Alzheimer's disease (AD) and Parkinson's disease (PD) are the two most common neurodegenerative conditions, impacting 7.2 million Americans combined (Alzheimer's Association, 2022). Mild cognitive impairment (MCI) is often viewed as a precursor to Alzheimer's disease, with a 22 % risk of conversion to clinically definite Alzheimer's dementia annually (Davis et al., 2018). There are no effective disease modifying therapies for these conditions to impact disease progression (Cummings, 2017).

In neurodegenerative conditions such as AD and PD there is evidence of metabolic changes impacting glucose uptake in the brain, potentially worsening cognitive and motor function (Augustin et al., 2018; Murakami et al., 2010). In AD there is evidence of reduced cerebral glucose metabolism and brain insulin resistance leading to impairment of brain energy metabolism (Augustin et al., 2018; Phillips et al., 2021). Whereas in PD there is a known defect in the respiratory chain complex I activity impacting mitochondrial oxidative phosphorylation in the brain (Phillips et al., 2018). Although the full mechanism of action is not completely understood, following a high fat, low carbohydrate (ketogenic) diet has shown the ability to provide neurons and astrocytes a primary energy source in ketones, as opposed to glucose (Augustin et al., 2018). It is hypothesized that using ketone bodies as a main energy source may provide clinical benefit in persons with AD and PD (Phillips et al., 2021, Phillips, Murtagh, Gilbertson, Asztely and Lynch, 2018).

The ketogenic diet is comprised of foods that are contain high fat, low carbohydrate, and adequate protein macronutrients to induce ketosis (Barzegar et al., 2021). Ratios of fats to carbohydrates vary from 3:1 to 4:1 depending on the patient's age and needs (Barzegar et al., 2021). There are a variety of ketogenic diets used in epilepsy, including the classic ketogenic diet, a medium-chain triglyceride diet, a modified Atkins diet, and a low glycemic index treatment as a means to reach ketosis (Barzegar et al., 2021).

Understanding the benefits of the ketogenic diet in neurologic conditions other than epilepsy is a dire need, particularly for neurodegenerative conditions where pharmacologic treatments are limited. The purpose of this integrative review is to clarify the potential benefits of the ketogenic diet for symptom management or disease modification in patients with Parkinson's disease, Alzheimer's disease, or MCI.