Overall , the current literature supports clearly the dose-response relationship between physical activity and the seven chronic conditions identified .
Moreover , higher levels of physical activity reduce the risk for premature all-cause mortality .
The current Canadian guidelines appear to be appropriate to reduce the risk for the seven chronic conditions identified above and all-cause mortality
This systematic review examines critically the scientific basis for Canada 's Physical Activity Guide for Healthy Active Living for adults .
Particular reference is given to the dose-response relationship between physical activity and premature all-cause mortality and seven chronic diseases ( cardiovascular disease , stroke , hypertension , colon cancer , breast cancer , type 2 diabetes ( diabetes mellitus ) and osteoporosis ) .
The strength of the relationship between physical activity and specific health outcomes is evaluated critically .
The Association between Cardiorespiratory Fitness and Impaired Fasting Glucose and Type 2 Diabetes Mellitus in Men
Type 2 diabetes is a common disease in industrialized countries . It is a major cause of cardiovascular disease and all-cause mortality ( 1 - 6 ) , and its prevalence has increased continuously over the past few decades ( 1 ) . The American Diabetes Association currently defines impaired fasting glucose as a fasting plasma glucose level from 6.1 to 6.9 mmol/L ( 110 to 125 mg/dL ) and type 2 diabetes as a fasting plasma glucose level of 7.0 mmol/L ( 126 mg/dL ) or more ( 1 ) . Data from several prospect i ve studies show an inverse association between physical activity and diabetes ( 7 - 13 ) . However , these studies are limited by the use of self-reporting of physical activity and presence of type 2 diabetes ( 7 - 12 ) . Self-reporting of physical activity tends to be imprecise , and type 2 diabetes is undiagnosed in about 50 % of the prevalent cases ( 14 ) . This leads to misclassification on both exposure and outcome measures ( 15 ) . These limitations may result in underestimation of the true association between sedentary habits and risk for type 2 diabetes . Impaired fasting glucose is a strong predictor of type 2 diabetes , cardiovascular disease , and other diabetic complications ( 6 , 16 - 18 ) . The underlying cause of impaired fasting glucose is unknown , and no prospect i ve study of the association between physical activity and impaired fasting glucose has been published . We examined the relation of cardiorespiratory fitness , objective ly determined by a maximal exercise test on a treadmill , to the incidence of impaired fasting glucose and type 2 diabetes . Cases of impaired fasting glucose and diabetes at baseline and follow-up were determined by using the American Diabetes Association 's current guidelines ( 1 ) . Methods Patients In our population -based prospect i ve study , we included 8633 men 30 to 79 years of age at baseline ( mean , 43.5 years ) who completed at least two medical evaluations at the Cooper Clinic in Dallas , Texas , from 1970 to 1995 . Patients come to the Cooper Clinic for preventive medical examinations and health promotion counseling . Many are sent by their employers for these services , some are referred by their personal physicians , and others are self-referred . More than 97 % of the patients are white , and most are employed in executive or professional occupations . More than 75 % are college graduates . Although study participants came from middle and upper socioeconomic strata , they were similar to other well-characterized population -based cohorts in terms of blood pressure , cholesterol level , body weight , and cardiorespiratory fitness ( 19 ) . The study was review ed and approved annually by the institutional review board at the Cooper Institute for Aerobics Research . Additional details of the study methods and population characteristics of the cohort have been published elsewhere ( 20 , 21 ) . Because clinical or sub clinical heart disease and other conditions associated with type 2 diabetes may alter the level of physical activity and thus cardiorespiratory fitness , we excluded men with an abnormal resting or exercise electrocardiogram or a history of heart attack , stroke , or cancer at the baseline clinical examination ( n=2350 ) . The baseline evaluation was performed after participants gave written informed consent for the initial medical examination and registration in the follow-up study . Examinations were done after patients had fasted for at least 12 hours and included personal and family health histories , a question naire on demographic characteristics and health habits  , a  physical examination  , an  exercise test , anthropometric measurement , electrocardiography , blood chemistry analyses , and blood pressure measurement . Technicians who followed a st and ard manual of operations administered all procedures . Impaired fasting glucose and type 2 diabetes were diagnosed according to American Diabetes Association criteria that define impaired fasting glucose as a fasting plasma glucose level of 6.1 to 6.9 mmol/L ( 110 mg/dL to 125 mg/dL ) and diabetes as a fasting plasma glucose level of 7.0 mmol/L ( 126 mg/dL ) or more ( 1 ) . Patients who did not meet these criteria but who reported a history of diabetes or current therapy with oral antidiabetic agents or insulin were also considered to have diabetes . We excluded patients who had diabetes at baseline according to any of these criteria ( n=377 ) .  Cardiorespiratory fitness was assessed with a maximal exercise test that followed a modified Balke protocol ( 22 ) . Details of treadmill speed and elevation have been described elsewhere ( 20 , 21 ) . Briefly , the test began with the patient walking on a horizontal treadmill at 88 m/min . After the first minute , the elevation increased to 2 % ; the elevation then increased 1 % each minute up to 25 minutes . For the few patients who were still able to continue , the elevation was held constant after 25 minutes and the speed was increased by 5.4 m/min until the patient reached volitional fatigue . Use of this protocol for the exercise test correlates highly ( r=0.92 ) with measured maximal oxygen uptake ( 23 ) . All patients in our study achieved at least 85 % of their age-predicted maximal heart rate ; average maximal heart rates ( SD ) in each age group were 186 11 beats/min for patients 30 to 39 years of age , 179 12 beats/min for those 40 to 49 years of age , 172 13 beats/min for those 50 to 59 years of age , and 162 17 beats/min for those 60 years of age or older .  