Introduction: Despite its efficacy, conventional center-based cardiac rehabilitation has several limitations which have led to the emergence of home-based programs and intensive cardiac rehabilitation as alternative methods for overcoming these limitations. Alternative methods for primary prevention have been recommended for similar reasons. Lifestyle modification is considered key to success in both primary and secondary prevention. Therefore, this primary prevention study aimed to investigate the efficiency of intensive lifestyle education and home-based programs involving unstructured exercise in urban forests to prevent coronary artery disease (CAD). The availability of urban forests as preventive exercise environments was also examined. Methods: Patients with risk factors for CAD participated in primary prevention using either FBEG (forest-based exercise group (FBEG, n = 11) or CBEG (center-based exercise group (CBEG, n = 17) for 12 weeks. The FBEG was provided with intensive residential lifestyle education and followed a home program that included performing exercise in an urban forest. The CBEG followed a conventional supervised exercise program at a fitness facility. Changes in body composition, cardiometabolic variables, and functional capacity were tested using a 2-way repeated ANOVA measurement. An independent t-test was used to examine the differences in weekly energy expenditure between the two groups. Results: Significant within-group differences were identified in body composition, cardiometabolic variables, and the 10-yr probability of CAD in both groups. However, the functional capacity, weekly energy expenditure, and attendance rate showed between-group differences, with superiority in the FBEG. Conclusion: Intensive lifestyle education and subsequent home-based programs with unstructured exercise in the forest were as effective as a conventional center-based program, with superiority in terms of the change of some variables. Intensive education on experiencing and habituating a healthy lifestyle seemed to play an important role in improving motivation.

Primary prevention is as important as secondary prevention owing to its economic efficacy and ease of disease management. However, primary prevention has attracted relatively little attention in clinical settings compared with secondary prevention. Even current fitness facility programs in Korea have predominantly focused on body shaping for healthy young people, and have provided little insight into the prevention of cardiovascular disease.

Lifestyle modification is as key to primary prevention as it is for secondary prevention. Lifestyle modification to prevent coronary artery disease (CAD) includes enhancing exercise and physical activity, improving dietary habits and emotional stability, smoking cessation, etc., to manage CAD risks [1, 2] The secondary prevention named as cardiac rehabilitation has traditionally been hospital-based with conventional regimens, but it has some barriers that make it difficult for patients to enroll and adhere to the program, despite its benefits and effectiveness [3]. For these reasons, emerging approaches, such as home-based rehabilitation and intensive cardiac rehabilitation, have been introduced to complement the weaknesses of conventional methodologies [2‒6]. Those approaches could be applied to primary prevention, too.

Of all the available interventions for improving CAD risk, exercise training plays an important role as a hub, providing many benefits, including an increase in endurance capacity and improvement in body composition, endothelial function, myocardial flow, and blood lipid profile [7‒9]. Most exercises are categorized as structured versus unstructured and indoor center-based versus outdoor activities. For more than a decade, the therapeutic value of forest environments has been debated in numerous studies, with some results suggesting that forest-based exercise or physical activity could be beneficial in controlling chronic health problems [10‒13]. Considering that more than 60% of the territory in Korea is an orographic area, urban forests close to the residence could be widely used as an area to perform preventive exercise for those who favor the natural environment and wish to engage in outdoor activities. This study aimed to compare the effectiveness of a forest-based unstructured exercise program by adopting intensive lifestyle education in CAD risk change with a conventional structured supervised center-based exercise program and to investigate the availability of urban forests in performing primary prevention programs.

Participants

The participants were recruited from Seosan City Health Center, where we advertised our primary prevention program. Eligibility to participate in the study included people who had CAD risk factors such as metabolic syndrome, prediabetes or diabetes, hypertension, and hyperlipidemia. Patients with current or a history of CAD were excluded. The participants fully understood all the processes of the study and voluntarily decided to participate. IRB approval for this study was obtained from the Committee of Kangbuk Samsung Hospital (KBSMC 2022-06-054-001). All participants were randomly assigned to either a forest-based exercise group (FBEG) or a center-based exercise group (CBEG). Table 1 presents the demographic characteristics of the participants. At first, 40 participants were randomly assigned to either FBEG(n = 20) or CBEG (n = 20). But 9 and 3 participants in FBEG and CBEG dropped, respectively, due to personal situation or refusal.

