Abstract: We included six RCTs, involving 204 preterm infants. Low-quality evidence showed that protein supplementation of human milk increased in-hospital rates of growth in weight (MD 3.82 g/kg/day, 95% CI 2.94 to 4.7; five RCTs, 101 infants; I² = 73%), length (MD 0.12 cm/wk, 95% CI 0.07 to 0.17; four RCTs, 68 infants; I² = 89%), and head circumference (MD 0.06 cm/wk, 95% CI 0.01 to 0.12; four RCTs, 68 infants; I² = 84%). There was no evidence of a clear difference in rate of growth of skin fold thickness between the supplemented and unsupplemented groups (triceps MD 0.06 mm/wk, 95% CI –0.09 to 0.21; one RCT, 20 infants; or subscapular MD 0.00 mm/wk, 95% CI –0.17 to 0.17; one RCT, 20 infants). Protein supplementation led to longer hospital stays (MD 18.5 days, 95% CI 4.39 to 32.61; one RCT, 20 infants; very low-quality evidence), and higher blood urea nitrogen concentrations compared to the unsupplemented group (MD 0.95 mmol/L, 95% CI 0.81 to 1.09; four RCTs, 81 infants; I² = 56%). Very low-quality evidence did not show that protein supplementation clearly increased the risk of feeding intolerance (RR 2.70, 95% CI 0.13 to 58.24; one RCT, 17 infants), or necrotizing enterocolitis (RR 1.11, 95% CI 0.07 to 17.12; one RCT, 76 infants), or clearly altered serum albumin concentrations (MD 2.5 g/L, 95% CI –5.66 to 10.66; one RCT, 11 infants), compared with the unsupplemented groups. No data were available about the effects of protein supplementation on long-term growth, body mass index, body composition, neurodevelopmental, or cardio-metabolic outcomes. Low-quality evidence showed that protein supplementation of human milk, fed to preterm infants, increased short-term growth. However, the small sample sizes, low precision, and very low-quality evidence regarding duration of hospital stay, feeding intolerance, and necrotising enterocolitis precluded any conclusions about these outcomes. There were no data on outcomes after hospital discharge. Our findings may not be generalisable to low-resource settings, as none of the included studies were conducted in these settings. Since protein supplementation of human milk is now usually done as a component of multi-nutrient fortifiers, future studies should compare different amounts of protein in multi-component fortifiers, and be designed to determine the effects on duration of hospital stay and safety, as well as on long-term growth, body composition, cardio-metabolic, and neurodevelopmental outcomes.

Plain-language summary: We found six randomised trials (trials in which each infant had an equal chance of being chosen to receive either treatment), involving 204 preterm infants. The search is up to date to February 2018. Low-quality evidence showed that the addition of extra protein to breast milk increased short-term rates of weight gain (five trials), length gain (four trials), and head growth (four trials). Low-quality evidence from one trial did not show a clear difference in the rate of growth of skin fold thickness (measure of fat under the skin) between the supplemented and unsupplemented groups. Very low-quality evidence from one trial reported that infants who received additional protein stayed in hospital longer, while very low-quality evidence from four trials observed higher blood urea nitrogen concentrations (measure of kidney function and protein breakdown) in these infants, compared to those who received no additional protein. Very low-quality evidence from one trial suggested that adding extra protein to expressed breast milk did not clearly increase the risk of necrotising enterocolitis (inflammation of the intestine) or feeding intolerance, or clearly alter serum albumin concentrations (a measure of blood protein levels). No data were available on the effects of adding extra protein to human milk on long-term growth, body fat, obesity, high blood sugar, or brain development. Adding extra protein to human milk for preterm infants may increase short-term growth. However, its effect on length of hospital stay, feeding intolerance, and necrotizing enterocolitis is uncertain, due to data limitations and very low-quality evidence. There were no data about effects on later health and development, or effects in low resource settings. Since protein supplementation of human milk is now usually done as a component of multi-nutrient fortifiers, future studies should compare different amounts of protein in multi-component fortifiers, and be designed to determine the effects on length of hospital stay, safety, long-term growth, body fat, obesity, high blood sugar, and brain development.