This is a really nice paper, it does provide a good background on brain metabolism and the known effects of ketone both to mild cognitive impairment and Alzheimer's.

TLDR: Ketones are the brain's PREFERRED energy source from birth all the way up to development of Alzheimer's. The theory is that insufficient brain glucose metabolism is the leading cause of MCI and AD (i.e. hyperinsulinemia). The brain is basically starving of energy in older adults. Exogenous ketones are beneficial to get energy to the starving brain, and pump some life into damaged cells.

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The concept that ketones could be of therapeutic value during brain aging hinges on demonstrating that there is a pre-symptomatic problem with brain glucose metabolism in people at risk of AD but who are still cognitively normal.

laid the foundation for our current understanding that ketones are an essential physiological brain fuel working in tandem with glucose to assure that brain energy requirements are being met on a daily basis, not just in infants or during starvation

The importance of this point became clear to us when we studied young women with mild insulin resistance due to PCOS. They had a brain glucose uptake deficit in the superior and middle frontal cortex of about 14%, i.e., a profile resembling that of people in their 70 and 80s (Castellano et al., 2015a). PCOS is a multi-factorial endocrine disease involving not only mild-moderate insulin resistance but also infertility and hyperandrogenism.

This is critical, PCOS is driven by hyperinsulinemia, so its highly correlated with brain energy deficiencies. So a women in her early twenties with PCOS has the brain energy of a 80 year old.....

In our women with PCOS, glucose uptake in several brain regions was significantly inversely correlated to both the mild insulin resistance and to fasting plasma glucose. Recent studies in young women with PCOS demonstrate that insulin resistance is associated not only with impaired brain glucose metabolism but also with altered white matter microstructure and cognitive performance (especially working memory) in young adults

Unlike in the human adult in whom the brain appears to use ketones only to compensate for periodic insufficiency in glucose supply, in the developing infant, ketones are essential both as a major fuel and also as the main substrate for brain lipid synthesis

The difference between the infants and adults is the introduction of carbohydrates (and that addiction), which drive insulin, which suppress ketones. The above quote is really just saying modern society is so carb addicted it's foundational interfered with everyones brain metabolism.

Postnatally, the brain’s dependence on ketones is made possible because infants are normally in a sustained state of mild ketosis (0.2–0.5 mM β-HBA). This neonatal ketosis is present regardless of whether the infant has just been fed or is in a post-prandial state, i.e., the ketosis is not a function of food restriction or hypoglycemia (Settergren et al., 1976). This contrasts with the adult human in whom 0.5 mM β-HBA in plasma is normally only achieved after 24–48 h fasting accompanied by hypoglycemia and hypoinsulinemia.

The link between sustained ketosis and MCT in breast milk is fascinating. Babies are always in keto!

Ketones: The Brain’s Preferred Fuel

Just read this whole section, really.

Both short-term PET and arterio-venous difference studies in humans show that brain glucose consumption decreases as ketone availability to the brain increases. These results suggest that ketones are actually the preferred energy substrate for the brain because they enter the brain in proportion to their plasma concentration irrespective of glucose availability; if the energy needs of the brain are being increasingly met by ketones, glucose uptake decreases accordingly.

The brain uses ketones first, and any unmet need is then given to glucose.

Under conditions of normal energy sufficiency and three meals per day, ketogenesis is supressed and glucose supplies >95% of the brain’s energy requirements; hence, glucose (or fuels derived from glucose, i.e., lactate or pyruvate) is the brain’s dominant but not actually its preferred fuel.

BINGO - This is the problem. 3 meals a day of carbohydrates is breaking our metabolism, and starving the brain (over years). Carbs drive insulin. Insulin suppresses ketones. Over time too much insulin prevents the brain from using glucose... starvation.

Hyperinsulinemia inhibits the normal ketogenic response (Bickerton et al., 2008), thereby putting the aging brain in double jeopardy of being deprived of both its primary fuels. We believe that this problem is at the root of the vicious cycle between deteriorating brain fuel uptake/availability and deteriorating brain function that leads to AD.

This is it. Hyperinsulinemia breaks the body, and stops ketosis. Everyone eating carbs constantly is racing for hyperinsulinemia. It is a slow and vicious process.

Our work shows that older people classified as cognitively normal by conventional neuropsychological tests corrected for age and education have significant brain atrophy, cortical thinning and lower brain glucose metabolism compared to cognitively normal younger adults

The decay happens far before people notice.