EAL

DM: Prevention of Type 2 Diabetes (2007)

Citation:

Study Design:

Class:

- Click here for explanation of classification scheme.

Quality Rating:

Research Purpose:

To evaluate any association between total physical activity and the incidence of type 2 diabetes in women, and to contrast benefits derived from walking versus more vigorous exercise as a predictor for risk of type 2 diabetes. A major focus was on walking since it is the most prevalent exercise among older adults, and because it is feasible, accessible, and relatively safe.

Inclusion Criteria:

Participants from Nurses' Health Study (female registered nurses, aged 30 to 55 in 1976) who responded to questionnaires in 1976
Based on provided medical history and health habits, were without a diagnosis of diabetes, cardiovascular disease, and cancer at that time
Completed questions on physical activity in 1986, 1992, and 1996.

Exclusion Criteria:

Diagnosis of diabetes before 1986. Women with type 1 diabetes (27), gestational diabetes only (40), or self-reported diabetes (183)
Women who reported on questionnaires in 1990 and 1992 that they were unable to walk.

Description of Study Protocol:

Recruitment: Not applicable, as data were collected from female registered nurses already enrolled and participating as a cohort in the Nurses' Health Study (see above for inclusion and exclusion of women from the Nurses' Health Study cohort into present physical activity/type 2 diabetes study).

Design: Cohort Study.

Blinding Used: Endocrinologist confirming the diagnosis of diabetes was blinded.

Intervention:

Assessment (by questionnaire) of participants in a cohort study for a possible correlation between the intensity of their physical activity and their possible development of type 2 diabetes. Data from the Nurses' Health Study were analyzed. Specifically, questions put forth in 1986 (and updated in 1988 and 1992) on physcial activity were analyzed from 70,102 female registered nurses (aged 40-65). These data were then correlated with the diabetes status of these women in 1999.

Statistical analysis: Person-years were calculated from the date that the 1986 questionnaire was returned, to the date of confirmed type 2 diabetes, death from any cause, or June 1, 1994 (whichever came first).

Incidence rate of type 2 diabetes was calculated by dividing number of cases by person-years for each category of physical activity.

Relative risks were calculated as the incidence rates in specific MET categories, divided by that in the lowest quintile (adjusted for 5-year age categories).

The linear trend across increasing categories of MET was accomplished by treating categories as a continuous variable and assigning a median score for the category as its value.

The cumulative average of MET scores from all available questionnaires represents long-term physical activity.

Pooled logistic regression (2-yr intervals) was used to adjust for confounding variables such as age, smoking status, alcohol consumption, menopausal status and postmenopausal hormone use, parental history of diabetes, and history of hypercholesterolemia or hypertension at baseline.

Data Collection Summary:

Timing of measurements: Questionnaire supplement sent if diagnosis of diabetes was indicated on biennial questionnaire. Physical activity questionnaires were obtained in 1986, 1988, and 1992. Questionnaire was tested for validity: Two-yr test-retest correlation for activity was 0.59. Correlation between physical activity from 1-week recalls and on questionnaire was 0.79. Correlation between activity diaries and questionnaire was 0.62.

Dependent Variables:

Risk of type 2 diabetes by quintile of metabolic equivalent task score, based on time spent per week on each of the 8 common physical activities

Independent Variables:

Medical history and health practices
Physical activity assessed by questionnaire. Data included amount of time spent walking, jogging, bicycling, calisthenics, aerobics, aerobic dance, rowing machine, lap swimming, playing squash racquetball tennis. Since walking was of particular interest, details related to walking pace (easy, normal, brisk) were collected and converted to METS. Vigorous activity was defined as equal to or greater than six METS, and included activities such as jogging, running, bicycling, calisthenics, lap swimming, playing squash, racquetball, or tennis. Walking was moderate intensity, yielding 2-4.5 METS.
Clinical symptoms and diagnosis of diabetes was determined by a separate questionnaire. Positive diagnosis was established if (1) excessive thirst, polyuria, weight loss, hunger were present along with a fasting plasma glucose of 140 mg/dL or a random plasma glucose of equal to or greater than 200 mg/dL OR (2) equal to or greater than 2 occasions of plasma glucose of 140 mg/dL, random plasma glucose levels equal to or greater than 200 mg/dL, and/or equal to or greater than 200 mg/dL after 2 hours or more from an oral glucose tolerance test, or (3) treatment with hypoglycemic medications.

