Dementia is a neurodegenerative disease characterized by progressive impairments in memory and executive function that is prevalent worldwide [1]. In 2019, the Global Burden of Disease group (GBD) reported that morality due to dementia was ranked fourth among individuals aged over 70 [2]. Globally, 57.4 million people are living with dementia [3]. As our society ages, recent data from the GBD estimates that approximately 152.8 million individuals will be affected by dementia in 2050 [3]. The occurrence of cognitive impairment and dementia is closely linked to increased caregiving challenges and thus results in a significant economic burden to families and society [4]. Due to the rising prevalence of dementia and the burden it imposes, numerous studies have sparked the investigation of the possible risk factors of dementia and effective ways to prevent cognitive decline at an early stage [5]. Over the last two decades, research has increasingly focused on exploring the relationship between cognitive impairment and tooth loss.

Evidence has suggested tooth loss is an important risk factor for cognitive decline [6,7]. Proposed underlying mechanisms include: 1) Reduction in occluding units could result in poorer masticatory function, which might lead to decreased peripheral sensory input to the areas of the somatosensory cortex that are associated with working memory, executive function, and processing speed [8]; 2) Loss of teeth may change food preference for sugary and easy-to-chew foods instead of certain types of food such as fruits and vegetables, which may cause inadequate nutrient intake and increased risk of malnutrition that in turn increase the risk of dementia [9], and 3) Periodontal pathogens may contribute to chronic neuroinflammation in dementia through the increase of inflammatory mediators in the systemic circulation [7,10]. However, the relationship between tooth loss and cognitive impairment remains unclear, as the variables involved, and their specific roles have yet to be fully elucidated.

Recently, researchers have proposed a potential connection between dietary inflammation and cognitive decline [11], [12], [13]. Dietary inflammation, which arises from the consumption of a diet high in processed foods, sugar, and unhealthy fats, can trigger the immune system and lead to the release of pro-inflammatory molecules like cytokines, which further contribute to damage in tissues and organs [14]. The relationship between tooth loss and dietary inflammation has been discovered; For one thing, individuals who suffer from more tooth loss tend to consume a diet with lower quality [15], mainly due to the impaired mastication function that affects their eating habits and food intake. The decline in dietary quality is associated with an elevation in dietary inflammation [16,17]. For another, the increased susceptibility to dietary inflammation further has an impact on the oral condition including tooth loss and periodontitis [18,19]. Since dietary inflammation has also been suggested to play a role in cognitive function, and tooth loss could lead to dietary changes and potentially dietary inflammation, it is plausible to assume that a pro-inflammatory diet may serve as a mediating factor between tooth loss and cognitive impairment. Additionally, dietary inflammation index (DII) is not only used as an evaluation method for dietary pattern, but also extensively utilized across different populations to forecast inflammation levels and systemic diseases, encompassing cardiovascular diseases and rheumatoid arthritis [20,21].

Serum albumin level has been identified as a powerful predictor of disease prognosis and is vulnerable to inflammation and inadequate protein and caloric intake in individuals with chronic disease [22]. A retrospective study reported that participants with a relatively lower baseline serum albumin concentration were more likely to experience mild cognitive decline and cardiovascular disease [23]. Similarly, a cohort study from Korea reported that lower serum albumin levels were associated with lower MMSE scores [24]. Moreover, an animal study has found serum albumin serves as an inhibitor in polymerization, and a cleaner in the removal, of β-amyloid, indicating a protective role on neuronal and glial cells [25]. In addition to the above evidence, some studies supported that the decreased serum albumin level can have an impact on the mortality rate, which was supported by the inflammation pathway [26]. This suggests that a lower immunity level may be implicated in the risk of inflammation, thereby having an adverse impact on cognitive function.

Therefore, in the current study, we hypothesized that:

Dietary inflammation pattern plays an indirect role in the association between tooth loss and cognitive function; and,

Serum albumin concentration acts as a moderator of the relationship between dietary inflammation and cognitive function.