Table 1.

Demographics of the study participants

FBEG (n = 11)CBEG (n = 17)
Age 67.9±8.3 65.4±3.9 
Gender (F/M) 9/2 16/1 
%BF, % 34.4±8.2 33.3±6.4 
Hypertension, n/N (%) 6/11 (54.5) 11/17 (64.7) 
Diabetes mellitus, n/N (%) 6/11 (54.5) 5/17 (29.4) 
Hyperlipidemia, n/N (%) 9/11 (81.8) 7/17 (41.1) 
FBEG (n = 11)CBEG (n = 17)
Age 67.9±8.3 65.4±3.9 
Gender (F/M) 9/2 16/1 
%BF, % 34.4±8.2 33.3±6.4 
Hypertension, n/N (%) 6/11 (54.5) 11/17 (64.7) 
Diabetes mellitus, n/N (%) 6/11 (54.5) 5/17 (29.4) 
Hyperlipidemia, n/N (%) 9/11 (81.8) 7/17 (41.1) 

CHD, coronary heart disease; FBEG, forest-based exercise group; CBEG, center-based exercise group; BF, body fat.

The participants assigned to the FBEG attended a one-week intensive lifestyle education program at the National Forest Healing Center in Youngju, Kyungbuk. The multidisciplinary staff comprised medical directors, program coordinators, nutritionists, psychologists, exercise trainers, and cooking specialists who accompanied the participants to give classes and run the education program. Table 2 lists the routines of the intensive education program. As indicated in Table 2, all participants followed their daily routine to familiarize themselves with a healthy lifestyle by engaging in theoretical lessons and practice in real daily life. The primary objective of this hands-on education was to experience and habituate to a healthy daily life that included higher physical activity, a prudent diet, and stress reduction.

Table 2.

Overview of the intensive education program routine

1st day2nd day3rd day4th day5th day6th day
06:00  Wake-up Wake-up Wake-up Wake-up Wake-up 
07:00  Meditation gymnastics Meditation gymnastics Meditation gymnastics Meditation gymnastics Meditation gymnastics 
08:00  Breakfast Breakfast Breakfast Breakfast Breakfast 
09:00  Class I CAD risk factor management Class I CAD risk factor management Class IV exercise education Class IV exercise education Forest tracking 
10:00  Class II psychologic intervention Class II psychologic intervention Practicum indoor exercise Practicum indoor exercise 
11:00  Class III nutritional education Class III nutritional education Class III nutritional education Class III nutritional education Review of weekly activity 
12:00  Lunch with cooking class Lunch with cooking class Lunch with cooking class Lunch with cooking class Closing 
13:00  
16:00  Forest tracking Forest tracking Forest tracking Forest tracking 
17:00  
18:00  Dinner Dinner Dinner Dinner 
19:00 Opening and pre-education Review of daily activity Review of daily activity Review of daily activity Review of daily activity 
20:00 
21:00 Relaxation/stretching Relaxation/stretching Relaxation/stretching Relaxation/stretching 
22:00 Sleep Sleep Sleep Sleep Sleep 
1st day2nd day3rd day4th day5th day6th day
06:00  Wake-up Wake-up Wake-up Wake-up Wake-up 
07:00  Meditation gymnastics Meditation gymnastics Meditation gymnastics Meditation gymnastics Meditation gymnastics 
08:00  Breakfast Breakfast Breakfast Breakfast Breakfast 
09:00  Class I CAD risk factor management Class I CAD risk factor management Class IV exercise education Class IV exercise education Forest tracking 
10:00  Class II psychologic intervention Class II psychologic intervention Practicum indoor exercise Practicum indoor exercise 
11:00  Class III nutritional education Class III nutritional education Class III nutritional education Class III nutritional education Review of weekly activity 
12:00  Lunch with cooking class Lunch with cooking class Lunch with cooking class Lunch with cooking class Closing 
13:00  
16:00  Forest tracking Forest tracking Forest tracking Forest tracking 
17:00  
18:00  Dinner Dinner Dinner Dinner 
19:00 Opening and pre-education Review of daily activity Review of daily activity Review of daily activity Review of daily activity 
20:00 
21:00 Relaxation/stretching Relaxation/stretching Relaxation/stretching Relaxation/stretching 
22:00 Sleep Sleep Sleep Sleep Sleep 

Exercise Program and Monitoring Daily Life.