Control Variables:

Age
Smoking
Alcohol use
History of hypertension
History of high cholesterol
BMI

Description of Actual Data Sample:

Initial N: 70,102 females

Attrition (Final N): Eight years of followup (534,928 person years), 1419 cases of type 2 diabetes.

Age: 30 - 55 (in 1976)

Ethnicity: Not mentioned

Other Relevant Demographics:

Anthropometrics: No associations between weight and exercise were used.

Location: 11 different states

Summary of Results:

After adjusting for age, smoking, alcohol use, history of hypertension, history of high cholesterol, and other covariates, the relative risk of developing type 2 diabetes across quintiles, from least to most physical activities, were: 1.0, 0.77, 0.75, 0.62, 0.54 (p<.001). After adjustment for BMI, relative risks were 1.0, 0.84, 0.87, 0.77, 0.74 (p<.002).

For participants who walked without vigorous exercise, the relative risk of developing type 2 diabetes across quintiles of MET walking scores were: 1.0, 0.91, 0.73, 0.69, and 0.58 (p<.001). After adjusting for BMI, relative risks were 1.0, 0.95, 0.80, 0.81, 0.74 (p = .01).

Author Conclusion:

Our data suggest that greater physical activity level is associated with a substantial reduction in risk of type 2 diabetes, including physical activity of moderate intensity and duration. We observed comparable magnitudes of risk reduction with walking and vigorous activity in this cohort, when total energy expenditures were similar. Both walking and vigorous exercise contribute significantly to reduced risk of type 2 diabetes in women, confirming the Center for Disease Control and Prevention, and the National Institute of Health advice to exercise for at least 30 minutes most if not all days of the week.

Funding Source:

Government:

NIH

Not-for-profit

Foundation associated with industry:

Reviewer Comments:

Strengths of this study include a large number of participants in a varied number of locations. This study was limited by the fact that all participants were in the same occupation and therefore lived in similar environments, had similar education levels, had similar life habits, behaviours, and attitutes. Another limitation was that all physical activity was self-reported; however, the authors did assess validity between documentation. The authors state that some cases of diabetes may have gone undiagnosed. The authors state that self-selection of heavy subjects for lower physical activity could partly account for attenuation of physical activity-diabetes relationship in regression models that include adjustment for BMI. The point was made that it is visceral fat, associated with insulin resistance (increased number and activity of glucose transporters like GLUT4), that is usually lost from participation in physical activity.

Quality Criteria Checklist: Primary Research
Relevance Questions
	1.	Would implementing the studied intervention or procedure (if found successful) result in improved outcomes for the patients/clients/population group? (Not Applicable for some epidemiological studies)	Yes
	2.	Did the authors study an outcome (dependent variable) or topic that the patients/clients/population group would care about?	Yes
	3.	Is the focus of the intervention or procedure (independent variable) or topic of study a common issue of concern to dieteticspractice?	Yes
	4.	Is the intervention or procedure feasible? (NA for some epidemiological studies)	Yes

