This study reported the prevalences of anemia and frailty and the two conditions combined in a large population of older Chinese adults. The prevalences of both conditions were higher at older ages and in individuals with lower education levels. In addition, anemia was significantly associated with frailty, where each 1 g/dL increase in hemoglobin concentration was related with 4% decrease in the odds of frailty after adjusting for several variables. As far as we know, this was the first paper addressing the association between anemia and frailty among community-dwelling adults aged 50 years and older in China.
While estimates of anemia prevalence differ considerably, with reported prevalence ranging from 2.9% to 61% in older men and from 3.3% to 41% in older women  , the prevalence was generally higher in men than in women and increased with advancing age[17,18]. Anemia prevalence was 14.1% for men and 10.2% for women aged 65 and older in the US National Health and Nutrition Examination Survey (NHANES 2013–2016). An Australian epidemiologic study had anemia estimates of 14.6% among men aged 70+ years. Thirty-eight percent of community-dwelling people aged 60 years and older had anemia in a small study in India. Likewise, 38.1% of older adults had anemia in the Singapore Longitudinal Ageing Studies (SLAS). Our analyses indicated that the prevalence of anemia was 31.0% (95%CI: 28.4-33.8%) in China, which was higher in men, older people, lower levels of education, and those lived in urban area and with higher wealth, being inverse of the results among older Mexican adults. It indicated that living area and wealth may modifies the probability of being in the different nutritional conditions, which related with anemia. In addition, the different population, sampling programs and hemoglobin test methods may also contribute to the difference between these studies.
Attention to the measurement and impact of frailty in older age has increased substantially over the past decade. The deficit accumulation approach has been well tested in different populations. For example, the overall weighted prevalence of frailty was 9.9% in the community-dwelling older population (60+ years) derived from the China Comprehensive Geriatric Assessment Study (CCGAS), based on the Comprehensive Geriatric Assessment Frailty Index. The physical frailty phenotype approach was used in an analysis of the China Health and Retirement Longitudinal Study (CHARLS), resulting in 7% of adults aged 60 years or older being classified as frail. In our study, the frailty index resulted in 14.7% (95%CI: 13.5-16.0%) of community-dwelling residents aged 50+ years being classified as frail, higher than the two studies mentioned which use somewhat different frailty criteria.
Anemia reduces the oxygen-carrying capacity, which can result in tissue hypoxia and lead to a number of poor outcomes, including reduced submaximal and maximal aerobic capacity, failing muscle strength, cognitive impairment and development of frailty[26-28], which related to vulnerability and some negative outcomes . Several previous studies have examined the interaction between anemia and frailty among older people in high income countries. A case-control study in Baltimore (USA) firstly explored the relationship between anemia and frailty, showing an inverse correlation between interleukin-6 (IL-6) and hemoglobin or hematocrit in the frail group, suggesting that frail subjects have evidence of inflammation and lower hemoglobin and Hematocrit levels. Data from the Women´s Health and Aging Studies (WHAS) I and II found that mildly low and low-normal hemoglobin levels were associated with increased frailty, and the risk of frailty increased at statistically significant levels for anemia adjusted for age, race, and education[30-31]. Another cross-sectional and longitudinal study in older Australian men also suggested that anemia may contribute to the development of frailty. Recent studies including both older men and women indicated that older anemic adults were more likely to be frail, with the association between lower levels of hemoglobin and number of frailty criteria showing dose-response effect[32-34]. However, another contrasting result suggested having anemia contributed to a weak but significantly lower chance of worsening frailty. In our study, we used 40 variables to construct a Frailty Index and observed that both anemia and lower concentrations of hemoglobin were associated with frailty.
Some studies have suggested that age-associated chronic inflammation is an explanatory factor in the relationship between anemia and frailty. In older adults, anemia and frailty may share a pathophysiological pathway with chronic inflammatory processes, resulting from immunosenescence-associated changes and increased oxidative stress[36-38]. Gabriele described a close connection between inflammaging, anemia, and frailty, where comorbidities and inflammaging contribute to anemia of chronic inflammation(ACI), which was the most frequent type of anemia in older adults. Considering the etiopathogenetic mechanisms of inflammation, some interventions such as dietetic approach and physical exercise that can moderate oxidative stress and chronic inflammation may prevent anemia, frailty and their negative impact on functional performance and quality of life. Another study reported that a high intake of dietary total antioxidant capacity (TAC) was inversely related with frailty, and the intake of green tea, vegetable and fruits which contributed to TAC was also associated with lower odds of frailty. Our results also indicated sufficient intake of vegetables and fruit and moderate to high levels of physical exercise had protective effects against frailty.
There were a few limitations in our study. Firstly, we used cross-sectional data from SAGE China Wave 1, it cannot provide causal direction in the relationship between anemia and frailty. Results from SAGE China Waves 2 and 3 may provide an opportunity to examine the direction of this relationship we identified. Secondly, we used self-report for some items to construct Frailty Index, which may be influenced by recall bias, although self-reported health questions are widely applied in population studies. Thirdly, the missing data for haemoglobin may have also contributed to selection bias. We analyzed the distribution of the missing data of Hb and found that total of missing values were randomly distributed across five income groups, but there were significant differences between rural and urban across five income groups, that might be the reason why higher wealth individuals had higher anemia rates. However, our study was based on a large, national probability sample of older adults of both genders in China. We do not expect these missing values to have impacted the results or interpretations. Furthermore, the results indicated a quantitative relationship between hemoglobin concentration and frailty.