The participants assigned to the CBEG also received education before starting the exercise program. This educational content was similar to that of the FBEG, but it lasted several days, and more theoretical lessons were provided in the classroom. The habituation practiced by the FBEG was not included in this program.

After 1 week of intensive lifestyle education for the FBEG, and brief education for the CBEG, the participants started one of the 12-week exercise programs which aimed to maintain healthy habits in daily life. The FEBG recommended that participant performed “tracking” in urban forests near their residences at least twice a week. Tracking is an unstructured exercise that mainly comprises walking and performing other gymnastics under natural conditions. The CBEG recommended visiting a fitness center (ACE Exercise Science Center) at least twice a week for structured supervised aerobic and resistance exercises using devices. Both groups were prescribed aerobic exercise intensity with a range of 50–70% heart rate reserve. The participants were strongly recommended to maintain an active life, even on the days they did not participate in the exercise program. Therefore, they were provided with a wearable watch (Fitbit, USA) to monitor their heart rate and caloric expenditure to check the appropriate intensity and collect weekly energy expenditure, respectively.

Participants in both groups communicated with staff members via smartphones. The staff monitored and recorded caloric expenditure data from each participant’s wearable watch. The nutritionist and psychologist shared information on diet patterns and emotional status with individuals once a week to counsel them on how to better maintain a healthy lifestyle. Participants in both groups were instructed to do exercise at least 2 times a week. The reinforcement to keep the exercise routine was given as feedback to the patients who skipped the exercise.

Study Design and Statistical Analysis

This study used a 2-way repeated ANOVA measurement to verify within- and between-group differences in the outcome data before and after the 12-week intervention. Independent t-tests were used to compare the average weekly energy expenditure in each group during the 12-week program. Statistical analyses were performed using MedCalc software. All quantitative outcome data are presented as the Mean ± SD and a p < 0.05 indicates statistical significance.

We assessed the participants’ cardiometabolic variables, functional capacity, Framingham absolute and relative risks, and weekly energy expenditure before and after the 12-week program in both groups. The changes in these variables are presented in Table 3.

Table 3.

