I am not an authority on the question of peanut allegies and breastmilk, but I did some looking into this issue last year.  Below are summaries of relevant articles that I found in the published literature.  It would appear that some peanut proteins are or can be present in breast milk generally 1 to 3 hours after maternal ingestion.  There may be more literaure that is not included, so you may want to conduct a more recent search. 
Judy Schreiber
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Summary of information on peanut allergies and breastmilk/in utero exposures
J. Schreiber
05/30/01

Vadas, P., Wai, Y., Burks, W., Perelman, B. 2001. Detection of peanut allergens in breast milk of lactating women. JAMA 285(13): 1746-48.

In a study of lactating women, peanut protein was detected in breast milk samples of 11 of 23 subjects.  Peanut protein appeared within 1 hour of ingestion in 8 of 11 subjects, within 2 hours in 2 of 11 subjects, and was delayed for 6 hours in 1 subject.  Peanut protein was cleared rapidly from breast milk except in one subject who showed protracted presence of peanut protein.   The authors note that since peanut allergy reactions typically occur on first known exposure (72 to 81% of all cases), and sensitization requires prior exposure to generate allergen-specific IgE, then occult sensitizing exposure must have occurred in the past.  Exposure to food allergens in breast milk originating from the maternal diet is thought to be responsible for occult sensitization.  The time courses of appearance of proteins from peanuts, eggs, milk and wheat are similar, generally from 1 to 3 hours after maternal oral ingestion. About 48% of lactating women in this study secreted peanut protein in their milk.  Similarly, 53 to 79% of women (other studies) secreted beta-lactoglobulin and ovalbumin into milk after ingestion of cow's milk and eggs, respectively. The atopic status of lactating women have not accounted for the variable detection of food proteins.

Nutrition During Lactation. 1991.  National Academy Press, Washington, D.C., pp. 168-9.

"When suspected food allergens were excluded from the diets of pregnant and lactating women whose families were at high risk for atopic dermatitis, the incidence of this condition was lowered among breastfed infants compared with its incidence in a group of controls.  

Recently, Hattevig and colleagues (1990) also found that maternal dietary restrictions for 3 months during lactation resulted in a lower rate of atopic dermatitis among the mothers' infants for 6 months.  Several case reports suggest that components of foods ingested by the mother pass into the milk and then cause allergic reactions in the infant, and that these problems can be prevented by excluding the allergens from the maternal diet.  

Nonetheless, the benefit of prophylactic use of allergen-restricted diets for the general population of pregnant and lactating women is not well established."

La Leche League International, 1997.  Allergies and the Breastfeeding Family.  Leaven, Volume 33(4): 75-77.  In: http://www.lalecheleague.org/llleaderweb/LV/LVAugSep97p75.html 

"When a breast fed baby is exposed to an allergen, small amounts of the offending substance may trigger a response: IgE levels rise and a severe reaction may occur.  Early and occasional exposure to cow's milk proteins sensitizes a baby so that even tiny amounts of cow's milk may act as booster doses in provoking an allergic response.  In families demonstrating milk allergies, a mother should avoid such foods and not offer them to baby.  Severe, perhaps even life-threatening reactions could occur.

Other common foods which cause reactions are wheat, corn, pork, fish and shellfish, peanuts, tomatoes, onions, cabbage, berries, nuts, spices, citrus fruits and juices, and chocolate.

Sometimes mothers feel that because a food could be a potential allergen, it best to avoid it entirely.  If there is no history of allergy to these foods in a mother's or father's family, this may be an unnecessary precaution. Eating foods a mother enjoys will help her find breast feeding more satisfying.  Only if a baby shows allergic symptoms should a mother consider avoiding certain foods. 

Once a family has experienced an allergic child, they are most concerned with avoiding allergies or mitigating them for subsequent children.  Studies by Chandra and others have shown that avoidance during pregnancy and lactation of foods to which family members show sensitivity will produce far fewer instances of allergy in later children.  Avoidance of large amounts of any food during pregnancy will lessen the likelihood of infant allergies to that food.  

Ewan, P. 1998.  Prevention of peanut allergy.  Lancet 352: 4-5.

"The UK Department of Health has circulated advice aimed at reducing the development of peanut allergy.  The advice, based on an expert-committee report, is that pregnant women "may wish" to avoid eating peanuts or foods containing peanut products if they of the father or siblings of the unborn child are atopic.  The same advice is given for the lactation period".

The author asks, is there evidence of sensitization in utero or via breast milk?  In pregnant rats, food proteins can be detected in serum and amniotic fluid.  In human beings, food proteins (egg and milk) are detectable in maternal serum, but there is no evidence of transfer to the fetal circulation.  Maternal IgE cannot cross the placenta. There is anecdotal evidence of transfer of allergen in breast milk after maternal ingestion, but whether such low concentrations can sensitize the infant for IgE production is not known. 

Fox, D. and Lack, G. 1998.  Peanut allergy.  Letter.  Lancet 352: 741.

The authors respond to Ewan, 1998 (above).  They report five children with peanut allergy in one week; in three cases the mothers believed that their peanut consumption (once every two to three weeks) during pregnancy had lead to their child's allergy; by contrast, in two cases the mothers had deliberately avoided peanuts during pregnancy.  They find 'scant evidence' that maternal avoidance of foods during pregnancy and lactation prevents specific allergic sensitization to these foods.  Indeed, they suggest that it is very low level exposure that may lead to sensitization.  They conclude, "Although the new recommendations on peanut consumption may do no harm, they could increase the prevalence of peanut allergy".



Judith S. Schreiber, Ph.D.
Senior Public Health Scientist
Office of the Attorney General
Bureau of Environmental Protection
The Capitol
Albany NY 12224
518-474-4819 desk
518-474-9267 sec
518-473-2534 fax
email [log in to unmask]  

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