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Physical Activity and Television Watching in Relation to Risk for Type 2 Diabetes Mellitus in Men
Frank B. Hu, MD;
Michael F. Leitzmann, MD;
Meir J. Stampfer, MD;
Graham A. Colditz, MD;
Walter C. Willett, MD;
Eric B. Rimm, ScD
Arch Intern Med. 2001;161:1542-1548.
ABSTRACT
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Background Television (TV) watching, a major sedentary behavior in the United States,
has been associated with obesity. We hypothesized that prolonged TV watching
may increase risk for type 2 diabetes.
Methods In 1986, 37 918 men aged 40 to 75 years and free of diabetes, cardiovascular
disease, and cancer completed a detailed physical activity questionnaire.
Starting from 1988, participants reported their average weekly time spent
watching TV on biennial questionnaires.
Results A total of 1058 cases of type 2 diabetes were diagnosed during 10 years
(347 040 person-years) of follow-up. After adjustment for age, smoking,
alcohol use, and other covariates, the relative risks (RRs) for type 2 diabetes
across increasing quintiles of metabolic equivalent hours (MET-hours) per
week were 1.00, 0.78, 0.65, 0.58, and 0.51 (P for
trend, <.001). Time spent watching TV was significantly associated with
higher risk for diabetes. After adjustment for age, smoking, physical activity
levels, and other covariates, the RRs of diabetes across categories of average
hours spent watching TV per week (0-1, 2-10, 11-20, 21-40, and >40) were 1.00,
1.66, 1.64, 2.16, and 2.87, respectively (P for trend,
<.001). This association was somewhat attenuated after adjustment for body
mass index, but a significant positive gradient persisted (RR comparing extreme
categories, 2.31; P for trend, .01).
Conclusions Increasing physical activity is associated with a significant reduction
in risk for diabetes, whereas a sedentary lifestyle indicated by prolonged
TV watching is directly related to risk. Our findings suggest the importance
of reducing sedentary behavior in the prevention of type 2 diabetes.
INTRODUCTION
EPIDEMIOLOGICAL evidence strongly supports a role of exercise in the
prevention of type 2 diabetes mellitus.1-8
However, less attention has focused on sedentary behaviors in relation to
risk for diabetes. Television (TV) watching represents a major sedentary behavior
in the United States; on average, a male adult spends approximately 29 hours
per week watching TV, and a female adult, 34 hours per week.9
Television watching results in lower metabolic rate compared with other sedentary
activities such as sewing, playing board games, reading, writing, and driving
a car.10 In several studies, time spent watching
TV has been strongly associated with weight gain and obesity in children11-12 and adults.13-15
The purpose of this study is to examine whether prolonged TV watching predicts
subsequent diabetes risk independent of physical activity in a prospective
cohort of men. We also examined total physical activity, vigorous exercise,
and moderate-intensity activity in relation to risk for type 2 diabetes in
this cohort.
SUBJECTS AND METHODS
SUBJECTS
The Health Professional's Follow-up Study (HPFS) began in 1986 when
51 529 US health professionals (dentists, optometrists, pharmacists,
podiatrists, osteopaths, and veterinarians), aged 40 to 75 years, answered
a detailed questionnaire that included a comprehensive diet survey and items
on lifestyle practice and medical history.16
Follow-up questionnaires were sent in 1988, 1990, 1992, 1994, and 1996 to
update information on potential risk factors and to identify newly diagnosed
cases of diabetes and other diseases. We excluded from the present analysis
men with a previous diagnosis of cardiovascular disease (n = 4639), cancer
(n = 1638), or diabetes (n = 1796) at baseline. Participants with diagnosed
cardiovascular disease or cancer at baseline were excluded because these diagnoses
may lead to change in physical activity levels. Participants who had missing
information on activity questions or reported implausible total energy intake
on the food frequency questionnaire17 (<3347
or >17 572 kJ/d) were also excluded (n = 5538). We followed up the remaining
37 918 men for incidence of type 2 diabetes during the subsequent 10
years of the study.
