Effects of caregiving burden on serum homocysteine and folate levels in spouses of patients with cognitive impairment

Abstract

Background:

Spousal caregivers (SCGs) of patients with cognitive impairment are predisposed to an elevated risk of cardiovascular disease and cognitive decline. Homocysteine and folate levels are potential modifiable biomarkers for these risks, given their associations with cardiovascular and cognitive health.

Objective:

This study aimed to examine the association of the caregiving burden with homocysteine and folate levels in SCGs.

Methods:

This study included 107 SCGs and their care recipients who visited the Chungnam National University Hospital between May 2020 and May 2023. The caregiving burden was quantified using the Zarit Burden Interview, and blood assays were performed after overnight fasting. We adjusted for the age and sex of SCGs and the clinical diagnosis of care recipients as covariates.

Results:

The caregiving burden of SCGs showed a significant positive correlation with their homocysteine level (β = 0.219, t = 2.165, p = 0.033). In contrast, caregiving burden and folate showed an inverse correlation (β = -0.207, t = -1.997, p = 0.049). Furthermore, the association between caregiving burden and homocysteine levels remained significant after including vascular risk score and physical activity level as covariates (p = 0.042).

Conclusions:

Our findings indicate that the caregiving burden of SCGs in individuals with cognitive impairment is associated with elevated homocysteine levels and low folate levels, potentially exacerbating the risk of cardiovascular disease and cognitive decline. These findings underscore the importance of monitoring homocysteine and folate as modifiable biomarkers to address and potentially mitigate health risks in SCGs.

Keywords

Alzheimer’s disease cognitive impairment folate homocysteine spousal caregiver

Introduction

Caregiving for patients with cognitive impairment has many physical and psychological effects on informal caregivers.¹ Spousal caregivers (SCGs) constitute the largest group of informal caregivers of patients with dementia² and often are the sole caregivers for patients.^2,3 In general, the caregiving burden increases cardiovascular risk in SCGs,^4–6 and the same has been reported in spouses of people with dementia.^7–9 In a longitudinal analysis, SCGs of patients with mild cognitive impairment (MCI) or Alzheimer's disease (AD) dementia showed an increased risk of hypertension compared with demographically equivalent non-caregiver controls.⁷ Furthermore, SCGs of patients with dementia displayed an increased risk of cognitive decline.^10–12 In a longitudinal study, SCGs of patients with dementia performed significantly worse in several cognitive domains than the controls.¹² The SCGs of patients with cognitive impairment are generally older adults, and the caregiving burden puts them at an even greater risk of cardiovascular disease and cognitive decline.

Therefore, the SCGs of patients with cognitive impairment are a potentially important screening population, and early interventions by identifying modifiable factors and blood-based biomarkers that can predict negative health outcomes in this population are crucial. Notably, a biomarker that can be easily tested and clinically relevant is essential. Homocysteine levels can not only be easily tested but have also been reported to be associated with both cardiovascular disease and cognitive decline.^13–17 Therefore, we aimed to determine their potential use as biomarkers. Homocysteine, a non-protein, sulfur-containing essential amino acid intermediate in methionine metabolism,¹⁸ acts on the vascular endothelium and smooth muscle cells to cause cardiovascular problems.¹⁵ Elevated plasma homocysteine levels are also a strong risk factor for the development of all-cause dementia and AD.¹⁷

And because folate is required for the conversion of homocysteine to methionine, sufficient folate intake and adequate folate levels lower homocysteine levels.¹⁹ Folate plays an important role in preventing cardiovascular diseases. It has been reported that folate has independent benefits unrelated to homocysteine, such as antioxidant activity.²⁰ A meta-analysis showed that folate intake and blood folate levels were inversely associated with the risk of coronary heart disease risk.²¹ Additionally, inadequate folate levels are associated with cognitive decline and dementia.²² Given this, in this study, we explored the association between caregiving burden and homocysteine and folate levels in SCGs. Investigating the association between caregiving burden and these blood-based biomarkers may help predict and prepare for negative health outcomes in SCGs. We also exploratively looked at differences in statistics based on sex since several previous studies on caregivers have shown differences in outcomes by sex.^4,23,24

