Elderly persons (aged 65 years and older) accounted for over 14% of the total Korean population in 2017, thereby qualifying Korea officially as an “aging society.” It is predicted that in 2025, the country will become a “super-aging society,” in which the elderly population accounts for over 20% of the total population [1]. Increased longevity leads to increased incidences of cancers, metabolic diseases, and degenerative diseases, all of which cause physical and psychological problems, as well as social issues related to increased medical costs [2]. Therefore, it is essential for the elderly to maintain their health, particularly since health maintenance makes it possible for individuals to alleviate physical and psychological disorders caused by diseases and for the national government to reduce direct or indirect revenue losses.

The health status of the elderly was influenced by various factors, such as physical and environmental changes during senescence [345]. Reportedly, an older person's dietary intake affects their physical and psychological health [6], and some studies have reported that the level of food satisfaction in the elderly is significantly related to depression, loneliness, living satisfaction, and social support [7].

Many relevant studies have reported that the higher the diet quality, the lower the risks of metabolic diseases and mortality. Typical tools used for the evaluation of diet quality include the Diet Quality Index (DQI) [89], Healthy Eating Index (HEI) [10], and Healthy Diet Indicator (HDI) [1112]. Also available is a recently developed DQI for Koreans (DQI-K) that is based on that country's dietary reference intakes [13].

One of the physiological changes occurring in elderly persons is a reduction of muscular strength and/or a loss of muscle mass; a condition referred to as sarcopenia. The reason sarcopenia is a problem for the elderly is that they need a proper level of muscular strength to maintain their minimal customary physical functions [14]. In the Health, Aging and Body-Composition Study, subjects with a low level of muscular strength who were tracked and observed over a 6-month period had an increased mortality rate [15]. The most significant determinant of muscular strength is skeletal muscle mass. During aging, muscle mass can be reduced as a result of decreased physical activity, increased disease susceptibility, lowered body mass index (BMI), and many other factors; thereby, the prevalence of sarcopenia increases. Dietary intake is also known to be an influential factor. In a previous study on the relationship between sarcopenia and dietary intake, daily intake energy, and the amount of protein intake were shown to be the main factors affecting sarcopenia [16]. Moreover, diet quality was also reported to be an influential factor [1718]. An analytic study on elderly sarcopenia based on a National Health and Nutrition Examination Survey (NHANES) observed that the lower the diet quality, the higher the mortality rate [19]. In addition, a study related to sarcopenia and meal quality has been conducted to analyze the relationship between the HEI of elderly (over 60 years old) in Australia [20], and meal patterns, The Mediterranean Diet Score (MDS) and DQI-International (DQI-I) have been evaluated in a study of subjects over 65 years old in Hong Kong [18]. Unfortunately, there is little available research on the relationship between sarcopenia and diet quality in elderly Koreans. In the present study, the DQI-K, a tool for evaluating the quality of diet that reflects the Korean diet style, has been used to analyze the relationship between sarcopenia and diet quality of diet in elderly Koreans. To that end, the study used data obtained through the 2008-2011 Korea National Health and Nutrition Examination Survey (KNHANES).

This study analyzed the 2008–2011 KNHANES data to assess the relationship between diet quality and sarcopenia in elderly Korean subjects. The data showed that sarcopenia prevalence among the assessed elderly Koreans was 37.2%. The results of the DQI-K analysis showed that, compared to the good diet-quality group, the bad diet-quality group had a higher OR of sarcopenia presence, and with a one-point increment in the DQI-K score, the OR of sarcopenia presence increased.

Sarcopenia can be defined in several ways, as its diagnostic criteria vary. Moreover, sarcopenia prevalence rates differ according to the specific criteria being used. In a study on sarcopenia diagnosis derived through the relationship between ASM divided to height and based on the 2008–2009 KNHANES, sarcopenia class 1 diagnosed based on the subject being less than 1 SD from the accounted for 18.6% of the total assessed subjects while sarcopenia class 2 mean diagnosed based on the subject being less than 2 SDs from the mean was 3.5% [23]. In the research by Chung et al. [24], who used the same KNHANES data and employed the same diagnostic method as the above-noted study, the sarcopenia prevalence rate for men was 42.0%, and that for women was 42.7%. This result was similar to that of the current study, in which men and women showed 38.1% and 37.2% sarcopenia prevalence, respectively. Therefore, as noted above, the sarcopenia prevalence rate will differ depending on the diagnostic calculation method used. The method employed in the present study was considered appropriate, given Newman et al.'s suggestion [25] that body-fat mass should be taken into consideration during sarcopenia diagnosis.

