1.Trans fat

 Trans fats are naturally found in animal products, including meat and dairy products, but even these are not as troublesome as industrially produced trans fats that are pumped into all kinds of packaged foods.

 Studies have shown that high consumption of trans fat also leads to earlier cognitive decline, lower brain volume, and poorer memory.

 2. Sweet drinks
 
 Sugary drinks like soda, sports drinks, energy drinks, and even fruit juices have little or no nutritional value.  Regular consumption of sugary drinks can lead to a variety of physical disabilities, including type 2 diabetes, high blood pressure, high cholesterol and yes, Alzheimer's disease or dementia.

 3. Refined carbohydrates

 Refined carbohydrates are products made from processed grains.  They don't necessarily taste sweet, but they break down very quickly into sugar in your body.  This is because the refining process removes all the fiber and nutrition from the original grain.  A meal rich in refined carbohydrates represents a high glycemic load that increases blood sugar.

 4. Processed and packaged foods
 However, it is important to cook with whole nutritious foods as often as possible, as the Western diet is known to cause fat accumulation around vital organs.  This, in turn, is associated with damage to brain tissue and a reduction in brain volume.  It can also disrupt the blood-brain barrier, the membrane responsible for protecting the brain from harmful substances.

 5. alcohol
 
 It's probably not surprising that alcohol can harm the brain, considering the amount of stupid things that people tend to do under the influence.  Getting drunk once in a while is unlikely to cause permanent damage, but alcoholism and binge episodes can.