Self-rated health (SRH) and cardiorespiratory fitness (fitness) are independent risk factors for all-cause mortality. The purpose of this report is to examine the single and joint effects of these exposures on mortality risk. The study included 18 488 men who completed a health survey, clinical examination and a maximal exercise treadmill test during 1987–2003. Cox regression analysis was used to quantify the associations of SRH and fitness with all-cause mortality. There were 262 deaths during 17 years of follow-up. There was a significant inverse trend (ptrend <0.05) for mortality across SRH categories after adjustment for age, examination year, body mass index, physical activity, smoking, alcohol consumption, abnormal ECG, hypertension, hypercholesterolaemia, cardiovascular disease, diabetes and cancer. Adjustment for fitness attenuated the association (p value =0.09). The authors also observed an inverse association between fitness and mortality after controlling for the same covariates and SRH (ptrend = 0.006). The combined analysis of SRH and fitness showed that fit men with good or excellent SRH had a 58% lower risk of mortality than their counterparts. SRH and fitness were both associated with all-cause mortality in men. Fit men with good or excellent SRH live longer than unfit men with poor or fair SRH.
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The WHO set a target for all countries to introduce, actively manage and monitor programmes that enhance healthy lifestyles.1 Self-rated health (SRH) is a relevant construct for a healthy lifestyle and is a question included in many epidemiological studies. SRH is a subjective measure that captures a person's perception of their overall health status. Individuals' perceptions of their health not only incorporate psychological, biological and social dimensions that are unavailable to the external observer, but also provide a dynamic assessment of their current health status while integrating a trajectory of their health.2,–,4
A healthy lifestyle can stabilise SRH and reduce mortality.3 5 6 Also, SRH is an independent statistical determinant of mortality due.7 Maddox and Douglass reported that a positive correlation existed between a patient's SRH and the physician's ratings.8 The patient's SRH was a strong predictor of future physician's ratings.8 In a cohort of patients with diabetes, SRH was a strong predictor of all-cause mortality and cardiovascular-related mortality.9 Even in patients with stable chronic heart failure, SRH had an inverse relationship with mortality.10 Yet, these studies did not control for physical activity (PA) or cardiorespiratory fitness (hereafter simply fitness), and both of these variables have a protective effect on mortality.11,–,13
Some earlier studies have examined the relationship between SRH and PA and its association with mortality.2 6 14,–,16 The Whitehall II study showed an equally strong protective effect of SRH on mortality in men and women, while controlling for self-reported PA.17 In addition to self-reported PA, a 2008 longitudinal analysis of the ilSIRENTE study found a strong association between Short Physical Performance Battery, SRH and mortality.18
However, none of the previous studies evaluated the relationship between SRH and mortality while controlling for measured fitness. In addition, we have found no previous studies that investigated the association between fitness and mortality while controlling for SRH. Therefore, the primary research aim of this study was to explore the protective effect of SRH on mortality after controlling for fitness and vice versa. A secondary aim was to investigate whether the effect modification was evident.
Materials and methods
This was a prospective observational study consisting of 18 488 men, aged 20–84 years, who completed a baseline examination at the Cooper Clinic, Dallas, TX, during 1987–2003. Study participants came to the clinic for periodic preventive health examinations. The cohort includes few members of minority groups. Most participants come from middle and upper socioeconomic strata (∼80% hold a college degree).19 The study protocol was reviewed and approved annually by the Institutional Review Board at the Cooper Institute.
All study participants completed a thorough health examination after completion of written informed consent. The medical evaluation, performed after participants had fasted overnight for ≥12 h, included a physical examination, anthropometric measurements, ECG, blood chemistry analyses, blood pressure assessment, maximal exercise test on a treadmill, self-report of health habits and recording of demographic characteristics. Additional details of the medical assessment are in previous reports.20,–,22
SRH was reported on the medical history. Participants were asked to rate their health based on a single, validated and widely used question,23,–,25 ‘How would you rate your overall health?’ The choices were ‘poor’, ‘fair’, ‘good’ and ‘excellent’. We combined poor and fair due to the small number of responses in these categories. Also, for the stratified and final analyses, the responses were dichotomised into ‘poor/fair’ and ‘good/excellent’ and were coded 0 and 1, respectively.9 23 26
Fitness was quantified as the duration of a symptom-limited maximal treadmill exercise test using a modified Balke protocol.20 27 Total treadmill endurance time was used as an index of aerobic power, with time on treadmill in this protocol highly correlated (r>0.92) with maximal oxygen uptake (VO2 max) in men.28 We classified men into three categories depending on their age-specific (20–39, 40–49, 50–59 and ≥60 years) treadmill time from the entire Aerobics Center Longitudinal Study (ACLS): ‘low (least fit 20%)’, ‘moderate (next fit 40%)’ and ‘high (most fit 40%)’. All participants included in the analyses achieved at least 85% of their age-predicted maximal heart rate (220 minus age in years).
