Various longitudinal studies have examined frailty trajectories using measurements at three or more time points, in which frailty is typically included as a continuous measure (mainly assessed with the FI).⁴⁰⁾ Four examples of such studies are shown in Table 1. Overall, these studies provide an overview of increasing frailty levels with age. However, the rate of change and the shape of the change curve (linear or quadratic) vary across studies.^22,29-31) Jenkins et al.³⁰⁾ aimed to replicate findings across five longitudinal cohort studies from Europe and the United States. The included aging cohorts had 11–20 years of follow-up. A linear model best characterized frailty progression, and frailty trajectories did not differ between men and women. Additional insights into frailty changes over the life course can be derived from the work of Raymond et al.,³¹⁾ as this study also included younger individuals. Interestingly, a turning point was observed at 65 years of age, with a faster increase in frailty levels after that age. The studies listed in Table 1 represent those on frailty trajectories in older populations. However, in the field of frailty trajectories, more specific research directions are evolving, such as work in specific patient populations (e.g., psychiatry and oncology),^41,42) latent frailty trajectories (i.e., groupings of people based on distinct frailty trajectories),⁴³⁾ frailty fluctuations (i.e., within-person deviations from general frailty trajectories),¹⁰⁾ the order in which frailty components develop,⁴⁴⁾ and terminal decline in frailty.^45,46)

The study of frailty transitions provides another way to examine longitudinal data. To study these transitions, frailty scores must be categorized. Especially for clinical care pathways, the frailty transition approach is helpful because categorization simplifies clinical decision-making, although the classifications may sometimes be arbitrary. Understanding transitions in frailty may help to identify optimal target populations for frailty interventions. Studies have suggested targeting pre-frailty for early interventions.⁴⁷⁾ Studies on transitions are predominantly performed using the frailty phenotype, which has clear categories (robust, pre-frail, and frail). However, theoretically, other frailty measures such as the FI may also be used for this purpose. In 2019, Kojima et al.⁴⁷⁾ performed a systematic review of transitions between frailty states in community-dwelling older adults, which included 16 studies related to this topic. Table 1 includes four examples of studies on frailty transitions. In general, these studies showed that frailty is a dynamic condition, as all studies reported that frailty worsening and frailty improvement were common, while large parts of the study samples remained in the same state at follow-up.^32-35) The study by Gill et al.³²⁾ was one of the earlier studies on frailty transitions. Over 4.5 years, the participants underwent assessments every 1.5 years; among these participants, at least 58% experienced a frailty transition during follow-up. Espinoza et al.³³⁾ examined potential predictors of frailty progression, in which fewer years of education, among other factors, was associated with frailty progression. However, only two measurements (baseline and follow-up) were performed in this study. More research over an extended time is needed to better understand the transitions in frailty later in life and the factors associated with the transitions.

Studies on frailty trends are relatively scarce as very few longitudinal cohort studies have a cohort-sequential design in which refresher samples of older adults from new generations are added to the study after several years. To our knowledge, only studies in the United States, the Netherlands, the UK, and Sweden have reported on this topic (Table 1).^36-39) All these studies focused on older adults in the same age group at different periods over a long time span, and together they cover the period from 1990 to 2020. For example, a study from the Netherlands (Longitudinal Aging Study Amsterdam [LASA]) showed that the prevalence of frailty among older adults aged 64–84 years increased from 21% in 1995–1996 to 28% in 2011–2012.³⁷⁾ Other studies have shown similar results, with increasing levels of frailty or higher frailty prevalence in more recent generations of older adults aged 65 years and over.^36,38,39) Blodgett et al.³⁹⁾ reported an increasing trend in frailty levels between 1999 and 2018 among middle-aged people (≥35 years) in the United States using data from the National Health and Nutrition Examination Survey. A study from Sweden reported that increasing frailty prevalence rates were mainly driven by increased frailty scores among individuals in the highest quartile of the frailty score.³⁸⁾ Further explanations for the increasing trend in frailty among older adults are unknown and should be explored in future research. Some of the above-mentioned studies also examined trends in the association between frailty and mortality and found that this association remained stable (i.e., in subsequent generations of older adults, the association between frailty and mortality did not become weaker or stronger).^36,38,39) Altogether, the results of trend studies indicate a negative prospect for the future, as there are no signs of a decline in the excess mortality rates of frailty.

Insight into risk factors for certain frailty trajectories, transitions, and trends may inform the development of better-targeted interventions to prevent or reduce frailty levels in older adults. Risk factors are not limited to lifestyle and clinical factors relevant later in life and may also involve behavioral and clinical conditions during early life stages, such as midlife and childhood.⁵⁾ A recent review of frailty trajectory studies conducted so far by Welstead et al.⁴⁰⁾ showed that the different methodologies of these studies make it difficult to draw conclusions regarding which factors contribute to frailty progression. Some risk factors were only identified in a single study.⁴⁰⁾ Nevertheless, factors such as low socioeconomic position and lack of physical activity likely contribute to frailty progression. This research field requires expansion, and results should be replicated using similar methodological approaches across studies and settings.

Based on the results of studies on the trajectories, transitions, and trends in frailty among older adults, some implications for clinical practice and public health can be derived. First, the results of the studies discussed in this review emphasize the importance of early identification of frailty. Identification of frailty at an early stage of a frailty trajectory is critical to promote early interventions.^8,48) Second, regular (e.g., yearly) assessments of frailty among older adults in routine clinical care may be important to monitor frailty changes. An individual reaching a certain frailty level may require additional evidence-based clinical actions.⁴⁹⁾ Change in frailty is a risk factor for mortality in later life,^50,51) although findings from previous studies show conflicting results.^51-53) Third, knowledge of frailty trajectories and transitions can inform the cost-effectiveness of future economic planning, which in turn, directly informs policy. Fourth, the findings from trend studies indicate the expected increased burden of frailty for healthcare systems and underscore the need for continued efforts to prevent or reduce frailty among older adults. Frailty prevalence is increasing in more recent generations of older adults, while frailty-associated mortality remains the same.^37,39) Finally, the studies presented in this review make clear the major research gaps, which limit our ability to formulate clear messages for healthcare policymakers. These gaps include the limited number of studies on risk factors for frailty trajectories, transitions, and trends and the lack of studies in low- and middle-income countries. A single study identified various risk factors for frailty progression; thus, replication of these findings is desperately needed.⁴⁰⁾ Although most of the world’s older population lives in low- and middle-income countries,⁵⁴⁾ nearly all evidence on frailty trajectories comes from high-income countries, especially European countries and the United States. Increased efforts are needed to overcome systemic issues that make frailty research in low- and middle-income countries challenging.⁵⁾