Tuesday June 1 2010
Individuals should still aim to maintain a healthy BMI
“Accepted medical wisdom that overweight people are more susceptible to diabetes, heart disease and high blood pressure is a myth,” the Sunday Express has reported.
This story is based on a study of the relationship between body mass index (BMI), current health, age and gender. Survey data were available for about 18,000 adults whose health was assessed by looking at how many prescription medications they took at the time.
Contrary to what the news headline may suggest, these results are not sufficient to challenge our current understanding of how being overweight or obese affects our health. A person’s self-reported use of medication may not fully reflect their state of health, and this method does not assess the type or severity of an illness.
Other limitations include the fact that the study assessed weight and health at only one point in time, and, therefore, cannot estimate what the longer-term effects of being overweight or obese might be. The authors themselves note, “it is likely that an increased BMI requires time before it results in an increased medication load”.
For now, most individuals should aim to maintain a BMI within the normal range.
Where did the story come from?
The study was carried out by researchers from Brigham Young University in the USA, and was funded by the university. It was published in the peer-reviewed International Journal of Obesity.
The Sunday Express reported this study. The newspaper failed to place the findings in their proper context or report on the many limitations of this research.
What kind of research was this?
This was a cross-sectional study looking at the association between body mass index (BMI) and current health. The researchers say that, although obesity is a significant health risk, health risk is not the same as current health status. They argue that it is important to determine the relationship between BMI and current illness. The researchers also wanted to look at how age and gender affect this relationship.
Because this type of study looks at two factors (in this case BMI and health) at one point in time, it cannot prove that one factor is the direct consequence of the other. For example, a person with a high BMI and poor health may either have developed the high BMI before or after they developed their poor health. Without establishing which factor came first, it is not possible to say which factor might be influencing the other.
What did the research involve?
The researchers used data from the National Health and Nutrition Examination Surveys (NHANES) carried out in the USA in 1988-1994, and 2003-2006. These surveys had collected various data, including individuals’ prescription medication use, gender, age and BMI. This study used data from 9,071 women and 8,880 men from these surveys to investigate the relationships between these factors.
The researchers used mathematical methods to make the samples they were assessing more representative of the US population as a whole (for example, in terms of age and gender).
NHANES staff had measured participants’ height and weight in order to calculate their BMI. People who were underweight (defined for this study as having a BMI of less than 19.5) were excluded from the analysis. Normal weight was defined as a BMI of 19.5 to 24.99, overweight as a BMI between 25 and 29.99, and obese as a BMI over 30.0. Adults aged 25 to 70 were included in the analysis for the current study, and were divided into three age groups: 25-39, 40-54 and 55-70 years.
Medication use was taken to be an indicator (proxy) of current health status. The researchers used two broad approaches to calculate this, the first of which classified people as either taking prescription medications or not, and the second analysing the total number of medications taken. The researchers only wanted to look at non-psychiatric medication, so they excluded any data on medications taken for mental illnesses (for example stimulants, anxiolytics, antidepressants, cholinesterase inhibitors, mood stabilisers, anticholinergic and antipsychotic medications).
Medication usage was compared between people of different weight categories, by age group and gender.
What were the basic results?
Overweight people generally did not take more medications than normal-weight people across the age groups and genders. Obese people aged 40 or over did take more medications than similarly aged normal-weight people, but this increase was much smaller in the 25-39 age group. Women took more medicines than men, but this difference was reduced in the 55-70 age group.
How did the researchers interpret the results?
The researchers conclude that “although obesity does not substantially affect current health in young people, it is likely that the increased medication loads in obese compared with normal-weight older people originates at least in part from an increased BMI starting at a younger age”.
They say that age, sex and onset of high BMI “all require consideration when using BMI to assess current health status”.
This study aimed to investigate the relationship between BMI, age, gender and current health. The results appear to suggest that, in younger people, a higher BMI may not be associated with significantly worse current health, as indicated by prescription medication use. However, this does not mean that a higher BMI does not affect future health and that “obesity ills are a ‘myth’”. The authors themselves note that “it is likely that an increased BMI requires time before it results in an increased medication load”.
The study has several other limitations:
- Because this type of study looks at two factors (in this case BMI and health) at one point in time, it cannot prove that one factor is the direct consequence of the other. For example, a person with a high BMI and poor health may have developed the high BMI after they developed their poor health, rather than the other way around. Without establishing which came first, it is not possible to say which factor might be influencing the other.
- Medication use was taken as an indicator (proxy) of current health. A person’s medication use may not fully capture their health status, for example, an individual may have undiagnosed illnesses for which they are not taking medication. In addition, this method does not assess type or severity of illness.
- Medication use was reported by participants, which may result in inaccuracies. However, the interviewers did ask to see medication containers to verify patient responses.
- Although the study looked at three factors that could affect current health (BMI, gender and age) there are many other factors that can affect health, such as socio-economic status and physical activity levels. These factors were not taken into account in the analyses and could be influencing the results.
These results are not sufficient to challenge our current understanding of the negative effects of being overweight or obese on our health. The finding that obese people over the age of 40 had significantly larger medication loads than normal-weight people conforms to the widely accepted theory that obesity is an important risk factor for ill health.