• We are sorry, but NCBI web applications do not support your browser and may not function properly. More information
Logo of cmajCMAJ Information for AuthorsCMAJ Home Page
CMAJ. Nov 28, 2000; 163(11): 1435–1440.
PMCID: PMC80410

The economic burden of physical inactivity in Canada

Abstract

Background

About two-thirds of Canadians are physically inactive. As a risk factor for several chronic diseases, physical inactivity can potentially be a substantial public health burden. We estimated the direct health care costs attributable to physical inactivity in Canada, the number of lives lost prematurely each year that are attributable to a sedentary lifestyle and the effect that a reduction of 10% in inactivity levels (a Canadian objective for 2003) could have on reducing direct health care costs.

Methods

We calculated summary relative risk (RR) estimates from prospective longitudinal studies of the effects of physical inactivity on coronary artery disease, stroke, colon cancer, breast cancer, type 2 diabetes mellitus and osteoporosis. We then computed the population-attributable fraction (PAF) for each illness from the summary RR and the prevalence of physical inactivity (i.e., 62%) and applied the PAF to the total direct health care expenditures for 1999 and to the number of deaths in 1995 associated with each disease to determine the health care costs and lives lost prematurely that were directly attributable to physical inactivity.

Results

About $2.1 billion, or 2.5% of the total direct health care costs in Canada, were attributable to physical inactivity in 1999. A sensitivity analysis (simultaneously varying each of the health care costs and PAF by ±20%) indicated that the costs could be as low as $1.4 billion and as high as $3.1 billion. About 21 000 lives were lost prematurely in 1995 because of inactivity. A 10% reduction in the prevalence of physical inactivity has the potential to reduce direct health care expenditures by $150 million a year.

Interpretation

Physical inactivity represents an important public health burden in Canada. Even modest reductions in inactivity levels could result in substantial cost savings.

Given the convincing scientific evidence that physical inactivity leads to a host of chronic degenerative conditions and premature death, the promotion of a physically active lifestyle is an important public health objective. The health benefits of physical activity have been widely publicized in the 1998 Canada's Physical Activity Guide to Healthy Active Living1 and the 1996 US Surgeon General's report on physical activity and health.2 However, the results of a survey carried out in 19973 suggest that 62% of Canadians are still not active enough to reap the health benefits of a physically active lifestyle. Similarly, only 34% of Canadians aged 25 to 55 years are meeting the recommendation in Canada's Physical Activity Guide to Healthy Active Living,1 which calls for an hour of low-intensity activity every day or 30–60 minutes of moderate-intensity activity or 20–30 minutes of vigorous-intensity activity 4 to 7 days a week.4 A recent public health objective of federal, provincial and territorial governments is a 10% reduction in the level of physical inactivity in Canada by 2003.5

The main risk factors for coronary artery disease (CAD), a primary cause of death in Canada, are cigarette smoking, high blood pressure, high blood cholesterol levels and physical inactivity, all of which have similar risk ratios.6 Recent prevalence estimates indicate that 28% of Canadians currently smoke,7 20% have high blood pressure,8 26% have high blood cholesterol levels9 and 62% are inactive.3 Thus, in the context of population health, an increase in physical activity has the greatest potential to effect a reduction in CAD. A recent study showed a 4.7% reduction in short-term (18 months) health care costs for each active day per week reported by participants.10 These results support those of earlier studies indicating significant health care savings associated with corporate fitness and health promotion programs.11,12 However, the results of analyses of short-term health care costs are difficult to interpret because it may take several years to accrue cardiovascular benefits from an active lifestyle.

Given the high prevalence of physical inactivity in Canada and its relation to degenerative conditions and premature death, the burden that sedentary living is placing on the economy through the health care system and its effect on longevity are presumed to be great. However, there have been only limited attempts to quantify the costs of physical inactivity in this country.13 Our primary objective in this study was to estimate the economic burden of physical inactivity in Canada. Our secondary objectives were to estimate the number of lives lost prematurely each year because of a sedentary lifestyle and the effect that a reduction of 10% in physical inactivity could have on reducing direct health care costs.