Validity Questions
1.	Was the research question clearly stated?		Yes
	1.1.	Was (were) the specific intervention(s) or procedure(s) [independent variable(s)] identified?	Yes
	1.2.	Was (were) the outcome(s) [dependent variable(s)] clearly indicated?	Yes
	1.3.	Were the target population and setting specified?	Yes
2.	Was the selection of study subjects/patients free from bias?		Yes
	2.1.	Were inclusion/exclusion criteria specified (e.g., risk, point in disease progression, diagnostic or prognosis criteria), and with sufficient detail and without omitting criteria critical to the study?	Yes
	2.2.	Were criteria applied equally to all study groups?	Yes
	2.3.	Were health, demographics, and other characteristics of subjects described?	Yes
	2.4.	Were the subjects/patients a representative sample of the relevant population?	Yes
3.	Were study groups comparable?		Yes
	3.1.	Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT)	N/A
	3.2.	Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline?	N/A
	3.3.	Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.)	N/A
	3.4.	If cohort study or cross-sectional study, were groups comparable on important confounding factors and/or were preexisting differences accounted for by using appropriate adjustments in statistical analysis?	Yes
	3.5.	If case control study, were potential confounding factors comparable for cases and controls? (If case series or trial with subjects serving as own control, this criterion is not applicable.)	N/A
	3.6.	If diagnostic test, was there an independent blind comparison with an appropriate reference standard (e.g., "gold standard")?	N/A
4.	Was method of handling withdrawals described?		No
	4.1.	Were follow-up methods described and the same for all groups?	Yes
	4.2.	Was the number, characteristics of withdrawals (i.e., dropouts, lost to follow up, attrition rate) and/or response rate (cross-sectional studies) described for each group? (Follow up goal for a strong study is 80%.)	No
	4.3.	Were all enrolled subjects/patients (in the original sample) accounted for?	???
	4.4.	Were reasons for withdrawals similar across groups?	???
	4.5.	If diagnostic test, was decision to perform reference test not dependent on results of test under study?	N/A
5.	Was blinding used to prevent introduction of bias?		Yes
	5.1.	In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate?	N/A
	5.2.	Were data collectors blinded for outcomes assessment? (If outcome is measured using an objective test, such as a lab value, this criterion is assumed to be met.)	Yes
	5.3.	In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded?	Yes
	5.4.	In case control study, was case definition explicit and case ascertainment not influenced by exposure status?	N/A
	5.5.	In diagnostic study, were test results blinded to patient history and other test results?	N/A
6.	Were intervention/therapeutic regimens/exposure factor or procedure and any comparison(s) described in detail? Were interveningfactors described?		Yes
	6.1.	In RCT or other intervention trial, were protocols described for all regimens studied?	N/A
	6.2.	In observational study, were interventions, study settings, and clinicians/provider described?	N/A
	6.3.	Was the intensity and duration of the intervention or exposure factor sufficient to produce a meaningful effect?	Yes
	6.4.	Was the amount of exposure and, if relevant, subject/patient compliance measured?	Yes
	6.5.	Were co-interventions (e.g., ancillary treatments, other therapies) described?	Yes
	6.6.	Were extra or unplanned treatments described?	N/A
	6.7.	Was the information for 6.4, 6.5, and 6.6 assessed the same way for all groups?	Yes
	6.8.	In diagnostic study, were details of test administration and replication sufficient?	N/A
7.	Were outcomes clearly defined and the measurements valid and reliable?		Yes
	7.1.	Were primary and secondary endpoints described and relevant to the question?	Yes
	7.2.	Were nutrition measures appropriate to question and outcomes of concern?	N/A
	7.3.	Was the period of follow-up long enough for important outcome(s) to occur?	Yes
	7.4.	Were the observations and measurements based on standard, valid, and reliable data collection instruments/tests/procedures?	Yes
	7.5.	Was the measurement of effect at an appropriate level of precision?	Yes
	7.6.	Were other factors accounted for (measured) that could affect outcomes?	Yes
	7.7.	Were the measurements conducted consistently across groups?	N/A
8.	Was the statistical analysis appropriate for the study design and type of outcome indicators?		Yes
	8.1.	Were statistical analyses adequately described and the results reported appropriately?	Yes
	8.2.	Were correct statistical tests used and assumptions of test not violated?	Yes
	8.3.	Were statistics reported with levels of significance and/or confidence intervals?	Yes
	8.4.	Was "intent to treat" analysis of outcomes done (and as appropriate, was there an analysis of outcomes for those maximally exposed or a dose-response analysis)?	N/A
	8.5.	Were adequate adjustments made for effects of confounding factors that might have affected the outcomes (e.g., multivariate analyses)?	Yes
	8.6.	Was clinical significance as well as statistical significance reported?	Yes
	8.7.	If negative findings, was a power calculation reported to address type 2 error?	N/A
9.	Are conclusions supported by results with biases and limitations taken into consideration?		Yes
	9.1.	Is there a discussion of findings?	Yes
	9.2.	Are biases and study limitations identified and discussed?	Yes
10.	Is bias due to study's funding or sponsorship unlikely?		Yes
	10.1.	Were sources of funding and investigators' affiliations described?	Yes
	10.2.	Was the study free from apparent conflict of interest?	Yes