Cardiometabolic and functional outcomes and the FRS

FBEG (n = 11)CBEG (n = 17)
prepostmean-diffprepostmean-diff
BW, kg 63.9±8.4 61.5±7.6 (−2.5) 57.8±5.8 57.5±5.0 (−0.29) 
%BF, % 34.5±8.2 31.9±8.8 (−2.5)a 33.7±6.4 32.5±6.9 (−1.17) 
HR (rest), bpm 77.2±9.2 64.1±8.8 (−13.0)a 69.4±9.5 65.7±6.2 (−3.8) 
SBP (rest), mm Hg 132.4±7.3 123.3±12.4 (−9.1)a 133.8±19.9 126±12.9 (−7.6) 
DBP (rest), mm Hg 76.4±7.3 76.9±9.5 (+0.5) 76.2±9.7 75.8±9.3 (−0.4) 
TC, mg/dL 187±43.1 179±42.1 (−8.0) 197.8±65.5 177.4±43.2 (−20.4) 
LDL-C, mg/dL 116.7±40.6 105.9±38.1 (−10.8)a 120.6±59.1 96.7±41.2 (−23.8) 
HDL-C, mg/dL 56.6±9.1 64.6±11.5 (+8.0)a 58.0±10.6 59.5±9.5 (+1.5) 
TG, mg/dL 104±36.3 94±47.6 (−9.1) 125.6±62.6 124.3±55.1 (−1.35) 
FBS, mg/dL 113.4±17.9 107±16.2 (−5.5) 106.9±17.1 112.4±29.7 (+5.5) 
HbA1C, % 5.9±0.6 5.7±0.4 (−0.14) 6.1±1.1 6.1±1.0 (−0.04) 
MET peak 9.4±1.8 11.6±1.5 (+2.2)b 7.6±3.1 8.1±3.4 (+0.5) 
FRS absolute risk, % 11.2±5.1 6.5±3.4 (−4.7)a 9.6±6.5 7.8±5.9 (−1.82) 
FRS relative risk, % 1.3±0.5 0.8±0.4 (−0.5)a 1.2±0.8 1.0±0.7 (−0.23) 
Activity energy expenditure, kcal/week 2,140±807c 1,166±613 
Attendance rate, % 97.5±2.9c 65.4±15.5 
FBEG (n = 11)CBEG (n = 17)
prepostmean-diffprepostmean-diff
BW, kg 63.9±8.4 61.5±7.6 (−2.5) 57.8±5.8 57.5±5.0 (−0.29) 
%BF, % 34.5±8.2 31.9±8.8 (−2.5)a 33.7±6.4 32.5±6.9 (−1.17) 
HR (rest), bpm 77.2±9.2 64.1±8.8 (−13.0)a 69.4±9.5 65.7±6.2 (−3.8) 
SBP (rest), mm Hg 132.4±7.3 123.3±12.4 (−9.1)a 133.8±19.9 126±12.9 (−7.6) 
DBP (rest), mm Hg 76.4±7.3 76.9±9.5 (+0.5) 76.2±9.7 75.8±9.3 (−0.4) 
TC, mg/dL 187±43.1 179±42.1 (−8.0) 197.8±65.5 177.4±43.2 (−20.4) 
LDL-C, mg/dL 116.7±40.6 105.9±38.1 (−10.8)a 120.6±59.1 96.7±41.2 (−23.8) 
HDL-C, mg/dL 56.6±9.1 64.6±11.5 (+8.0)a 58.0±10.6 59.5±9.5 (+1.5) 
TG, mg/dL 104±36.3 94±47.6 (−9.1) 125.6±62.6 124.3±55.1 (−1.35) 
FBS, mg/dL 113.4±17.9 107±16.2 (−5.5) 106.9±17.1 112.4±29.7 (+5.5) 
HbA1C, % 5.9±0.6 5.7±0.4 (−0.14) 6.1±1.1 6.1±1.0 (−0.04) 
MET peak 9.4±1.8 11.6±1.5 (+2.2)b 7.6±3.1 8.1±3.4 (+0.5) 
FRS absolute risk, % 11.2±5.1 6.5±3.4 (−4.7)a 9.6±6.5 7.8±5.9 (−1.82) 
FRS relative risk, % 1.3±0.5 0.8±0.4 (−0.5)a 1.2±0.8 1.0±0.7 (−0.23) 
Activity energy expenditure, kcal/week 2,140±807c 1,166±613 
Attendance rate, % 97.5±2.9c 65.4±15.5 

BW, body weight; BF, body fat; HR, heart rate; SBP, systolic blood pressure; DBP, diastolic blood pressure; TC, total cholesterol; LDL-C, low density lipoprotein cholesterol; HDL-C, high density lipoprotein cholesterol; TG, triglyceride; FBS, fasting blood sugar; HbA1C, hemoglobin A1C; MET, metabolic equivalent; FRS, Framingham Risk Score.

aSignificant difference within group (p < 0.05).

bSignificant difference within and between group (p < 0.05).

cSignificant difference between group (p < 0.05).

Cardiometabolic Outcomes

The percentage of body fat (BF) was significantly reduced in both groups (p < 0.05). Although there were no differences in BF loss between the two groups, the FBEG showed a numerically larger body weight and % BF reduction. Resting heart rate and systolic blood pressure decreased significantly after the 12-week program in both two groups (p < 0.05) but with no between-group difference. Table 4 shows the changes in heart rate during the graded exercise testing. In both groups, the heart rate decreased significantly at all stages after the 12-week program (p < 0.05). There was no significant difference between the two groups, but the heart rate decreased much more in the FBEG group. The levels of LDL-cholesterol and HDL cholesterol changed significantly in both groups (p < 0.05); however, there was no difference between the two groups. Changes in HDL cholesterol and weekly energy expenditure were well correlated (Fig. 1, 2).

Table 4.

