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Infants born very preterm represent a nutritional emergency due to interruption in placental supply of essential macronutrients and micronutrients. In-utero, the fetus receives high intakes of amino acids (AA), and carbohydrates, and relatively lower intakes of lipid than that received from breastmilk. High uptake and turnover of AA reflect a high demand for nitrogenous substrate in order to accrete lean tissue. While understanding of AA physiology in utero in humans remains limited, the existing data combined with animal studies suggest that up to half of all transplacental AA are oxidised rather than accreted, in other words, AA supply is far greater than protein demands.1 The fetus accretes ~2 g/kg/day of protein in mid/late pregnancy, but protein accretion decreases rapidly after birth. Thereafter, term born infants receive a protein intake (from breastmilk) that is far lower than almost every other mammal. Emerging data suggest that higher protein intakes at this stage may increase the later risk of obesity and abnormal metabolic programming.
In newborn preterm infants, AA requirements estimated from third trimester fetal growth are high. Enteral feeds take time to establish, and parenteral nutrition (PN) containing AA, carbohydrates, lipids and micronutrients on admission is considered a standard of care in most neonatal intensive care units (NICUs) in order to meet needs for growth of the brain and body. While this is considered safe, the optimal intake of AA at which to start, and the maximum recommended intake remains uncertain as nutritional practices must avoid toxicity caused by supplying nutrients in excess of metabolic capacity. International guidelines exist, but the evidence base is limited, and the relationship between early weight gain and long-term outcomes are largely observational. In the absence of long-term data, short-term weight gain, body composition and nitrogen retention remain important outcomes, but none of these is considered functional, …
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