Many people might have set themselves the New Year’s resolution to “get fit” – but what does that really mean? Both the amount of abdominal fat and level of inflammation have been previously linked to an increased risk of metabolic diseases. The authors of a recent PLOS ONE study investigated the association between fitness and waist circumference as well as the association between fitness and low-grade inflammation, and whether there was a correlation with body mass index (BMI).
They found that low fitness is associated with a larger waist size and a higher degree of inflammation, regardless of BMI. This suggests that fitness levels, and not just BMI, should be considered when assessing a patient’s health.
I discussed the implications of this finding with lead author Anne-Sophie Wedell-Neergaard, a researcher at The Centre of Inflammation and Metabolism and the Centre for Physical Activity Research in Denmark.
What drew you to studying fitness?
AW: As a medical doctor I sometimes experience feeling powerless when patients with lifestyle-related diseases – i.e., Type 2 diabetes and cardiovascular disease –regret their way of living and express a desire to get healthier. It is frustrating that the patients have not been informed, motivated and supported to make the necessary change in lifestyle in time to prevent development of lifestyle diseases.
In particular, I had one experience some years ago when I was working at an emergency department where a heavily obese middle-aged woman was hospitalized. She was struggling with heart failure, terrified, and she desperately asked me for help. At that time I knew that I could relieve her symptoms, but she had a very poor chance of recovery. This was one reason that my interest in this field was sparked, and I wanted to investigate what patients and people in general can do to prevent development of lifestyle diseases.
You found an inverse correlation between waist circumference and fitness, regardless of BMI. Why is it risky to draw conclusions about metabolic fitness based solely on BMI?
AW: From the current literature we know that it might not be the total amount of body fat, but rather the amount of abdominal fat mass that leads to an increased risk of metabolic diseases. Thus, an overweight individual can have high cardiorespiratory fitness levels and low abdominal fat mass and therefore be at a low risk of developing lifestyle diseases, but with a low cardiorespiratory fitness levels and high abdominal fat mass there will be a high risk of developing lifestyle diseases.
Additionally, a normal BMI can lull both patients and doctors into a false sense of security. If a normal-weight individual has low cardiorespiratory fitness and a high amount of abdominal fat mass, this person also has a higher risk of developing lifestyle diseases than other individuals with the same BMI. Cardiorespiratory fitness and body fat distribution therefore need to be taken into consideration as a supplement to BMI when assessing the risk of developing diseases.
Why did you measure waist circumference for this research?
AW: The golden standard for measuring visceral fat mass is MRI, which is very accurate, but also costly and time consuming. Estimating visceral fat mass from waist circumference is a cheap and quick, but less accurate method. However, in previous studies it was found that visceral fat masses estimated from waist circumference are highly associated with measures of visceral fat mass from MRI. Since our research study was based on data from 10,976 individuals, waist circumference was chosen as a measure of abdominal fat mass to diminish time spent and economical expenses.
Was there anything that surprised you about your findings?
AW: We were very excited when we saw the outcomes of the study. The robustness and clearness of our findings especially surprised us, since all investigated associations were highly significant, independently of BMI. The results of the study are generating the hypothesis that an increase in cardiorespiratory fitness leads to a reduction in visceral fat mass and thereby a reduction in low-grade inflammation. However, if you take a closer look at our findings they suggests that cardiorespiratory fitness has an anti-inflammatory effect in addition to the effect driven by the reduction in abdominal fat mass. This needs to be investigated further in future studies.
What were the limitations for your study?
AW: The study was based on cross-sectional data, which is good for generating a hypothesis, but limiting in its ability to draw conclusions about causalities, because all data are measured simultaneously. Furthermore, the low participation rate creates a non-response bias and limits the generalizability of our findings. In addition, we also create bias by excluding “unhealthy” participants from the exercise test. Thus, the associations we found need to be confirmed in other studies in order to draw conclusions on causality and generalizability.
What implications might this study have for public health?
AW: In this study, cardiorespiratory fitness was found to be inversely associated with both abdominal adiposity and low-grade inflammation independent of BMI. These data suggest that although good health previously has been equated with a normal BMI, we need to take cardiorespiratory fitness levels into account. High fitness levels may lead to a reduction in abdominal fat mass and low-grade inflammation, within the same BMI category, and therefore play an important role in metabolic health.
Reference: Wedell-Neergaard A-S, Eriksen L, Grønbæk M, Pedersen BK, Krogh-Madsen R, Tolstrup J (2018) Low fitness is associated with abdominal adiposity and low-grade inflammation independent of BMI. PLoS ONE 13(1): e0190645. https://doi.org/10.1371/journal.pone.0190645
Image Credit: Jamie, Flickr; Anne-Sophie Wedell-Neergaard