Heart rate responses during exercise stress test

FBEGCBEG
prepostmean-diffprepostmean-diff
1st stage 89.3±9.1 82.7±11.7 (−7.2)a 80.6±13.5 76.4±10.4 (−4.2) 
2nd stage 100.7±10.4 87.2±11.2 (−13.5)a 85.6±15.0 85.5±10.9 (−0.1) 
3rd stage 105.6±11.0 91.9±12.9 (−13.7)a 95.2±11.7 89.2±11.8 (−6.0) 
4th stage 112.5±9.5 101.1±10 (−11.4)a 102.2±13.4 97.7±12.9 (−4.5) 
5th stage 121.2±11.1 108.3±10.1 (−12.9)a 109.7±13.2 102.1±14.0 (−7.6) 
6th stage 126.4±12.3 114.3±9.3 (−12.1)a 119.4±12.2 113.5±11.1 (−5.8) 
7th stage 131.7±11.2 120.8±11.7 (−10.8)a 127.7±13.6 121.4±13.3 (−6.3) 
8th stage 142.2±12.0 132.7±9.6 (−9.4)a 139.2±14.6 134.7±14.5 (−4.5) 
FBEGCBEG
prepostmean-diffprepostmean-diff
1st stage 89.3±9.1 82.7±11.7 (−7.2)a 80.6±13.5 76.4±10.4 (−4.2) 
2nd stage 100.7±10.4 87.2±11.2 (−13.5)a 85.6±15.0 85.5±10.9 (−0.1) 
3rd stage 105.6±11.0 91.9±12.9 (−13.7)a 95.2±11.7 89.2±11.8 (−6.0) 
4th stage 112.5±9.5 101.1±10 (−11.4)a 102.2±13.4 97.7±12.9 (−4.5) 
5th stage 121.2±11.1 108.3±10.1 (−12.9)a 109.7±13.2 102.1±14.0 (−7.6) 
6th stage 126.4±12.3 114.3±9.3 (−12.1)a 119.4±12.2 113.5±11.1 (−5.8) 
7th stage 131.7±11.2 120.8±11.7 (−10.8)a 127.7±13.6 121.4±13.3 (−6.3) 
8th stage 142.2±12.0 132.7±9.6 (−9.4)a 139.2±14.6 134.7±14.5 (−4.5) 

FBEG, forest-based exercise group; CBEG, center-based exercise group.

aSignificant difference within group (p < 0.05).

Fig. 1.

Change of HDL. Group 1: forest-based exercise group (FBEG). Group 2: center-based exercise group (CBEG). HDLpre: HDL level in both FBEG and CBEG before intervention. HDLpost: HDL level in both FBEG and CBEG after intervention.

Fig. 1.

Change of HDL. Group 1: forest-based exercise group (FBEG). Group 2: center-based exercise group (CBEG). HDLpre: HDL level in both FBEG and CBEG before intervention. HDLpost: HDL level in both FBEG and CBEG after intervention.

Close modal
Fig. 2.

Relationship between weekly energy expenditure and HDL level. HDLpost-pre: the amount of HDL change after intervention.

Fig. 2.

Relationship between weekly energy expenditure and HDL level. HDLpost-pre: the amount of HDL change after intervention.

Close modal

Functional Capacity and Daily Energy Expenditure

Functional capacity refers to cardiorespiratory endurance and was the only factor that showed significance in both within- and between-group differences, with superiority in the FBEG. As previously mentioned, we monitored energy expenditure data from all participants’ wearable watch via smartphone communication. Average weekly energy expenditure during the 12-week period was significantly higher in the FBEG than in the CBEG. The FBEG was superior in the change in MET levels (Fig. 3), which is supposed to be associated with higher energy expenditure in this group. There was a positive correlation between weekly energy expenditure and changes in MET in all participants in this study (Fig. 4).

Fig. 3.

Changes in MET. Group 1: forest-based exercise group (FBEG). Group 2: center-based exercise group (CBEG). METpre: MET level in both FBEG and CBEG before intervention. METpost: MET level in both FBEG and CBEG after intervention.

Fig. 3.

Changes in MET. Group 1: forest-based exercise group (FBEG). Group 2: center-based exercise group (CBEG). METpre: MET level in both FBEG and CBEG before intervention. METpost: MET level in both FBEG and CBEG after intervention.

