Obesity (body mass index (BMI) of >30 kg/m2) is associated with low-grade chronic inflammation. Studies which have used a technique called Mendelian randomization (MR) have demonstrated that the relationship between body mass index and inflammatory proteins is causal. However, the timing of the onset of this inflammation is unclear. Inflammation could be a phenomenon which starts later in life as a result of a sustained high BMI over the life-course. Alternatively, it could be that a high BMI causes inflammation even in young children. Understanding the timing of this relationship is an important public health question as it could help in understanding the health impact of overweight/obesity in early life.