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.
|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.
|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.
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.