Methods

Participants

This study included SCGs and their care recipients who visited the Geriatric Neuropsychiatric Clinic of the Chungnam National University Hospital, Daejeon, Korea, between May 2020 and May 2023. The detailed study protocol and inclusion criteria have been previously reported.²⁵ The SCGs were between 55 and 90 years old. All SCGs had no history of MCI or dementia at the time of enrollment and were not taking cognitive enhancer. We categorized the care recipients into three groups based on their clinical diagnosis: normal cognition (NC), MCI, and dementia. NC was defined by a global Clinical Dementia Rating (CDR) of 0. NC care recipients presented to the clinic with anxiety disorders or insomnia. The MCI group was defined according to the diagnostic criteria of the National Institute of Aging and Alzheimer's Association guidelines.²⁶ The MCI group maintained independence in their overall daily functioning. Dementia was diagnosed according to the Diagnostic and Statistical Manual of Mental Disorders DSM-IV (American Psychiatric Association, 1994). The exclusion criteria for SCGs and care recipients were as follows: (1) concomitant brain or systemic diseases that could significantly affect their mental status, (2) participants with poor vision or hearing that would interfere with the performance in neuropsychological tests, and (3) participants who were illiterate. All participants were assessed by an experienced clinical psychologist and a research nurse. The study was approved by the Ethics Review Committee of Chungnam National University Hospital (committee's reference number 2020-05-002). Participants were fully informed about the study outline, and informed consent was obtained in writing.

Clinical assessment

The caregiving burden of the SCGs was measured using the Korean version of the Zarit Burden Interview (ZBI).²⁷ The ZBI is one of the most commonly used tools for measuring caregiving burden.²⁸ The ZBI consists of 22 questions on a 5-point scale (0 = ‘never,’ 4 = ‘almost always’), with higher scores indicating greater caregiving burden.²⁸ Furthermore, the Korean version of the ZBI has also been validated for measuring caregiving burden.²⁷

To explore the association between caregiving burden and the SCGs’ lifestyle factors, we obtained data on physical activity and sleep. We measured the SCGs’ physical activity using the Korean version of the International Physical Activity Questionnaire (IPAQ).²⁹ As low physical activity is associated with the development of cardiovascular disease, we also utilized the IPAQ score as a covariate. We used the Korean version of the Pittsburgh Sleep Quality Index (PSQI) to obtain information on the sleep quality of the SCGs.³⁰ The reliability and validity of the Korean version of the PSQI have been previously confirmed.³⁰ The symptoms of depression in SCGs, which are thought to be closely related to the caregiving burden, were measured using the Korean version of the Geriatric Depression Scale (GDS-KR), a self-report instrument.³¹ A higher GDS score indicates more severe depressive symptoms, and the reliability and validity of the GDS-KR have been validated.³¹ Vascular risk score (VRS) in the SCGs was determined in proportion to the number of comorbid vascular risk factors (diabetes mellitus, hypertension, stroke, TIA, hyperlipidemia, and coronary artery disease).

Blood sampling and laboratory assessments

Blood samples were obtained from the subjects via venous blood collection in the early morning after an overnight fast. Samples were collected in SST tubes and tested for homocysteine using a chemiluminescent microparticle immunoassay and for folate using a radioimmunoassay.^32,33

Statistical analysis

To determine the association of the caregiving burden of SCGs with homocysteine and folate levels, we used linear regression analysis with the ZBI score as the independent variable and blood-based biomarkers as the dependent variable. Furthermore, we adjusted the SCGs’ age, sex, and the clinical diagnosis of care recipients (dementia versus non-dementia). In further analyses, we adjusted for VRS and SCGs’ IPAQ scores in addition to the aforementioned covariates. To explore the moderating effect of sex on the association between ZBI score and blood-based biomarkers, we performed multiple linear regression analyses with the interaction term (ZBI × sex) as the independent variable and the blood-based biomarkers as the dependent variable. Finally, we analyzed the association between caregiving burden and other lifestyle factors. Participants with missing data for the variables that we wanted to analyze were excluded from the analysis. All analyses were performed using SPSS 21 software (SPSS, Inc.; Chicago, IL, USA), and p-values of < 0.05 (two-sided) were considered statistically significant.

