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Background Official guidelines from the Centers for Disease Control and Prevention assert that the majority of health benefits from physical activity are obtained by walking 2 miles (3.2 km) briskly most days of the week (the energy equivalent of running 8 to 12 km per week). The objective of our study was to examine the dose–response relation in women between risk factors for coronary heart disease, particularly the concentration of high-density lipoprotein (HDL) cholesterol, and vigorous exercise at levels that exceed the official guidelines.
Methods The number of kilometers run per week reported by 1837 female recreational runners in a national cross-sectional survey was compared with medical data provided by the women's physicians.
Results In these cross-sectional data, plasma HDL cholesterol concentrations were higher by an average (±SE) of 0.133±0.020 mg per deciliter (0.003±0.0005 mmol per liter) for every additional kilometer run per week, an amount nearly identical with that previously reported for men (0.136±0.006 mg per deciliter [0.004±0.0002 mmol per liter] per kilometer per week). Among women who ran less than 48 km per week, mean plasma HDL concentrations were significantly higher with each 16-km increment in distance. Women who ran more than 64 km per week had significantly higher mean concentrations of HDL cholesterol than did women who ran less than 48 km per week. They were also significantly more likely to have HDL cholesterol concentrations greater than 100, 90, or 80 mg per deciliter (2.6, 2.3, or 2.1 mmol per liter) than were women running less than 64 km per week. HDL cholesterol concentrations increased significantly in relation to the number of kilometers run per week in premenopausal women who were not using oral contraceptives and in postmenopausal women, whether they were receiving estrogen-replacement therapy or not.
Conclusions Substantial increases in HDL cholesterol concentrations were found in women who exercised at levels exceeding current guidelines; higher HDL cholesterol concentrations could provide added health benefits to these women.
Whether these findings in men also apply to women is unknown. Heart disease is less likely to develop in women than in men,7 and women have different levels of risk factors for heart disease8 and possibly different physiologic responses to physical activity.9,10,11 Physical activity appears to reduce the risk of coronary heart disease in women as well as in men, however.12,13 The reduction may be due in part to higher levels of HDL cholesterol. In women as in men, plasma HDL cholesterol concentrations appear to increase with physical activity14,15,16,17,18,19,20,21,22,23 and to prevent coronary heart disease.24,25,26 However, little is known about the dose–response relation between the level of exercise and HDL cholesterol in women or about the way menstrual status and hormone use may affect this relation.9,10,27,28 We therefore examined the dose–response relation between the reported distance run per week and risk factors for coronary heart disease in 1837 female runners who participated in the National Runners' Health Study. The specific aim of this study was to determine whether health benefits accrue in women at levels of exercise that exceed current minimal guidelines.
Methods
Using a two-page questionnaire, distributed nationally at races and to subscribers of a major running magazine (Runner's World, Rodale Press, Emmaus, Pa.), we solicited information on demographic characteristics (age, race, and education); running history (age when the participant began running at least 12 miles [19.2 km] per week, average weekly distance run and number of marathons run over the preceding five years, best marathon and 10-km running times); weight history (highest and current weight; weight when the participant started running; lowest weight as a runner; circumferences of the chest, waist, and hips; and bra-cup size); menstrual history (whether currently having periods and age at menarche); hormone use (birth-control pills, postmenopausal estrogen replacement, or progesterone); diet (whether vegetarian; weekly intake of alcohol, red meat, fish, fruit, vitamin C, vitamin E, and aspirin); current and past cigarette use; history of heart attack and cancer; and medications taken to treat high blood pressure, thyroid conditions, high cholesterol levels, or diabetes. The questionnaire also requested permission to obtain the participants' height, weight, plasma cholesterol and triglyceride concentrations, blood pressure, and heart rate at rest from their physicians. Consent to participate in the study was obtained from all study subjects, and the study design was approved by the committee for the protection of human subjects.
