Pro football players may be more likely to die from degenerative brain diseases and heart problems than baseball players but the reasons are unclear, a new study suggests.

The differences may seem obvious. Repeated head blows have been linked with a wasting brain disease in football players. Also, girth can contribute to heart problems, and football players are generally bigger and heavier than baseball players.

But the researchers emphasized that they lacked information on family history, genetics and lifestyle that all affect risks for specific diseases and death.

Some studies have suggested NFL players may live longer than the general population, but the researchers said comparing athletes from two elite sports provides a better perspective on risks that may be inherent to football or baseball.

They focused on 6,100 athletes born before 1965 who competed for at least five seasons in the NFL or Major League Baseball and who died between 1979 through 2013. Among NFL players, there were 517 deaths at an average age of 60. That compares with 431 deaths at age 67 on average among baseball players.

The researchers, led by Marc Weisskopf from Harvard's public health school, wrote that their results may be "limited to NFL players in the playing years considered because there have been changes in sports characteristics over time, such as helmet use, training regimen, and smoking prevalence." They said more studies are needed to determine reasons for the differences they found.

Their study was published Friday in JAMA Network Open.

Zachary Kerr, a sports injury researcher at the University of North Carolina, called the study important but said it leaves many questions unanswered, including whether young amateur athletes face similar risks. Kerr co-authored a journal editorial.

Brain diseases caused or contributed to 39 NFL deaths compared with 16 deaths among baseball players. That amounts to a nearly three times greater risk for NFL players, results that echo an earlier study comparing brain disease deaths in NFL players with the general population. Heart disease caused or contributed to 498 deaths among NFL players, more than double the 225 deaths among the baseball group.

Brain diseases in the data included Alzheimer's disease and other dementias but there was no information on chronic traumatic encephalopathy, the degenerative disease that has been found in brain autopsies of some former NFL players. Memory problems are among possible signs of the disease, and some cases may be misdiagnosed as Alzheimer's disease. Some studies have suggested that head blows also may increase risks for Alzheimer's.

FOLLOW NBC HEALTH ON TWITTER & FACEBOOK