Methods

We determined the main diseases that are known to be significantly related to physical inactivity from comprehensive reviews on the topic,2,14 together with more recent peer-reviewed articles obtained through searches on MEDLINE. First, we quantified the association between physical inactivity and chronic diseases known to be associated with physical inactivity. We used estimates from previously published meta-analyses15,16 for CAD, colon cancer and breast cancer, and we obtained the relative risks (RRs) attributable to physical inactivity from large prospective epidemiologic studies for stroke,17,18,19,20,21,22,23,24,25 hypertension,26,27,28,29 type 2 diabetes mellitus27,30,31,32 and osteoporotic fractures33,34 (Table 1). We pooled the RR estimates from each chronic disease group using a general variance-based method of meta-analysis based on the estimate of RR and the 95% confidence intervals (CI) reported in each study35 (details available from the authors). For 2 studies24,36 we estimated the 95% CI from the 90% CI reported by the authors.

Table thumbnail
Table 1

To estimate the proportion of chronic disease and of premature death in Canada that could theoretically be prevented by eliminating physical inactivity we calculated population-attributable fractions (PAF). The PAF is an estimate of the effects of an individual risk factor on a given disease. The PAF for each disease was calculated as [P(RR – 1)] / [1 + P(RR – 1)], where P is the prevalence of physical inactivity in the population and RR is the relative risk for the disease in an inactive person. The prevalence of physical inactivity in Canada was estimated using data from the Physical Activity Monitor Survey3 by the Canadian Fitness and Lifestyle Research Institute. The survey was based on telephone contact with a representative but weighted sample of 1875 Canadians, selected to reflect roughly the proportion of the Canadian population in each of the provinces. The results indicated that 62% of Canadians aged 18 years or more were inactive, defined as reporting less than 12.6 kJ/kg of body weight per day of physical activity. Thus, for the purpose of our study, we accepted 62% as the prevalence of physical inactivity.

Economic costs of physical inactivity

We estimated the direct health care costs of treating diseases related to physical inactivity and the fraction of the total costs that were attributable to physical inactivity from recent sources. We obtained the estimated health care costs for 1999 from analyses of the Canadian Health Expenditures Database37 maintained by the Canadian Institute for Health Information. The database provides information using broad categories of spending and sources of funding; however, information on the cost of treating specific diseases is not available. We obtained this information from the Economic Burden of Illness in Canada, 1993 (EBIC).38 We extracted the costs of treating specific diseases associated with physical inactivity from the EBIC and inflated them to 1999 values using the estimates of global expenditures from the 1999 Canadian Health Expenditures Database.37 The EBIC provided costs specific to CAD and stroke. We approximated the costs of treating the other chronic diseases from general categories as follows. The EBIC provided a cost for treating diabetes; we estimated the cost of treating type 2 diabetes using the proportion of total cases of diabetes that are type 2 (92.5%).39 The EBIC provided the cost of treating cancer; we estimated the cost of treating colon cancer using the incidence of colon cancer relative to all cancers in Canada (8.6%).40 The EBIC provided the cost of treating cancer specific to women; we estimated the cost of treating breast cancer using the incidence of breast cancer relative to all female-specific cancers (71.4%).40 The EBIC reported the total cost of treating cardiovascular diseases; we estimated the cost of treating hypertension by multiplying the costs associated with treating hypertension relative to total cardiovascular disease costs in the United States (5.7% of hospital costs, 50.6% of drug costs and 28.7% of physician costs).41 The costs of treating osteoporosis associated with physical inactivity could not be directly determined with this method. It has been reported that $1.3 billion is spent annually on the treatment of this disease in Canada,42 and we adopted this value for the present analysis.

To determine the influence of variations in PAF and health care costs, we performed a 2-way sensitivity analysis similar to that of Birmingham and colleagues43 in their study of the cost of obesity. We simultaneously varied each PAF and disease-specific health care cost by ±20% of the mean estimate.

Premature death due to physical inactivity

We obtained the number of deaths and cause of death among adults (20 years of age and over) in Canada in 1995 from Statistics Canada data44 and multiplied the number of deaths from the main inactivity-related diseases (CAD, stroke, colon cancer, breast cancer and type 2 diabetes) by the PAF to estimate the number of deaths attributable to physical inactivity. The number of deaths due to colon cancer was not presented directly; only deaths from colorectal cancer were given. We, therefore, estimated the number of deaths due to colon cancer using the incidence of colon cancer relative to total colorectal cancers (67.1%).40

Savings from reduction in physical inactivity

We estimated the economic savings associated with a 10% reduction in physical inactivity levels by recalculating the PAFs, assuming the prevalence of inactivity to be 56% (i.e., 62% – 6.2% = 55.8%) rather than 62%. We then calculated the savings by taking the difference between the costs derived with the 2 inactivity prevalence rates.

