Lesson 4: Infants and Toddlers - Breast Milk and Formula Feeding

Introduction

Thus far I have gone over literature regarding preconception and pregnancy considerations for childhood health and more specifically childhood weight outcomes. Here I will begin discussing considerations for infants and toddlers (generally describing the first 24 months of life).

Once born, an infant will primarily eat, sleep, pee/poop, cry, and repeat. While an infant may not perform many life functions at this point, there are still several ways that caregivers can influence the infant’s health and later weight management. These options expand as infants become toddlers. I will discuss what the research suggests regarding breastfeeding and formula feeding here, and I will discuss additional topics over the following three lessons.

Breastfeeding

There has been a substantial amount of research regarding breastfeeding and its impact on various aspects of weight management.

Impact on flavor preference and picky eating

Similar to the Lesson 2 where maternally consumed flavors transfer to the amniotic fluid and can influence the fetus, while breastfeeding maternally consumed flavors can transfer to breast milk and impact the growing infant.^{(Spahn, 2019)} Thus, while breastfeeding it may be helpful to prioritize healthier food options and minimize exposure to non-nutritive sweeteners (as these have been shown to transfer to breastmilk).^{(Sylvetsky, 2018)} This may help the child accept more flavors when foods are eventually introduced and lead to them being less picky with eating as they continue to grow. On that note, some evidence indicates that formula-fed babies have more difficulty accepting new flavors due to formula having a uniform flavor without variation (these infants may even prefer foods with a flavor similar to the formula).^{(MacLean, 2017)}

Note: Breastmilk has many bioactive compounds that confer potential health benefits beyond the nutrition from its macronutrient and micronutrient composition. As a couple of examples:

Human milk has ~1,500 types of micro RNA molecules which may impart biological properties to help explain the various health benefits seen with breastfeeding.^{(Wehbe, 2021)} These remain intact as they are delivered in exosomes.
A 2020 study noted that human milk oligosaccharides (“HMOs”) are present at 5-15 g/L in mature human milk and there are genetic mutations that impact the composition of the HMOs.⁽^{Lagström, 2020)} In this study the HMO diversity was negatively correlated with preconception body mass index (“BMI”) as well as height and weight z-scores* during the first 12 months of life in infants of “secretor” mothers (“secretor” means α1-2-fucosylated HMOs were present, this represented ~87% of the sample).
- Thus, elevated preconception BMI associated with decreased HMO diversity in breastmilk as well as increased length and weight gain in breastfed infants of secretor mothers.

*Z-scores represent standard deviations; a z-score of 0 indicates the value is at the mean, while a z-score of 1.00 indicates the value is 1 standard deviation above the mean. These are used when discussing various growth measurements in children (ie, height or length, weight, and BMI) since the absolute values are supposed to change with age; by using z-scores it is easier to indicate how a child’s growth compares to peers of similar age and gender. This is discussed in more detailed regarding childhood BMI in upcoming lessons.

Impact of maternal body mass index (“BMI”) status on breast milk composition

This has been examined in a few recent analyses:

A 2020 systematic review (“SR”) examining the impact of maternal obesity on breastmilk macronutrient composition included 31 studies, finding overall no differences in protein or carbohydrate content beyond the colostrum stage and finding slightly increased fat content in mature milk of women with obesity (2.73 grams/liter more fat).^{(Leghi, 2020)}
- The authors acknowledged that as fat content in milk increases up to 3-fold from the beginning to the end of a breastfeeding session it is possible the differences may have been due to subtle changes in collection times.
However, a 2021 analysis of 169 breastfeeding mothers (83 with normal weight, 86 with obesity) analyzed the amino acid composition of the breast milk in relation to infant size and intake.^{(Saben, 2021)} The authors found that women with obesity had breast milk with branched-chain amino acid and aromatic amino acid content that was ~28-53% higher than women with normal weight, histidine content that was 35% lower than women with normal weight, and a few other amino acids that differed in concentration at some but not all of the tested time points between 0.5-6 months. The free amino acid intake was associated with weight-for-length z-score, the fat mass index (similar to BMI but just considering body fat), and the fat-free mass index (similar to BMI but just considering fat-free mass), thus implying the differences in breast milk composition may have influenced differences in infant body composition.
- Thus, even though the prior analysis listed above found no difference in protein content as a whole, there do seem to be differences in the amino acid profiles that make up the protein.
A 2021 review highlighted differences in breast milk between women with normal weight and women with overweight & obesity, finding several differences in amino acids, lipids, oligosaccharides, acyl-carnitines, and other substances, many of which can theoretically increase the risk of cardiometabolic diseases, but at this time there is no strong evidence that these lead to any differences in infant/child health outcomes.^{(Milliken-Smith, 2021)}

Therefore, even if the macronutrients as a whole are similar there do seem to be differences in several classes of compounds, though it’s not clear to what degree this leads to health differences.

Note: It is unclear to what degree breast milk composition differences influence health outcomes in the long run. There are several studies that show benefits of breastfeeding in various aspects of a healthy lifestyle, but frequently these studies do not separate individuals based on maternal BMI status at the time that breastfeeding was performed.