Results

Characteristics of the participants

The demographic characteristics of the study participants are summarized in Table 1. Of the 107 SCGs, 9 (8.4%) were from the NC group, 33 (30.8%) were from the MCI group, and 65 (60.7%) were from the dementia group. Almost all SCGs were the sole care providers, with only three having two care assistants and four having one. The mean ZBI score was significantly higher in the SCGs from the dementia group than those from the non-dementia group (p < 0.001), and the mean GDS score was also significantly higher in the SCGs from the dementia group (p < 0.001).

Table 1.

Comparisons of demographic characteristics, clinical assessment, lifestyle factors, and blood-based biomarkers between SCGs of non-dementia patients and SCGs of dementia patients.

Variables		Non-dementia SCG (n = 42)	Dementia SCG (n = 65)	Total (n = 107)	p
SCG factors
Age (y)		71.74	73.80	72.99	0.135
Sex, Male (%)		13 (31.0%)	24 (36.9%)	37 (34.6%)	0.531
Education, y		9.21	8.62	8.85	0.507
Living with other family members		10 (23.8%)	8 (12.3%)	18 (16.8%)	0.146
Caring alone		42 (100%)	58 (89.2%)	100 (93.5%)	0.007*
MMSE (SCGs)		25.24	24.86	25.01	0.512
BMI		25.14	24.28	24.62	0.208
VRS (%)		19.1%	22.1%	20.87%	0.352
DM		11 (26.2%)	15 (23.1%)	26 (24.3%)	0.717
HTN		20 (47.6%)	35 (53.8%)	55 (51.4%)	0.534
Hyperlipidemia		14 (33.3%)	26 (40.0%)	40 (37.4%)	0.491
Coronary artery disease		2 (4.8%)	7 (10.8%)	9 (8.4%)	0.242
Stroke		1 (2.4%)	3 (4.6%)	4 (3.7%)	0.556
TIA		0 (0%)	0 (0%)	0 (0%)
Care-recipient factors
MMSE		23.29 (n = 41)	16.65	19.22 (n = 106)	<0.001*
Global CDR					<0.001*
0		9 (21.4%)	0 (0%)	9 (8.4%)
0.5		30 (71.4%)	20 (30.8%)	50 (46.7%)
1		3 (7.1%)	30 (46.2%)	33 (30.8%)
2		0 (0%)	15 (23.1%)	15 (14.0%)
Clinical assessments
ZBI		24.07	42.65	35.36	<0.001*
GDS		9.64	15.15	12.99	<0.001*
Lifestyle factors
IPAQ (METs)		2362.64	2142.82	2229.92	0.671
PSQI		8.21	8.83	8.59	0.378
Blood-based biomarker
Homocysteine (μmol/L)	male^†	16.42	17.97	17.42	0.323
Homocysteine (μmol/L)	female^‡	13.95	13.53	13.7	0.653
Folate(nmol/L)	male	9.26	10.12	9.82	0.717
Folate(nmol/L)	female	15.06	15.33	15.22	0.900

*p < 0.05, ^†3 missing data, ^‡5 missing data; SCG: spousal caregiver; MMSE: Mini-Mental Status Exam; VRS: Vascular Risk Score; TIA: transient ischemic attack; CDR: Clinical Dementia Rating; ZBI: Zarit Burden Interview; GDS: Geriatric Depression Scale; IPAQ: International Physical Activity Questionnaire; PSQI: Pittsburgh Sleep Quality Index.

Association of caregiving burden with homocysteine and folate levels

Greater caregiving burden was associated with higher homocysteine levels when adjusted for the age and sex of SCGs and the clinical diagnosis of care recipients (β = 0.219, t = 2.165, p = 0.033; Table 2; Figure 1). The positive correlation between caregiving burden and homocysteine remained significant even after adjusting for VRS and IPAQ score as additional covariates (β = 0.209, t = 2.065, p = 0.042). In contrast, the relationship between SCGs’ caregiving burden and folate levels was inversely correlated (β = -0.207, t = -1.997, p = 0.049), but after further adjustment for VRS and IPAQ, the correlation was not significant (p = 0.087).