Results

The summary RR estimates (and 95% CI) for physical inactivity for the various chronic diseases are given in Table 2. The RR estimates range from 1.2 (95% CI 1.0–1.5) for breast cancer to 1.9 (95% CI 1.6–2.2) for CAD. The PAF values suggest that 11.0%–35.8% of the cases of the various diseases might be eliminated if those who were sedentary became physically active.

Table thumbnail
Table 2

The estimated health care costs attributable to physical inactivity in 1999 are presented in Table 3. In total, about $2.1 billion was estimated to have been spent on health care that was directly attributable to physical inactivity. This amount represents 2.5% of the total health care costs in that year (calculated at $86.0 billion37). The sensitivity analysis indicated that the health care costs for the major chronic diseases attributable to inactivity may have been as low as $1.4 billion and as high as $3.1 billion. The total cost attributable to physical inactivity represents 25.5% of the cost of treating CAD, stroke, hypertension, colon cancer, breast cancer, type 2 diabetes and osteoporosis in that year. The highest costs attributable to physical inactivity were associated with CAD ($891 million), osteoporosis ($352 million), stroke ($345 million) and hypertension ($314 million).

Table thumbnail
Table 3

There were 207 408 deaths from all causes among Canadian adults in 1995,44 of which 35.8% were due to the main diseases known to be associated with physical inactivity, namely, CAD, stroke, colon cancer, breast cancer and type 2 diabetes. Table 4 shows the estimated number of deaths attributable to physical inactivity for each of these diseases and for all causes. If physical inactivity were completely eliminated in Canada, we could theoretically increase life expectancy and save 21 340 lives that are lost prematurely each year — 10.3% of the total deaths among adults.

Table thumbnail
Table 4

Recalculating the direct health care costs attributable to physical inactivity with a reduction of 10% in the prevalence of inactivity (56% v. 62%) yielded a cost of $1.97 billion. Thus, a 10% reduction would result in savings of about $150 million per year in direct health care expenditures.

Interpretation

Our results indicate that $2.1 billion, or about 2.5% of the total direct health care costs in Canada in 1999, are attributable to physical inactivity. A similar figure was recently reported for the United States ($24 billion or 2.4% of the US health care expenditures).45 We found that 33% of deaths from CAD, colon cancer and type 2 diabetes could hypothetically be prevented by eliminating physical inactivity. Similarly, it has been estimated that about one-third of the deaths from CAD, colon cancer and diabetes in the US are attributable to inactivity.46

The cost of obesity in Canada was estimated to be $1.8 billion in 1997.43 When this value is inflated to 1999 dollars using the increase of 10.3% that occurred in total health care expenditures between 1997 and 1999 (from $78.0 billion to $86.0 billion), the cost of obesity would be $2.0 billion. Thus, our cost estimates for physical inactivity are similar to those for obesity. Given the significant association between physical inactivity and obesity, a portion of the health care costs attributable to obesity is also attributable to physical inactivity.47 However, it is unlikely that the costs attributable to inactivity and obesity are simply additive; the relative contributions of physical inactivity and excessive caloric intake to obesity have not been determined. More research is needed to determine the total costs attributable to physical inactivity, taking into account the overlapping costs of inactivity-related obesity.

Estimates from our study suggest a saving of $150 million per year with a reduction of 10% in the prevalence of physical inactivity.5 Thus, even a modest reduction would have a significant effect on the health of Canadians. However, one would not expect sweeping health care savings immediately following a reduction in the level of inactivity because the benefits of a physically active lifestyle accrue over a lifetime.