Several factors influence the presence of sarcopenia, including age, BMI, marriage and smoking status, and participation in physical activities; additionally, it has been reported that dietary intake is also an important factor [26]. Research has been done on the relationships between sarcopenia risk factors and dietary factors using the 2008–2009 KNHANES. According to this research, total energy intake, carbohydrate intake, protein intake, and fat intake are significantly high in older individuals without sarcopenia [27]. In this study, the sarcopenia and non-sarcopenia groups had significantly different education and smoking status. Although there was no significant difference in protein intake of those 2 groups, the sarcopenia group had a significantly lower protein intake per kilogram of body weight. According to the Korean Dietary Reference Intakes for Koreans (KDRI), the protein reference intake for adults and elderly persons is 0.91 g/kg/day. The subjects with sarcopenia in our study had a protein intake of 0.83 g/kg/day, significantly lower than that in those without sarcopenia and lower than the KDRI reference value. Reportedly, the muscle protein in the body undergoes increased catabolism during aging [28], and the protein intake of elderly subjects has shown a significant positive relationship with their fat-free mass [29]. For this reason, nutrition-management guidelines for elderly persons with sarcopenia emphasize increased protein intake in senescence. The research team of the Society for Sarcopenia, Cachexia, and Wasting Disease recommended increased energy intake, balanced protein intake (1–1.5 g/kg/day), and amino acid supplementation in their 2010 nutrition recommendation for sarcopenia management [30]. The European Society for Clinical Nutrition and Metabolism (ESPEN) has recommended that elderly persons without sarcopenia take in protein at a rate of 1.2–1.4 g/kg/day in order to prevent disease [31]. Recently, the Korean Geriatrics and Korean Nutrition societies recommended that elderly Koreans (aged 65 and older) should take in protein at a rate of 1.2 g/kg/day.

Since people consume several types of nutrients when eating a variety of foods during routine daily living, it is crucial to understand the relationships between nutrients and metabolism status. Compared to diets in other countries, the Korean diet has many side dishes and diverse cooking methods; thus, diet-quality evaluation is especially important in Korea. When evaluating the diet quality of elderly subjects, specific groups of people with various geriatric diseases and symptoms may be evaluated, and thereby, a particular disease or symptom may be shown to be associated with diet quality. Analytic research on the risk of hip fracture in four DQI-I groups among a large cohort of elderly Chinese urban dwellers showed that those who experienced hip fracture scored lower on the DQI-I (54 points) than those without hip fracture (60 points), with the group with high diet quality having a significantly reduced risk of hip fracture compared to the group with low diet quality [32]. In prospective research to examine the association of DQI-I with frailty in Chinese community-dwelling men and women aged ≥ 65 years, participants with a high DQI-I score had a low incidence of frailty. The authors suggest that high adherence to a good quality diet is associated with a low OR of developing frailty [33]. In research on the relationship between DQI of sarcopenia patients and their mortality rate, which used data from the 4th NHANES, the patients' DQI-I was 62.8 points, and the death hazard ratio was 0.53 (95% CI: 0.38–0.82) when their diet quality was good, even after adjusting for age, sex, BMI, living habits, and disease covariates [19].

Given the results of the above-noted studies, a poor diet quality seems to influence the older person's quality of life and various risk factors, such as fracture and frailty. In this study, we compared diet quality and sarcopenia in groups based on obesity and age. In elderly subjects aged 65–74 years, there were no different DQI-K scores for overweight subjects with or without sarcopenia and sarcopenic. However, it was shown that sarcopenic overweight elderly subjects aged 75 years and older consumed a lower quality diet than the non-sarcopenic overweight participants. Especially, overweight sarcopenic elderly persons had higher levels of all components of the DQI-K, with daily protein intake and daily whole grains intake being significantly high. Sarcopenic obesity is a high-risk elderly syndrome that is mainly observed in elderly populations at risk of synergistic complications related to sarcopenia and obesity. Previously, sarcopenic obesity has been associated with a relatively higher risk of incident disability, comorbidities, and mortality [34]. Thus, introducing dietary interventions to optimize health status, especially for sarcopenic obese elderly persons in Korea, should be considered.

