SummaryBackgroundMaternal obesity and excessive weight gain during pregnancy are associated with higher birth weight and increased risk of childhood obesity.ObjectiveThis study investigated the effect of a high‐protein and low‐glycaemic‐index (HPLGI) diet during pregnancy on offspring body composition and metabolic health.MethodsWe conducted a dietary intervention study in pregnant women with a pre‐pregnancy BMI of 28–45 kg/m2 who were randomly assigned to an HPLGI diet or a moderate‐protein moderate‐glycaemic‐index (MPMGI) diet. A total of 208 offspring born to these women were followed‐up from birth to 5 years of age.ResultsNo differences were found on BMI z‐scores at different ages; however, offspring born to women on the HPLGI diet exhibited 0.43 mmol/L higher glucose levels (p = 0.017) at birth compared with the MPMGI diet. At 3 years of age, HPLGI offspring had 0.09 mmol/L lower levels of HDL‐cholesterol (p = 0.018) and 16% higher levels of triglycerides (p = 0.044). At 5 years of age, they had 0.25 mmol/L higher total cholesterol levels (p = 0.027) and 0.27 mmol/L higher LDL‐cholesterol levels (p = 0.003) compared with the MPMGI diet.ConclusionAn HPLGI diet during pregnancy may lead to adverse metabolic outcomes in the offspring, necessitating further investigation into long‐term health implications.