Our methods have several limitations. The EBIC data were for 1993, and we assumed that the relative proportions of expenditures for each illness did not change dramatically between 1993 and 1999. Furthermore, the EBIC reported expenditures for major categories of disease, such that expenditures for colon cancer, breast cancer, type 2 diabetes and hypertension had to be estimated from prevalence and incidence data or from expenditure data from the United States. However, these limitations likely had only a marginal effect on our estimates, and the use of these methods allows a comparison of results to those for obesity derived using similar methods.43

Our estimates of the economic costs of physical inactivity are likely conservative. We calculated only the direct health care costs of inactivity and made no attempt to estimate indirect costs, which include lost productivity due to premature death and disability due to illness. In addition to the diseases included in our analysis, physical inactivity has been associated with dyslipidemia, anxiety, depression, poorer quality of life and premature admission to an institution or geriatric care.2 However, the effect sizes for these conditions and situations are generally small, and there is little consensus on these issues. Like most behaviours, physical activity is difficult to measure accurately. Thus, in using estimates of physical activity rather than objective measures of physical fitness, the studies included in the meta-analysis likely underestimated the health effects associated with an active lifestyle.

Conversely, we have also not accounted for the potential costs of physical activity promotion. The implementation of nation-wide intervention programs and campaigns to promote physical activity is an expensive prospect; the costs are undoubtedly lower than the health care expenses associated with treating inactivity-related illnesses, however.

Our estimates are based on RRs from prospective longitudinal studies, which may not always translate into actual benefits when tested in randomized controlled trials. A fundamental assumption is that changes in physical activity (though promotion) will result in changes in disease risk. Although there are randomized controlled trials on the effects of physical activity on risk factors for disease (such as blood lipid levels), there is little information on whether physical activity interventions can change one's risk for disease per se. However, 2 longitudinal prospective studies48,49 have shown that increases in physical fitness or physical activity levels can reduce the risk of death from all causes.

Given the limitations in the data and the lack of randomized controlled trials to evaluate the long-term effectiveness of exercise interventions, more research on the effects of changes in physical activity levels on health care costs is needed. In a public health context, the finding that physical inactivity accounts for about 2.5% of the current direct health care costs is very important. The costs attributable to cigarette smoking in Canada were estimated to be 3.8% of total health care costs in 1992.50 Given the considerable efforts that have been aimed at curbing the prevalence of smoking in Canada, public health campaigns directed at increasing physical activity in the population should be no less aggressive and persistent.

Footnotes

This article has been peer reviewed.

Acknowledgements: This project was supported in part by the Canadian Society for Exercise Physiology and Health Canada.

Competing interests: None declared.

Reprint requests to: Dr. Peter T. Katzmarzyk, School of Kinesiology and Health Science, York University, 4700 Keele St., North York ON M3J 1P3; fax 416 736-5774; ac.ukroy@zramztak