Figure 1.

A plot to demonstrate the association between SCGs’ caregiving burden and serum homocysteine. SCG: Spousal caregiver; ZBI: Zarit Burden Interview.

Table 2.

Regression analyses for ZBI score versus blood-based biomarker and self-report questionnaires.

	ZBI total
	Modified R²	F (p)	β	t	p ^a	p ^b
Blood-based biomarker
homocysteine	0.195	6.946 (<0.001)	0.219	2.165	0.033*	0.042*†
folate	0.104	4.073 (0.004)	−0.207	−1.997	0.049*	0.087‡
Clinical assessments
GDS	0.303	12.518 (<0.001)	0.480	5.241	<0.001*
IPAQ (METs)	−0.029	0.270 (0.897)	−0.020	−0.175	0.862
PSQI	0.054	2.505 (0.047)	0.188	1.749	0.083

For all participants (n = 107).

*p < 0.05; ZBI: Zarit Burden Interview; GDS: Geriatric Depression Scale; IPAQ: International Physical Activity Questionnaire; PSQI: Pittsburgh Sleep Quality Index.

^a adjusted for age and sex of SCGs and the clinical diagnosis of care recipients (dementia / non-dementia). All regression results (modified R², F value, β, t value, and p value) are presented in the table for this adjustment.

adjusted for age and sex of SCGs, clinical diagnosis of care recipients (dementia / non-dementia), vascular risk score, and IPAQ score. Only p value is presented in the table for this adjustment.

^†Model fit: modified R² 0.216, F (p) 5.464 (< 0.001).

^‡Model fit: modified R² 0.111, F (p) 3.184 (0.007).

The moderating effect of sex on the association between caregiving burden and blood-based biomarkers

The moderating effect of the SCGs’ sex on the association between caregiving burden and homocysteine level was not significant (p = 0.405), but that on the association between caregiving burden and folate levels was significant (p = 0.034; Table 3). Subsequent subgroup analyses showed that the inverse correlation between caregiving burden and folate levels was significant only in female SCGs (B = -0.143, t = -2.840, p = 0.005; Figure 2) and not in male SCGs (B = 0.032, t = 0.450, p = 0.654; Figure 2).

Figure 2.

A plot to demonstrate sex subgroup analysis of the association between SCGs’ caregiving burden and folate. SCG: Spousal caregiver; ZBI: Zarit Burden Interview.

Table 3.

Interaction analyses of the moderating effect of sex on the association between caregiving burden and blood-based biomarkers.

Variables	B	SE	t	p^a
Caregiving burden x sex → homocysteine	0.034	0.041	0.836	0.405
Caregiving burden x sex → folate	0.175	0.082	2.144	0.034*

*p < 0.05.

Adjusted for age of SCGs and the clinical diagnosis of care recipients (dementia versus non-dementia).

Association of caregiving burden with self-report measures and lifestyle factors

As expected, increased caregiving burden showed a significant correlation with higher GDS scores (β = 0.480, t = 5.241, p < 0.001; Table 2). However, there was no significant association between caregiving burden and IPAQ (p = 0.862) or PSQI scores (p = 0.083).

Discussion

In the present study, we investigated the association between the caregiving burden experienced by SCGs of individuals with cognitive impairment and their physiological correlates, particularly homocysteine and folate levels. Our findings showed that a greater caregiving burden was associated with higher homocysteine and lower folate levels in the SCGs. These results suggest that elevated homocysteine and lower folate levels may be the mechanisms underlying the elevated risk of cardiovascular disease and cognitive decline in SCGs.

