Introduction
Traditionally, health research and healthcare services have focused on the identification and treatment of disease, while the development and distribution of knowledge about how to improve and/or maintain functional capacities during ageing is limited [
1,
2]. Except for the decrease in life expectancy due to the Covid-19 pandemic [
3], life expectancy has increased in recent years while years of life lived in good health has not increased at the same rate [
4].
The global population aged ≥ 65 years is projected to increase from 9.7% in 2022 to 16.4% in 2050 [
5] (p8), therefore it is important to follow up on the World Health Organization (WHO) summons to invest in data to monitor healthy ageing across the life course [
6]. According to the WHO, healthy ageing is the process of maintaining functional abilities to enable well-being in older age [
6]. A helpful tool in this transition is the concept of “intrinsic capacity”, defined as an individual’s mental and physical capacities, further divided into the five domains locomotion, cognition, vitality, mental health and sensory. Intrinsic capacity was coined by WHO in 2015 [
7], and in 2017 the first of WHO’s “Integrated Care for Older People” (ICOPE) guidelines were published. These guidelines provide a framework for screening, assessment and management of decline in intrinsic capacity.
Interventions suggested in ICOPE include nutrition and physical activity counselling, self-care and management skills and treatment of individual health conditions. Interventions should not be used in isolation, but rather “considered and applied together” [
8]. Indeed, ICOPE behavior interventions mimic general country guidelines for healthy living that also include multiple modifiable behaviors to improve the healthiness of the population. Prior studies have shown that the number of health-promoting behaviors of an individual is associated with improved functioning in old age in the intrinsic capacity domains locomotion [
9,
10], cognition [
10,
11] and mental well-being [
10,
12]. On the same note, health-promoting behaviors have been associated with lower mortality [
13], less disability [
13] and improved quality of life of older adults [
14].
There is increasing awareness of the inadequacies of guidelines for treating single diseases, as individuals may present with multiple conditions that occur together [
15]. Similarly, functional decline may occur in more than one domain of intrinsic capacity. Visser et al. [
10] show the benefit of not only looking at multiple health-promoting behaviors together, but also including multiple functional domains when assessing the effect of health-promoting behavior on intrinsic capacity.
In this study we aimed to examine if multiple behaviors known to improve health in individuals ≥ 65, such as dietary habits and physical activity, were associated with improved intrinsic capacity at follow-up ten years later.
Discussion
In this study we aimed to explore if adherence to health-promoting behaviors in older adulthood was associated with improved intrinsic capacity at ten year follow up. We found that even when controlling for known predictors of lower adherence to guidelines for healthy living and lower functional ability, closer adherence to guidelines for healthy living was associated with higher intrinsic capacity ten years later. These findings support the WHO’s ICOPE guidelines [
8] that it is beneficial to take multiple health-promoting behaviors into account when working to improve an individual’s intrinsic capacity. Establishing these behaviors early may have additional benefits, as they are associated with reduced prevalence of long-term conditions that can lead to lower intrinsic capacity during ageing, shown in our data by the lower intrinsic capacity in the participants with long-term conditions. Reducing risk of non-communicable diseases and/or disease severity and improved metabolic health are likely causal mechanisms behind these findings.
Unsurprisingly, we found that adherence to fruit and vegetable intake and physical activity level guidelines was low. This is in line with dietary intake data from Norway which show that intake of fruit and vegetables have increased since the time of HUNT3 [
29]. Also following trends seen in Norwegian diets, the intake of milk and fatty fish reported by our HUNT cohort is higher than more recent data in Norway showing a decreased intake of these foods in the years since HUNT3 [
29]. Similarly, since HUNT3 an increase in physical activity levels has been reported in the Norwegian population, including in older adults [
30].
Looking at the association between individual components of the HLI and intrinsic capacity, the strongest associations were found with social interaction, fruit and vegetable intake and physical activity levels. This is in line with a study by Jia et al. [
11] that found that diet, physical activity and social contact were most strongly associated with memory function. For the other components of adherence to dietary guidelines, fatty fish intake and vegetable oils tends towards being a positive influence on intrinsic capacity, while dairy showed no such association. Both fatty fish and vegetable oils are strongly associated with improved health in multiple domains [
31,
32], while the data on dairy is more mixed though in favor of consumption [
33,
34]. The strongest association was seen for social interaction, which is likely due to people lacking social support networks having fewer resources to play on in the face of health problems, and that lonely people are more likely to demonstrate less health-promoting behaviors [
35,
36]. Looking at multiple behaviors and multiple domains of intrinsic capacity together is important as individuals partake in multiple behaviors and can have multiple functional impairments. Seen together it is easier to observe benefits of health-promoting behavior for healthy ageing. Increasing knowledge of the benefits of health-promoting behaviors, while not minimising the consequences of non-adherence, has been suggested as a way to increase occurrence of health-promoting behaviors in the older population [
37].