ASSESSMENT OF PHYSICAL ACTIVITY
Physical activity was assessed using mailed questionnaires at baseline
and every 2 years thereafter. Subjects were asked to report the average amount
of time they spent per week on each of the following activities: walking,
jogging, running, bicycling, calisthenics or use of a rowing machine, lap
swimming, squash or racquetball, and tennis. They were also asked about their
usual walking pace, specified as easy or casual (<2 miles/h), normal (2-2.9
miles/h), brisk (3-3.9 miles/h), or striding ( 4 miles/h). From this information,
weekly energy expenditure in metabolic equivalent hours (MET-hours) was calculated.10 We defined any physical activity requiring 6 MET-hours
or greater (a 6-fold or greater increase above resting metabolic rate) as vigorous. These activities included jogging, running, bicycling,
calisthenics or use of a rowing machine, lap swimming, squash or racquetball,
and tennis. In contrast, walking requires an energy expenditure of only 2
to 4.5 MET-hours, depending on pace, and was therefore considered to be a
moderate-intensity activity.
The reproducibility and validity of the physical activity questionnaire
was evaluated in a subsample (n = 238) of participants in the HPFS cohort.18 The Pearson correlation between moderate plus vigorous
physical activity, assessed by means of diaries for 4 weeks across different
seasons, and that reported on the questionnaire was 0.58. The correlation
between vigorous activity score, assessed by means of the questionnaire, and
resting pulse was -0.45; for pulse after stopping, the correlation was -0.41.
In a separate study on a population aged 20 to 59 years recruited from a university
community (n = 103), the correlation between physical activity score on a
similar questionnaire and maximum oxygen consumption was 0.54.19
In a subsample of participants in the HPFS cohort (n = 466), high-density
lipoprotein (HDL) cholesterol level increased by 0.06 mmol/L (2.4 mg/dL) for
each increment of 20 MET-hours per week (P<.01).20
Starting from 1988, participants reported their average weekly time
spent watching TV (including videotapes) on the biennial questionnaires. The
1988 questionnaire included 6 response categories (ranging from 0-1 to >40
h/wk). Subsequent questionnaires included 13 response categories (ranging
from 0 to >40 h/wk). In the present analyses, 5 categories were coded consistently
across all questionnaires (0-1, 2-10, 11-20, 21-40, and >40 h/wk). In a subsample
of participants in the HPFS (n = 466), average hours of TV watching were significantly
associated with higher levels of leptin and low-density lipoprotein (LDL)
cholesterol and with lower levels of HDL cholesterol and apolipoprotein A-I.20
DIAGNOSIS OF TYPE 2 DIABETES
A supplementary questionnaire regarding symptoms, diagnostic tests,
and hypoglycemic therapy was mailed to men who indicated on any biennial questionnaire
that they had been diagnosed with diabetes. A case of diabetes was considered
confirmed if at least 1 of the following was reported on the supplementary
questionnaire: (1) 1 or more classic symptoms (excessive thirst, polyuria,
weight loss, hunger) plus 1 fasting plasma glucose level of at least 7.8 mmol/L
(140 mg/dL) or random plasma glucose of at least 11.1 mmol/L (200 mg/dL);
(2) at least 2 elevated plasma glucose concentrations on different occasions
(fasting, 7.8 mmol/L [140 mg/dL]; random, 11.1 mmol/L [200
mg/dL]; and/or 11.1 mmol/L [200 mg/dL] after 2 hours of oral glucose
tolerance testing) in the absence of symptoms; or (3) treatment with hypoglycemic
medication (insulin or oral hypoglycemic agent). Because of potential associations
between weight and physical activity, no body weight criteria were used in
the classification of type of diabetes for these analyses. Our criteria for
diabetes classification are consistent with those proposed by the National
Diabetes Data Group21 for 1986-1996. The validity
of self-report of diabetes has been verified in a subsample of 71 men from
the HPFS cohort. A physician blinded to the information reported on the supplementary
questionnaire and reviewed the medical records according to the diagnostic
criteria. Of the 71 patients, 12 had incomplete records, eg, absent laboratory
data (n = 2), or 1 set only of laboratory data (n = 9). Among the remaining
59 cases, the diagnosis of type 2 diabetes was confirmed in 57 (97%). One
patient denied the diagnosis and another lacked evidence of diabetes in his
submitted records. Similarly, 98% of diabetic cases reported by the supplementary
questionnaire were confirmed by medical record review in a subsample of participants
(n = 62) in the Nurses' Health Study.22
STATISTICAL ANALYSIS
Person-time for each participant was calculated from the date of return
of the 1986 (physical activity) or 1988 (TV watching) questionnaires to the
date of confirmed type 2 diabetes, death due to any cause, or January 1, 1996,
whichever came first. Incidence rates of type 2 diabetes were obtained by
dividing the number of cases by person-years in each category of physical
activity or average time spent on watching TV. Relative risks (RRs) were computed
as the incidence rate in a specific category of MET score (ie, MET-hours per
week) or TV watching divided by that in the reference category, with adjustment
for 5-year age categories. Tests for linear trend across increasing categories
of MET score or average time spent watching TV were conducted by treating
the categories as a continuous variable and assigning the median score for
the category as its value. Both MET score or time spent watching TV were updated
every 2 years.
