Background There is a growing body of evidence showing that early life events are associated with increased risk of cardiovascular and metabolic diseases later in adult life. However, there is a paucity of data in this field from Asian populations. In this study, we examined the association of birth order with obesity risk and cardiometabolic outcomes in young adults in Thailand.

Methods Participants were the offspring from a birth cohort study in Chiang Mai (northern Thailand), who were followed up at ~20.5 years of age. Clinical assessments included anthropometry, blood pressure, fasting blood samples and carotid intima-media thickness. Insulin sensitivity was estimated using homeostatic model assessment of insulin resistance (HOMA-IR). Participants were stratified into two groups: first-borns and later-borns. Health outcomes between groups were compared using multivariable models adjusting for important confounders, in particular maternal body mass index (BMI).

Results A total of 559 participants were studied: 316 first-borns (46% males) and 243 later-borns (47% males). Adjusted models showed anthropometric differences, with first-borns being 2.3 kg heavier (p=0.023) with a BMI 0.86 kg/m² greater (p=0.019) than later-borns. Thus, rates of obesity were higher in first-borns than in later-borns (6.6% vs 2.9%), so that first-borns had an adjusted relative risk of obesity 3.3 times greater than later-borns [95% CI 1.42 to 7.88; p=0.006]. There were no observed differences in cardiovascular or metabolic parameters assessed, including HOMA-IR.

Conclusion As observed in other populations, first-borns in Thailand had greater BMI and an increased risk of obesity in young adulthood. However, we observed no other cardiometabolic differences between first- and later-borns.