Age, year of baseline examination, BMI (body mass index = weight (kg)/height (m)2), PA, smoking, alcohol consumption, abnormal ECG, hypertension, hypercholesterolaemia and incident cancer, cardiovascular disease and diabetes were controlled in multivariate analyses. Height and weight were measured using a standard physician's scale and stadiometer. Information on smoking habits (never, former, current), alcohol intake (drinks per week) and PA (low, moderate, high) was obtained through questionnaires. The measure of alcohol intake was described in detail in a previous paper.13 Blood pressure was measured by auscultation after at least 5 min of sitting quietly. The participants were classified as having a chronic medical condition if they had one or more of the following physician-diagnosed diseases or conditions: abnormal ECG, cancer, cardiovascular disease, diabetes, hypertension and hypercholesterolaemia.
All participants were followed for mortality from the baseline examination to the date of death or 31 December 2003. Deaths among study subjects were identified from the National Center for Health Statistics National Death Index.
Baseline characteristics of the population were calculated for SRH categories: poor/fair, good and excellent. Cox proportional-hazards regression was used to estimate the hazard ratios (HRs), 95% CIs and mortalities (deaths per 10 000 person-years of follow-up adjusted for age and examination year), according to categories of SRH and fitness. Within the first statistical model, age and year of baseline examination were controlled across SRH and fitness categories. The second model was further adjusted for the other aforementioned covariates. In the final SRH and fitness models, fitness and SRH were additionally adjusted for, respectively. The proportional hazards assumption was examined by comparing the cumulative hazard plots grouped on exposure; no appreciable violations were noted. Next, we evaluated the possible effect modification across age (<60 vs ≥60 years old), smoking status (non-smoker vs smoker), BMI (<25 vs ≥25) and chronic medical condition (with vs without). Finally, we assessed the joint association of SRH and fitness on mortality. Unfit men with a poor/fair SRH were the referent group. All analyses were performed with SAS statistical software, version 9.1 (SAS).
There were 262 deaths during an average 7.7 years of follow-up. The baseline characteristics of the study population were analysed according to SRH categories (table 1). Men with excellent SRH were slightly older than those with poor/fair or good SRH (p value <0.001), had a lower BMI (p value <0.001) and more likely to be in the high fitness group (p value <0.001 for both).
SRH and fitness were both shown to have a crude protective effect on mortality with a HR of 0.72 and 0.59, respectively. After adjusting for age and examination year (table 2), both SRH (ptrend ≤ 0.001) and fitness (ptrend < 0.001) continued to be inversely associated with mortality. These relationships remained significant when controlling for the other covariates in the second model. In the final model, when controlling for all covariates and fitness, the trend across SRH groups was only marginally significant (p=0.09). The strong inverse gradient for mortality across fitness groups remained in the final model after adjustment for SRH (ptrend = 0.006).
We also examined the association of SRH and mortality in specific subgroups of the population (figure 1). Participants 60 years of age and older who rated their health as good or excellent had a significantly lower mortality than participants in the same age group who ranked their health as poor or fair. Participants diagnosed with one or more chronic medical conditions, such as diabetes or cardiovascular disease, and ranked themselves in good or excellent health had a lower mortality risk than their counterparts who ranked their health as poor or fair. There were no significant differences for SRH groups for the other comparisons.
We then evaluated the combined influence of SRH and fitness on mortality (figure 2). Men who were fit (moderate or high fit) and had good or excellent SRH had a 58% lower risk of mortality compared to the referent group (unfit men with poor/fair SRH).
To our knowledge, this is the first study to examine the combined associations of SRH and fitness on mortality. Our primary finding is that SRH and fitness independently predicted mortality after adjusting for several confounders and for each other, although this was of marginal significance for SRH after adjusting for fitness. In addition, men with good or excellent SRH, combined with high fitness, had a 58% lower risk of dying when compared with those with poor or fair SRH and unfit.