Our finding is particularly notable in that it showed a significant association between caregiving burden and homocysteine levels, even after adjusting for several key factors that influence homocysteine levels. In addition to age, sex, and physical activity, we adjusted for traditional vascular risks, such as hypertension and hyperlipidemia, all of which are known to have a significant impact on homocysteine levels.³⁴ This suggests that the caregiving burden may elevate homocysteine levels through mechanisms other than the aforementioned covariates. We propose two possible mechanisms for the association between caregiving burden and homocysteine levels. First, elevated homocysteine levels associated with increased caregiving burden may be due to inadequate intake of nutrients such as folic acid and vitamin B12. A systematic review found that caregivers have higher rates of malnutrition and inadequate diet, and caregiving intensity is one of the contributing factors.³⁵ Inadequate protein, vitamin, and folate intakes have been reported, particularly among elderly family caregivers.³⁶ Previous studies of caregivers of people with dementia have also reported poor diet and nutrition.^37,38 Similarly, previous work from our group has shown that the severity of cognitive decline in patients is associated with malnutrition in the SCGs.²⁵ The results of the current study are also consistent with these findings. The average folate levels of our study subjects (aged 55–90 years) were 9.82 nmol/L and 15.22 nmol/L for men and women, respectively, which were actually much lower than the average for Koreans over 50 years old (14.05–15.62 nmol/L and 20.35–21.06 nmol/L for men and women, respectively).³⁹ These findings suggest that SCGs of patients with cognitive impairment may be deficient in folate levels. The inverse correlation between SCGs’ caregiving burden and folate levels also supports this mechanism. Moreover, depressive symptoms may have mediated the elevated homocysteine levels associated with the increased caregiving burden. Several studies have reported family caregivers’ vulnerability to depressive symptoms.^40,41 A systematic review found that subjective caregiving burden was strongly associated with depressive symptoms and that this association was greater in caregivers of people with dementia,⁴² which is consistent with our findings. A positive correlation between depressive symptoms and homocysteine levels has also been well-documented,^43,44 and mental stress has been reported to elevate homocysteine levels.⁴⁵ An animal study supported the idea that elevated homocysteine levels are more of a consequence than a cause of depression,⁴⁶ which supports our proposed mechanism. In addition, a cross-sectional study reported that the more depressed the family caregivers of people with dementia were, the worse their nutritional status.⁴⁷ This means that the two mechanisms we propose may act synergistically.

The moderating effect of sex on the association between caregiving burden and folate levels was significant, and the inverse association between caregiving burden and folate levels was significant only among female SCGs. While caregiving burden scores did not differ significantly by sex (p = 0.697), and the variance was comparable between men and women (Levene's test p = 0.980), these data were not shown in the main text. This suggests that the observed sex-specific differences are unlikely to result from inherent differences in ZBI scores or selection bias. Instead, these findings point to other potential mechanisms underlying the interaction between sex, caregiving burden, and biomarker changes. One possible explanation for this difference lies in dietary factors. The inverse relationship between caregiving burden and folate levels may be mediated by dietary changes, and previous studies have shown that serum folate responses to dietary intake vary by sex due to differences in lean body mass, with women exhibiting greater variance.⁴⁸ This difference may have contributed to the insignificance of the results for male SCGs in our cohort. However, this sex-specific difference needs to be revisited using a larger sample size.

SCGs have been reported to be at increased risk of cognitive decline as they care for people with dementia.^10–12 And previous studies have reported that elevated homocysteine levels and insufficient folate are associated with cognitive decline and dementia.^17,22 Our study is significant in that it demonstrates the potential of homocysteine and folate as biomarkers for negative health outcomes in SCGs of patients with cognitive impairment and suggests possible mechanisms. However, our study could not confirm whether homocysteine and folate act as mediators in this relationship. This is because the study specifically focused on SCGs who were cognitively intact at the time of enrollment, as the cohort was designed to evaluate the longitudinal impact of caregiving on cognitive decline. This recruitment criterion may have constrained the current cross-sectional analysis from fully determining the mediating role of homocysteine and folate in the relationship between caregiving burden and cognitive decline. Nevertheless, still there is a possibility that the true impact may have been underestimated. Therefore, careful interpretation of these findings is necessary, and future longitudinal studies will be essential to better elucidate these relationships over time and confirm the mediating role of these biomarkers. Despite these limitations, our results hold statistical strength as we adjusted for several confounding factors affecting blood-based biomarkers.