We used pooled logistic regression to adjust estimated incidence rate
ratios simultaneously for potential confounding variables. In this approach,
independent 2-year blocks of person-time of follow-up are pooled for regression
analysis, and the dependence of the incidence rates on time is modeled nonparametrically
with indicator variables. D'Agostino et al23
have shown that the pooled logistic model is asymptotically equivalent to
the Cox regression when the time intervals are short and the probability of
outcome in the intervals is low. Our covariates included age (40-44, 45-49,
50-54, 55-59, 60-64, 65-69, and 70 years), smoking (never, past, or current
[1-14, 15-24, and 25 cigarettes per day]), alcohol consumption (0-4, 5-9,
10-14, 15-29, and 30 g/d), parental history of diabetes, and history of
hypercholesterolemia or hypertension at baseline. In additional analyses,
we included body mass index (BMI [calculated as weight in kilograms divided
by the square of height in meters], in quintiles) in the model to examine
the degree to which the relation with physical activity was mediated through
BMI.
To examine whether the effects of physical activity on diabetes were
modified by important covariates, we conducted multivariate analyses according
to categories of age (<65 or 65 years), family history of diabetes
(no or yes), smoking (never or ever), and BMI (<25.0, 25.0-29.9, or 30.0
kg/m2). To examine independent effects of physical activity and
TV watching, we estimated RRs of diabetes according to joint classifications
of these 2 variables. In this analysis, both variables were classified into
quartiles rather than 5 categories to have sufficient power.
RESULTS
During 10 years (347 040 person-years) of follow-up, we documented
1058 newly diagnosed cases of type 2 diabetes. As described elsewhere,15 physically more active men tended to be leaner and
were less likely to be current smokers. Increasing total physical activity
score was strongly associated with progressively reduced risk for type 2 diabetes
(Table 1). The age-adjusted RRs
across quintiles of MET score from total physical activity were 1.00, 0.76,
0.61, 0.55, and 0.47 (P for trend, <.001). Further
adjustment for smoking, parental history of diabetes, and other covariates
did not appreciably change these RRs. This inverse gradient remained strong
even after adjusting for BMI (RRs across quintiles of MET score were 1.00,
0.82, 0.72, 0.66, and 0.62; P for trend, <.001).
Adjustment for dietary intakes of fats and cereal fiber did not appreciably
change the results.
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Table 1. Relative Risks for Type 2 Diabetes According to Quintiles
of Total Physical Activity Score Among US Male Health Professionals, 1986-1996*
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To minimize potential bias from subclinical disease, we conducted additional
analyses in which we excluded cases of type 2 diabetes that occurred during
the first 2 years of follow-up. The multivariate RRs (without BMI) across
quintiles of physical activity score were 1.00, 0.88, 0.75, 0.69, and 0.57
(P for trend, <.001). The inverse association
between total physical activity score and diabetes risk was persistent in
subgroup analyses according to age (<65 or 65 years), family history
of diabetes, smoking (never or ever), and BMI (<25.0, 25.0-29.9, or 30.0
kg/m2) (Table 2). In
particular, the increased risks associated with family history of diabetes
and obesity were substantially mitigated by increasing physical activity levels.
To address the possibility that medical surveillance may have varied according
to physical activity level, we conducted an analysis restricted to subjects
reporting at least 1 symptom of diabetes at diagnosis (n = 595). Results from
this subgroup were similar to those for the entire cohort (multivariate RRs
without BMI in the model across quintiles of MET score were 1.00, 0.66, 0.65,
0.57, and 0.49; P for trend, <.001).
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Table 2. Relative Risks of Type 2 Diabetes According to Quintiles of
MET-Hours from Total Physical Activity Among Various Subpopulations of US
Male Health Professionals, 1986-1996*
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After adjustment for age and other covariates, we observed a significant
inverse association between MET score for walking and risk for type 2 diabetes.