Average maximal heart rates in each age group exceeded the age-predicted rate ( 220 beats/min age in years ) , which indicates that the exercise test can be considered maximal performance . We defined level of fitness by  total time on the treadmill  at the baseline examination , as in our previous studies ( 20 , 21 ) . Treadmill times were placed in frequency distributions for specific age groups ( 30 to 39 , 40 to 49 , 50 to 59 , or 60 or more years of age ) . The least fit 20 % of the participants in each age group were classified as low fitness , the next 40 % as moderate fitness , and the remaining 40 % as high fitness . The respective cut-points for  total treadmill time  in the low- , moderate- , and high-fitness groups were 945 seconds or less , 946 to 1259 seconds , and 1260 seconds or more for patients 30 to 39 years of age ; 849 seconds or less , 850 to 1020 seconds , and 1021 seconds or more for patients 40 to 49 years of age ; 750 seconds or less , 751 to 1035 seconds , and 1036 seconds or more for patients 50 to 59 years of age ; and 644 seconds or less , 645 to 953 seconds , and 954 seconds or more for patients 60 years of age or older . These cut-points at the 20th and 60th percentiles to define fitness levels were used in previous studies ( 20 , 21 ) and were selected before analysis for our investigation . However , we calculated these cut-points with patients in the current study , from which unhealthy persons were excluded . Therefore , they differ somewhat from the cut-points derived from the entire cohort of the Aerobics Center Longitudinal Study ( 21 ) . For some analyses , such as the models that included change in fitness from baseline to follow-up , cardiorespiratory fitness was expressed as maximal metabolic units ( metabolic equivalents [ METs ] , calculated as the working metabolic rate/resting metabolic rate ; 1 MET is equivalent to an oxygen uptake of 3.5 mL1 kg1 ) achieved on the exercise test . In other analyses , time on the treadmill was used as a continuous variable . Serum sample s were analyzed by using automated techniques in a laboratory that participates in the Centers for Disease Control and Prevention Lipid St and ardization Program . Blood pressure was measured by using auscultatory methods with a mercury sphygmomanometer . We defined high blood pressure as systolic blood pressure of at least 140 mm Hg , diastolic blood pressure of at least 90 mm Hg , or a history of hypertension . Height and weight were measured with a st and ard physician 's scale and stadiometer , and body mass index was calculated as weight in kg/height in m2 . Waist circumference was measured with a st and ard anthropometric tape . Statistical Analysis We used SAS statistical software for data analyses ( 24 ) . The incidence of impaired fasting glucose was calculated for men with normal fasting glucose at baseline , and the incidence of diabetes was based on data from all 8633 patients . For analyses with impaired fasting glucose as the outcome , we excluded 1122 men who had impaired fasting glucose at baseline and an additional 69 men who had normal fasting plasma glucose at baseline but developed diabetes during follow-up . Rates of impaired fasting glucose or diabetes were calculated by dividing the number of incident cases during the study period by the number of person-years over the same period . We defined the study period as the interval between the baseline examination and the last follow-up visit . We used logistic regression to estimate the association between dependent variables and independent variables after adjustment for possible confounding factors . We used general linear models to study the cross-sectional association of fitness level and parental history of diabetes ( 24 , 25 ) . To account for the possible cohort effect of baseline year , we examined the relation between incident cases and baseline year and found no association . We used tests for ordinal linear trend to evaluate the possible relation of higher treadmill time with risk for impaired fasting glucose or diabetes after dividing the sample into the three fitness groups . All P values are two-sided , and those less than 0.05 were considered statistically significant . Role of the Funding Source The funding agencies did not participate in the collection , analysis , or interpretation of data presented in this report or in the decision to su bmi t the manuscript for publication . Results During an average follow-up of 6.1 4.8 years ( range , 1 to 24.8 years ) that included 52 588 person-years , 593 men developed impaired fasting glucose and 149 developed diabetes . Of the men with incident diabetes , 139 ( 93 % ) were not aware of
Overall , the current literature supports clearly the dose-response relationship between physical activity and the seven chronic conditions identified .
Moreover , higher levels of physical activity reduce the risk for premature all-cause mortality .
The current Canadian guidelines appear to be appropriate to reduce the risk for the seven chronic conditions identified above and all-cause mortality
This systematic review examines critically the scientific basis for Canada 's Physical Activity Guide for Healthy Active Living for adults .
Particular reference is given to the dose-response relationship between physical activity and premature all-cause mortality and seven chronic diseases ( cardiovascular disease , stroke , hypertension , colon cancer , breast cancer , type 2 diabetes ( diabetes mellitus ) and osteoporosis ) .
The strength of the relationship between physical activity and specific health outcomes is evaluated critically .