Docosahexaenoic acid (DHA) is an omega-3 fatty acid found abundantly in the brain and is essential for normal neuronal function. Its potential use in Alzheimer's disease (AD) is based on both biological plausibility and preclinical research. DHA is thought to support membrane fluidity, synaptic function, and reduce neuroinflammation, all of which are disrupted in AD. Epidemiological studies have suggested that higher dietary intake of DHA is associated with reduced risk of cognitive decline and Alzheimer's disease. Animal studies have shown that DHA can reduce amyloid-beta accumulation and improve cognitive performance.

However, randomized controlled trials (RCTs) in humans have produced mixed results. While some small studies have observed cognitive benefits in individuals with mild cognitive impairment or early AD, larger and more rigorous RCTs have generally failed to demonstrate significant benefits of DHA supplementation in slowing cognitive decline or improving symptoms in established Alzheimer's disease. Meta-analyses and systematic reviews conclude that while DHA is safe and biologically plausible, evidence for its effectiveness in treating or preventing AD is limited and inconsistent.

In summary, there is scientific rationale and some early evidence supporting DHA's use for Alzheimer's disease, but current clinical data do not robustly validate its efficacy. The overall evidence is moderate to weak.