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 .
Physical Activity, Obesity, and Risk for Colon Cancer and Adenoma in Men
Westernization or industrialization leads to an increase in rates of colon cancer , which is the second leading cause of malignant death in the United States [ 1 ] . Although the precise causes of colon cancer remain unclear , a diet high in red meat or animal fat and low in fruits and vegetables appears to increase the risk for this malignancy [ 2 , 3 ] . It is perhaps less well recognized that an inverse association between physical activity and risk for colon cancer has been seen in studies of occupational activity only [ 4 - 12 ] and of both job-related and recreational activity [ 13 - 23 ] . In addition , many studies have found an association between body mass and elevated risk for colon cancer in men ; this association is weaker in women [ 24 - 33 ] . The fact that the association is stronger in men suggests that the abdominal distribution of adiposity typical in men may be an important component of enhanced risk . More limited evidence suggests that height , which may be a proxy for a person 's net energy intake during childhood and adolescence , is related to a higher risk for colon cancer [ 33 - 35 ] . We examine the association between physical activity , obesity , and attained height in relation to risk for colon cancers and their precursory adenomas in a large cohort of male health professionals in the United States . Waist and hip circumferences were available for a subcohort of the study population . We address the hypotheses that physical inactivity , obesity , and height increase the risk for colon cancer and adenoma independently of each other and of diet , and that the abdominal pattern of obesity is an additional independent risk factor . Methods Study Population The Health Professionals Follow-up Study [ 36 ] was started in 1986 ; in that year , 51 529 male dentists , optometrists , osteopaths , podiatrists , pharmacists , and veterinarians in the United States between 40 and 75 years of age responded to a mailed question naire . They reported on their leisure-time physical activity ( described below ) ; current body weight ; weight at age 21 years ; height ; history of cancer and other medical conditions ; parental history of various diseases , including colorectal cancer ; and use of aspirin and other nonsteroidal anti-inflammatory medications . They also reported dietary and alcohol intake using a vali date d [ 37 , 38 ] , semi-quantitative food-frequency question naire . We mailed an optional question naire in 1987 to assess  waist and hip circumferences . In 1988 , 1990 , and 1992 , we up date d exposure information and ascertained newly diagnosed medical conditions and history of colonoscopy or sigmoidoscopy , including the indications for endoscopy : bleeding in stool , positive results from tests for occult fecal blood , abdominal pain , diarrhea or constipation , family history of colorectal cancer , routine screening without symptoms , or follow-up [ 39 ] . Most of the deaths in the cohort were reported by family members or by the postal system in response to the follow-up question naires . We also used the National Death Index to identify deaths among nonrespondents [ 40 ] . Assessment of Physical Activity The 1986 question naire included a section about mainly recreational or leisure-time physical activity . The reliability and validity of question naires design ed to assess physical activity have been investigated [ 41 - 43 ] . A question naire such as the one used in our cohort was evaluated in a cohort of U.S. nurses and was found to provide useful information [ 44 ] , and similar results were found during a similar validation study done within the Health Professionals cohort ( Chasan-Taber S. Personal communication ) . Participants reported the average time per week spent doing each of eight moderate and vigorous activities , choosing from among 10 possible responses that ranged from 0 minutes to 11 or more hours per week . The specific activities listed were walking or hiking outdoors ( including walking during golf ) ; jogging ( slower than 10 minutes/mile ) ; running ( 10 minutes/mile or faster ) ; bicycling ( including that done on a stationary machine ) ; lap swimming ; tennis , squash , or racquetball ; and calisthenics or rowing . In addition , each respondent reported the number of flights of stairs he climbed daily and his usual walking pace . The reported time spent at each activity per week was multiplied by its typical energy expenditure requirements expressed in metabolic equivalents ( METs ) [ 45 ] to yield a MET-hour score . One MET , which is the energy expended by sitting quietly , is equivalent to 3.5 mL of oxygen uptake per kilogram of body weight per minute for a 70-kg adult . For example , 1 hour per week of running contributed 10.2 MET-hours , 1 hour of tennis contributed 6 MET-hours , and 1 hour of walking at a moderate pace contributed 3.2 MET-hours . Body weight was excluded from the derivation of energy expenditure from physical activity to avoid confounding the expenditure variable by body weight . If more than one published intensity level