Importantly, at this point I have not come across any evidence that women with obesity should avoid breastfeeding due to concerns of this yielding a detrimental health affect to their children. Additionally, to my knowledge all of the large medical organizations and groups that recommend breastfeeding for health benefits do not to my knowledge make any sort of exception for women with obesity.

Impact of breastfeeding on child weight status

So what does the research actually show regarding breastfeeding and future obesity risk? It’s hard to know for sure as almost all of the literature has been observational in nature. The observational evidence overall does show a significant benefit for prolonged breastfeeding with respect to decreasing the risk of childhood obesity.^{(Qiao, 2020)} However, any time observational analyses are examined it is important to consider correlation vs. causation or even reverse causality. To this end, I will provide overviews below of two different publications; one is a large analysis of several different studies (all observational other than 1 randomized controlled trial), and the other is a large interventional trial with an interesting and informative method of analysis, that may provide some clarity. You can click on the expandable box below to see overviews of these two publications; I provide a summary paragraph below.

Click here for an overview of two publications discussing the impact of breastfeeding on childhood weight

A 2021 analysis of the studies included in a SR conducted for the 2020-2025 Dietary Guidelines for Americans considered a subset of the various studies evaluating the association of breast milk consumption with subsequent overweight or obesity.^{(Dewey, 2021)}

Thus, when considering paired-sibling studies there is inconsistent evidence of a benefit to any breastfeeding while there is essentially no evidence suggesting prolonged breastfeeding is helpful for decreasing the risk of developing overweight and obesity.

A 2018 study performed a breastfeeding intervention in several maternity hospitals and compared infant growth in individuals who received the intervention and were breastfed >12 months compared to those who did not, comparing analytic approaches from an observational point of view (based on breastfeeding outcomes) as well as an intention-to-treat point of view (based on the actual intervention itself).^{(Kramer, 2018)} This is a bit complicated, but the main takeaway is that the results differed by the analytic method.

The intention-to-treat analysis based on the actual intervention as well as the subset of babies that were breastfeeding beyond 12 months showed faster growth during the first 2-3 months of life with decreasing differences after this and near equivalence in growth at age 12 months.
However, the observational analysis showed that babies breastfed <12 months had faster growth from 6 months of age onward.

Thus, the two analytic methods generated different results. This is likely due to a few of factors:

Breastfed babies who naturally are growing faster (or who initially grow more slowly and then have a regression to the mean and accelerate in growth) will demand more breast milk and these families will be more likely to supplement feeding with formula.
- Thus, in the end it will appear that babies who were given formula grow more quickly, but this is a consequence of their natural predisposition to grow at a faster rate.
Breastfed babies who naturally are growing more slowly (or who initially grow more quickly and then have a regression to the mean and decelerate in growth) will likely need less breast milk, making continued breastfeeding either.
- Thus, in the end it will appear that babies who were given breast milk grow more slowly, but this is a consequence of their natural predisposition to grow at a slower rate.
Parents who have more difficulty distinguishing hunger cues (perhaps due to having an infant who is fussier than typical) will more often introduce formula; this will lead to excess nutritional intake when the infant is fussy but not hungry and can lead to accelerated growth.
- Thus, in the end it will appear that babies who were given formula grow more quickly, but this is a consequence of parents having a more difficult time distinguishing hunger cues, not because the formula inherently caused faster growth.

Summary:

Observational data indicates a significant benefit of breastfeeding for weight management outcomes in childhood, but when using more rigorous paired-sibling analyses benefits are not always seen, and when they are seen they are typically smaller. This implies there may be other factors that lead to the observed associations. One caveat is that the paired-sibling analyses have small sample sizes and it is possible this is why they do not detect stronger effects.

The second publication shows why the observational data may be misleading. When breastfeeding is going well this may coincide with infants growing more slowly after the first few months of life.

This is because slower growing infants need less total milk.

When breastfeeding is not going well such that formula is introduced this may coincide with infants growing more quickly after the first few months of life.

This is because faster growing infants need more total milk, which entices parents to provide formula.
Additionally, infants with a fussier temperament will make it more difficult to distinguish hunger cues, potentially leading to them being fed more than necessary, which will lend itself to formula supplementation and faster growth.
- I discuss more about picking up on infant hunger and satiety cues with responsive feeding in Lesson 6.

Ultimately, this can lead to observational data indicating formula leads to faster growth and breastfeeding leads to slower growth when in reality there may not be any significant difference between the two over the first year of life. Overall, there may be a small benefit in favor of breastfeeding for long-term weight management outcomes, but it is likely smaller than typically thought.

Tip: If you feed your infant with a bottle and you feel it is difficult to determine when your baby is satiated such that you typically feed the entire bottle when this may not be necessary, consider making the bottle opaque (ie, by wrapping it in foil). This will make it easier to pick up on the infant’s natural satiety cues rather than trying to ensure the infant empties the bottle.

