High protein intake from meat as complementary food increases growth but not adiposity in breastfed infants: a randomized trial

Background: High intake of cow-milk protein in formula-fed infants is associated with higher weight gain and increased adiposity, which have led to recommendations to limit protein intake in later infancy. The impact of protein from meats for breastfed infants during complementary feeding may be different.

Objective: We examined the effect of protein from meat as complementary foods on growths and metabolic profiles of breastfed infants.

Design: This was a secondary analysis from a trial in which exclusively breastfed infants (5–6 mo old from the Denver, CO, metro area) were randomly assigned to receive commercially available pureed meats (MEAT group; n = 14) or infant cereal (CEREAL group; n = 28) as their primary complementary feedings for 5 mo. Anthropometric measures and diet records were collected monthly from 5 to 9 mo of age; intakes from complementary feeding and breast milk were assessed at 9 mo of age.

Results: The MEAT group had significantly higher protein intake, whereas energy, carbohydrate, and fat intakes from complementary feeding did not differ by group over time. At 9 mo of age mean (± SEM), intakes of total (complementary feeding plus breast-milk) protein were 2.9 ± 0.6 and 1.4 ± 0.4 g kg−1  d−1, 17% and 9% of daily energy intake, for MEAT and CEREAL groups, respectively (P < 0.001). From 5 to 9 mo of age, the weight-for-age z score (WAZ) and length-for-age z score (LAZ) increased in the MEAT group (ΔWAZ: 0.24 ± 0.19; ΔLAZ: 0.14 ± 0.12) and decreased in the CEREAL group (ΔWAZ: −0.07 ± 0.17; ΔLAZ: −0.27 ± 0.24) (P-group by time < 0.05). The change in weight-for-length z score did not differ between groups. Total protein intake at 9 mo of age and baseline WAZ were important predictors of changes in the WAZ (R2 = 0.23, P = 0.01).

Conclusion: In breastfed infants, higher protein intake from meats was associated with greater linear growth and weight gain but without excessive gain in adiposity, suggesting potential risks of high protein intake may differ between breastfed and formula-fed infants and by the source of protein.


Alex’s Notes: To be honest, I’m not entirely sure why I thought this study was interesting; I don’t know anything about babies or infant nutrition. Nonetheless, the title caught my eye – probably because of the wording “high protein intake” combined with “increases growth but not adiposity.” The body only has so much tissue, and if it isn’t fat that is growing then it must be lean body mass, which you should know is metabolic currency, even for babies.

My initial thoughts going into this study stemmed from a habit to consider things in evolutionary context. Ancestrally, what would toddlers and babies eat after breastfeeding, or as an adjunct to it? Without looking to the internet for an answer, animal flesh seems like a pretty good guess. That and perhaps soft fruits or other easily ingested vegetation. In retrospect I suppose that meat isn’t really edible by an infant, but there is more to a carcass than muscle and the child could have easily had elevated protein intakes via blood and broth.

The study at hand consisted of 42 exclusively breastfed infants between 5-6 months old that were assigned to receive 1-2 servings per day of either commercially prepared pureed meats or fortified infant cereals for 12 weeks. Fruits, vegetables, yogurt, and cheese were allowed ad libitum as developmentally appropriate.

Importantly, no adverse effects were reported for either feeding regime. In both groups, total energy intake protein, fat, and carbohydrates increased over time. Protein and fat was consistently greater in the meat group, while carbohydrates were consistently greater in the cereal group. It must be noted that this is with regards to the complementary feeding and does not include breast milk consumption. The average dietary intake can be seen in the table below.













** Energy contributions of protein, carbohydrates, and fat averaged over the 12 weeks from complementary feeding.

When we consider complete dietary intake (breast milk included), it becomes obvious that energy intake did not differ between the groups, which reinforces the notion that breastfed infants are able to effectively regulate energy intake based on their physiological signals. Interestingly, the total protein intake for the meat group was about 2.7g/kg per day versus 1.4g/kg per day for the cereal group. I’ll come back to this. As a percentage of total energy intake, when breast milk was included there was no longer a difference in fat intake, but protein in the meat (cereal) group was 17% (8%) and carbohydrates were 46% (55%).

In terms of growth, infants are measured in terms of length-for-age (LAZ), weight-for-age (WAZ), and weight-for-length (WLZ) z-scores. Overall, nothing happened with WLZ which indicates proportional growth in all study subjects. However, both LAZ and WAZ were significantly greater in the meat group compared to the cereal group, and in fact, over the study period these actually decreased in the cereal group! Moreover, it was found that protein intake (g/kg/day) was a significant predictor of changes in WAZ and nearly significant (p=0.09) for LAZ. To quote the authors,

“Data from the current study suggest that dietary quality, including the amount of protein intake, may positively affect linear growth in older breastfed-only infants in the United States. In contrast, for those infants who consumed the conventional, cereal-based diet, the LAZ steadily decreased over the intervention period (Figure 1), which suggested that the cereal-based diet may not have optimally supported linear growth in these breastfed infants.”

And this is where things get upsetting. The researchers did not do any body-composition testing and instead hypothesize that the increased LAZ was what drove the increased WAZ, which would suggest that the extra weight came from lean mass and not fat mass. This was actually supported by the lack of a significant difference in WLZ between the groups, as well as a lack of difference in BMI and waist circumference. Still, this can’t be verified without actual body-composition measurements.

Another interesting question that arises is what would happen in formula-fed infants? Breast milk may as well be an elixir of growth, and this study supports the notion that getting your child to a more carnivorous state with milk to wash it down leads to “lean-gains.” This was despite the fact that both groups ate similar calories and demonstrated the ability to self-regulate energy intake. However, formula-fed infants appear to be less able to self-regulate. This is critical because other studies have demonstrated that higher protein intakes at six months of age is correlated with weight gain and fat mass in formula-fed infants. Taken together with the current study, it stands to reason that higher protein intakes are beneficial to growth provided the infant is able to self-regulate energy intake and offset the increased protein with a reduction in carbohydrates, which for whatever reason formula-fed infants are unable to do.

So the takeaway is two-fold:

  1. Breast-feed your kid
  2. Start him on meat as soon as feasible

Oh, and before I forget, remember how the meat group had 2.7g/kg/day of protein intake? It is kind of funny how research is now finding that 2.2-3g/kg/day of protein to be ideal for a range of body composition, fitness, and overall health goals in adults.

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