The overall number of studies that were identified as potentially relevant from the database searches was 1,724, of which 857 were duplicates and 867 progressed to title and abstract screening. A total of 82 articles were retrieved for full-text screening. Nine papers8–16 were selected to be part of this review (Fig. 1).
Table S1 (Additional file 2) presents the main characteristics and findings of the nine selected papers. The first study was published in 2015 and the remaining ones were published between 2017 and 2019. One of the studies was a randomised control trial and the others were prospective cohorts. The articles included were all in English and were conducted in Canada, Scotland, England, Ireland, Finland, and Australia.
The participants in most of the articles were middle-aged. Four included participants who were aged 50 and above,11–14 one involved participants aged 45–50,16 and one had participants aged 60–77.10 Three availed of data from both young and middle-aged adults, with age ranges of 18–75, 20–69 and 25–64.8,9,15 One study only reported results for women16 and another provided separate results for men and women with different baseline chronic conditions,15 whereas the rest of the studies included combined results for both men and women.
All eight cohort studies8,9,11−16 availed of self-reported methods to collect data on the level of physical activity of participants. Follow-up length ranged from 10 to 32 years, except for one study with a 2-year follow-up.12
One study was a randomised control trial.10 The intervention group received an intensive multidomain intervention for two years. For the physical activity aspect, participants in the intervention group received training that was guided by physical therapists using individually tailored programs. They used a modified version of the Dose Response to Exercise Training (DR EXTRA) study protocol. Participants received muscle strength training 1–3 times per week, aerobic exercise 2–5 times per week and posture improving exercises 1–3 times per week, as well as receiving nutritional guidance and cognitive training. However, the control group received regular general health advice. The groups were allocated randomly using computer-generated allocation.
Some of the studies provided evidence on subgroups of those with multimorbidity depending on baseline characteristics, such as the number of conditions at baseline,10–12 or the type of condition participants had been diagnosed prior to study.15 The number and type of chronic conditions included varied between studies. Some of the studies chose to limit the number of conditions or groups of conditions that were included, whereas one study had up to 40 conditions. Illnesses were measured by self-reporting the conditions through questionnaires or through data retrieved from medical records.
3.1 Risk of bias assessment
The eight prospective cohort studies were assessed using the Newcastle-Ottawa Quality Assessment Scale for Cohort Studies6 (Additional File 3, Table S2). Three studies8,9,13 scored six, three11,12,16 scored seven, and two14,15 scored eight out of nine points. The majority of the studies failed to have a follow-up rate of at least 80%, with just three of the eight studies achieving that.
The randomised control trial10 was assessed using the US National Heart, Lung, and Blood Institute’s Scale for Controlled Intervention Studies,7 and scored 12 out of 14 points, with little risk of bias (Additional File 3, Table S3).
3.2 Relationship between physical activity and multimorbidity prevention and management
All nine studies reported results of incident multimorbidity for healthy participants at baseline (11–19).8–16 No evidence of relationship between physical activity and multimorbidity was observed in five of these studies;8–10, 12,16 however, other four found an increase in risk of multimorbidity among those less physically active.11,13−15 In the study by Singer et al.,13 the risk of incident multimorbidity from doing moderate physical activity compared with vigorous physical activity was 1.22 (95% confidence interval (CI): 1.16–1.28), increasing to 1.57 (95% CI: 1.46–1.68) and 1.60 (95% CI: 1.45–1.76) for those doing mild or no physical activity, respectively. In the study by Singh-Manoux et al.,14 participants who engaged in less than 2.5 hours/week of moderate or vigorous physical activity were more likely to have multimorbidity at follow-up than those above the threshold (1.21, 95% CI: 1.01–1.45). In the study by Mounce et al.,11 those in the low/sedentary category were more likely to have multimorbidity at follow-up than those who were highly active (1.43, 95% CI: 1.02–2.00). Wikström et al.15 found that me initially disease-free and who took part in low levels of physical activity at baseline were more likely to have multimorbidity at follow-up than those who were highly active (1.34, 95% CI: 1.03–1.73), as were women (1.62, 95% CI: 1.14–2.30).
Additionally, four studies investigated the risk of developing or worsening multimorbidity within subgroups with chronic conditions at the baseline.10–12, 15 In the randomised control trial conducted by Marengoni et al.,10 participants in the intervention group who had one or more chronic diseases at baseline had a lower risk of developing or worsening multimorbidity at follow-up compared to the control group (0.80, 95% CI: 0.64–0.99). In the study by Mounce et al.,11 those with any number of chronic conditions at baseline who were in the low/sedentary category were more likely to developing or worsening multimorbidity at follow-up than those in who were highly active (1.19, 95% CI: 1.06–1.35). Wikström et al.15 found that men with diabetes at baseline and who took part in low levels of physical activity had greater risk of multimorbidity in comparison to those that reported high levels of physical activity (1.80, 95% CI: 1.29–2.53). However, they did not find evidence of association among women with diabetes and men and women with cardiovascular diseases at baseline. One other study did not find evidence of developing or worsening multimorbidity according to physical activity behaviour or people with chronic conditions at baseline.12