Breastfeeding and feeding behaviors

A 2018 study evaluating breastfeeding up to age 36 months and outcomes at age 3-5 years included almost 3,000 children; benefits were seen for measures of dietary intake and eating behaviors when breastfeeding up to 12 months of age.^{(Borkhoff, 2018)} Breastfeeding greater than 1 year was associated with lower iron and vitamin D levels as well as an increased risk of dental caries. The authors note that breastfeeding may be helpful or hurtful regarding the development of appetite cues and self-regulation with regarding to feeding:

Compared to bottle-emptying with bottle-fed infants, breastfeeding allows infants to naturally stop feeding when they feel full, which may help with self-regulation.
On the other hand, some people will use breastfeeding to soothe their infants when the infants seem upset, and this is associated with longer breastfeeding duration and may disrupt self-regulation as the infants begin to associate feeding for comfort as opposed to for resolving a state of hunger.

As breastfeeding may theoretically be beneficial or detrimental for the development of self-regulation with feeding, it is not too surprising that a 2021 review of feeding experiences during infancy and their impact on later child eating behavior noted that breastfeeding does not directly influence appetite and energy regulation in later years.^{(Pang, 2021)}This is one case where the average may show no impact but individually there may still be differences.

Formula feeding

The breastfeeding section above discussed some of the literature comparing breastfeeding and formula feeding for weight outcomes, but that section did not discuss the type of formula that you can choose to use. There are many different infant formulas out there that you can choose to purchase, and most are fairly similar in composition. There are not any great analyses to my knowledge that compare the many different brands on child weight outcomes. What has been seen in the literature is that it seems to be the overall protein content and quality that makes a difference for weight outcomes. A 2019 SR found that term infant protein supplementation leads to increased weight gain without a similar increase in linear growth, while a 2022 SR/MA found that higher-protein formulas contribute to greater weight gain in the first 6 months of life, both of which may increase the risk of obesity long-term.^{(Pimpin, 2019; Ren, 2022)} For this reason infant formulas are typically relatively low in protein content. Having said that, while they have similar carbohydrate and fat content relative to breast milk, they still have more protein than breast milk (beyond the initial stages of lactation).^{(Tang, 2018; D’auria, 2020; Kouwenhoven, 2022; Ren, 2022)}

A couple of studies by the same research group additionally found that extensively-hydrolyzed protein formulas led to less weight gain (and fewer instances of rapid weight gain (discussed further in Lesson 7)) than regular cow’s milk protein formulas.^{(Mennella, 2018; Mennella, 2019)} Unfortunately, the extensively-hydrolyzed protein formulas are more expensive, and for that reason there needs to be considerably more research showing a long-term benefit from using them for me to recommend them for this purpose.

A 2020 study that developed a virtual infant computer simulation found that when providing infants formula according to different sets of recommended feeding volumes, this will tend to lead to the infant developing excess weight prior to 6 months of age assuming the feeding volumes are not decreased to accommodate for excessive growth.^{(Ferguson, 2020)} Thus, it can be helpful to track growth with at general check-ups and adjust feeding accordingly.

Note: Perhaps more important than the type of infant formula is the act of preparing the infant formula correctly. A 2019 publication detailed an experiment performed in 2012-2013 where adults had to prepare 12 bottles of 2, 4, 6, or 8 ounces of formula.^{(Altazan, 2019)} Overall, considering an accurate result to mean the correct amount of formula was used within 5%, 3% of bottles were underprepared, 19% of bottles were accurate, and 78% of bottles were overprepared. On average each bottle had 11% more formula powder than desired. Per a mathematical model the authors utilized this would correspond to an extra weight gain of ~118 grams per month, which would eventually lead to a 4% increase in the infant’s body fat percentage. A separate study using a survey found that 46.3% of parents self-reported at least 1 error when preparing formula.^{(Appleton, 2022)}

Thus, taking care to prepare formulas correctly should be helpful and when caregivers do not do so this may explain some of the association of formula feeding with an increased risk of obesity.

Conclusion

The key points from this lesson regarding breast and formula feeding include:

Breastfeeding can introduce various flavors and may help prevent picky eating.
There are some differences in breastmilk of women with and without obesity but at this point there is no indication that this changes the health impact of breastmilk overall.
Breastfeeding seems to decrease the risk of obesity relative to formula feeding, but this observed association may be due to reverse causality or other lifestyle factors:
- Infants who are predisposed to grow faster will likely demand more milk; this may make caregivers prone to providing them formula and thus can drive an association between formula feeding and increased weight gain.
- Infants who are generally more content and ok with less milk will likely be easier to breastfeed; thus they may be less likely to receive formula and this will appear as though breastfeeding is associated with less weight gain.
When using infant formulas it is desirable to choose a low protein version (which is standard) and to ensure the formula is prepared correctly.
Among the standard infant formulas there is no strong evidence base suggesting specific brands or ingredients. The much more expensive extensively-hydrolyzed formulas may decrease the risk of excess weight gain but more research is needed regarding this possibility.

In the next lesson I will discuss complementary feeding, which describes the nutrition that infants and then toddlers receive in addition to nutrition from breast milk or formula.

Click here to proceed to Lesson 5

References

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Lesson 4: Infants and Toddlers – Breast Milk and Formula Feeding

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