In a DQI-I-based research study into diet quality and sarcopenia among elderly subjects in Hong Kong, the sarcopenia group had a significantly lower index of diet quality [18]. This result was similar to that of the present study, in which the higher the DQI-K, the greater the odds of sarcopenia presence. The items considered in the evaluation of diet quality included nutrient amounts, food intake, nutrient balance, and nutrition adequacy. The tools that have been used to evaluate diet quality in many related studies include the DQI, HEI, and HDI, which were developed by applying the US guidelines for dietary living, the US nutrition reference intakes, and the WHO guidelines for chronic disease prevention [89101112]. It has been reported that the DQI is related to the cardiovascular disorder prediction index [35], metabolic syndrome [36], and cancer mortality rate [37]. However, the DQI fails to reflect the dietary patterns of all countries or groups, and therefore, modified DQI models for specific countries and groups have been developed [3839]. In Korea, the DQI-K, a modified version of the DQI, was developed to reflect the specific nutrition reference intakes of Koreans. An increase in DQI-K has been correlated with an increase in mortality [13]. Therefore, it is necessary to evaluate diet quality according to specific groups or diseases in Korea. In the present study, elderly individuals with sarcopenia had significantly high DQI-K scores. On that basis, overall diet quality is considered to be a significant factor that can influence the development of sarcopenia. In particular, in Koreans aged 75 and older, a poor diet quality increased the odds of sarcopenia presence, even after adjusting for other factors that can influence the development of sarcopenia. Accordingly, in order to improve the diet quality of those aged 75 and older, it is necessary to develop detailed guidelines on what comprises a good diet and to establish a dietary living improvement policy suitable for the community of interest. Additionally, in previous research into the dietary patterns of those aged 85 years and older according to protein intake levels, the group whose protein intake was less than 1 g/kg/day showed no significant relationship between sarcopenia and dietary pattern, but in the group whose protein intake was at least 1 g/kg/day, the presence of sarcopenia was dependent on dietary pattern [40]. Given that result and the results of the present study, the association of diet quality and sarcopenia is considered to be influenced by the level of protein intake. In order to develop detailed guidelines for sarcopenia-appropriate nutrition management for the elderly, it will be necessary to complete further research into the diet quality of the Korean elderly according to protein intake.

This study had several limitations. This study analyzed the relationship between sarcopenia and diet quality in subjects aged 65 and older using 2008–2011 KNHANES data obtained via a cross-sectional design. Therefore, it will be necessary to conduct prospective research into this relationship by using domestic open tracking and observation data in order to reveal the causal relationship fully. As well, there is a need to conduct comparative research into the influence of the DQI-K and the factors that trigger sarcopenia. Another limitation of our study is that dietary data were obtained using the 24-hour recall method, which does not reflect the subjects' dietary habits fully. Nevertheless, this study is meaningful in that it made use of the DQI-K (developed to apply nutrition reference intakes of Koreans to the DQI, the validity of which had already been proved in many other countries), and, thereby, revealing that it was suitable for examining the relationship between diet quality and sarcopenia in elderly Koreans. Finally, another limitation is that muscle strength was not considered during the diagnosis of sarcopenia. Nevertheless, the strength of our research is that it shows that the DQI-K, which reflects the traditional Korean diet, can be used to examine the association of diet with sarcopenia in elderly persons. Thus, the DQI-K can provide basic data that is useful in the assessment of dietary education strategies against sarcopenia.

In all stages of a lifecycle, a balanced diet is essential. Elderly persons can have difficulty with maintaining a good quality diet due to a variety of personal, social, and environmental factors. The results of this study show that an appropriate and balanced dietary intake can help prevent sarcopenia in elderly persons. In particular, the results of this study will be useful fundamental data when formulating a dietary intervention plan that will prevent sarcopenia among Koreans aged 65 years and older.