Korea is experiencing the fastest increase in the proportion of individuals aged 65 years and older among OECD countries, attributed to an extension of average life expectancy and a sharp decline in birth rates [1]. In 2014, the elderly population aged 65 and older accounted for 12.7% of the total population, a figure expected to rise to 17.5% by 2022, marking Korea’s entry into an aging society [1]. Projections indicate that this percentage will exceed 20% by 2025, heralding the onset of a super-aged society [2]. Notably, in 2022, medical expenses for those aged 65 years and older accounted for 43.4% of the total national health insurance expenditure, with this proportion continuing to increase [3]. Therefore, analyzing and addressing health-related issues among the elderly is crucial, as it can alleviate the economic burden from a societal perspective while enhancing the quality of life for individuals. Consequently, frailty is considered a critical issue.

Aging and frailty are related yet distinct concepts, with numerous studies addressing these differences. Aging refers to the gradual decline in physiological functions due to genetic, environmental, and physiological factors that naturally occur in all living organisms [4,5]. It is characterized by an increase in chronic diseases, a decline in physical function, cognitive impairment, hormonal changes, and a decrease in immune function, all of which progress gradually over time [5]. In contrast, frailty is a state of increased vulnerability to external stressors associated with aging and is characterized by reduced resilience [4]. Frailty arises under specific conditions and is primarily influenced by physical capabilities, nutritional status, activity levels, and chronic diseases [6]. Previously, frailty was viewed as an inevitable consequence of aging; however, it is now understood that frailty is a condition observed only in some elderly individuals and is not an unavoidable result of aging [4]. Furthermore, numerous studies [7-11] have demonstrated that frailty is not only preventable but also that some individuals who have progressed to a frail state can recover from their previous condition. This underscores the importance of proactive intervention and management. The most representative assessment of physical frailty, the Fried frailty phenotype, is evaluated using five criteria: unintentional weight loss, weakness or poor grip strength, self-reported exhaustion, slow walking speed, and low physical activity (PA) levels [4]. Grip strength, one of the diagnostic criteria for the frailty phenotype, is a highly reliable indicator of muscle strength that can assess overall physical function (endurance, muscle strength, and balance) [12]. It serves as a crucial variable for verifying health status and outcomes of health behaviors.

Most domestic studies examining the relationship between grip strength and health in elderly individuals have focused on the association between grip strength and specific diseases. However, there is a notable lack of large-scale studies investigating the impact of PA and dietary intake on the prevention and maintenance of grip strength decline. Additionally, when examining health behaviors that influence grip strength, such as PA and nutrient intake, it is important to consider household type and sex. Previous studies have identified insufficient PA and imbalanced dietary intake in single-person households, with notable sex-based differences [13-18].

Therefore, we utilized data from the Korea National Health and Nutrition Examination Survey (KNHANES) from 2014 to 2019 to analyze differences in PA and nutrient intake levels based on grip strength, which serves as an indicator of frailty and overall muscle strength. The analysis focused specifically on elderly individuals living in single-person households (ESH), with a particular emphasis on sexbased categorization. The primary objective of this study was to examine the differences in PA and nutrient intake levels based on grip strength among ESH of both sexes. Our findings highlight the importance of PA and nutrient intake in maintaining and enhancing muscle strength and provide foundational data for developing strategies to prevent frailty and promote healthy living in ESH while considering sex differences.

Disease management alone is insufficient for addressing the health needs of the rapidly growing elderly population. Physical frailty is defined as a syndrome marked by declines in strength, endurance, and physiological function, leading to increased vulnerability to dependence and mortality due to various factors [26]. Frailty, therefore, is not a disease but a geriatric syndrome associated with adverse health outcomes such as falls, hospitalization, functional decline, fractures, incident disability, and mortality, driven by multiple contributing factors [27-29]. Fortunately, frailty can often be prevented, and even in cases where it progresses to a frail state, partial recovery is possible. This highlights the importance of shifting focus from disease management to strategies aimed at maintaining physical function and delaying the onset of frailty through health behavior improvements. In response to these challenges, this study analyzed differences in PA and nutrient intake according to grip strength among ESH based on data from the KNHANES from 2014 to 2019. The study aimed to identify variations in PA and nutrient intake levels according to grip strength across sexes among ESH.

In summary, the findings indicated that among patients with ESH, men did not exhibit differences in PA levels based on grip strength, except for the number of days engaged in strength training. Conversely, women with higher grip strength demonstrated higher levels of PA across various domains, including time spent on moderate-intensity recreational activities, travel, and days engaged in strength training. The PA elements that produced consistent results across sexes were the number of weekly walking days and the number of days engaged in strength training. For walking, no significant differences in participation days were observed according to grip strength across sexes. However, higher grip strength was associated with more days of participation in strength training in both men and women.

Although the regression analysis did not yield statistically significant results, engagement in strength training at least once per week was positively associated with grip strength in men. Men with greater grip strength tended to participate in more days of strength training per week, suggesting a potential practical impact of strength training on improving grip strength. Additionally, the strong association between engaging in strength training at least once per week and increased grip strength in women, which persisted even after adjusting for smoking and drinking habits, underscores the crucial role of strength training in enhancing grip strength. This finding is consistent with the observation that women with higher grip strength also participated in more strength training days.

This study identified only simple differences without elucidating the underlying reasons. Due to the limited information in this field, these results cannot be directly compared with those of other studies. However, for men, factors beyond PA, such as nutrient intake or other health behaviors, may have a more significant impact on grip strength. It is generally observed that women tend to experience a more substantial decrease in PA levels with age compared to similarly aged men [30]. This tendency may be even more pronounced in ESH, where PA levels may be more restricted. Additionally, recent research [31] has highlighted sex differences in the effect of PA on grip strength decline. Regular high levels of PA slowed the rate of grip strength decline in both men and women; however, this effect was less pronounced in women than in men. Consequently, several studies have recommended that women engage in higher levels of PA to maintain muscle strength [30,32,33]. According to Nelson et al. [34], higher muscle strength is associated with increased PA, and this effect is more pronounced in women than in men. This suggests that muscle strength serves as the primary motivator and driving force for PA.