The average distance run per week was computed by averaging the reported weekly distance over the preceding five years. The amount of alcohol consumed per week was calculated by multiplying the number of various drinks consumed by 0.48 oz (14 ml) per 12-oz (350-ml) bottle of beer, 0.48 oz per 4-oz (120-ml) glass of wine, and 0.60 oz (18 ml) per drink of hard liquor.29 The body-mass index was calculated as the weight in kilograms divided by the square of the height in meters. Bra-cup sizes were coded on a 4-point scale: 1 (A cup), 2 (B cup), 3 (C cup), or 4 (D cup or larger). Bra size was included because it is a component of chest circumference that has been found to be related to the HDL cholesterol concentration.30 Data on HDL cholesterol were obtained from the medical records of 2667 female runners. We excluded 151 runners who had histories of cancer or heart attacks, 52 who smoked, 400 who followed vegetarian diets, and 227 who were taking medications that could potentially affect the concentrations of plasma lipoproteins. This left 1837 nonvegetarian, nonsmoking runners who had no history of heart disease or cancer and who were not using medications that might affect lipoprotein concentrations.
Multiple regression analyses were used to test for overall linear relations between the distance run and risk-factor levels and to assess the significance of these relations after adjustment for other variables.31 Two-sample t-tests were used to compare mean risk-factor levels among the distance categories.31 We tested linear trends for proportions using the estimated trends and standard errors for linear contrasts among the five distance categories.31
Results
Table 1 shows that women who ran greater weekly distances tended to adopt types of behavior reflecting greater health consciousness: they consumed fewer weekly servings of beef, lamb, or pork and had higher weekly intakes of fruit and vitamin E. Age, education, and intake of fish, alcohol, aspirin, and vitamin C were all unrelated to running distance. As expected, the runners in the longer-distance categories were more likely to have participated in at least one marathon or 10-km race over the preceding five years and to have completed those races more quickly. Longer distances run were also associated with lower heart rates at rest and more years spent running at a level of 12 miles or more per week. There were 447 women (24.3 percent) who reported no longer having menstrual periods, 176 women (9.6 percent) who reported receiving postmenopausal estrogen-replacement therapy, 73 women (4.0 percent) who reported taking progesterone, and 236 women (12.8 percent) who reported using oral contraceptives. Menstrual status and hormone use were unrelated to distance run per week (Table 1).
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= 0.133±0.020 mg per deciliter [0.003±0.0005 mmol per liter] for each additional kilometer per week). As Table 2 shows, mean plasma concentrations of HDL cholesterol were significantly higher with each 16-km increment in the weekly running distance up to 48 km run per week and at or above 64 km per week. The higher HDL cholesterol concentration could not be attributed to age; education; menstrual status; intake of alcohol, aspirin, red meat, fish, fruit, or vitamin C or E; or use of progesterone, estrogen, or birth-control pills (i.e., the regression slope was = 0.128±0.019 mg per deciliter [0.003±0.0005 mmol per liter] per kilometer per week after adjustment for these variables). Figure 1 shows that the HDL cholesterol concentration was increased with greater running distances in premenopausal women who were not using oral contraceptives ( = 0.140±0.024 mg per deciliter [0.004±0.0006 mmol per liter] per kilometer per week), in postmenopausal women who were not taking estrogen ( = 0.136±0.040 mg per deciliter [0.004±0.001 mmol per liter] per kilometer per week), and in postmenopausal women taking estrogen ( = 0.148±0.066 mg per deciliter [0.004±0.002 mmol per liter] per kilometer per week) after adjustment for the variables indicated above. Among women using birth-control pills, the regression slope for HDL cholesterol plotted against distance run ( = 0.080±0.050 mg per deciliter [0.002±0.001 mmol per liter] per kilometer per week) was neither significantly greater than zero (P = 0.11) nor significantly less than the regression slope for women not using birth-control pills (P = 0.27).
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= -0.005±0.001 per kilometer per week), systolic blood pressure ( = -0.06±0.02 mm Hg per kilometer per week), and diastolic blood pressure ( = -0.028±0.013 mm Hg per kilometer per week). All three variables remained significantly related to distance after we adjusted for the women's age, education, menstrual status, hormone use, and diet. Levels of neither low-density lipoprotein cholesterol ( = -0.03±0.04 mg per deciliter [0.0008±0.001 mmol per liter] per kilometer per week) nor triglycerides ( = -0.049±0.074 mg per deciliter [0.0006±0.0008 mmol per liter] per kilometer per week) showed any relation to distance run by the women. Adiposity, as measured by the body-mass index, decreased significantly with longer distance ( = -0.036±0.003 per kilometer per week); the circumferences of the waist ( = -0.091±0.009 cm per kilometer per week), hip ( = -0.097±0.009 cm per kilometer per week), and chest ( = -0.052±0.007 cm per kilometer per week); and bra cup ( = -0.005±0.001 cup size per kilometer per week).
