Back in 2014, Paul Anderson and his colleague, Ralph Bovard, set out to study the health habits of cross-country skiers by surveying those who participated in the American Birkebeiner (“the Birkie”), a cross-country ski marathon in Wisconsin.
“We wanted to understand and characterize the role of social support for exercise among Birkie skiers,” Anderson explained in an email this month. “Dr. Bovard and myself are both Birkie skiers and we have both experienced how the Birkie motivates us and others to stay fit throughout the year. Also, we wanted to document the health status in a population of people who follow recommended guidelines for exercise and diet in order to underscore the value of preventive health measures.”
Anderson and Bovard sent their survey out to nearly 24,000 race participants, getting a 23% response rate.
“When over 5,000 people take the time to answer a long survey, it shows a high level of interest in the Birkie itself and the culture surrounding it,” Anderson wrote.
In a paper recently published in BMC Research Notes (available for free/open access here), the pair and their colleagues compared the health and behavior of Birkie skiers to that of a representative swath of Americans. That data had been collected as part of the National Health and Nutrition Examination Survey and the Behavioral Risk Factor Surveillance System.
Perhaps unsurprisingly, by many metrics the Birkie participants were healthier than the “average American.” For instance, they had an average body mass index (BMI) of 23.8, vs. 26.7 for the U.S. population sample. And only 3.23% had a BMI over 30, which is said to indicate obesity, compared to 27.7 for the U.S. population sample.
BMI is a problematic index, as it is based only on height and weight. Muscle is heavy, so lean but strong people often have high BMI scores even though they have little fat on their bodies.
“BMI has upwards of a 40 percent error rate with approximately 20 percent of these being false positives and 20 percent being false negatives,” Bovard explained. “It is a 180-year-old metric created by a Belgian astronomer named Adolphe Quetelet who was trying to find a formula that would be able to calculate & predict the dimensions of the ‘average man.’”
You can calculate your BMI here to understand the measurement.
“The BMI is a metric used to evaluate populations, but it isn’t very good at characterizing individuals,” Anderson wrote in an email. “We used it because it remains the standard language for describing body habitus in the health literature.”
In other words: the research team knew that it might not be a great measure, but it’s the standard tool of the trade, so still made sense to use in order to compare the Birkie participants with other populations.
Other data from the study indicate that even as a flawed measurement, the BMI data is showing something real. The Birkie survey participants had healthier habits than the average American. (And they weren’t shy about saying so: 99.1% of respondents estimated that they were “as fit/more fit than their peers.”)
Just 3.1% of the survey participants smoked. More than 65% of them reported sleeping on average more than seven hours per night, compared to 35.3% in the the U.S. population sample. Birkie skiers reported eating more fruits and vegetables, and only one-fifth as many of them reported doing no physical activity in their leisure time compare to the the U.S. population sample – where 25.5% of Americans report this to be true.
There are four healthy lifestyle characteristics that public health professionals see as, combined, leading to a 40-80% reduction in stroke risk. They are exercising at least 150 minutes a week (2 ½ hours), not smoking, eating at least five fruits and/or vegetables a day, and keeping a BMI under 25.
It’s quite rare for Americans to hit all those marks, and in the U.S. population sample, just 3% did. But 15% of Birkie survey participants reported having all of the four healthy characteristics.
“We were trying to make a point about cardiovascular health,” Anderson wrote of this comparison. “Some researchers have suggested that too much exercise can be harmful for cardiovascular health, but in general, their conclusions are based on small studies.”
As shown by the U.S. population sample, the main heart health risks to most Americans come from too little activity, not too much. And the Birkie skiers didn’t, on average, report doing so much training that the risks would come from the other side. In fact, less than 40% of the respondents reported getting more than 2 ½ hours of exercise a week, although Anderson warned about taking that number too literally.
“We may not have asked the question in a way that people normally think about their exercise and it may have been confusing,” he wrote. “On the other hand, it may tell us that people can come out to do this race on a very modest fitness regimen. This was self-reported exercise and this number should be viewed with caution. In future studies, I would really like to measure physical activity using a wearable device and an online data repository (e.g. Garmin Connect).”
