Cognitive impairment (CI) can range from mild cognitive impairment (MCI), which is commonly regarded as the transition stage between an expected cognitive decline due to aging and dementia, to more advanced and serious forms of dementia, such as Alzheimer’s disease (Pankratz et al., 2015). The global prevalence of cognitive impairment was estimated to range between 1 and 42% depending on the setting (clinical vs. population-based) and classification used (Ward et al., 2012; Sachdev et al., 2015; Petersen, 2016). In India, various studies have reported wide variation in the prevalence of MCI among different populations ranging from 3.5 to 63.2% (Das et al., 2007; Kumar and Sudhakar, 2013; Sharma et al., 2013; Sengupta et al., 2014; Khullar et al., 2017; Kaur et al., 2018).

Since MCI in itself is a transient phase, not all individuals experiencing MCI progress to dementia (Aretouli et al., 2010; Ganguli et al., 2011). While some MCI patients might develop advanced dementia, others may regain normal cognitive function with time (Aretouli et al., 2010; Ganguli et al., 2011) and intervention (Meng et al., 2021). Though studies have acknowledged the complexity of defining loss of cognitive functions, especially the subclinical expressions of CI, into discrete categories (Ritchie et al., 2001; Richardson et al., 2019), cognitive impairment, for research purposes, has often been classified into three categories, i.e., mild, moderate and severe CI (Murray et al., 2006; Choe et al., 2008).

Cognitive impairment is etiologically complex. Apart from socio-demographic parameters like gender, education level, occupational status, and family medical history, several neurologic, systemic, genetic, biochemical, and psychiatric factors are believed to contribute to the development and progression of CI (Lopez, 2013; Singh et al., 2023). Of these, abnormal lipid levels and hypercholesterolemia are emerging as important modifiable risk factors for CI and other psychiatric conditions (Cicconetti et al., 2004; Khullar et al., 2017; Chaudhary et al., 2022). Further, as estrogen levels are known to play a vital role in neurocognitive function, adverse reproductive history/events and hormonal imbalances are also being investigated for their role in cognitive decline among women (Li et al., 2016; Shimizu et al., 2019; Song et al., 2020).

Despite clinical reports linking blood lipid levels and reproductive trajectory with cognitive health, the relationship between cognitive impairment and lipid levels or women’s reproductive trajectory in community settings remains understudied. Since blood lipid levels, as well as reproductive trajectories, are greatly affected by the ecological and cultural milieu of a community, community-specific studies exploring the relationship between CI and blood lipids or CI and women’s reproductive trajectory are crucial to designing effective interventions. Looking at the paucity of community-based studies, especially from India, the present study aims to explore the association of CI with blood lipid levels in both sexes and reproductive events/trajectory among women of an endogamous North Indian Population.

Abnormal lipid levels and adverse reproductive events have been reported to influence cognitive functions and play a role in the development of cognitive impairments (Van Exel et al., 2002; Devore et al., 2004; Power et al., 2018; Lv et al., 2019; Ning et al., 2020; Song et al., 2020; Gong et al., 2022). However, very few studies have methodically investigated their relationship in community settings. Further, even lesser attention has been given to the gender-specific relationship between blood lipids and CI (Ancelin et al., 2013, 2014; Svensson et al., 2019; Choe et al., 2021; Bakeberg et al., 2021a). It is important to highlight that sex is a crucial factor to be considered when conducting cognitive assessment studies (Reekes et al., 2020; Bakeberg et al., 2021a). Literature suggests that women have better verbal and non-verbal reasoning skills, such as language, fluency, memory, and decision-making, when compared to their male counterparts (Li and Singh, 2014). Other studies have also emphasized on sex-specific approach to studying CI because of physiological differences between males and females, which eventually cause cerebrovascular and cognitive decline (Miller et al., 2013; Reekes et al., 2020; Sundermann et al., 2021). Considering these research gaps, the present study attempted to investigate the relationship between blood lipid levels and CI among both males and females and reproductive trajectory and CI among females of an endogamous North Indian Population.

In the present study, median TG and VLDL-C levels were significantly higher among individuals with normal cognition than those with mild CI. This observation is in concordance with some of the previous reports where TG levels have been reported to be significantly higher in control groups (normal cognition group) than in case groups (MCI group; Dimopoulos et al., 2007; Lepara et al., 2009; He et al., 2016). However, contrary findings have also been reported (De Frias et al., 2007; Sims et al., 2008). Since the adverse implications of pathologically high TG can easily outweigh the benefits of high TG in cognitive functions, population-based studies, as well as clinical trials, are warranted to establish population-specific healthy ranges of TG. Again, despite significant differences in the median TG levels between normal cognition and mild CI groups, adjusted logistic regression models failed to find any significant association between TG and CI (in overall as well as sex-stratified analyses). Further research is required to explicate the relationship between TG and CI.