The higher HDL cholesterol concentrations and the lower values for pulse rate, blood pressure, body-mass index, and body circumferences in the women who ran longer distances could not be attributed to a choice by the initially leaner women to run farther (i.e., self-selection based on adiposity). As part of the questionnaire, runners reported their initial weights and body-circumference values when they started running. We used these to calculate the change in runners' body-mass-index and body-circumference values since they started to run (Table 2). Longer weekly distance was associated with greater reductions in body-mass index ( = -0.014±0.003 per kilometer per week) and the circumferences of the waist ( = -0.031±0.008 cm per kilometer per week), hip ( = -0.030±0.008 cm per kilometer per week), and chest ( = -0.014±0.006 cm per kilometer per week) since running was begun. When adjusted for the initial body-mass index, running greater weekly distances continued to be associated with an increase of 0.134±0.021 mg per deciliter (0.003±0.0005 mmol per liter) in the HDL cholesterol concentration per kilometer run and decreases of 0.004±0.001 in the ratio of total cholesterol to HDL cholesterol and of 0.04±0.02 mm Hg in systolic blood pressure per kilometer run. Similar values were obtained when the initial circumferences of the waist, hip, and chest were used for adjustment.
Discussion
The number of female runners that we studied was large and adds substantially to the total number in all previously published cross-sectional studies of running and lipoproteins in women.32 Because of the large sample, this study had the statistical power to test for incremental increases in HDL cholesterol concentrations at levels of physical activity that exceed the current recommendations of the CDC. Mean plasma HDL cholesterol concentrations increased significantly with each 16-km increment in weekly running distance up to 48 km per week and at or above 64 km per week. Running at least 64 km per week was also associated with significantly lower body-mass-index values and waist and hip circumferences than running fewer kilometers. These data suggest that additional health benefits accrue to women who run longer distances (up to 64 km [40 miles] per week). The difference of 9.6 mg per deciliter (0.25 mmol per liter) in the mean HDL cholesterol concentration between the groups running the shortest distances (0 to 15.9 km per week) and the longest (>64 km per week) may represent a 29 percent reduction in the risk of coronary heart disease and a 45 percent reduction in mortality from cardiovascular disease.33
Our results appear to contradict the assertion in the CDC's guidelines that there is little additional benefit to increasing physical activity beyond the level attained by walking 2 miles (3.2 km) briskly most days of the week.1 However, a meta-analysis of the studies cited in the CDC consensus statement has shown a linear decline in morbidity and mortality due to cardiovascular disease with increasing levels of leisure-time physical activity.3 These analyses provide no evidence that the benefits of increasing activity diminish at higher levels of activity. Extrapolating the results of the meta-analysis to long-distance runners is problematic, however, since few of the studies cited by the CDC involved very active men or women.
Both premenopausal and postmenopausal female runners would be expected to have higher concentrations of HDL cholesterol than their sedentary counterparts, since previous cross-sectional studies have reported significant effects of exercise in both groups of women.15,16,17,18,19,20,22,34 However, it is unclear from previous studies whether the magnitude of the effect of exercise is the same in the two groups.9,10 In our study, menstrual status did not affect the increases in HDL cholesterol induced by exercise. Postmenopausal estrogen replacement also did not reduce the magnitude of the increase in the mean HDL cholesterol concentration with each additional kilometer run. It is unclear whether the concentrations of HDL cholesterol in women using oral contraceptives increased as their weekly running distances increased. Data from other studies suggest, however, that exercisers who use oral contraceptives may have smaller increases in HDL cholesterol than those who do not.27
The National Runners' Health Study also included 8290 male runners, whose risk factors for coronary heart disease have been reported separately.6 The increase for women in HDL cholesterol per kilometer run is nearly identical to that reported for men (0.136±0.006 mg per deciliter [0.004±0.0002 mmol per liter] per kilometer per week). For each additional kilometer run, the decreases in systolic blood pressure, diastolic blood pressure, and body-mass index in men and women were also similar. As compared with men, women runners had significantly smaller decreases per kilometer run per week in the plasma concentrations of low-density lipoprotein cholesterol and triglycerides and in the ratio of total cholesterol to HDL cholesterol.