Close modal
Fig. 4.

Relationship between weekly energy expenditure and MET level. METpost-pre: the amount of HDL change after intervention. Week-Kcal: average weekly energy expenditure.

Fig. 4.

Relationship between weekly energy expenditure and MET level. METpost-pre: the amount of HDL change after intervention. Week-Kcal: average weekly energy expenditure.

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Attendance Rate

The average attendance rate of the recommended exercise frequency during the 12-week program was also compared between the two groups. A significant difference between the two groups in terms of the FBEG.

Framingham Risk Score

To predict the probability of CAD in 10 years, we adopted the algorithm suggested by Wilson et al. [14]. The absolute and relative risks of the participants significantly improved after the 12-week program in both groups, with no significant differences between the groups. The absolute risk of CAD within 10 years decreased significantly in both the groups (Fig. 5).

Fig. 5.

Changes in the FRS absolute risk (%). Group 1: forest-based exercise group (FBEG). Group 2: center-based exercise group (CBEG). FRS-CADpre: Framingham Risk Score of 10-year CAD probability before intervention. FRS-CADpost: Framingham Risk Score of 10-year CAD probability after intervention.

Fig. 5.

Changes in the FRS absolute risk (%). Group 1: forest-based exercise group (FBEG). Group 2: center-based exercise group (CBEG). FRS-CADpre: Framingham Risk Score of 10-year CAD probability before intervention. FRS-CADpost: Framingham Risk Score of 10-year CAD probability after intervention.

Close modal

This study aimed to compare the efficiency of two primary prevention programs in preventing CAD: one focused on intensive lifestyle education with ensuing unstructured exercise in an urban forest (FBEG), and the other was conducted with a conventional structured and supervised exercise program in a fitness facility (CBEG). We further tested whether the intensive cardiac rehabilitation-type program could also be applied to primary prevention. Accordingly, FBEG mimics intensive cardiac rehabilitation, whereas CBEG follows conventional hospital-based rehabilitation. We added unstructured exercises to the FBEG program in an urban forest to investigate its usability as a primary prevention method.

The decreased heart rates in both groups were attributed to physical activity and exercise. Lowering of the heart rate following endurance training has been well documented, and the mechanism has been demonstrated to involve an increase in parasympathetic tone [15‒17]. It is also well known that endurance training leads to heart rate reduction, even during submaximal exercise, owing to higher parasympathetic activity [18]. In this study, heart rate at every stage of the Exercise Stress Test decreased after the program (Table 4). Comparing both resting and submaximal heart rates between the two groups, the FBEG showed more changes between the pre- and post-tests. This implies that the unstructured forest tracking FBEG was likely to be more effective as an endurance-type activity, resulting in a decrease in both resting and submaximal heart rates.

One of the outstanding results of endurance exercise is the improvement in endothelial function. Shear stress resulting from important components of exercise affects vascular nitric oxide concentration, which is responsible for vascular relaxation [19]. Specifically, exercise stimulates the affinity of l-arginine to the endothelium, and augments the expression of both nitric oxide synthase and extracellular superoxide dismutase, which collectively contribute to the improvement of vascular endothelial function [20‒22]. It is also likely that the significantly decreased systolic blood pressure observed in this study, as well as the decreased heart rate mentioned above, were partially due to this mechanism.

In this study, the lipid profiles showed a normal tendency to change, similar to the results of previous studies. It was assumed that the desirable changes in lipids in this study might have occurred as a result of not only physical activity, but also a sound diet. Nutritional education and counseling during intensive lifestyle programs and psychological reinforcement helped the participants to maintain a healthy diet. Notably, HDL-C, a well-known protective agent against atheroma, was significantly increased, especially in the FBEG.

The results of prior studies have strongly supported the positive correlation between training volume and HDL-C increases [23‒25]. In this study, the average activity energy expenditure per week was significantly higher in the FBEG than in the CBEG. Furthermore, the relationship between HDL-C change and weekly energy expenditure in all participants in this study supports the findings of the aforementioned studies.