The multivariate RRs across quintiles of walking score were 1.00, 1.02, 0.80,
0.76, and 0.72 (P for trend, <.001). This inverse
association remained significant after adjustment for vigorous exercise (RRs
were 1.00, 1.06, 0.86, 0.82, and 0.80; P for trend,
.006). Independent of the number of hours spent walking, walking pace was
strongly associated with risk for diabetes. Compared with men whose usual
walking pace was easy or casual, multivariate RRs were 0.68 for normal pace,
0.46 for brisk pace, and 0.39 for very brisk pace (P
for trend, <.001).
Walking and vigorous exercise were associated with comparable risk reductions
for equivalent energy expenditure. When the walking and vigorous activity
scores were entered into the model as continuous variables simultaneously,
RRs associated with an increase in energy expenditures of 10 MET-hours per
week were 0.89 (95% confidence interval [CI], 0.82-0.96) for walking and 0.88
(95% CI, 0.85-0.92) for vigorous exercise.
Men who spent more time watching TV were more likely to smoke and drink
alcohol and less likely to exercise (Table
3). They were substantially heavier and more likely to have hypertension
and hypercholesterolemia. These men also had higher intake of total energy,
total and saturated fats, red meat, processed meat, French fries, refined
grain products, snacks, and sweets or desserts and lower intakes of fish,
vegetables, fruits, and whole grains.
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Table 3. Age-Standardized Characteristics According to Average Number
of Hours Watching Television per Week in the HPFS at Baseline in 1988*
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After adjustment for age, average time spent watching TV was strongly
associated with increased risk for diabetes (Table 4). The RRs across categories of average hours spent watching
TV per week (0-1, 2-10, 11-20, 21-40, and >40) were 1.00, 1.62, 1.61, 2.22,
and 3.35 (95% CI, 1.71-6.55, respectively; P for
trend, <.001). After further adjustment for smoking, alcohol use, physical
activity, and other covariates, the positive association persisted (RR comparing
extreme categories, 2.87; 95% CI, 1.46-5.65; P for
trend, <.001). The significant positive association persisted even after
adjustment for BMI (RR comparing extreme categories, 2.31; 95% CI, 1.17-4.56; P for trend, .01). Further simultaneous adjustment for
intakes of saturated fat, monounsaturated fat, polyunsaturated fat, trans-fatty acids, and cereal fiber did not appreciably
change the results (Table 4).
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Table 4. Relative Risks for Type 2 Diabetes According to Categories
of Television Watching, HPFS 1988-1996
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In multivariate analyses, we observed independent effects of TV watching
and physical activity levels (Figure 1).
Compared with men who were in the most active (>46 MET-hours per week) and
the lowest TV watching category (<3.5 h/wk), those who were in the least
active (<10 MET-hours per week) and most sedentary category (>15 h/wk watching
TV) had a significantly increased risk for type 2 diabetes (RR, 2.92; 95%
CI, 1.87-4.55; P for interaction, .90). When total
physical activity score and time spent watching TV were simultaneously included
in a multivariate model (without BMI), an increment of 2 h/d spent watching
TV was associated with a 20% (95% CI, 8%-32%) increase in risk for diabetes,
whereas an increment of 18 MET-hours per week (equivalent to very brisk walking
for 40 minutes per day) was associated with a 19% (95% CI, 13%-24%) reduction
in risk.
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Multivariate relative risks (RRs) for type 2 diabetes mellitus according
to categories of metabolic equivalent hours (MET-hours) per week and average
weekly time spent watching television (TV). Adjusted for the same covariates
as in Table 1 (body mass index
not included in the model).
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COMMENT
In this large prospective cohort of men, greater leisure time physical
activity was associated with reduced risk for type 2 diabetes. In contrast,
a sedentary lifestyle, as indicated by time spent watching TV, was significantly
associated with an increased risk for diabetes, independent of the effects
of physical activity and body weight.