Our data contradict the idea that the generally higher HDL cholesterol concentrations in women (as compared with men) limit the potential for these levels to increase with exercise and that this ceiling effect accounts for the smaller increases in HDL cholesterol concentrations in women who exercise than in men.35 In the current study, the increase in HDL cholesterol per kilometer run in women was almost identical to that in the men, despite their higher mean concentration of HDL cholesterol (63.6±0.4 mg per deciliter [1.64±0.01 mmol per liter], vs. 51.8±0.2 mg per deciliter [1.34±0.005 mmol per liter] in the men). Moreover, the data suggest that running increases HDL cholesterol throughout the upper range of the HDL cholesterol distribution. Women who ran 64 or more kilometers per week were significantly more likely to have HDL cholesterol concentrations above 100, 90, and 80 mg per deciliter than women who ran less. Moreover, the percentage of women with HDL cholesterol concentrations above these levels increased significantly as weekly running distance increased. The lack of a ceiling effect in the response of HDL cholesterol to exercise was also suggested in a previous clinical trial in men.36
The cross-sectional associations of this report do not prove that running greater distances causes these reductions in risk factors. Well-designed randomized, controlled intervention studies have proved causality in men, but the data are less conclusive in women.35 We controlled for the body-mass index when the women started running in order to eliminate the possibility that leaner women who had more favorable lipoprotein profiles may have chosen to run farther (i.e., a self-selection bias with respect to adiposity). When adjusted for base-line body-mass index, the associations of distance with the HDL cholesterol concentration, the ratio of total cholesterol to HDL cholesterol, blood pressure, and measures of adiposity all remained significant. It is possible that women with initially high levels of HDL cholesterol chose to run greater distances (self-selection due to high HDL cholesterol concentrations). Two separate studies have shown that base-line HDL cholesterol concentrations in sedentary men predict their weekly running distance during exercise training.36,37 Higher HDL cholesterol concentrations at base line may be a marker for men genetically endowed with types of muscle fiber that make running easier.36,37,38
The analyses reported here, in which we used measurements of risk factors for coronary heart disease that were supplied by the women's physicians, show detectable improvements in risk factors as commonly measured in medical practice. Thus, the associations between the distance run and risk factors for coronary heart disease are valid even when based on clinical measurements, which would be expected to be less precise than measurements made under research conditions. The added measurement error increases the likelihood of a false negative result, rather than a false positive one, and therefore yields conservative results.39
Because less than 27 percent of women in this country meet the current CDC guidelines for physical activity,40 increasing physical activity on the part of women is an important goal. It is unfortunate that the CDC recommendations have been widely interpreted to mean that there are few or no additional benefits to increasing vigorous exercise beyond the recommended levels. The findings of this study suggest that women obtain additional benefits at higher levels of exercise than currently recommended, benefits that accrue regardless of menstrual status or the use of postmenopausal estrogen replacement.
Supported in part by grants (HL-45652 and HL-55640) from the National Heart, Lung, and Blood Institute and a grant (DE-AC03-76SF00098) from the Department of Energy at the University of California.
I am indebted to Davina Moussa, who provided much technical assistance, and to Amby Burfoot and Vern Walther of Runner's World magazine for their assistance in contacting many of the runners who participated in this study.
Source Information
From the Life Sciences Division, Lawrence Berkeley National Laboratory, Bldg. 934, Berkeley, CA 94720, where reprint requests should be addressed to Dr. Williams.
References
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indicates P<0.05 for the comparison with the 0-to-15.9-km category; § indicates P<0.05 for the comparisons with the 0-to-15.9-km and 16-to-31.9-km categories; ¶ indicates P<0.05 for the comparisons with the 0-to-15.9-km, 16-to-31.9-km, and 32-to-47.9-km categories; * indicates P<0.05 for the comparison with the 16-to-31.9-km category only; and 
indicates P<0.05 for the comparisons with the 0-to-15.9-km and 32-to-47.9-km categories. To convert values for cholesterol to millimoles per liter, multiply by 0.02586. To convert kilometers to miles, multiply by 0.6214.