It is well known that diabetes or a prediabetic state can be controlled via lifestyle modification. Along with diet, regular exercise or physical activity is an important intervention to control blood sugar levels as muscle contraction activates GLUT4 to regulate glucose intake [26]. Additionally, insulin sensitivity increases with exercise training [27]. In this study, we did not obtain a contestable result regarding the change in fasting blood glucose and glycosylated hemoglobin, although there was a slight decrease in the FBEG. It is believed that the daily lives of all individuals should have been more focused on glucose control and vigorously supervised for it.

Functional capacity (FC) refers to cardiorespiratory fitness (CRF), which is defined as the ability to perform exercise or physical activity for a prolonged period, and is represented as either the VO2 peak or MET peak. High CRF levels are associated with a reduction in death from all causes, including CAD [28, 29]. Because CRF is a clinically vital sign for predicting mortality, more efforts should be focused on increasing the level of CRF in both primary and secondary prevention [29]. In this study, MET was the only variable that showed significant within- and between-group differences. The FBEG was superior in the change in MET levels, which is believed to be associated with higher energy expenditure in this group.

The dose-response relationship can be applied to improve CRF. The habitual physical activity and exercise are supported to increase higher levels of CRF [28, 29]. More than 1,000 kcal/week is recommended as a reasonable target exercise volume for most adults [30]. The amount of exercise or physical activity plays an important role in patients with cardiac diseases. One study revealed that an energy expenditure of >2,000 kcal/week resulted in regression of atherosclerotic legions. In this regard, the energy expenditure of 2,140 kcal/week in FBEG was thought to be sufficient to improve CAD risk factors, whereas the energy expenditure of 1,166 kcal/week in the CBEG was at the standard level for most adults. This is highly probable because most of the risk factors improved in this study, showing more desirable results with FBEG. It is worth noting that the relative risk compared to average age-gender matched persons in FBEG decreased from 1.3 times to 0.8 times, which meant the participants became less risky than their age-gender matched population. It is thought that FBEG accomplished the goal of primary prevention well, while their counterparts returned to the same average level as their age- and sex-matched counterparts.

There are several limitations that deserve comment. First, the study inherently has limitations in its research design. Because after an initial study design including total 40 patients, there were insufficient participants which made insufficient statistical power to conclude. This number showed the real situation of underused CBCR. We expected that this FBCR model could be an alternative model to overcome. Second, our study was performed in a specific region in Korea and all were relatively healthy Korean patients. So there might be cautions to interpret our study result. We need a bigger randomized trial using bigger provinces and a higher number of patients from various ethnicities.

In summary, the results of the present study indicate that intensive lifestyle education with subsequent exercise in an urban forest is more effective than conventional facility-based exercise programs. It is thought that intensive habituation in the FBEG program led participants to maintain a healthy daily life. The significantly higher attendance rate in the FBEG indicates that the participants in this group were more motivated to maintain healthy behaviors than their counterparts. Another important point is that the presence of urban forests around participants’ residences also played a valuable and alternative exercise environment, and their availability for primary prevention was indicated. In fact, a higher motivation level, higher attendance rate, higher energy expenditure using forest-based physical activity, and increased functional capacity seemed to interact closely to improve CAD risk factors. In future studies, we intend to apply the FBEG-type program to real-world intensive cardiac rehabilitation to investigate its efficiency and availability.

All participants were enrolled voluntarily, and written informed consent was obtained from each participant at every visit. The study protocol followed the principles outlined in the Declaration of Helsinki and was approved by the Korean National Research Institute of Health and Institutional Review Board of Kangbuk Samsung Hospital (IRB No. KBSMC 2022-06-054-001).

The authors declare no conflicts of interest.

This study was funded by a grant from the Korea Forestry Promotion Institute.

Jong-Young Lee: data curation, methodology, validation, and writing the original draft. Kee-Chan Joo and Peter H Brubaker: formal analysis, investigation, and writing the original draft. Dae-Sik Yoon and Kyung-Su Choi: conceptualization, writing the review, and editing.

The data that support the findings of this study are not publicly available due to the concern of compromising the privacy of research participants but are available from Jong-Young Lee (Kangbuk Samsung Hospital, jyleeheart@naver.com) upon reasonable request.

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