Unfortunately, we were not able to assess participants intake of other foods known to influence health, such as whole grains, red and processed meat, ultra-processed foods and other types of seafood than fatty fish as these food items were not assessed in HUNT3.
Tavassoli et al. [
38] found highest levels of impairment in the domains of locomotion, cognition and vitality when assessing intrinsic capacity in primary care services in France. This is similar to our results that also found locomotion and vitality to be two of the three domains with most impairment. In our sample, hearing impairment was more prevalent than impairment in both vitality and cognition, which may be due to different methods of measuring these domains but also that individuals with cognitive impairments may be more prevalent in a primary care setting than in HUNT participants. However, when cognition was measured in HUNT4 70 + participants could have the assessment health care team come to their home, likely improving participation in the more impaired population. Still, our cohort that took part in both HUNT3 and HUNT4 70 + has a lower level of cognitive impairment than what was found for all participants in HUNT4 70+ [
39], and this could be due to how repeat participants in cohort studies are known to have better health outcomes than non-responders [
40]. In our study, the majority of non-responder were participants that died between the two HUNT surveys. These participants were generally older, and had more missing data on the variables included in the HLI.
The questions used to assess health-promoting behaviors were short and simple, indicating that even a short questionnaire on adherence to healthy behaviors may be adequate to assess an older individual’s health-promoting behaviors with the potential to improve their long-term intrinsic capacity. This could help design questionnaires to be used in primary care practice, with a low burden on both participants and clinicians. WHOs ICOPE screening regime for intrinsic capacity has already been shown to be feasible to use in clinical practice [
38]. While the individual has a responsibility to manage their health, differences in living conditions and socioeconomic factors mean that not everyone has the same opportunities for good health. For best care, it is important for healthcare workers to be aware of the stigma that can arise if ill health is blamed on a lack of personal responsibility and an inability to live healthily [
41].
Strengths and limitations
Strengths of this study are the use of the HUNT cohort, with a large sample and long follow-up time; the multiple behaviors that compose our HLI; and that besides the sensory impairment and mental health domains of intrinsic capacity all domains were measured by validated tests which have been proposed to be used by clinicians to assess patients. There is a clear temporal association between the HLI and ICI variables as the HLI measurements were collected ten years prior to the ICI.
Limitations include the self-reported behavior data, especially as these have not been validated for our cohort’s age group, and that the HLI is dominated by dietary factors. Additionally, for the domain of sleep we were only able to look at participant’s symptoms of insomnia, which may be insufficient as a measure of sleep quality. Previous studies have found relationships between other markers of sleep quality, like self-reported sleep quality and hours spent in bed, and functional capacities in the elderly [
42]. It is also possible that health-promoting behaviors have changed during the ten-year period between HUNT3 and HUNT4 70+. While we controlled our models for the presence of long-term conditions, the severity of the conditions was not assessed, and the participants may have suffered from additional health conditions not included in the questionnaires. Finally, at the time of HUNT3, some participants may already have lowered functional capacities due to ill health that make them less able to comply with health-promoting guidelines.
Our cohort is from a high-income country with a relatively good health status. However, when accounting for functional measures the burden of the ageing population may be similar to lower income countries, although occurring later in life [
43]. The HLI encompasses public health challenges, such as low fruit and vegetable intake and low levels of physical activity, that are not isolated to high income countries [
44,
45]. Therefore our findings may be applicable to different populations.
Still, it is likely that the participants in our cohort are healthier, more health aware and have a higher socioeconomic status than the general population [
46]. This is indicated by the high HLI and ICI of our participants and may have affected our results. Health-promoting behaviors are associated with a longer lifespan, and older age is associated with more functional impairments which could lead to an underestimation of the effect of health-promoting behaviors on intrinsic capacity through reverse causality.
Conclusion
Participating in multiple health-promoting behaviors was associated with intrinsic capacity ten years later. Behaviors with the strongest associations with their intrinsic capacity were consuming more plant-based foods and participating in physical activity. The measurements used for assessing the health-promoting behaviors were simple, showing that even short questionnaires may provide useful information to a clinician to assess their patients. This could be used to help design short questionnaires to be used in clinical practice for assessing health-promoting behaviors. Still, due to the design of the study, we cannot conclude that lower adherence to healthy guidelines is not due to already present functional impairments and this should be elucidated in further studies.
Acknowledgements
The Trøndelag Health Study (HUNT) is a collaboration between HUNT Research Centre (Faculty of Medicine and Health Sciences, Norwegian University of Science and Technology NTNU), Trøndelag County Council, Central Norway Regional Health Authority, and the Norwegian Institute of Public Health.
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