Abstract. Low birth weight (LBW) as a result of restricted fetal growth increases the risk for later metabolic diseases and adiposity. However the relationship between LBW and postnatal growth and adult body composition has not been fully investigated. We have used sheep to determine the effects of LBW on postnatal growth and body composition at maturity. LBW was induced by twinning and placental embolization. At birth, LBW lambs were 38% lighter than controls (2.8 ± 0.2 vs 4.4 ± 0.3 kg, P<0.05), but had caught up in bodyweight by 8 weeks after birth. At ~2.3 years, bodyweights were not different between groups, but there were reductions in absolute (-8%) and relative (-17%) brain weights of LBW sheep (P<0.05) compared to controls. X-ray absorptiometry showed that the mature LBW sheep, compared to controls, had greater amounts of lean muscle (38.1 ± 1.3 vs 35.3 ± 0.5 kg, P<0.05) and tended to have more body fat (12.2 ± 1.2 vs 9.6 ± 0.9 kg; P=0.1); at autopsy abdominal fat mass was greater in LBW sheep (3.06 ± 0.26 vs 2.20 ± 0.25 kg, P<0.05). Plasma leptin concentrations were not different between groups. We conclude that, in sheep, LBW is associated with early postnatal catch-up in body weight, but body composition is permanently altered such that, relative to controls, adiposity is increased and brain weight is decreased. Key words: Adiposity, Fetal growth restriction, Body composition, Birth weight, Leptin (J. Reprod. Dev. 51: [59][60][61][62][63][64][65][66][67][68] 2005) he postnatal effects of low birth weight (LBW) are of considerable interest because in recent years LBW has been associated with an increased risk of several adult-onset diseases [1][2][3]. LBW, especially that caused by restricted fetal growth, is often followed by accelerated postnatal growth [4], and recent studies indicate that this catch-up growth can amplify the risk for cardiovascular and metabolic dysfunction. Individuals who had low birth weights but exhibited increased growth rates in terms of weight, height or body mass index (BMI) have been shown to be at a greater risk for abdominal adiposity [5], hypertension [2, 3] and non-insulin dependent diabetes mellitus [6] than those who failed to catch up [7].As well as differing postnatal growth rates, body c o m p o si t i o n c a n d i f f e r b e t w e e n L B W a n d appropriately grown individuals. At birth, LBW infants have reduced body fat, lean mass and bone m i n e r a l c o n t e n t ( B M C ) c o m p a r e d w i t h appropriately grown newborns [8]. However, these reductions are related to lower body weights, and body composition is not different between LBW and normally grown infants of similar weights [8]. After birth, age-related changes in body composition differ between LBW and appropriately grown infants. For example, the postnatal increase in BMC in LBW infants is less than that of appropriately grown infants [9], and adults of low birth weight have reduced total BMC and reduced bone mineral density (BMD) [10]. Adiposity is also affected, such that body fat content...