In addition, based on a study that examined average homocysteine and folate levels in Korean population,³⁹ we found that elevated homocysteine and decreased folate levels in SCGs were significant in our data. In our study, the mean homocysteine level was 17.42 μmol/L and 13.70 μmol/L for men and women, respectively, which were significantly higher than the average for Koreans aged 50 years and older (11.75 ∼ 12.44 μmol/L and 8.66 ∼ 9.57 μmol/L for men and women, respectively).³⁹ This suggests that the homocysteine levels are indeed higher in the SCGs of patients with cognitive impairment.

Given the adaptable nature of homocysteine and folate through lifestyle and dietary adjustments, our findings advocate a proactive approach to managing the health of SCGs. Interventions aimed at reducing homocysteine levels could feasibly include dietary counseling to increase the intake of folic acid and vitamin B12³⁴ and mental health screening and management. Such interventions, tailored to the unique needs and circumstances of SCGs, are expected to significantly mitigate the increased risk of cardiovascular disease and cognitive decline in this cohort.

Our study has a few limitations. First, we included only patients and caregivers who visited the tertiary hospital. Caregivers and patients with severe-to-profound severity were less likely to be included in the sample because of the difficulty in visiting the hospital. This may have weakened the strength and significance of the associations between variables. Second, since our data were analyzed at a cross-sectional time point, we were unable to determine temporal associations between caregiving burden and blood-based biomarkers. Biomarkers at one point in time are likely to be influenced by the total caregiving burden accumulated over time. Therefore, future longitudinal analyses are needed to better understand these relationships over time and to develop effective interventions for SCGs.

In conclusion, this study showed that when caring for patients with cognitive impairment, close attention should be paid to their SCGs, who may be potential patients. The next step involves conducting longitudinal studies to determine whether early interventions to lower homocysteine levels and ensure adequate folate intake can prevent cardiovascular disease and cognitive decline. These future studies will be crucial in developing clear guidelines for the SCGs of patients with cognitive impairment.

Footnotes

Acknowledgments

The authors thank all the participants and their families for their participation in this study. We would also like to thank the research assistants and study staff.

ORCID iDs

Ho Yeong Jeong

Shin Young Park

Jeong Lan Kim

Sang Yoon Kim

So Yeon Jeon

Ethical considerations

This study was approved by the Ethics Review Committee of Chungnam National University Hospital (committee's reference number 2020-05-002).

Consent to participate

The study was approved by the Ethics Review Committee of Chungnam National University Hospital (committee's reference number 2020-05-002). Participants were fully informed about the study outline, and informed consent was obtained in writing.

Consent for publication

Not applicable

Author contributions

So Yeon Jeon (Conceptualization; Data curation; Funding acquisition; Resources; Visualization; Writing – review & editing); Ho Yeong Jeong (Conceptualization; Formal analysis; Methodology; Visualization; Writing – original draft; Writing – review & editing); Shin Young Park (Methodology; Writing – review & editing); Jeong Lan Kim (Data curation; Writing – review & editing); Sang Yoon Kim (Conceptualization; Data curation; Resources; Writing – review & editing).

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (grant number RS-2023-00210380); and Chungnam National University Hospital.

Declaration of conflicting interests

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Data availability

The data supporting the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.

References

Fonareva

Oken

. Physiological and functional consequences of caregiving for relatives with dementia. Int Psychogeriatr 2014; 26: 725–747.

Brodaty

Donkin

. Family caregivers of people with dementia. Dialogues Clin Neurosci 2009; 11: 217–228.

Ornstein

Wolff

Bollens-Lund

, et al. Spousal caregivers are caregiving alone in the last years of life. Health Aff (Millwood) 2019; 38: 964–972.

Haley

Roth

Howard

, et al. Caregiving strain and estimated risk for stroke and coronary heart disease among spouse caregivers: differential effects by race and sex. Stroke 2010; 41: 331–336.

Capistrant

Moon

Berkman

, et al. Current and long-term spousal caregiving and onset of cardiovascular disease. J Epidemiol Community Health 2012; 66: 951–956.