Our findings extend the literature showing that regular physical activity
is associated with a substantial reduction in risk for type 2 diabetes.1-5,7, 22
Our results also suggest that the apparent beneficial effect of exercise is
not confined to high-risk groups (eg, subjects with 1 risk factors such
as obesity and family history of diabetes). Contrary to the belief that fitness
and physical activity might offset the adverse effects of obesity,24 we found that men who were obese and physically active
had a substantially increased risk for diabetes compared with those who were
lean and inactive (Table 2), although
obese and inactive men were at highest risk. In addition, we found that equivalent
energy expenditure from brisk walking or vigorous exercise may confer comparable
benefits. These findings are consistent with emerging evidence to support
the benefits of moderate-intensity activities in the prevention of diabetes
and cardiovascular disease.8, 25-27
Since walking is an activity that is highly accessible, readily adopted, and
rarely associated with exercise-related injury, these findings may have important
public health implications.
The beneficial effects of vigorous exercise and walking on risk for
type 2 diabetes are partly mediated by body weight and body fat distribution.
Leaner individuals have a reduced risk for diabetes,28-30
and physical activity facilitates weight loss and weight maintenance.31 Furthermore, exercise may lead to loss in visceral
fat,32 which is strongly associated with insulin
resistance and the related metabolic syndrome. To the extent that exercise
causes individuals to have lower BMI than they would otherwise, adjustment
for BMI in regression models constitutes statistical overcorrection and results
in underestimation of the true beneficial effect of physical activity.
In our study, prolonged TV watching was strongly associated with risk
for diabetes. These findings do not necessarily imply that TV watching per
se causes type 2 diabetes; rather, they suggest that a sedentary lifestyle
substantially affects future risk for diabetes. There are at least 2 explanations
for the observed positive association between TV watching and diabetes risk.
First, TV watching is directly related to obesity and weight gain,11-15,33
probably due to lower energy expenditure (ie, less physical activity) and
higher caloric intake. Second, participants who spent more time watching TV
tended to eat more red meat, processed meat, snacks, refined grains, and sweets
and fewer vegetables, fruits, and whole grains. Such an eating pattern, which
is directly related to commercial advertisements and food cues appearing on
TV,34-35 may adversely affect
diabetes risk. In our previous study of 466 men in the HPFS, average hours
of TV watching was significantly associated with increased levels of leptin
and LDL cholesterol and lower levels of HDL cholesterol and apolipoprotein
A-I, independent of physical activity levels.20
Because our cohort did not undergo uniform screening for glucose intolerance,
some diabetes cases may have been undiagnosed. However, misclassification
would be expected to be small compared with that in the general population
because of health professionals' ready access to medical care. For example,
more than 85% of men in our study visited a physician for a physical examination,
sigmoidoscopy, or colonoscopy at least once between 1988 and 1990. In addition,
when the analyses were restricted to symptomatic cases of type 2 diabetes,
the findings were similar, suggesting that surveillance bias according to
activity level is unlikely. The diagnostic criteria for type 2 diabetes have
recently changed36 such that lower fasting
glucose levels (>7.0 mmol/L [>126 mg/dL]) would now be considered diabetic.
We used the criteria proposed by the National Diabetes Data Group21 because all of our cases were diagnosed before January
1996. If new criteria were used, some nondiabetic subjects would have been
classified as diabetic. However, this is unlikely to explain our results,
because inclusion of diabetics in the nondiabetic group would have attenuated
the associations we observed.
CONCLUSIONS
Our data provide further evidence that higher levels of physical activity,
including moderate-intensity exercise such as walking, are associated with
a substantial reduction in risk for diabetes. In contrast, sedentary lifestyle
indicated by prolonged TV watching is directly related to diabetes risk. Although
these findings lend further support to current guidelines37-38
that promote physical activity, they also suggest the importance of reducing
sedentary behavior in the prevention of diabetes.
AUTHOR INFORMATION
Accepted for publication October 3, 2000.
Supported by research grants CA 55075 and HL 35464 from the National
Institutes of Health, Bethesda, Md, and partly by a Research Award from the
American Diabetes Association, Alexandria, Va (Dr Hu).
Corresponding author: Frank B. Hu, MD, Department of Nutrition, Harvard
School of Public Health, 665 Huntington Ave, Boston, MA 02115 (e-mail:
frank.hu{at}channing.harvard.edu).
From the Departments of Nutrition (Drs Hu, Leitzmann, Stampfer, Willett,
and Rimm) and Epidemiology (Drs Leitzmann, Stampfer, Colditz, Willett, and
Rimm), Harvard School of Public Health, and the Channing Laboratory, Department
of Medicine, Harvard Medical School and Brigham and Women's Hospital (Drs
Stampfer, Colditz, Willett, and Rimm), Boston, Mass.
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