Childhood obesity is characterised by excess body fat and is commonly assessed using body mass index (BMI), calculated by dividing weight in kilograms by height in metres squared. For children, BMI is interpreted using age- and sex-specific growth charts. Those between the 85th and 95th percentiles are classified as overweight, while those at or above the 95th percentile have obesity. Severe obesity is defined as a BMI at least 120% of the 95th percentile. However, BMI cannot distinguish fat from muscle or bone, so clinical assessment is important, particularly during periods of rapid growth.
Childhood obesity has multiple causes, including dietary habits, physical inactivity and genetic predisposition. Its consequences can extend into adulthood, increasing the risk of persistent obesity, diabetes, cardiovascular disease and hypertension. It is also associated with reduced self-esteem, stress and depressive symptoms. Childhood and adolescence therefore provide important opportunities to establish healthy habits that may protect long-term health.
Predicting obesity risk
Research led by the Murdoch Children's Research Institute identified three factors associated with weight outcomes during adolescence: the child's BMI, the mother's BMI and the mother's educational attainment. Each additional BMI unit in children aged 6–7 was associated with threefold higher odds of weight problems at ages 14–15, while each additional maternal BMI unit increased those odds by 5%. Maternal university education was associated with lower odds of early childhood overweight and obesity and greater likelihood of resolving weight problems by adolescence.
The prevalence of overweight or obesity at ages 14–15 was 13% among children with none of these three risk factors at ages 6–7, compared with 71% among those with all three. The study, using data from the Longitudinal Study of Australian Children, suggested that these readily measured factors could help clinicians identify children who may benefit from targeted prevention. However, they indicate risk rather than determine an individual child's future weight.
Rethinking the childhood BMI rebound
Emerging research also challenges the longstanding theory of adiposity rebound, which describes BMI declining after infancy before rising again from around age four to six. Earlier studies associated an earlier rebound with greater adiposity later in life, prompting concerns that it could signal future obesity.
However, an analysis led by Professor Andrew Agbaje at the University of Eastern Finland, using data from 2,410 children aged 2–19 in the US National Health and Nutrition Examination Survey (2021–2023), suggests that this BMI increase may not represent a true rebound in body fat. While BMI returned to its average level at age two by approximately age six, waist-to-height ratio (WHtR), a measure associated with body fat, remained below its age-two level throughout childhood and adolescence.
The findings suggest that increasing muscle and other lean tissue may partly explain the rise in BMI during early childhood. BMI alone can therefore misrepresent changes in body composition. Nevertheless, these findings challenge the traditional interpretation of adiposity rebound rather than establish that BMI increases are always harmless. Further research is needed to determine the implications for clinical practice.
Preventing childhood obesity
Parents can support healthy growth by prioritising sustainable behaviours rather than focusing on a child's appearance or weight. Because obesity can be a sensitive subject, discussions should be supportive and non-stigmatising, emphasising benefits such as improved energy, wellbeing and sporting performance.
Practical strategies include:
- Encouraging physical activity: Incorporate walking, outdoor play, cycling and sports into daily routines, favouring enjoyable activities children can maintain throughout life.
- Improving the home food environment: Make nutritious foods readily available, offer fruit and vegetables as snacks, and prepare meals at home where possible.
- Establishing healthy sleep routines: Maintain consistent bedtimes and keep electronic devices out of bedrooms. Monitor screen time and exposure to harmful dieting advice or unrealistic body-image messages.
- Reducing sugary drinks and highly processed foods: Limit fizzy drinks, sweetened teas, fruit punches, energy drinks and heavily processed snacks. Encourage water, unsweetened sparkling water and plain milk, and teach children to interpret nutrition labels.
- Finally, consider supplementation: Despite our best efforts, various factors may prevent children from getting all the nutrients their growing bodies need. When appropriate, a child-friendly multivitamin, such as a chewable teddy bear-shaped tablet or an easy-to-swallow liquid, may help fill nutritional gaps in their diet. Supplements should complement, not replace, a balanced diet, and should be chosen according to the child’s age and individual needs.
Overall, childhood obesity prevention requires a balanced approach that combines healthy family habits, supportive environments and appropriate clinical assessment. BMI remains useful for screening, but it should not be interpreted in isolation. Recognising individual growth patterns, body composition and established risk factors can help clinicians and families distinguish normal development from excess body fat and direct support towards children who need it most.