Fitness, the new age health prescription, has now been linked to an increased life span. New research suggests that maintaining or improving one's fitness levels could enhance longevity. This is possible even if there is no significant change in body weight.  While previous studies have linked weight gain and obesity to a higher mortality risk, the focus has usually been on Body Mass Index (BMI)  

The new study, however, suggests that maintaining or improving fitness was associated with a longer life expectancy, even after controlling for BMI change.

This is good news for people who are physically active but can't seem to lose weight, said Duck-chul Lee, the lead researcher and a physical activity epidemiologist at the Department of Exercise Science at the University of South Carolina's Arnold School of Public Health in Columbia.

You can worry less about your weight as long as you continue to maintain or increase your fitness levels, he added.

The study explained that every unit of increased fitness - Metabolic Equivalent of Task (MET) - over six years, was associated with a 19 percent lower risk of heart disease and stroke-related deaths. However, the researchers also found no association between changes in body fat percentage or body weight and death risk.

In addition, men who remained physically fit in their 40s (and maintained that level of fitness for a decade) increased their chances of avoiding a number of medical complications by 30 percent, in comparison to men who were slack at 40, according to the results of a long-term prospective study.

Lee and his colleagues, however, highlighted the need for studies that measured changes in fitness and BMI over time, as these could have a bearing on mortality rates. He also noted that such studies were, unfortunately, uncommon.

It is important to study the combined effects of fitness and BMI on mortality as both clinical indicators impact the development of health recommendations and policies, the researchers wrote, adding that the impact of fitness on obesity (and vice versa) in terms of mortality was unclear and complicated and continuously debated.

The researchers examined independent and combined associations of changes in fitness and BMI with all-cause and examined Cardio Vascular Disease (CVD) mortality in a cohort of 14,345 men from the Aerobics Center Longitudinal Study. The participants were adults, mostly white and middle or upper-class. The researchers determined factors over 11 years of follow-up and the relative risks among men who lost, maintained or gained fitness over six years.

The values for BMI were collected by maximal treadmill tests to estimate physical fitness (maximal METs), as well as height and weight measurements; changes in BMI and physical fitness levels were recorded over a six-year period.

In addition, factors such changes in BMI, age, family history, initial fitness levels, lifestyle factors like smoking and prior medical conditions were also accounted for. The researchers found that 914 people died from all-causes, while 330 died from CVD. Many baseline characteristics differed, significantly, between those who died and those who survived past the six-year period.

To begin with, those who died were older than the survivors (50.7 versus 43.2), had less METs (with regard to cardio-respiratory fitness - 10.5 versus 12), had higher systolic and diastolic blood pressures (126/83 versus 121/81), higher fasting glucose levels, higher total cholesterol, a higher percentage of smokers and a higher percentage of having a parent with CVD.

Compared with those who lost fitness, those who remained fit had a Hazard Ratio (HR) for all-cause death of 0.70 in the fully adjusted model and also in the model variously adjusted for BMI and MET changes. For CVD death, the HRs was 0.72 and 0.73, respectively, for the two models.

Those who gained fitness over the course of time had a greater reduction: HR 0.60 and 0.61 for the two models in all-cause death and 0.56 and 0.58 for both models in CVD death.

One possible explanation for these results could be that about 90 percent of the men were either normal weight or overweight at the beginning of the study. Among obese people, changes in BMI might have a significant effect on death risks. Therefore, it remains unclear as to whether these results would apply to severely obese people, Lee explained.

On a final note, because the study was mostly done with white middle and upper-class men, it would be difficult to extrapolate the results to other racial and socioeconomic groups. However, Lee hazarded that women would likely have similar results as the men in the study.