Alzheimer's Disease: Causes and Risk Factors

Causes of Alzheimer's Disease

The exact cause of Alzheimer's disease is unknown. However, the disease is associated with abnormal proteins that accumulate in the brain and damage nerve cells. Genetic, environmental, and lifestyle factors all play a role together.

• Amyloid plaques. Protein fragments called beta-amyloid accumulate outside brain cells and form plaques. These plaques block communication between nerve cells and trigger inflammation. Amyloid buildup begins years before Alzheimer's symptoms appear. When the brain is unable to clear amyloid, the accumulation increases.
• Tau tangles. The tau protein normally stabilizes microtubules inside nerve cells. In Alzheimer's, tau changes abnormally and forms thread-like structures that tangle inside the cell (neurofibrillary tangles). This blocks the transport of nutrients and other substances within the cell and leads to cell death.
• Brain atrophy (Shrinkage). As nerve cells die, the brain shrinks. Brain regions associated with memory and learning (particularly the hippocampus) are especially affected. Brain imaging shows a reduction in brain tissue and enlargement of the ventricles.
• Neurotransmitter imbalance. Levels of a chemical messenger called acetylcholine decrease significantly. Acetylcholine is critically important for memory and learning. Cholinesterase inhibitors used in Alzheimer's treatment aim to increase acetylcholine levels.
• Inflammation and oxidative stress. Chronic inflammation in brain tissue and oxidative damage caused by free radicals destroy nerve cells. Microglia (the brain's immune cells) become overactive and can contribute to cell damage.
• Vascular factors. Reduced blood flow to the brain increases Alzheimer's risk. High blood pressure, diabetes, high cholesterol, and heart disease damage the brain's blood vessels. Some cases represent a combination of Alzheimer's and vascular dementia (mixed dementia).
• Genetic factors. Early-onset Alzheimer's has a strong genetic component. Mutations in the APP, PSEN1, and PSEN2 genes can cause early-onset Alzheimer's. In late-onset Alzheimer's, the APOE-e4 gene variant is associated with increased risk. Those carrying one copy of APOE-e4 are at elevated risk, while those carrying two copies are at even higher risk. However, carrying APOE-e4 does not guarantee Alzheimer's; it only indicates an increased risk.

Risk Factors for Alzheimer's Disease

Some factors increase the risk of developing Alzheimer's disease. Some risk factors are modifiable (lifestyle-related), while others are not (age, genetics).

• Age. Age is the greatest risk factor for Alzheimer's disease. Risk increases significantly after age 65. While prevalence is around 5% between ages 65-74, it approaches 33% in those over age 85. However, Alzheimer's is not an inevitable consequence of normal aging.
• Family history and genetics. Having a first-degree relative (parent or sibling) with Alzheimer's increases risk. The stronger the family history, the higher the risk. Carriers of the APOE-e4 gene variant are at higher risk. Gene mutations that cause early-onset Alzheimer's are rare but carry nearly 100% risk.
• Down syndrome. People with Down syndrome typically develop Alzheimer's symptoms in their 40s or 50s. The extra copy of chromosome 21 that causes Down syndrome increases the number of copies of the gene responsible for amyloid production.
• Female sex. Women develop Alzheimer's at higher rates than men. This is partly explained by women's longer life expectancy, but hormonal factors may also play a role. The loss of estrogen after menopause has been associated with increased risk.
• Mild cognitive impairment (MCI). MCI refers to memory or thinking problems that are greater than normal aging but do not meet the criteria for dementia. A significant proportion of people with MCI go on to develop Alzheimer's, though not all do. Amnestic MCI (memory-related) in particular increases Alzheimer's risk.
• Head trauma. Serious head trauma or repeated concussions can increase Alzheimer's risk. Traumatic brain injury can accelerate amyloid accumulation. Caution is needed in high-risk groups such as boxers, football players, and military personnel.
• Cardiovascular risk factors. Factors that affect heart health also affect brain health. High blood pressure, high cholesterol, diabetes, obesity, and smoking all increase Alzheimer's risk. These factors damage the brain's blood vessels and impair the clearance of amyloid.
• Education level and cognitive reserve. Lower levels of education may increase Alzheimer's risk. Higher education and lifelong learning build "cognitive reserve"; meaning more synaptic connections in the brain. People with greater cognitive reserve show better resistance to brain damage.
• Social isolation. Lack of social interaction may increase Alzheimer's risk. Social activities stimulate the brain and preserve cognitive function. Loneliness and depression are also risk factors.
• Sleep disorders. Chronic sleep deprivation or sleep apnea may increase Alzheimer's risk. During sleep, the brain clears amyloid and other waste products. Sleep disorders interfere with this clearance process and increase amyloid accumulation.
• Sedentary lifestyle. Lack of physical activity increases Alzheimer's risk. Exercise increases blood flow to the brain, promotes neuroplasticity (the formation of new neural connections), and reduces inflammation.
• Alcohol use. Excessive alcohol consumption leads to brain damage and increased dementia risk. However, it has been suggested that light to moderate alcohol consumption (particularly red wine) may be protective, though the evidence on this remains mixed.
• Air pollution. Some research suggests that exposure to air pollution may increase Alzheimer's risk. Particulate matter can reach the brain and cause inflammation.