A unique finding of this study was the effect of fitness on the relationship between SRH and mortality. SRH, alone, has a protective effect on mortality, but when fitness was added to the model, SRH's protective effect was reduced to marginal significance. This is similar to results reported from an Australian cohort that observed a 45% higher all-cause mortality risk for participants with good SRH who reported a low exercise level compared with the individuals with good SRH and moderate exercise.6 However, our finding of a joint association is not congruent with other studies that reported an independent relationship between SRH and other forms of PA with mortality.15 18
Previous studies on SRH and mortality show a mortality risk 1.4–3.0 times higher for respondents with poor SRH compared with individuals who ranked their health as good.2 4 6 10 17 29 In these studies, the prevalence of smoking (22%) in the good SRH group was similar to our findings of 25%.2 A systematic review of 27 studies found that when controlling for cardiovascular risk factors such as ECG, diet history and blood pressure, SRH was still a significant predictor of all-cause mortality.4 This result is similar to the result presented in figure 1, indicating that participants with chronic conditions experienced a lower all-cause mortality risk when they reported their health as good, compared to individuals with poor SRH.
Our results on the association between fitness and all-cause mortality are consistent with previous reports, but extend the literature by accounting for SRH.18 A recently published meta-analysis30 summarised 33 studies linking fitness to all-cause mortality. Fitness was assessed as maximal aerobic capacity and expressed in metabolic equivalents (METs). The study concluded that the pooled risk for all-cause mortality decreased 13% per 1-MET increment. Also, compared with participants with low fitness, those with high fitness had a 41% lower risk for all-cause mortality in their categorical analyses.30 Other reports originating from the ACLS also support our findings of a protective effect of fitness on all-cause mortality with approximately 30–70% lower mortality risk in highly fit individuals.19 20 22 31,–,34 While examining the effect of fitness on all-cause mortality, these studies controlled for potential confounders such as BMI, blood pressure, smoking status and the presence of chronic illness, but not for SRH. Our findings of joint effects of fitness and SRH indicate that future studies should consider controlling for SRH when investigating the relationship between fitness and mortality.
Although no previous studies have examined the joint associations between SRH and objectively measured fitness with mortality, a study in Finland did report the combined effects of SRH and self-perceived fitness on mortality in 1340 men. The results depict an inverse dose–response relationship between SRH and mortality.2 While controlling for self-perceived fitness, the risk for mortality increased by 92% for men reporting poor SRH compared to those reporting good health.2 In our study, men reporting poor health showed a 1.3 times higher risk of all-cause mortality compared to men with good SRH. This dissimilarity in findings may be due to the objective measure of fitness used in the current study, which has less misclassification than self-perceived fitness.
The mechanism between SRH and mortality is not well established. One plausible explanation is the afferent information that conveys messages from the organism to the brain.35 These messages are usually not brought to consciousness because they function at lower levels of the central nervous system. However, this afferent information is perceived by the individual as sensations, feelings and emotion and is the sense that reflects the physiological condition of the entire body.7 35 Another theory explaining a person's perceptions of their health involves a family of proteins called cytokines.7 Cytokines are involved in inflammation processes and play a major role in infectious conditions and also the pathogenesis of many chronic diseases. Research is beginning to show that the inflammatory process and certain cytokines are associated with tiredness, impaired sleep, depressive mood and poor appetite.7 36
To the best of our knowledge, this investigation is the first study to examine the combined effect of SRH and fitness on mortality. The present study expands on previous findings by showing strong independent associations of SRH and fitness with mortality. This investigation was conducted using the ACLS, a large prospective study with a relatively long follow-up period, objective measures of fitness and other potential health covariates, and thorough baseline clinical examinations by a physician to detect underlying disease conditions. One limitation of the current study is the homogeneity of sociodemographic factors in our population. Although this increases internal validity by controlling for potential confounders, caution must be taken in generalising the results too broadly. Along with this limitation, a time trend limitation may exist. Due to the span of 17 years of baseline data collection, a higher expectation concerning individual health might have been present in 2003 compared with earlier examinations.7
We found that both SRH and cardiorespiratory fitness were strong independent predictors of all-cause mortality. Participants with one or more chronic diseases/conditions such as abnormal ECG, cancer, cardiovascular disease, diabetes, hypertension or hypercholesterolaemia had a lower mortality risk when they reported good/excellent SRH, compared with peers who reported poor/fair SRH. The combined association of good or excellent SRH and moderate or high fitness had a 58% lower mortality risk when compared with men who had poor or fair SRH and were unfit. Health professionals might need to pay attention to patients' SRH and assess their fitness level as well for risk stratification.
The authors thank all the Cooper Clinic physicians and technicians for collecting the baseline data, and staff at the Cooper Institute for data entry and data management.
Funding This study was supported by National Institutes of Health grants AG06945, HL62508, R21 DK088195 (to XS from the National Institute of Diabetes and Digestive and Kidney Diseases), K05 CA136975 (to JRH from the Cancer Training Branch of the National Cancer Institute) and an unrestricted research grant from The Coca-Cola Company.
Competing interests None.
Provenance and peer review Not commissioned; externally peer reviewed.