was available for a given activity , the moderate or general MET value was chosen . An average MET value was assigned to the categories that listed more than one activity , such as rowing or calisthenics , and squash or racquetball . Assessment of Anthropometric Variables  Each man reported his current weight and height and his weight at age 21 years on the 1986 question naire . In 1987 , we mailed an optional question naire once to obtain additional exposure information , including body circumference measurements . We instructed each participant to measure ( to the nearest quarter inch ) his waist at the umbilicus and his hips at the largest circumference between the waist and thighs while st and ing and without measuring over bulky clothing [ 46 ] . We provided a tape measure and an illustration to help st and ardize the measurements . Sixty-five percent of the cohort responded . We used the Quetelet index ( kilograms/height in meters2 ) as a measure of total adiposity , waist-to-hip ratio to measure relative distribution of fat , and waist circumference to estimate total abdominal fat . Although the waist-to-hip ratio has been used more widely , waist circumference provides an estimate of absolute abdominal adiposity , the component most closely related to important metabolic abnormalities , including hyperinsulinemia , hypertension , and high triglyceride levels . To remove extraneous variation in the waist circumference because of height ( taller men tend to have larger waist circumferences due to their larger body size rather than to obesity ) , we adjusted waist for height using residual analysis [ 47 ] . We first regressed waist on height using multiple linear regression and then added the residual to the average waist size ( 37.4 inches ) for a man of average height ( 70 inches ) to convert this measure back to the initial scale . This conversion created for each man a st and ardized waist circumference unconfounded by height . We evaluated the precision of self-reported anthropometric measures among 123 cohort members who were part of a dietary validation study [ 46 ] . Trained technicians paid the sub study participants two visits , approximately 6 months apart , to measure current weight and waist and hip circumferences . The Pearson correlation between self-report and the average of the technicians ' two measurements was 0.97 for weight , 0.95 for waist circumference , 0.88 for hip circumference , and 0.69 for waist-to-hip ratio . The men 's self- measurements of their waist circumferences were 0.36 inches greater , their self- measurements of hip circumferences were 0.78 inches smaller , and their self- measurements of weight were 2.3 pounds less than the technician 's measurements . Identification of Patients with Colorectal Cancers In 1988 , 1990 , and 1992 , we asked each participant whether cancer had been diagnosed during the previous 2 years . The response rate to the follow-up question naires was 94 % through 31 January 1992 . When a participant ( or a decedent 's next-of-kin ) reported a diagnosis of cancer of the colon or rectum , we sought permission to obtain hospital records and pathology reports . A study physician , blinded to exposure information , review ed all medical records received and extracted data about histologic type , anatomic location , and stage of the cancer . Proximal colon cancers were defined as those from the cecum to and including the splenic flexure , and distal colon cancers were defined as those in the descending and sigmoid colon . We confirmed 249 new cases of colorectal adenocarcinoma ( excluding carcinoma in situ ) , 90 % by medical records and the remainder with corroborating information about diagnosis and treatment from the cohort member . Two hundred three cancers were in the colon and 46 were in the rectum . Identification of Patients with Colorectal Adenomas and Controls Because more than 90 % of the adenomas were diagnosed during endoscopic procedures for screening or for unrelated gastrointestinal conditions , we restricted the adenoma analysis to men who had had a colonoscopy or sigmoidoscopy .  This was done to reduce the potential for detection bias . Most procedures were sigmoidoscopies ; thus , we analyzed only adenomas of the distal colorectum . Although we did not examine proximal colon adenomas , this should not have biased inferences for the distal colorectum . However , different causes for proximal adenomas may exist . A total of 12 879 men who did not meet any of the exclusion criteria ( see Data Analysis ) reported having had an endoscopy between 1986 and 1992 . In 1993 , we sent a mailing to a r and om sample of 200 controls ( men who reported negative results from an endoscopy ) to confirm that they did not have adenomas . After one mailing , 140 ( 70 % ) controls granted us permission to review the medical records of their endoscopic procedure ; none had an unreported , histologically confirmed adenoma . We were able to recontact 96 % of the men who reported a diagnosis of polyp , and we received medical records in response to more than 96 % of the requests sent to medical record departments ,
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 .