It is
well established that the transition from exclusive breastfeeding to eating
family/table foods is a period of increased risk for infants (Sellen, 2007).
During this time period, the infant shifts from consuming only “sterile” milk
to eating household foods with all their potential pathogenic hitchhikers. This
transition, more popularly known as weaning, increases an infant’s exposure to
environmental pathogens, especially those that may contaminate food or water.
Historically,
weaning has been a period of increased morbidity and mortality risk for infants
(Sellen, 2007). Managing these risks, while promoting optimal growth for the
infant, has been the subject of considerable debate (Sellen 2007), and there
appears to be considerable ecological variability in optimal weaning time and
trajectory (McDade and Worthman, 1998). Some the differences in timing may be
driven by the availability of suitable weaning foods. Other environmentally
important factors are the pathogen load of the environment, and the relative
risk of infection versus growth faltering from lack of calories.
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| Figure 1: The weanling's dilemma illustrated. Image from McDade and Worthman 1998; PMID:9717139
|
One
possible solution to these challenges may be the use of kiss-feeding, or
pre-mastication of the supplemental foods by the mother. These foods are then
transferred from the mother’s mouth to the infant’s mouth via what is
essentially a kiss.
When I
decided to write this post, I had no idea that there had some controversy over
Alicia Silverstone publically stating that she kiss feed her son. Considerable
pixels were spilled in the discussion of this, and it was framed largely as
“disgusting” or “unhealthy” by the media. But while the idea of kiss feeding
may sound gross to some, in terms of the weanling’s dilemma as described above,
it is actually a pretty good strategy.
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| Figure 2: Birds demonstrate how it is done. I was going to use Alicia Silverstone's photos, but her site has a no use policy. You can check out her video at www.youtube.com/watch?v=c6tEKLT5G2M. This image is from: originalwallpers.com. |
During
kiss feed, the mother pre-masticates the food for the infant. The infant is
then feed via a “kiss”, where the mother pushes the chewed food into the
infant’s mouth. Depending on the consistency of the starting food and the
availability of teeth, the mother may heavily pre-chew the food before the
kiss. The chewing not only breaks down the food so they infant can swallow and
digest it, but also has a secondary, but unknown function. The process of
chewing the food exposes the food to the mother’s saliva and the mother’s mucosal
immune system. While the mother is chewing, her immune system is responding to
potential antigens in the food, some of which may be pathogens. As the food is
transferred, so too are many of the immune factors in her mouth, likely along
with a healthy dose of oral bacteria. Both the mother’s innate immune system,
in the form of secretory IgA and other factors, such as soluble CD14+ cells
(macrophages), and her adaptive immune system, will be transferred with the
food. At feeding, the kiss-feed infant already has a jump start on any
potential pathogens, and this may reduce the risk or severity of infection. The
mother’s immune system is also responding to pathogens in the food, and may
systemically upregulate her adaptive immune response targeting these pathogens;
these in turn will be incorporated into the milk (Hanson et al., 2003). Her
oral exposure to these pathogens may increase the immune factors in her milk,
further protecting the infant. In populations with limited access to soft or
appropriate weaning foods, this may be ideal means of providing the infant with
supplemental calories while supporting the immune system and possibly reducing
overall infection risk.
Sadly,
this hypothesis has never been investigated. Testing this would be pretty invasive
– measure the pathogens in the food, the pathogens and specific immune cells
and related immunoproteins in the masticated food given to the infant, the
immune factors (including pathogen specific) in the milk, and probably the same
factors in the saliva and plasma of the mothers and infants. But while data may not exist to definitively
support the hypothesis, it remains a biologically plausible model for
protecting infants during supplementation, especially for populations with
limited access to soft weaning foods.
Author’s note: I know human milk is not
sterile. It has wonderful beneficial bacteria in it, and sometimes bacterial
and viral pathogens. This is used here as a relative term compared to the risks of
weaning foods.
References
Hanson LA, Korotkova
M, Lundin S, Haversen L, Silfverdal SA, Mattsby-Baltzer I, Strandvik B, Telemo
E. 2003. The transfer of immunity from mother to child. Annals of the New York
Academy of Sciences 987:199-206.
McDade T, Worthman C. 1998. The weanling's dilemma reconsidered:
a biocultural analysis of breastfeeding ecology. J Dev Behav Pediatr
19(4):286-299.
Sellen D. 2007. Evolution of infant and young child feeding:
implications for contemporary public health. Annu Rev Nutr 27:123-148.