Research on the relationship between walking and muscle strength is limited, and the findings are inconsistent. A systematic review [35] reported that brisk walking improved muscle strength in individuals aged 60 and above; however, few studies focused specifically on men. Conversely, a 10-week brisk walking study in women aged 50 and older [36] did not observe significant changes in muscle strength. Ahn et al. [37] found a significant relationship between walking and grip strength in women but not in men. These discrepancies may be attributed to differences in measurement methods (direct measurement, surveys, daily walking, and walking interventions) and may reflect sex-related differences in body composition that influence the effectiveness of walking exercise.

In our previous study [38], which examined the relationship between grip strength and PA levels in middle-aged and older men, we found that the association between PA levels and grip strength was more pronounced in the older age group. Specifically, the group with the highest grip strength engaged in significantly more days of strength training than the group with the weakest grip strength. Thus, PA, particularly strength training, is widely recognized as the primary strategy for maintaining grip strength and preventing its decline. This is supported by a systematic review by de Labra et al. [39], which examined the effects of physical exercise interventions in frail older adults, and a randomized trial by Cameron et al. [40], which assessed the effects of a multifactorial interdisciplinary intervention in reducing frailty in older people. Both studies confirmed that strength training significantly improves muscle strength in older adults. Notably, isolated training involving various muscle groups was more effective than single-muscle training. Multicomponent exercises that focus on resistance, balance, and flexibility are particularly effective in improving muscle strength. These multicomponent exercise programs significantly affect not only muscle strength but also fall reduction, mobility, balance, functional ability, and body composition [41-44], making them the most effective interventions for enhancing overall health in older adults [45,46].

To prevent frailty, older adults must maintain high levels of PA as early as possible. Dodds et al. [47] found that leisure PA levels divided into tertiles among middle-aged and older adults showed that those in the highest tertile had higher grip strength than those in the lowest tertile. This suggests that active leisure PA during middle age has a cumulative effect on grip strength in older individuals, indicating that maintaining active leisure PA can help prevent a decline in grip strength in later years. Furthermore, recent studies [8,9] on the preventive and management effects of PA on frailty progression have shown that moderate or high levels of PA reduce the progression or degree of frailty in individuals in their 70s. However, in individuals in their 80s, these effects were minimal or nonexistent.

There were differences in nutrient intake between groups of elderly individuals based on grip strength criteria (men: ≥26 kg, women: ≥18 kg). Specifically, men in ESH with higher grip strength consumed more protein, whereas women in ESH with higher grip strength consumed more total fat, MUFA, and PUFA. Although it could not be determined whether these differences were due to women considering fatty acid composition when selecting their diet, this information may be valuable for future nutritional education of the elderly. The regression analysis revealed a positive, although not statistically significant, association between protein intake and grip strength in men. Despite this, the fact that men with higher grip strength had significantly greater protein intake suggests the potential influence of proteins on grip strength, although this effect may have been diluted in the adjusted model. In contrast, in women, a protein intake exceeding the KDRI was significantly associated with increased grip strength, and this association remained strong even after adjusting for energy intake, highlighting the crucial role of proteins in enhancing grip strength. Additionally, while consuming at least 75 mg/day of vitamin C was significantly associated with higher grip strength in men, this association was not statistically significant after adjusting for energy intake, suggesting that the effect of vitamin C may be confounded by other dietary factors. In women, the significant association between vitamin C intake and grip strength became nonsignificant after adjusting for energy intake, although the higher vitamin C intake observed in the stronger grip group suggested a potential positive effect, warranting further investigation. Overall, the influence of specific nutrients such as proteins and vitamin C on grip strength varies between men and women, with these factors showing a stronger association with grip strength in women.

The results of this study should be interpreted with the following limitations: 1) although we evaluated PA levels and nutrient intake based on grip strength in ESH, we did not ascertain the onset of grip strength deterioration, which could have provided further insight into the timing of these changes; 2) PA levels were derived from self-reported survey data, which are susceptible to recall and social desirability biases; and 3) nutrient intake analysis relied on the 24-hour recall method, which may not fully capture the overall dietary habits of an individual. Additionally, the generalizability of our results is constrained, as the study was confined to ESH in Korea, which may limit the applicability of these findings to other age groups or elderly individuals in multi-person households.

To address these limitations, future research should employ longitudinal data to establish causal relationships among PA, nutrient intake, and grip strength over time. Intervention trials should be designed to develop and evaluate targeted programs aimed at improving PA and nutrient intake based on grip strength levels. These studies must incorporate objective measurement tools, such as accelerometers, food diaries, or biomarker analyses, to enhance data accuracy.

Nevertheless, the strength of this study lies in its detailed analysis of PA and nutrient intake according to grip strength, segmented by sex, within the context of ESH. Most existing studies on the relationship between PA, nutrient intake, and grip strength have not considered variations in household type or sex. Our findings revealed significant sex differences in the impact of PA and nutrient intake on grip strength among the ESH, highlighting the need for targeted interventions to address these disparities.

In this study, we found that there were sex differences in PA and nutrient intake levels according to grip strength in patients with ESH. This suggests that strategies for preventing frailty and maintaining muscle strength should be tailored according to sex. To improve overall grip strength and address frailty, women with ESH require programs that increase PA across various domains and ensure a balanced fat intake based on fatty acid composition. Conversely, men with ESH require programs that focus on increasing strength training and protein intake.