Nevertheless, the amount of exercise the Birkie participants were doing was, compared to the general public, very much in a range that should promote health and decrease risk of cardiac events.
“There may be some evidence that highly trained endurance athletes (especially males) are more likely to experience cardiac arrhythmias than those who exercise moderately,” Anderson wrote. “[But] In general, the scientific evidence continues to find that the benefits of physical activity far outweigh the risks.”
That’s in line with a recent study from Finland. There, researchers found that even after adjusting for confounding factors like total amount of physical activity, age, smoking status, and BMI, male cross-country skiers had a 16-18% reduction in risk of death for any reason after 30 years of follow-up. The original study participants had been selected in the mid-1980’s.
In the Birkie study, it’s harder to draw concrete conclusions because these confounding factors were not controlled for in any way. So when it comes to trying to disentangle causes and effects – are Birkie participants healthy because they exercise? Do they exercise because they already had low self-reported rates of physical limitations? – there are no clear answers.
Overall, the Birkie survey respondents were quite different than the general population: predominantly male, on average 48 years old, with more than 70% of survey respondents married and more than 65% employed full time, vs. 53.1% married and 29.4% employed full-time in the surveillance system data. Nearly 90% of the Birkie participants had a four-year college degree, well above the 31% rate for the U.S. population sample.
“Nordic skiing can be costly and our study participants reported higher socioeconomic status, higher levels of educational attainment, and more leisure time for training,” the authors wrote in their paper. “The Nordic origins of cross-country skiing and the trails in the United States where it can be practiced currently attract predominantly white participants although there are exceptions to this trend.”
These are factors that are correlated to many health characteristics and behaviors. As a result, it’s hard to tell if differences in health between the Birkie participants and the general population sample are due to the Birkie and cross-country skiing in particular, or are simply artifacts of the different demographic and socioeconomic characteristics of the samples.
“A cross-sectional survey of a population can’t really be used to test a hypothesis, but it can tell you where to look next,” Anderson wrote in an email. “I’d like to design a study that compares two similar groups of people who are randomly selected from the population except for one thing: one group would participate in the Birkie culture, the other group would not. We would measure and study both groups before and at the end of the study period and observe how the groups differed. Our guess (hypothesis) is that the group exposed to the Birkie culture would do slightly better at starting and sticking with an exercise regimen.”
A few studies have been published in Sweden and Finland tracking the long-term status of cross-country skiers.
“We are highly interested in creating our own longitudinal study of Nordic skiers in the U.S. to compare with these populations,” Anderson wrote. “The task is harder for us because we do not have national health registries like those in Norway, Finland, or Sweden. These registries allow researchers to mine individual medical records and hospital records to track disease occurrence over time. Nevertheless, we can do the work but we have to find other ways of getting accurate health data on a large number of people over time without losing too many in follow-up.”
The research team is planning further studies, but for now they can say that for one reason or another, the type of person who participates in the Birkie is fairly healthy compared to the average American. As Anderson had hoped, this “underscore the commonly observed coexistence of vigorous physical activity and higher levels of other beneficial health behaviors,” he and his co-authors wrote in their paper.
If public health and policy experts could find ways to help more Americans be like Birkie participants, perhaps through informal or organized exercise groups, there would be many benefits.
“Our basic point is that individuals in the U.S. have primarily been told to exercise in order to be healthy, but the importance of social support (i.e. working out with a community or group) has been less emphasized,” Anderson eplained. “This is changing, I think. I do believe that programs and events that simply expose less active folks to fitness events (5K, fun walks for a cause, the MS 150) can be potent mechanisms by which they see people having fun, enjoying health, and feeling ‘alive.’ These events can also help new participants meet others and join running groups, ski clubs, bike shop teams, close to their homes, schools, and workplaces.”
Stay tuned for Part II of this interview, which is more specifically about the role of social support in forming and keeping exercise habits.
Interested in this study? Anderson wrote, “We are also very interested in learning what Birkie skiers wonder about and doing some studies that answer their questions too. They should feel free to email me at Paul.J.Anderson@HealthPartners.com with their ideas.”