What does the body actually do for the brain, and why are most cognitive symptoms physiological?

What systems affect cognitive function?

Cognitive function depends on several interconnected systems:

• Metabolic. The brain consumes 20% of the body's glucose. Blood sugar instability, insulin resistance, and metabolic dysfunction directly affect cognition. The link between metabolic health and brain health is now well-established [PMID: 39200352]
• Hormonal. Estrogen supports memory and mood (its decline in perimenopause directly affects cognition), testosterone influences focus and motivation, thyroid affects nearly every aspect of cognitive function, cortisol patterns affect attention and memory consolidation
• Vascular. Cerebral blood flow, vascular health, and the integrity of small vessels in the brain matter for cognition. Cardiovascular disease is also brain disease
• Inflammatory. Chronic inflammation affects neural signaling speed, neurotransmitter production, and contributes to neurodegeneration over decades
• Sleep. Memory consolidation happens during sleep, particularly during deep and REM stages. Disrupted sleep architecture impairs cognition independently of all other factors
• Gut. 90% of serotonin and roughly 50% of dopamine are produced in the gut. Gut inflammation and dysbiosis affect mood and cognition through what's called the gut-brain axis
• Nutritional. Omega-3 fatty acids (DHA in particular), B vitamins (especially B12 and folate), vitamin D, magnesium, choline, and iron all support cognitive function. Deficiencies in any of these can mimic primary cognitive disorders

What labs map cognitive risk and current function?

A useful cognitive workup typically includes:

• Metabolic markers. Fasting insulin and HOMA-IR (a calculated insulin sensitivity score), HbA1c
• Full thyroid panel. TSH (the brain's signal to the thyroid), free T4, free T3, reverse T3, and antibodies
• Sex hormones, especially relevant in perimenopause and andropause
• Inflammatory markers. hs-CRP (a general inflammation marker), homocysteine
• Nutrient markers. Vitamin D, B12 with methylmalonic acid (which catches functional B12 deficiency), omega-3 index (the proportion of EPA and DHA in red blood cell membranes), ferritin (the iron storage marker)
• Cardiovascular markers. ApoB and Lp(a), since cerebrovascular health is part of cognitive health
• ApoE genotype. A one-time test that stratifies Alzheimer's risk
• Depending on presentation: organic acids panels for neurotransmitter metabolism, advanced markers including p-tau and amyloid blood tests (in research and increasingly clinical use)

Why is cognitive decline not inevitable with aging?

Cognitive decline isn't a feature of aging that simply happens. The pathological processes that lead to dementia begin 20 to 30 years before symptoms appear, which means the window for prevention is the 40s and 50s, not the 70s when memory problems start. The 2024 Lancet Commission on dementia prevention identified 14 modifiable risk factors associated with roughly 45% of dementia cases worldwide [Source: Lancet Commission 2024, Dementia prevention, intervention, and care]. The risk factors span the life course: education in early life; midlife factors including hearing loss, traumatic brain injury, hypertension, alcohol misuse, obesity, depression, physical inactivity, diabetes, smoking, and high LDL cholesterol; late-life factors including social isolation, vision loss, and air pollution exposure.

What gets accepted as normal aging is, in significant part, identifiable, modifiable dysfunction. The diseases that drive cognitive decline (Alzheimer's, vascular dementia, mixed dementia) develop slowly and are influenced by factors that can be addressed.

How does ApoE4 affect Alzheimer's risk?

ApoE genotype is the strongest known genetic risk factor for late-onset Alzheimer's disease in people of European descent [PMID: 38710950]. The gene comes in three variants (ε2, ε3, ε4), and people inherit two copies, one from each parent.

The risk pattern:

• One copy of ε4 (heterozygous): roughly 3 to 4-fold higher risk of late-onset Alzheimer's
• Two copies of ε4 (homozygous): roughly 9 to 15-fold higher risk, with an estimated 60% chance of developing Alzheimer's dementia by age 85 [Source: NIH, citing Fortea et al. 2024]
• The ε2 variant is associated with reduced risk relative to ε3

Roughly 25% of the population carries at least one ε4 copy. About 2% are homozygous for ε4.

Carrying ApoE4 doesn't determine outcome. Many ε4 carriers reach old age without developing Alzheimer's, particularly those who aggressively address modifiable risk factors. The genotype changes the risk-benefit calculus on prevention strategies; it doesn't dictate the future.

What are the four healthspan domains, and why do they connect?

The four domains that matter most for healthspan, the years of healthy functional life:

• Cardiovascular
• Metabolic
• Cognitive
• Physical function (strength, balance, mobility)

These aren't independent. Insulin resistance drives cardiovascular disease, and both drive cognitive decline. Cardiovascular health is brain health: vascular dementia is the second most common form of dementia, and it's substantially preventable through cardiovascular risk reduction. Sarcopenia (the age-related loss of muscle mass) reduces physical function and increases falls, which increases head injury risk and brain trauma.

Optimizing one domain tends to support the others. Strength training affects all four. Sleep affects all four. Metabolic health affects all four. The interventions that protect cognition are largely the same interventions that protect cardiovascular health, body composition, and physical capability.

The deeper picture

Cognitive complaints are some of the most context-dependent presentations to evaluate. The right workup, interpreted in the right clinical context, often identifies a driver that's quite responsive to intervention. The brain has remarkable capacity for recovery when underlying physiology is addressed. Dr. Paul approaches cognitive presentations with this kind of comprehensive evaluation as part of standard precision medicine care.