Regulated by Orexin and the Sleep-Wake Cycle", Jae-Eun Kang, et. al.
12) Neurology, 73(8):589-595, August 25, 2009, "Association of higher diastolic blood pressure levels with cognitive impairment"
13) Neuron, 2009 Oct 29, 64(2):213-26, "Dual palmitoylation of NR2 subunits regulates NMDA receptor trafficking"
14) Prevention, Jan 2010, p. 14.
15) Alcoholism: Clinical and Experimental Research, Published Online: 11 Apr 2006, 16(3):539-541, "Alcohol, Nitric Oxide, and Neurotoxicity: Is There a Connection? a Review"
16) International Journal of Obesity, 2009, 33:893 898; "Overweight in midlife and risk of dementia: a 40-year follow-up study"