In the present study, the median HDL-C level was found to be higher among individuals with moderate/severe CI than those with normal cognition. In odds ratio analysis, low HDL-C was found to be protective against CI among females but not among males. Also, females in the fourth quartile of HDL-C were at significantly increased risk of CI than those in the second and third quartiles. Overall, these observations hint toward low HDL-C being protective and high HDL-C being a risk for CI among females (suggestive significance). Though these findings are largely in contradiction with previous reports (Reitz et al., 2010; Hottman et al., 2014; Vitali et al., 2014; Bates et al., 2017), Ancelin et al. (2014), in their study among elderly individuals, reported an increased risk of decline in cognitive functions among females having high HDL-C than those having low HDL-C. A recent study conducted among females with Parkinson’s disease (PD) has also found elevated HDL-C to significantly contribute to cognitive decline (Bakeberg et al., 2021b). The mechanism behind the higher risk of CI among women having higher HDL-C remains to be clarified, yet, studies have suggested the role of genetic vulnerability related to HDL-C, estrogen receptors, or metabolizing enzymes that could affect HDL-C responsiveness (Ancelin et al., 2014).

Other lipid variables, viz. TC and LDL-C, were not found to be associated with CI in either sex. Previous studies investigating the role of plasma lipid levels in cognitive functions have reported inconsistent findings (Mielke et al., 2005; Reitz et al., 2008; Morley and Banks, 2010; Li et al., 2018). The relationship between plasma lipids and cognitive function, the role of gender in modulating this relationship, as well as the mechanism by which plasma lipids affect cognitive function are still not fully understood.

Coming to the role of reproductive trajectory in cognitive impairment, menopausal women and women with five or more live births were found to be at a higher risk of CI. Previous studies have also reported menopause (Weber et al., 2014; Conde et al., 2021) and higher numbers of children (Beeri et al., 2009; Read and Grundy, 2017) as risk factors for cognitive decline.

Women have been shown to experience cognitive deficiencies during the menopausal transition, notably in areas like working memory, attention, slower processing speeds, and verbal memory (Miller and Cronin-Golomb, 2010; Lin et al., 2018). There is growing evidence that oestrogen has a major neuroprotective and neurotrophic effect on the central nervous system (Conde et al., 2021). The decline in oestrogen levels among post-menopausal women can be one of the factors behind the observed decline in cognitive functions. Menopausal symptoms like vasomotor symptoms, insomnia, and fatigue are also likely to affect the cognitive health of menopausal women (Conde et al., 2021).

Further, the association between cognitive impairment and the higher number of live births may be explained by both biological mechanisms and social/behavioral factors. Pregnancy and childbirth trigger a wide range of alterations in endocrine activities and metabolic processes, including changes in blood lipoprotein level, increased obesity, and central adiposity (Lahmann et al., 2000; Lain and Catalano, 2007; Soma-Pillay et al., 2016). These factors, independently and/or via other metabolic and cardiovascular diseases, may be associated with increased odds for CI. Interestingly, some recent studies have highlighted that the higher number of children affects the cognitive health of both men and women, indicating gender indifference (Ning et al., 2020; Song et al., 2020; Gong et al., 2022). Therefore, social/behavioral explanations appear to be more plausible. Having numerous children and raising them in a society with fewer social supports may result in significant psychological stress throughout the reproductive years and beyond, which may have an adverse effect on brain health (Wilson et al., 2006). A higher number of children is also associated with lower economic status and poor education, which, in turn, may affect brain health (Andel et al., 2006).

There are some important limitations of the study that must be mentioned. First, MMSE has been used for CI screening; while it is a reliable screening tool, it is not the gold standard test for diagnosing CI. Second, this is a single-site study; a multi-site study would have been more suitable because environmental factors can modify the relationship between the lipid variables and CI. Another limitation is the lack of hormonal data among women, which would have further substantiated the study’s findings.

The present study hints toward a gender-specific relationship between blood lipid parameters (HDL-C in particular) and CI. While none of the blood lipids were found to be associated with CI among males, low HDL-C was protective against CI among females (suggestive significance). Future studies investigating the mechanisms underlying the interactions between plasma lipids and cognition will stimulate new approaches to the treatment and prevention of cognitive disorders. Furthermore, menopause and the higher number of live births were found to be risk factors for CI among females. Pregnancy, childbirth as well as the menopausal transition are crucial reproductive milestones and are associated with significant hormonal changes and stress that may, in turn, be linked to cognitive deterioration.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving human participants were reviewed and approved by Departmental Ethics Committee, Department of Anthropology, University of Delhi. The patients/participants provided their written informed consent to participate in this study.

KS and ND designed and executed the research project and conceptualized the manuscript. KK designed and executed the statistical analyses and wrote the first draft of the manuscript. VC and IL reviewed and critiqued the statistical analysis and manuscript preparation. KS, ND, and IL thoroughly reviewed and critiqued the final version of the manuscript. All authors contributed to the article and approved the submitted version.

This study was supported by the Research and Development Grant, University of Delhi, New Delhi and Department of Biotechnology, Ministry of Science and Technology, Government of India under grant BT/PRI14378/MED/30/535/2010.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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