News release

From: Oxford University Press

Study suggests late teens and early 20s, rather than childhood, is critical period for intervention in body fat levels to stop harmful changes in heart structure and future heart disease

· Analysis using DXA scans suggests that while across ages 9-24 years overall body fat and abdominal fat is protective, age 17 appears to be a crossover age where this fat becomes harmful

· Author warns extreme caution should be used in reducing BMI in children simply because they are overweight, unless it is confirmed that their excess weight is due to fat mass, to avoid an unpleasant consequence of low muscle mass-induced poor heart growth

New research published in The European Journal of Endocrinology shows that increased total body fat mass and abdominal fat mass from late adolescence to early 20s, but not from the childhood period before this, predicts progressively worsening cardiac structural and functional outcomes by young adulthood.

Author Andrew Agbaje, physician and associate professor of clinical epidemiology and child health at the University of Eastern Finland, Kuopio, Finland, says the study results suggest that while across the whole age period 9-24 years overall body fat and abdominal fat is protective for the growing heart, age 17 appears to be a crossover age where this increased fat becomes harmful to heart health.

Professor Agbaje says: “The prevention of obesity and excess body fat in childhood is of course important for child health - but if missed, adolescence, and more specifically the period from around 17 years, could be the golden period to target with healthy lifestyle interventions to lower body fat and avert potentially harmful changes to heart structure, and increase the chances of healthy adulthood.”

He adds: “Age 17 years is very critical for young people, as it is also the time when adolescents can start to become independent from parents, often moving away from home, which changes their diet and lifestyle habits significantly – most frequently in a bad direction. This social change may significantly increase fat mass accumulation and thus pose future heart risk.”

Previous studies among children and adolescents have shown that higher body mass index (BMI) predicts the risk of higher cardiac mass, which is a marker of premature cardiac damage and subsequent cardiovascular diseases in adulthood. However, since BMI, a measure of obesity, fails to distinguish between fat mass and muscle mass, it remains unclear if the relationship of higher BMI with cardiac mass could be interpreted as an adverse effect of childhood obesity.

Cross-sectional studies with dual-energy Xray absorptiometry (DXA) measures of fat mass and muscle mass have reported that fat mass in childhood was not associated with higher cardiac mass. On the contrary, the cross-sectional studies reported a positive relationship between muscle mass and cardiac mass, which is masked and misinterpreted by the BMI as an obesity problem.

Critically, there is no longitudinal study in the world that has examined the earliest signs and consequences of increased total body fat and abdominal fat from childhood on a growing heart using a gold standard method such as DXA for assessing body composition.

This new study tracked 1,803 children aged 9 years and followed them up for 15 years duration until age 24 years from the University of Bristol’s Children of the 90s cohort. DXA scan was used to measure total body fat mass, abdominal (trunk) fat mass, and muscle mass at age 9 years, and repeated at ages 11, 15, 17, and 24 years. An echocardiography scan was conducted to measure cardiac structure and function at age 17 and was repeated at age 24 years. The participants' fasting blood glucose, insulin, and cholesterol levels were measured at ages 17 and 24 years; and blood pressure, heart rate, socioeconomic status, family history of cardiovascular disease, smoking status, accelerometer measure of sedentary behaviour and physical activity, as well as inflammation, were accounted for in the analyses.

The study found important differences in how total fat mass, abdominal fat mass and lean mass affected the heart across childhood and adolescence. Specific studies found that, in the period from age 17 years, increased total fat and abdominal fat were linked with structural changes to the heart that can put it under pressure to pump normally and increase risk of future disease; however, increased lean mass in childhood and adolescence were more likely to result in regular ‘physiological’ heart development and avoid disease.

Put another way - an increase in cardiac mass is good if lean mass is the predictor. This is natural because the heart is essentially a muscular organ, and means the heart is likely to develop normally.

But if the cardiac mass increases because of fat, then it is dangerous, because there is no natural space for fat in the heart. Therefore, increased fat mass exerts an additional load by attaching itself around the heart (epicardial fat) and within the coronary vessels of the heart, which clogs blood circulation. As a result, the heart overworks to pump more blood throughout the body, thereby increasing its size during this compensatory process. Research among adults has shown that every 1% increase in fatty muscle fraction increases the risk of future heart disease by 7%.

In this new study, abdominal fat mass accumulation had a two-fold worse effect on the heart compared to total body fat mass. It was also observed that the potential pathway through which fat mass alters cardiac structure is via increased systolic blood pressure (11% contribution), inflammation (7%), and low-density lipoprotein (or bad) cholesterol (8%). Increased systolic blood pressure and lipids in adolescence have been independently associated with premature cardiac damage in adolescents.

Professor Agbaje says: “Our current findings also strongly confirm that BMI is a poor marker for measuring fat mass in children and adolescents because children have 4 times more lean mass than fat mass, and BMI cannot distinguish between fat mass and muscle mass. This study helps us understand that accumulated total body fat and abdominal fat in late adolescence, but not in childhood, may adversely affect the growing heart. Maintaining both total body fat and abdominal fat mass proportions around childhood values may significantly contribute to a healthy heart, since the ratio of muscle mass to fat mass decreases from 4 at age nine years to 2.4 by mid-twenties. However, of course, any lifestyle changes that can be used to lower body fat in adolescence, as young people pass this critical age of 17 and beyond, would be beneficial.”

He adds: “We need to exercise extreme caution in reducing BMI in children simply because they are overweight, unless we have confirmed that their weight is due to fat mass, to avoid an unpleasant consequence of low muscle mass-induced poor heart growth. Accumulation of excess fat, especially abdominal fat mass, can be prevented through regular exercise, healthy food choices, limiting screen time, and early screening and management.”

Prof Agbaje’s team has developed a free and simple online calculator that is useful from the age of 5 years to adulthood, which can detect the risk of excess total body and abdominal fat using the waist circumference-to-height ratio.

The study was conducted by the Universities of Bristol and Exeter in the UK, and the University of Eastern Finland.