Wednesday December 4 2013
Obesity can make exercise harder
The Times has reported that the idea you can be ‘fat and fit’ is “a big fat myth”.
The term ‘fat and fit’ refers to the hypothesis that if you are obese, but all other metabolic factors such as blood pressure, are within recommended limits, then your obesity will not have a harmful effect on your health.
This new research brought together the results of 12 studies into the effects of different weight and metabolic health factors on risk of cardiovascular disease and mortality.
A key finding was that people who were obese yet metabolically healthy still had an increased risk of premature death compared to metabolically healthy people who had a recommended weight. A similar increase in risk was not seen in overweight metabolically healthy people – so it may be possible to be “a little bit fat and fit”. It could also be the case that some people in this category have a higher BMI due to muscle bulk rather than fat.
But before all you ‘skinnies’ out there start feeling smug, the study also found that people who are metabolically unhealthy regardless of their weight are also at increased risk of premature death.
So if you have a body mass index (BMI) in the recommended range (between 19 and 25) but you are smoking 40 cigarettes a day then you shouldn’t fool yourself that your health is not at risk.
This study does not change the advice to maintain a healthy weight (BMI between 19 and 25), quit smoking if you smoke, moderate your consumption of alcohol, eat healthily and take regular physical exercise to reduce the risk of many illnesses.
Where did the story come from?
The study was carried out by researchers from Mount Sinai Hospital, Toronto and was funded by the Leadership Sinai Centre for Diabetes.
The study was published in the peer-reviewed medical journal Annals of Internal Medicine.
The media reporting was reasonable, highlighting the need to be metabolically healthy even if you are of normal weight.
However, they largely ignored the finding that being metabolically healthy and overweight, so long as you are not obese, did not increase the risks. This omission is somewhat surprising, as the idea that you could be a little bit fat and fit would be welcome news to many readers expecting to put on a few pounds over the festive period.
What kind of research was this?
This was a meta-analysis of studies from 1950 to 2012 which aimed to see what effect BMI and metabolic status had on the likelihood of cardiovascular disease. A meta-analysis brings together the results of a number of similar studies to find patterns that might not be seen in small studies. A limitation of this kind of study is that it might not control for all the factors that might bias the results, in this case smoking or taking medication.
What did the research involve?
The researchers searched for articles in the database PubMed from 1950 to 2012. They assessed them for quality using the Newcastle-Ottawa Scale.
They categorised the participants’ metabolic status according to waist circumference, fasting triglyceride level, HDL cholesterol, LDL cholesterol, blood pressure and fasting blood sugar.
Then they looked at outcomes for people who were metabolically healthy and normal weight compared to people in the following five different categories:
- metabolically healthy and overweight (BMI of 25 to 29)
- metabolically healthy and obese (a BMI of 30 or above)
- metabolically unhealthy and normal weight
- metabolically unhealthy and overweight
- metabolically unhealthy and obese
Outcomes included insulin resistance (which can lead to diabetes), fatal cardiovascular events and nonfatal cardiovascular events which were defined as:
They performed statistical analyses using various combinations of the included studies, for example only looking at studies of more than 10 years duration. They also performed statistical analysis to account for differences between the studies. They calculated an overall risk score for any of these events occurring in comparison with the risk for metabolically healthy people of normal weight.
What were the basic results?
Out of 1,443 studies identified, 12 studies met the inclusion criteria, covering 72,567 people.
Compared to metabolically healthy normal weight people, the risk for all-cause mortality and/or cardiovascular events combined was:
- Metabolically healthy overweight people had a similar risk (relative risk (RR) 1.10; 95% confidence interval (CI), 0.90 to 1.24). In the studies that had at least 10 years follow up, this risk remained similar (RR, 1.21; 95%CI, 0.91 to 1.61).
- Overall metabolically healthy obese people had a similar risk (RR, 1.19; CI, 0.98 to 1.38). However, when only the studies with 10 years of follow up were analysed (therefore looking at long-term risk), the risk was significantly higher (RR, 1.24; CI, 1.02 to 1.55).
- Metabolically unhealthy people of normal weight had increased risk (RR, 3.14; CI 2.36 to 3.93).
- Metabolically unhealthy overweight people were at increased risk (RR 2.7; CI 2.08 to 3.30).
- Metabolically unhealthy obese people were at increased risk (RR 2.65; CI 2.18 to 3.12).
They found that blood pressure, waist circumference and insulin resistance increased and HDL cholesterol decreased as BMI increased in both metabolically healthy and unhealthy people.
They also added the data from two studies looking at long term follow up (10 and 11 years) and estimated an absolute risk of 0.7% for metabolically healthy obesity.
How did the researchers interpret the results?
The authors concluded that: “compared with metabolically healthy normal-weight individuals, obese persons are at increased risk for adverse long-term outcomes even in the absence of metabolic abnormalities, suggesting that there is no healthy pattern of increased weight. Considering a worldwide prevalence of approximately 200 million people with metabolically healthy obesity, the absolute risk increase of 0.7% over 10 to 11 years… translates to 1.4 million incident deaths or cardiovascular events over this time”.
This meta-analysis provides further evidence about the known risk factors for cardiovascular disease and mortality.
What this study adds is the indication that people who are metabolically unhealthy regardless of their weight are at increased risk.
However, interestingly, no increase in risk was seen for the category of people who are metabolically healthy though overweight.
A strength of this meta-analysis is the large sample size. However, the results should be interpreted with caution as:
- The studies did not use the same criteria for assessing metabolic status.
- The studies did not use the same criteria for assessing physical activity level and this was not included in the analysis.
- The analysis revealed the level of risk seen was quite different in each individual study so adding all the results together could be misleading.
- They found that this could be somewhat accounted for by length of follow up and smoking status but they did not independently look at the risk of smoking.
- The researchers point out that medication was not taken into account (such as antihypertensives or lipid-lowering medication).
- It is not known whether the BMI changed over time and what effect this may have.
While putting on a few pounds over the recommended limits did not appear to have a significant effect on health outcomes it is still important to be vigilant.
It is easy to fall into a pattern of bad habits and what was once a few pounds can grow into a few stones, putting you into the obese category.
Overall, this study does not change the advice to maintain a healthy weight (BMI between 19 and 25), stop smoking, eat healthily and take regular physical exercise to reduce the risk of many illnesses.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter.