Soon after birth, multiple microorganisms including coliforms, bacteroides, E. coli and Bifidobacteria colonize the infant's sterile gut.1 These intestinal flora are crucial for development of the infant's immune system and to reinforce the barrier function of the intestinal mucosa, helping to reduce the likelihood of the attachment of unfavorable microorganisms, as well as the entry of allergens.
But a balance of microbes is important. Modern medicine has presented ways to eradicate or effectively treat illness caused by many previously pathogenic microorganisms. However, with the eradication of many infectious diseases, the incidence of atopic and immune-related diseases has increased. This epidemiological observation has been explained by the "hygiene hypothesis", which suggests that an indiscriminately broad reduction in microbial burden by advances in anti-infective medication and focus on hygiene is contributing to an immunological imbalance in the intestine. Thus, an excessively hygienic environment in early childhood is thought to contribute to higher risk for atopy including eczema or atopic dermatitis, allergic conjunctivitis, allergic rhinitis and asthma, and other autoimmune diseases.2
Adapted from Bach, 2002.3
The importance of exposure to microbes for development of a healthy immune system has been well documented. In the breastfed infant, Bifidobacteria are thought to play a major role in maintaining a healthy balance and strong immune system. Bifidobacteria and other beneficial microorganisms can also be given as a supplement to provide similar beneficial effects. The intentional and selective inclusion of beneficial microorganisms in the human diet is the basis for the concept of probiotics.4,5
Probiotics and their function in the GI tract
Probiotics are defined as live nonpathogenic microorganisms in the food supply that, when given in adequate amounts, are capable of conferring a health benefit to the host by modifying the balance of the intestinal flora. Understanding of the role of probiotics in health is certainly not new – Louis Pasteur and Elie Metchnikoff were two of the earliest proponents of the importance of beneficial microorganisms in human life back at the beginning of the 20th century. Today, however, there is increasing awareness of the health benefits of probiotics at every age, and vehicles to deliver the benefits of probiotics are in constant development.
Probiotics can be found in both supplement form and as components of foods and beverages. Bacteria and yeasts have been used for centuries to ferment foods. Most people are familiar with the role of specific strains of bacteria in producing yogurt and other cultured dairy products. Only in recent years, though, has the significance and value of "live cultures" been transformed from a laboratory understanding to a promoted product feature in both food (e.g., yogurt) and supplement form.
The health benefits of probiotics relate to the way these microorganisms interact with the GI tract. The GI tract and flora play a significant role in overall health. The intestine acts as a gatekeeper, allowing nutrients to be absorbed for use by the body while keeping out toxins and pathogenic bacteria and viruses. The gut flora breaks down vitamins and indigestible substances to produce fatty acids that contribute to the intestinal barrier while helping to maintain a balance of favorable and unfavorable bacteria.6
Probiotics given as a supplement are a means of delivering beneficial active elements to the GI tract, which can help balance the flora. However, in order to actually deliver these beneficial effects, the probiotic must be active and viable within the delivery vehicle (i.e., the supplement or food) and stable through the acidic environment of the stomach until it reaches its target in the GI tract. Once in the GI tract, the probiotic must be able to effectively contribute to the balance of the gut flora, bolster the immune system and influence metabolic activities.4
The most commonly studied and used probiotics are lactic acid producing bacteria including Bifidobacteria and Lactobacilli, and the potential benefits of probiotics appear to be strain-specific and dose-dependent.4
How are probiotics beneficial to the infant's immune system and overall health?
Probiotics have a supportive role to play in the infant's overall health. Probiotics support healthy immunity in a number of ways, primarily by helping to maintain the intestinal barrier function and by modulating the intestinal immune response.
Probiotics appear to enhance the intestinal barrier and immune function by:
- competing for nutrients and receptor sites along the intestinal wall
- producing lactic acid that contributes to maintaining an acidic environment that is unfavorable to potential pathogens5,7
- helping to decrease gut permeability to assist in non-specific immune defense8
- increasing the levels of immunoglobulins such as secretory IgA, considered the most important immunoglobulin in the gut due to its role in immune response.
The intestinal flora of infants is dominated by Bifidobacteria; in breastfed infants Bifidobacteria make up 80 to 90% of the flora, and in formula-fed infants, Bifidobacteria are the predominant microorganism but at approximately one-tenth the numbers recorded in the flora of the breastfed infant.1
Like probiotics, "prebiotics" are receiving increasing attention as health supplements. Prebiotics differ from probiotics in that they contain no live microbes but do support the growth of probiotics in the intestine. Prebiotics are non-digestible dietary substances that promote the growth of beneficial flora including Bifidobacteria and Lactobacilli, likely by producing an acid intestinal environment, thereby supporting the beneficial effect of these probiotics. Breastmilk contains a number of prebiotic substances such as milk oligosaccharides that can favor the growth of the probiotics Bifidobacteria and Lactobacilli.10
Window of Opportunity
The fact that the body's immune system is still developing during infancy means that probiotics can have special implications for infant health. Due to the predominance of Bifidobacteria in the intestinal flora of the breastfed infant, the general good health of these infants is often attributed to the differences in flora between breastfed and formula-fed infants, and notably, to the role that may be played by Bifidobacteria.
The breastfed baby is assured of a high concentration of Bifidobacteria in their intestinal flora. For the formula-fed infant, supplementation with probiotics such as Bifidobacteria can result in an intestinal flora that is closer to that of breastfed infants.1,10
As the "hygiene hypothesis" is more fully understood, parents may be looking for ways to support their child's immune system through exposure to beneficial bacteria. Probiotics, in the form of either supplements or foods, depending on the age of the child, can help provide the immune system support they are looking for.10
- Yoshioka H, Iseki K, Fujita K. Development and Differences of Intestinal Microflora in the Neonatal Period in Breast-Fed and Bottle-Fed Infants. Pediatrics 1983;72(3):317-21.
- Strachan DP. Family size, infection and atopy: the first decade of the "hygiene hypothesis". Thorax 2000;55 (Suppl 1):S2-10.
- Bach JF. The effect of infections on susceptibility to autoimmune and allergic diseases. NEJM 2002;347:911-20.
- Szajewska H, et al. Probiotics in Gastrointestinal Diseases in Children: Hard and Not-So-Hard Evidence of Efficacy. Journal of Pediatric Gastroenterology and Nutrition 2006;42:454-75.
- Fooks L, Gibson G. Probiotics as modulators of the gut microflora. Br J Nutr 2002;88(Suppl 1):S39-S49.
- International Food Information Council. IFIC Foundation Functional Foods Fact Sheet: Probiotics and Prebiotics. June 2006.
- Gibson GR, McCartney AL, Rastall RA. Prebiotics and resistance to gastrointestinal infections. Br J Nutr 2005;93(Suppl 1):S31-S34.
- Fioramonti J, Theodorou V, Bueno L. Probiotics: what are they? What are their effects on gut physiology? Best Pract Res Clin Gastroenterol 2003;17:711-24.
- Fukushima Y, et al. Effect of a probiotic formula on intestinal immunoglobulin A production in healthy children. International Journal of Food Microbiology 1998;42:39-44.
- Chen CC, Walker WA. Probiotics and Prebiotics: Role in Clinical Disease States. Advances in Pediatrics 2005;52:77-113.