Monday February 15 2010
The body mass index (BMI) is not a perfect measure of body fat
“Being fat can help you live longer in old age,” the Daily Express reported. It said the death rate for people aged 70 to 75 years is lowest for those classed as overweight, while those who are obese have the same risk as people of ‘normal’ weight. According to the paper, being underweight is linked with the highest risk of death.
This is a well-conducted study, reported accurately by the newspapers, but it does have some limitations, which the researchers have highlighted. In addition, the body mass index (BMI) itself is not a perfect measure of body fat and is not sensitive to the age-related changes in body fat distribution.
It is difficult to interpret these findings for everyday life. However, the study highlights an issue that will require further consideration and research, particularly in light of other criticisms of the BMI measure. It is worth highlighting that, regardless of BMI, both men and women who were more active were less likely to die than their sedentary counterparts.
Where did the story come from?
The research was carried out by Dr Leon Flicker and colleagues from the Western Australia Centre for Health and Ageing and other academic and medical institutes across Australia. The study was funded by the National Health and Medical Research Council of Australia and the Australian government. The paper was published in the peer-reviewed Journal of the American Geriatrics Society.
What kind of research was this?
This cohort study investigated survival rates and BMI in Australian adults aged between 70 and 75 years.
The researchers looked at overall deaths and cause-specific deaths (cardiovascular disease, cancer, chronic respiratory disease) in the group using standard cohort methodology. This is a reasonable method of looking for associations when randomising people to an exposure would not be possible or ethical.
What did the research involve?
The researchers say previous studies have concluded that BMI in the overweight range is not a risk factor for all-cause mortality (death by any cause) in older people. However, they acknow- ledge that methodological differences between the studies limit their comparability. In this study, they wanted to find the BMI associated with the lowest mortality risk in older people and to see whether this differed between men and women.
The participants were obtained from two previous studies in Australia: the Health in Men Study (HIMS), and the Australian Longitudinal Study of Women’s Health (ALSWH). The HIMS, which began in 1996, is a randomised controlled trial of men aged 65 to 79 in Perth and is investigating screening for abdominal aortic aneurysm. The ALSWH is a longitudinal study following three groups of women (young, middle aged and older) across key stages of their lives, collecting information on determinants of health, health outcomes and service use.
For this study, the women in the oldest cohort (70 to 75 years) were invited to participate. From the HIMS and ALSWH studies, the researchers chose to include the most comparable groups of men and women. This resulted in 4,031 men aged 70 to 75 at baseline (when they started the study) from the HIMS and 5,042 women aged 70 to 75 from metropolitan and urban areas from the ALSWH.
Both the HIMS and the ALSWH had collected information on height and weight as well as demographic (age, education, marital status), lifestyle (smoking, alcohol, exercise) and health details. Participants were followed for 10 years or until their death (whichever was sooner). Date and causes of death were obtained from the Australian Bureau of Statistics and grouped into three main categories: cancer, cardiovascular disease and chronic respiratory disease.
Cox’s regression analysis (a statistical method of survival analysis) was used to estimate the association between survival time from entry into the study to the date of death or end of follow-up (31 December 2005). This method is needed to account for the people who would still be alive at the end of the study (i.e. would not be followed up until their date of death). It also adjusts for factors that could confound a relationship between BMI and survival, for example the lifestyle and demographic factors that have known associations with mortality.
What were the basic results?
During a mean (average) follow-up of 8.1 years for men and 9.6 years for women, 1,369 and 939 deaths occurred respectively. For both men and women the risk of death was lowest for those classified as overweight according to their BMI measure when they started the study. Smoking was a moderate confounder, so the analyses were adjusted for smoking. There was also an association between being sedentary and gender. Women who were sedentary were twice as likely to die during follow-up as those who exercised, while men who were sedentary were only 28% more likely to die. Because of this, the results were presented for sedentary and active individuals.
Overall, underweight people were more likely to die (1.76 times) than those of normal weight, while overweight people were less likely to die (0.87 times). There was no difference in death rates between those who were obese at baseline and those of normal weight. However, there was a greater risk of mortality with extreme obesity. Men and women in the non-sedentary group were less likely to die than their sedentary counterparts, regardless of BMI.
The lowest risk of all-cause mortality was consistently seen in those classified as overweight. When assessing the associations with cause-specific mortality (cancer, cardiovascular disease and respiratory disease), a similar pattern was seen with the lowest risks in men classified as overweight at baseline.
How did the researchers interpret the results?
The researchers conclude that their results support the claims by other studies that "the BMI thresholds for overweight and obese are overly restrictive for older people". They say that overweight older people are not at greater mortality risk than those who are normal weight.
This large cohort study concludes that being overweight (according to World Health Organization [WHO] BMI thresholds) is associated with reduced mortality compared to normal BMI. Gender does not alter this relationship. The effect of being sedentary was different for men and women, with the protective effect of exercise being greater in women. The study is large and of good quality. In addition, the researchers highlight the potential weaknesses that are largely unavoidable in a study of this design:
- They acknowledge that reverse causality is a problem with cohort studies, in that it is difficult to tease apart the complex relationships between health and BMI and how this affects mortality. Older people who become ill may lose weight before they die in which case, it is the illness that is linked to the death, not the weight loss. However, the researchers say they tried to control for this by comparing subjects who were relatively healthy with those who had chronic diseases or who smoked. They did not find a large effect on the link between BMI and mortality.
- They note that height and weight were collected only at one point (at study entry). It is unlikely that people had the same weight throughout the entire study and this would not have been captured through this methodology.
- They add that BMI itself is not a perfect measure of body fat and it is age- and sex-dependent. It is also not sensitive to the age-related changes in body fat distribution.
- Importantly, the researchers note the death rate in these cohorts was lower than expected in this age group. This is likely to be because people who do not respond may be doing so because of ill health. They say that the results here may not apply to older, frail people at risk of death.
This large cohort study has confirmed the results of previous research, and the researchers say that according to the BMI thresholds set by the WHO, older people considered to be ‘overweight’ are not at a greater mortality risk.