“Obese children ‘more likely to have heart attack or stroke in adulthood',” warns The Guardian.
The news is based on a large review from researchers at Oxford University who collated a number of smaller studies carried out over the past few decades, involving a total of 49,220 children, that were looking at whether childhood obesity could increase cardiovascular risk factors (factors that can increase the risk of heart disease, heart attacks and strokes).
They were shocked to find that many obese children did have risk factors you would normally expect to see in much older adults, such as high blood pressure and cholesterol, high fasting insulin levels (which are often a precursor to type 2 diabetes) and thickened heart muscles (a sign of damage to the heart).
A related editorial, which, like this study, was published in the British Medical Journal, said that this review “provides a stark illustration of the probable threat that childhood obesity poses to disease burden in the population”. A fact reinforced by the recent publication of a report into children in England that found that a fifth of year six children (aged around 11-12) were found to be obese.
The editorial argued that “opportunistic measurement of BMI (body mass index) and co-morbidities may be a useful first step in helping families move towards tackling childhood obesity”.
Where did the story come from?
This review was carried out by researchers from the University of Oxford. The research authors report the review received no specific funding. The study was published in the peer-reviewed British Medical Journal.
The story was reported fairly accurately by the media, but a number of clarifications need to be made.
The Guardian stated that obese children ‘have a 30%-40% higher chance of either suffering a stroke or developing heart disease’, but these types of outcomes were never studied by the research. The 30%-40% figure was in fact an extrapolation, carried out by the researchers, based on previous studies looking at cardiovascular risk factor. Having said that, the figure cited is likely to be reasonably accurate.
The Independent used the headline ‘calls for GPs to measure BMI’. The study itself made no such recommendation (although it did recommend that further research into cardiovascular risks in obese children was warranted). The recommendation regarding GPs was actually made in the editorial accompanying the review. The two authors called for opportunistic measurement of BMI and co-morbidities (such as routine GP appointments) in an attempt to tackle childhood obesity.
What kind of research was this?
These are both well-established methods of pulling together information from a range of previous studies.
However, the strength of the conclusions of the research are dependent on the quality and homogeneity (sameness) of the studies that these methods pool together.
What did the research involve?
The researchers reviewed studies examining an objective measure of weight and at least one or more of the following CVD risk measures:
- systolic blood pressure – the pressure of the blood when the heart beats to pump blood out
- diastolic blood pressure – the pressure of the blood when the heart rests in between beats
- HDL (good) or LDL (bad) cholesterol
- total cholesterol
- triglyceride (fats)
- fasting glucose, fasting insulin and insulin resistance levels – abnormalities in levels are often the first sign of the onset of metabolic conditions such as type 2 diabetes
- thickness of the walls of arteries in the neck (a measure of hardening of the arteries)
- left ventricular mass (a thickening of the heart muscle)
They only included studies carried out on healthy children aged between five and 15 years enrolled after 1990 and the researchers limited the search to only include studies that were carried out in highly developed countries and published between 2000 and 2011.
Studies that were carried out in schools or outpatient and community settings were only included if they had the following study designs:
- randomised controlled trial (RCT)
- case-control study
- prospective or retrospective cohort
- cross sectional
Studies were excluded if they included children diagnosed with another chronic physical or mental medical condition or another condition associated with overweight (such as asthma or sleep apnoea). Studies in inpatient settings or that used pharmacological treatment were also excluded.
The quality of the trials was assessed by the researchers (using a ‘risk of bias’ tool) and results were pooled for studies that reported at least one unhealthy category of BMI as well as the normal BMI category.
Overweight was defined as a BMI of 25 to 30 and obesity was defined as BMI of 30 or more, which are the internationally agreed definitions.
What were the basic results?
The researchers identified 63 relevant studies that included 49,220 children across 23 countries. Of the 63 included studies, only 24 had data suitable for the meta-analysis. The main findings from this study were:
- compared with normal weight children, systolic blood pressure was higher by 4.54mm Hg (99% confidence interval 2.44 to 6.64) in overweight children and by 7.49mm Hg (99% confidence interval 3.36 to 11.62) in obese children. Similar associations were also found between groups for diastolic blood pressure
- obesity was found to negatively affect the concentrations of all blood lipids (total cholesterol, triglycerides)
- fasting insulin and insulin resistance were significantly higher in obese children but not in overweight children
- obese children had a significant increase in left ventricular mass (often used as a marker for heart disease) compared with normal weight children
How did the researchers interpret the results?
The researchers conclude that having a BMI outside the normal range significantly worsens cardiovascular risk measures in school-aged children. This effect, that is already substantial in overweight children, increases further in obesity and could be larger than previously thought.
The researchers say there is a need to establish whether it is possible to produce an acceptable and standardised ‘cardiovascular risk estimation tool’ based on the factors discussed above. This could then be used to assess whether an individual child’s risk warranted medical intervention.
Overall, this study has been well conducted and provides evidence to support the growing literature that overweight and obese children have raised risk parameters for cardiovascular disease compared with normal weight children. The findings are important as there is a focus on the immediate health risks of children that are obese or overweight.
The researchers note some limitations to their review including:
- despite the authors’ efforts to have children that were as similar as possible, there were still some factors that varied between the children (such as ethnicity, pubertal status and age)
- the influence of age and pubertal status were not accounted for as too few papers reported this data, therefore it is unclear if these factors influenced the results and were underlying the association
- there was a high level of variation between some of the studies for some risk measures, which makes pooling of results difficult
It is important to note that this research does not provide a causal link, though one may exist, only that there is an association in observational studies.