“Fitness, not fat, determines life expectancy”, reads the headline of The Daily Telegraph . People “who are physically fit despite being obese suffer half the death rate of lean but unfit people”, the newspaper explains. Both the Telegraph and the Daily Mail report on the latest argument on the subject of Britain’s ongoing battle against obesity.
This story is based on a study of 2,603 US adults and found that higher fitness levels were associated with lower death rates in both fit normal weight and obese individuals. It does not suggest that overweight people were any healthier than similarly fit normal-weight people. A balanced diet and regular physical activity remain the best ways to promote a healthy lifestyle.
Where did the story come from?
Dr Xuemei Sui and colleagues from the universities of South Carolina, Buffalo, and North Texas, carried out this research. The study was funded by National Institutes of Health and was published in the peer-reviewed Journal of the American Medical Association.
What kind of scientific study was this?
This was a cohort study designed to look at the relationship between fitness, body fat and death rate. The researchers followed up a group of 2,603 adults aged 60 years or older (80% were male), who were enrolled in the Aerobics Centre Longitudinal Study. Between 1979 and 2001, the participants in the study were recruited and they completed a baseline physical examination and were included as members of the study if they achieved 85% or greater of their age-predicted heart rate during treadmill examination, and had a body mass index (BMI) of 18.5 or greater.
The researchers collected other measurements either through physical examination of those participating in the study or by asking about their clinical history. This included the participant’s current health and past medical problems (e.g. past heart attacks or strokes); cholesterol and blood sugar levels; blood pressure; ECG readings (heart traces); smoking and lifestyle; percentage body fat (determined from a weighing system considering measurement of fat density and skin-folds) and fat free mass (weight minus fat mass). They grouped the participants into categories of body fat (based upon BMI) and fat free mass and into categories of fitness based upon their maximum exercise performance on the treadmill.
They followed the participants from the date of their baseline examination until the end of the study in December 2003, and followed up information about any deaths. The researchers then calculated the risk of death depending on factors such as body fat (adiposity), fitness level, smoking, age, and other medical conditions at baseline.
What were the results of the study?
The average length of follow up for all participants combined was 12 years. During this period, there were 450 deaths in the sample of 2,603 people. Those who died tended to be older, with more cardiovascular risk factors, and with lower levels of fitness. There was a trend – which may have arisen by chance – towards increasing death rate with increasing BMI category, adjusted for age, sex and year of baseline examination. There was however a significant increase in death rate with high waist measurement (an indication of high abdominal obesity), when compared with normal waist circumference. There was no difference for those with a normal proportion of fat in their bodies compared to those with high fat levels. Higher death rates were also associated with abnormal ECGs at baseline and chronic medical conditions.
When they looked at fitness, the researchers found that there was a significant trend towards decreasing death rate as fitness improved. They also found that this trend for fitness was seen even after adjustment for other factors of BMI, waist circumference, body fat, general health, or smoking. The significance of waist measurement on death rate was not maintained when adjusted for fitness; however, the significance for BMI was. Those in the highest fitness groups tended to have fewer cardiovascular risk factors such as diabetes, high blood pressure, or high cholesterol.
What interpretations did the researchers draw from these results?
The researchers conclude that fitness and BMI are both predictors of all-cause mortality in elderly people aged over 60. Although other measures of body fat, such as waist measurement (abdominal obesity), were significant predictors of death rate, these effects were not maintained after adjusting for fitness. They say that their findings provide more evidence of the complex relationship between body size, fitness, and long-term survival, and conclude that “enhancing functional capacity should allow older adults to achieve a healthy lifestyle and to enjoy longer life in better health”.
What does the NHS Knowledge Service make of this study?
Age, body weight and fat, general health and fitness, and mortality are intricately related, and it is difficult to pick out one factor that has more bearing on health risk than another. This is a large and carefully conducted study in which the authors have attempted to unravel the relationships of several related factors on mortality.
- Although the study found that fitness was an independent predictor of death, and that abdominal obesity and certain other measures of body fat (fat free mass and percent body fat) were not, it did find that BMI was a predictor of mortality, with increasing BMI being associated with increasing death rate. The results of this study are therefore not in contradiction with health advice offered to the public, that a healthy diet and exercise to reduce BMI are beneficial for a healthy life.
- Although people with larger waist measurements were more likely to die during the study period, this was not independent of their fitness levels.
- The study found that higher levels of fitness were associated with lower death rates in both fit normal weight and obese individuals; it does not suggests that the more overweight people were any healthier than similarly fit, normal weight individuals. In this study, the percentage of people in the fitter categories who were of normal weight was far greater than those who were overweight or obese.
- This study has only examined those aged over the age of 60 over an average of 12 years. We cannot therefore reliably generalise these findings to younger people. The study also examined predominately white people of a high socioeconomic group, and likewise the results may not necessarily be the same in other populations.
- Women made up only 20% of the cohort, which would suggest that these observations between weight and fitness are most relevant in men.
- Due to the small number of deaths, the researchers had to consider all-cause mortality, and therefore no conclusions can be drawn about what effect fitness may have on a particular cause of death.
- The study excluded any participants who were classed as underweight, or who had not achieved their age-predicted fitness levels on exercise testing, or who were unable to complete an exercise test at all. The mortality findings may have been different if these groups had been included.
- The measure of fitness in this study – age- and gender-adjusted duration on the exercise treadmill – is only one measure of cardiovascular fitness. Joint mobility, muscle strength, and freedom from chronic aches and pains are all-important factors contributing to quality of life in ageing and were not considered by this study.
Sir Muir Gray adds...
, but another study in JAMA showed that although obesity had a smaller effect on mortality than was thought, it had a major impact on the prevalence of disability; the headline of the editorial asked “Is Disability Obesity's Price of Longevity?” Our aim is to add life to years not only years to life.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
Daily Mail, 5 December 2007
The Daily Telegraph, 5 December 2007
Links to the science
JAMA 2007; 298:2507-2516.