Lee

Colditz

Berkman

, et al. Caregiving and risk of coronary heart disease in US women: a prospective study. Am J Prev Med 2003; 24: 113–119.

Shaw

Patterson

Ziegler

, et al. Accelerated risk of hypertensive blood pressure recordings among Alzheimer caregivers. J Psychosom Res 1999; 46: 215–227.

Vitaliano

Scanlan

Zhang

, et al. A path model of chronic stress, the metabolic syndrome, and coronary heart disease. Psychosom Med 2002; 64: 418–435.

Von Känel

Mausbach

Patterson

, et al. Increased framingham coronary heart disease risk score in dementia caregivers relative to non-caregiving controls. Gerontology 2008; 54: 131–137.

10.

Norton

Smith

Østbye

, et al. Greater risk of dementia when spouse has dementia? The Cache County Study: [See editorial comments by Dr. Peter P. Vitaliano, pp 976–978]. J Am Geriatr Soc 2010; 58: 895–900.

11.

Vitaliano

Murphy

Young

, et al. Does caring for a spouse with dementia promote cognitive decline? A hypothesis and proposed mechanisms. J Am Geriatr Soc 2011; 59: 900–908.

12.

De Vugt

Jolles

Van Osch

, et al. Cognitive functioning in spousal caregivers of dementia patients: findings from the prospective MAASBED study. Age Ageing 2006; 35: 160–166.

13.

Baszczuk

Kopczyński

. Hyperhomocysteinemia in patients with cardiovascular disease. Postępy Hig Med Dośw 2014; 68: 579–589.

14.

Pang

Liu

Zhao

, et al. Homocysteine induces the expression of C-reactive protein via NMDAr-ROS-MAPK-NF-κB signal pathway in rat vascular smooth muscle cells. Atherosclerosis 2014; 236: 73–81.

15.

Ganguly

Alam

. Role of homocysteine in the development of cardiovascular disease. Nutr J 2015; 14: 6.

16.

Schaffer

Verdoia

Cassetti

, et al. Relationship between homocysteine and coronary artery disease. Results from a large prospective cohort study. Thromb Res 2014; 134: 288–293.

17.

Seshadri

Beiser

Selhub

, et al. Plasma homocysteine as a risk factor for dementia and Alzheimer's disease. N Engl J Med 2002; 346: 476–483.

18.

Djuric

Jakovljevic

Zivkovic

, et al. Homocysteine and homocysteine-related compounds: an overview of the roles in the pathology of the cardiovascular and nervous systems. Can J Physiol Pharmacol 2018; 96: 991–1003.

19.

LeBoeuf

. Homocysteine and Alzheimer's disease. J Acad Nutr Diet 2003; 103: 304.

20.

Verhaar

Stroes

Rabelink

. Folates and cardiovascular disease. Arterioscler Thromb Vasc Biol 2002; 22: 6–13.

21.

Wang

Z-M

Zhou

Nie

Z-L

, et al. Folate and risk of coronary heart disease: a meta-analysis of prospective studies. Nutr Metab Cardiovasc Dis 2012; 22: 890–899.

22.

Reynolds

. Folic acid, ageing, depression, and dementia. BMJ 2002; 324: 1512–1515.

23.

Pöysti

Laakkonen

M-L

Strandberg

, et al. Gender differences in dementia spousal caregiving. Int J Alzheimers Dis 2012; 2012: 162960.

24.

Xiong

Biscardi

Astell

, et al. Sex and gender differences in caregiving burden experienced by family caregivers of persons with dementia: a systematic review. PLoS One 2020; 15: e0231848.

25.

Jeon

Kim

. Caregiving for a spouse with cognitive impairment: effects on nutrition and other lifestyle factors. J Alzheimers Dis 2021; 84: 995–1003.

26.

Albert

DeKosky

Dickson

, et al. The diagnosis of mild cognitive impairment due to Alzheimer’s disease: recommendations from the National Institute on Aging-Alzheimer’s Association workgroups on diagnostic guidelines for Alzheimer’s disease. Focus 2013; 11: 96–106.