References

1. Canada's physical activity guide to healthy active living. Ottawa: Health Canada; 1998.
2. Physical activity and health: a report of the Surgeon General. Atlanta (GA): National Center for Chronic Disease Prevention and Health Promotion, Centers for Disease Control and Prevention, Department of Health and Human Services; 1996.
3. Craig CL, Russell SJ, Cameron C, Beaulieu A. Foundation for joint action: reducing physical inactivity. Ottawa: Canadian Fitness and Lifestyle Research Institute; 1999.
4. Canadian Fitness and Lifestyle Research Institute. Meeting guidelines. Progress in Prevention bulletin 31. Ottawa: The Institute; 1998.
5. Federal, Provincial and Territorial Fitness and Recreation Committee. Physical inactivity: a framework for action. Ottawa: Health Canada; 1996.
6. Grundy SM, Balady GJ, Criqui MH, Fletcher G, Greenland P, Hiratzka LF, et al. Guide to primary prevention of cardiovascular diseases. A statement for healthcare professionals from the Task Force on Risk Reduction. American Heart Association Science Advisory and Coordinating Committee. Circulation 1997;95:2329-31. [PubMed]
7. Statistical report on the health of Canadians. Ottawa: Statistics Canada; 1999. Cat no 82-570-X1E.
8. Joffres MR, Hamet P, Rabkin SW, Gelskey D, Hogan K, Fodor G, et al. Prevalence, control and awareness of high blood pressure among Canadian adults. CMAJ 1992;146:1997-2005. [PMC free article] [PubMed]
9. MacDonald S, Joffres MR, Stachenko S, Horlick L, Fodor G, for the Canadian Heart Health Surveys Research Group. Multiple cardiovascular disease risk factors in Canadian adults. CMAJ 1992;146:2021-9. [PMC free article] [PubMed]
10. Pronk NP, Goodman MJ, O'Connor PJ, Martinson BC. Relationship between modifiable health risks and short-term health care charges. JAMA 1999;282:2235-9. [PubMed]
11. Shephard RJ, Corey P, Renzland P, Cox M. The influence of an employee fitness and lifestyle modification program upon medical care costs. Can J Public Health 1982;73:259-63. [PubMed]
12. Bly JL, Jones RC, Richardson JE. Impact of worksite health promotion on health care costs and utilization. Evaluation of Johnson & Johnson's Live for Life program. JAMA 1986;256:3235-40. [PubMed]
13. Shephard RJ. Economic benefits of enhanced fitness. Champaign (IL): Human Kinetics; 1986.
14. Bouchard C, Shephard RJ, Stephens T. Physical activity, fitness and health. Champaign (IL): Human Kinetics; 1994.
15. Berlin JA, Colditz GA. A meta-analysis of physical activity in the prevention of coronary heart disease. Am J Epidemiol 1990;132:612-28. [PubMed]
16. Shephard RJ, Futcher R. Physical activity and cancer: How may protection be maximized? [review]. Crit Rev Oncog 1997;8:219-72. [PubMed]
17. Abbott RD, Rodriguez BL, Burchfiel CM, Curb JD. Physical activity in older middle-aged men and reduced risk of stroke: the Honolulu Heart Program. Am J Epidemiol 1994;139:881-93. [PubMed]
18. Agnarsson U, Thorgeirsson G, Sigvaldason H, Sigfusson N. Effects of leisure-time physical activity and ventilatory function on risk of stroke for men: the Reykjavik Study. Ann Intern Med 1999;130:987-90. [PubMed]
19. Evenson KR, Rosamond WD, Cai J, Toole JF, Hutchinson RG, Shahar E, et al. Physical activity and ischemic stroke risk. The Atherosclerosis Risk in Communities Study. Stroke 1999;30:1333-9. [PubMed]
20. Gillum RF, Mussolino ME, Ingram DD. Physical activity and stroke incidence in women and men. The NHANES I Epidemiologic Follow-up Study. Am J Epidemiol 1996;143:860-9. [PubMed]
21. Kiely DK, Wolf PA, Cupples LA, Beiser AS, Kannel WB. Physical activity and stroke risk: the Framingham Study. Am J Epidemiol 1994;140:608-20. [PubMed]
22. Lee IM, Hennekens CH, Berger K, Buring JE, Manson JE. Exercise and risk of stroke in male physicians. Stroke 1999;30:1-6. [PubMed]
23. Lee IM, Paffenbarger RS Jr. Physical activity and stroke incidence: the Harvard Alumni Health Study. Stroke 1998;29:2049-54. [PubMed]
24. Salonen JT, Puska P, Tuomilehto J. Physical activity and risk of myocardial infarction, cerebral stroke and death: a longitudinal study in eastern Finland. Am J Epidemiol 1982;115:526-37. [PubMed]
25. Wannamethee G, Shaper AG. Physical activity and stroke in British middle aged men. BMJ 1992;304:597-601. [PMC free article] [PubMed]
26. Folsom AR, Prineas RJ, Kaye SA, Munger RG. Incidence of hypertension and stroke in relation to body fat distribution and other risk factors in older women. Stroke 1990;21:701-6. [PubMed]
27. Haapanen N, Miilunpalo S, Vuori I, Oja P, Pasanen M. Association of leisure time physical activity with the risk of coronary heart disease, hypertension and diabetes in middle-aged men and women. Int J Epidemiol 1997;26:739-47. [PubMed]
