Introduction
The phenomenon of the increase in the elderly population is one of the crucial economic, social, and health challenges for healthcare service providers [1]. According to the latest census in 2016, the number of elderly people in Iran was 7 450 000 people, which has now reached about 8 million people (9.9%) and it is predicted that in 2050, about 30% of the country’s population will be elderly people [2]. Aging is a sensitive period of life and paying attention to the issues and needs of this stage of life is considered a social necessity [3] Self-care is a learned regulatory function in humans, which is based on the ability of people, and the elderly by acquiring self-care skills affect their comfort and performance abilities [4, 5]. The nutrition factor has a great impact on the rate of death, disability and quality of life of the elderly and has been neglected in developing countries [6, 7]. This study was conducted to determine the effect of self-care on the nutritional status of the elderly in Tehran City, Iran in 2019.
Methods 
The present study is descriptive-analytical and correlation type, which was conducted in 2018 on the elderly of Tehran. To determine the sample size, the formula was used and considering the index of design effect and drop, 420 people were estimated. The sampling method in this study was multi-stage. Urban community health centers affiliated with Shahid Beheshti University of Medical Sciences include 54 centers (east health center, 24 centers, north center, 21 centers, and Shemiranat health network, 11 centers). Each of the health centers was considered as a class, then sampling was done by choosing one center from each class and selecting available samples from these four classes. The inclusion criteria included the willingness to participate in the research, age 60 years and older, minimal reading and writing literacy, ability to communicate and cooperate in the study, not suffering from cognitive impairment, and the exclusion criterion included incomplete completion of the questionnaire. 
Data were collected through a demographic information questionnaire, self-care ability scale for the elderly (SASE), and mini nutritional assessment (MNA) questionnaire.
The self-care questionnaire of the elderly has 40 questions based on the Orom self-care factor to evaluate the self-care ability of the elderly. The answer to each question was based on the Likert scale, often, sometimes, rarely, and never. Based on the score, the person is placed in three levels of self-care, weak, average, and desirable [8].
The mini nutritional assessment (MNA) questionnaire of the elderly is an 18-item questionnaire, including body mass index (BMI), dietary intake, general assessment, and personal assessment. Based on the score, people were divided into three groups with normal nutrition, at risk of malnutrition, and suffering from malnutrition [9].
The method of data collection was interview and recording information in the questionnaire. Data were analyzed by SPSS software version 16 using descriptive and inferential statistical tests of Spearman correlation and structural equation model via using model fit indices under EQS6/1 software.
Results
In this study, 420 elderly people in Tehran City were studied; the collected data were analyzed by SPSS 16 and EQS statistical software version 6.1. First, the normality of the data was checked by evaluating the skewness and kurtosis of the study variables, and the results were within the acceptable range. (Between 2 and -2), then data were analyzed. Among the elderly people studied, 133 people (31.7%) were over 70 years old and 115 people (27.4%) were between 60-64 years old. The majority of the elderly people included 217 men (51.7%). The highest mean and standard deviation of nutritional status and self-care ability status was obtained in the elderly who were between 74-70 and 69-65 years old, respectively (21.47 ± 3.98) (87.51 ± 6.64). Elderly people with university education have the highest mean and standard deviation of self-care ability (88.97 ± 6.54) and the lowest mean and standard deviation of nutrition status (14.39 ± 3.75) compared to other elderly people who had a diploma and under diploma education. According to the findings, 34.3% of the studied elderly had an average level of self-care ability and 65.7% had an optimal level. Also, the nutritional status of 65% of the elderly was at a normal level, while 24.4% of the elderly were undernourished and 8.6% suffered from malnutrition. Pearson’s correlation coefficient showed that all dimensions of self-care have a statistically significant relationship with nutrition scores. Nutrition scores have the lowest correlation with daily self-care and nutrition scores have the highest correlation with physical self-care.
To investigate the effect of self-care on the nutritional scores of the elderly, structural equation modeling was used. The tested model of the research in the standard mode shows the intensity of the impact of the variables on each other. According to the model (Figure 1), self-care has a 71% impact on the nutritional scores of the elderly, and in other words, for one unit of change in the standard deviation of self-care, 71% of the change in the standard deviation of the nutritional scores of the elderly occurs. The structural model shows the relationship between the hidden variables of the research by displaying standardized factor loadings and fit indices. According to the fit indices obtained in the model, the chi-square ratio to the degree of freedom is less than 4. Also, RMR and RMSEA are less than 0.08, and AGFI, GFI, CFI, MFI, IFI, NNFI, and NFI indices have allocated values of 0.90 and above, all of which indicate the appropriate fit of the model.
Discussion
The results of this study showed that the majority of the elderly have a good level of self-care ability and nutritional status, and individual-social factors can be related to their self-care ability levels. Therefore, targeted planning to improve the quality of life and especially the quality of nutrition and control some modifiable variables in the field of self-care and nutrition has been essential. Although reducing the incidence of malnutrition in the elderly is not easily possible, increasing knowledge in this field can help reduce its prevalence and increase the level of self-care ability in this age group. In this context, emphasizing the supporting role and consultations of the geriatric health group can have a significant effect on improving the self-care ability and nutritional status of this age group. Therefore, trying to reduce the incidence of malnutrition and increasing the use of the self-care ability is a comprehensive action that requires the effort of many decision-makers in the field of elderly health.

Ethical Considerations
Compliance with ethical guidelines
This study is derived from the research project (Code: IR.SBMU.PHNS.REC.1398.010). Participants completed a written informed consent before entering the study. How to conduct the study and complete the questionnaires was explained to all the elderly. 

Funding
This article is a part of the Master's thesis of Mr. Reza Nowrozi in the Department of Public Health, School of Health and Safety, Shahid Beheshti University of Medical Sciences. The researcher is grateful to the esteemed Research Vice-Chancellor of Shahid Beheshti University of Medical Sciences for the material and spiritual support in this research.

Authors' contributions
Executive director of the study: Reza Novrouzi; Study design: Mohtasham Ghaffari; Editing of the article: Maryam Sabouri; Project consultant: Tayyebe Marashi; Data analysis and article editing: Sakineh Rakhshanderou.

Conflicts of interest
The authors declared no conflict of interest.

Acknowledgements
The researchers consider it necessary to thank all the educational and research officials of Shahid Beheshti University of Medical Sciences and the health and safety faculty of Shahid Beheshti University of Medical Sciences and all the officials of urban health service centers affiliated with Shahid Beheshti University of Medical Sciences who provided the financial and moral support necessary to conduct this study as well as the elderly participating in the study.