27.

Yoon

Robinson

. Psychometric properties of the Korean version of the Zarit Burden Interview (K-ZBI): preliminary analyses. J Soc Work Res Eval 2005; 6: 75.

28.

Hébert

Bravo

Préville

. Reliability, validity and reference values of the Zarit Burden Interview for assessing informal caregivers of community-dwelling older persons with dementia. Can J Aging 2000; 19: 494–507.

29.

Chun

. Validity and reliability of Korean version of international physical activity questionnaire short form in the elderly. Korean J Fam Med 2012; 33: 144.

30.

Shin

Kim

. The reliability and validity testing of Korean version of the Pittsburgh Sleep Quality Index. J Converg Inf Technol 2020; 10: 148–155.

31.

Kim

Park

Lee

, et al. Standardization of the Korean version of the geriatric depression scale: reliability, validity, and factor structure. Psychiatry Investig 2008; 5: 232.

32.

Refsum

Smith

Ueland

, et al. Facts and recommendations about total homocysteine determinations: an expert opinion. Clin Chem 2004; 50: 3–32.

33.

Longo

Herbert

. Radioassay for serum and red cell folate. J Lab Clin Med 1976; 87: 138–151.

34.

Schneede

Refsum

Ueland

. Biological and environmental determinants of plasma homocysteine. Semin Thromb Hemost 2000; 26: 263–280.

35.

Teixeira

. Nutritional status of formal and informal caregivers-A systematic review. Biomed J Sci Tech Res 2023; 50: 41386–41400.

36.

Koponen

Nykänen

Savela

R-M

, et al. Inadequate intake of energy and nutrients is common in older family caregivers. Nutrients 2021; 13: 2763.

37.

Tombini

Sicari

Pellegrino

, et al. Nutritional status of patients with Alzheimer’s disease and their caregivers. J Alzheimers Dis 2016; 54: 1619–1627.

38.

Snyder

Vitaliano

. Caregiver psychological distress: longitudinal relationships with physical activity and diet. Am J Alzheimers Dis Other Demen 2020; 35: 1533317520904554.

39.

Song

Park

H-Y

. Folate, vitamin B12, and homocysteine status in the Korean population: data from the 2013–2015 Korea National Health and Nutrition Examination Survey. Epidemiol Health 2024; 46: e2024007.

40.

Karimi Moghaddam

Rostami

Zeraatchi

, et al. Caregiving burden, depression, and anxiety among family caregivers of patients with cancer: an investigation of patient and caregiver factors. Front Psychol 2023; 14: 1059605.

41.

Chakraborty

Jana

Vibhute

. Caregiving: a risk factor of poor health and depression among informal caregivers in India-A comparative analysis. BMC Public Health 2023; 23: 42.

42.

del-Pino-Casado

Rodriguez Cardosa

López-Martínez

, et al. The association between subjective caregiver burden and depressive symptoms in carers of older relatives: a systematic review and meta-analysis. PLoS One 2019; 14: e0217648.

43.

Almeida

McCaul

Hankey

, et al. Homocysteine and depression in later life. Arch Gen Psychiatry 2008; 65: 1286–1294.

44.

Folstein

Liu

Peter

, et al. The homocysteine hypothesis of depression. Am J Psychiatry 2007; 164: 861–867.

45.

Sawai

Ohshige

Kura

, et al. Influence of mental stress on the plasma homocysteine level and blood pressure change in young men. Clin Exp Hypertens 2008; 30: 233–241.

46.

Chengfeng

Wei

Xinxing

, et al. Hyperhomocysteinemia is a result, rather than a cause, of depression under chronic stress. PLoS One 2014; 9: e106625.

47.

Rullier

Lagarde

Bouisson

, et al. Psychosocial correlates of nutritional status of family caregivers of persons with dementia. Int Psychogeriatr 2014; 26: 105–113.

48.

Winkels

Brouwer

Verhoef

, et al. Gender and body size affect the response of erythrocyte folate to folic acid treatment. J Nutr 2008; 138: 1456–1461.