28. Paffenbarger RS Jr, Wing AL, Hyde RT, Jung DL. Physical activity and incidence of hypertension in college alumni. Am J Epidemiol 1983;117:245-57. [PubMed]
29. Pereira MA, Folsom AR, McGovern PG, Carpenter M, Amett DK, Liao D, et al. Physical activity and incident hypertension in black and white adults: the Atherosclerosis Risk in Communities Study. Prev Med 1999;28:304-12. [PubMed]
30. Hu FB, Sigal RJ, Rich-Edwards JW, Colditz GA, Solomon CG, Willett WC, et al. Walking compared with vigorous physical activity and risk of type 2 diabetes in women: a prospective study. JAMA 1999;282:1433-9. [PubMed]
31. Manson JE, Rimm EB, Stampfer MJ, Colditz GA, Willett WC, Krolewski AS, et al. Physical activity and incidence of non-insulin-dependent diabetes mellitus in women. Lancet 1991;338:774-8. [PubMed]
32. Manson JE, Nathan DM, Krolewski AS, Stampfer MJ, Willett WC, Hennekens CH. A prospective study of exercise and incidence of diabetes among US male physicians. JAMA 1992;268:63-7. [PubMed]
33. Farmer ME, Harris T, Madans JH, Wallace RB, Cornoni-Huntley J, White LR. Anthropometric indicators and hip fracture. The NHANES I Epidemiologic Follow-up Study. J Am Geriatr Soc 1989;37:9-16. [PubMed]
34. Gregg EW, Cauley JA, Seeley DG, Ensrud KE, Bauer DC. Physical activity and osteoporotic fracture risk in older women. Study of Osteoporotic Fractures Research Group. Ann Intern Med 1998;129:81-8. [PubMed]
35. Petitti DB. Meta-analysis, decision analysis, and cost-effective analysis. New York: Oxford University Press; 1994.
36. Lee IM, Paffenbarger RS Jr, Hsieh C. Physical activity and risk of developing colorectal cancer among college alumni. J Natl Cancer Inst 1991;83:1324-9. [PubMed]
37. Canadian Institute for Health Information. National health expenditure trends, 1975–1999. Ottawa: The Institute; 1999.
38. Moore R, Mao Y, Zhang J, Clarke K. Economic burden of illness in Canada, 1993. Ottawa: Health Canada; 1997. Available: www.hc-sc.gc.ca/hpb/lcdc/publicat/burden/ (accessed 20 Aug 2000).
39. US Centers for Disease Control and Prevention. National diabetes fact sheet: national estimates and general information on diabetes in the United States. Atlanta (GA): Centers for Disease Control and Prevention, Department of Health and Human Services; 1997.
40. National Cancer Institute of Canada. Canadian cancer statistics 1999. Toronto: The Institute; 1999.
41. American Heart Association. 1999 heart and stroke statistical update. Dallas: The Association; 1998.
42. Goeree R, O'Brien B, Pettitt D, Cuddy L, Ferraz M, Adachi J. An assessment of the burden of illness due to osteoporosis in Canada. J Soc Obstet Gynaecol Can 1996;July(Suppl):15-24.
43. Birmingham CL, Muller JL, Palepu A, Spinelli JJ, Anis AH. The cost of obesity in Canada. CMAJ 1999;160:483-8. Available: www.cma.ca/cmaj/vol-160/issue-4/0483.htm [PMC free article] [PubMed]
44. Statistics Canada. Mortality, summary list of causes, 1995. Ottawa: Minister of Industry; 1997. Cat no 84-209-XPB.
45. Colditz GA. Economic costs of obesity and inactivity. Med Sci Sports Exerc 1999;31:S663-7. [PubMed]
46. Powell KE, Blair SN. The public health burdens of sedentary living habits: theoretical but realistic estimates. Med Sci Sports Exerc 1994;26:851-6. [PubMed]
47. Grundy SM, Blackburn G, Higgins M, Lauer R, Perri MG, Ryan D. Physical activity in the prevention and treatment of obesity and its comorbidities. Med Sci Sports Exerc 1999;31:S502-8. [PubMed]
48. Blair SN, Kohl HW, Barlow CE, Paffenbarger RS Jr, Gibbons LW, Macera CA. Changes in physical fitness and all-cause mortality: a prospective study of healthy and unhealthy men. JAMA 1995;273:1093-8. [PubMed]
49. Paffenbarger RS Jr, Hyde RT, Wing AL, Lee IM, Jung DL, Kampert JB. The association of changes in physical-activity level and other lifestyle characteristics with mortality among men. N Engl J Med 1993;328:538-45. [PubMed]
50. Single E, Robson L, Xie X, Rehm J. The economic costs of alcohol, tobacco and illicit drugs in Canada, 1992. Addiction 1998;93:991-1006. [PubMed]

Articles from CMAJ : Canadian Medical Association Journal are provided here courtesy of Canadian Medical Association
PubReader format: click here to try

Formats:

Related citations in PubMed

See reviews...See all...

Cited by other articles in PMC

See all...

Links

  • Cited in Books
    Cited in Books
    PubMed Central articles cited in books
  • PubMed
    PubMed
    PubMed citations for these articles

Recent Activity

Your browsing activity is empty.

Activity recording is turned off.

Turn recording back on

See more...