Two studies published yesterday shed light on some of the factors that might influence children's chances of getting asthma and allergies.
According to a British study, babies whose mothers ate apples and fish during pregnancy have a reduced risk of asthma and allergies. The researchers, from the University of Aberdeen, studied 1,212 children of women who filled out questionnaires about their diets during pregnancy.
In another study, this one by researchers from the Arizona Research Center in Tucson, children who were exposed to a toxic substance produced by bacteria had a lower chance of developing asthma and eczema, an allergic skin condition, by the age of 3.
The substance, called endotoxin, is present in higher levels in older homes, homes with carpeting and those with a musty smell.
Both studies were presented at the American Thoracic Society conference in San Francisco.
Why would apple and fish consumption by pregnant moms reduce allergy and asthma incidence in babies?
It is thought that the apples expose unborn offspring to flavonoids, compounds found in fruits, vegetables and certain beverages such as red wine and tea. They have diverse, beneficial, biochemical and antioxidant effects and have been reported to have antiviral, anti-allergic, anti-platelet and anti-inflammatory activities.
Fish reportedly offers protection through omega-3 fatty acids, a class of "healthy" fat found in certain fish oils. Salmon and other cold-water fish tend to have high concentrations of omega-3.
An expectant mother's exposure to vitamins E and D and zinc could also reduce the offspring's risk of developing wheezing and asthma.
Why would exposure to endotoxin reduce asthma and allergy in young children?
This relates to a theory commonly known as the hygiene hypothesis. First described in 1989, it was suggested that the rise in allergy cases may be linked to declining family sizes, reduced exposure to germs and higher standards of cleanliness resulting in less exposure to bacteria, which contain endotoxins.
Exposure to endotoxins in the first years of life is believed to help a child's immune system mature.
However, with the reduced exposure to common germs, the immune system is under-stimulated. As a result, it starts reacting to relatively harmless substances such as pollen, house dust and animal dander, leading to improper immune reactions that set the stage for the development of allergies.
How much fish and how many apples did the mothers have to eat?
Moms who ate fish once a week or more during pregnancy gave birth to babies who were less likely to have eczema than children of mothers who never ate fish. Children of mothers who ate the most apples were less likely to ever have wheezed or to have asthma at the age of 5, compared with children of mothers who had the lowest apple consumption, according to the British research.
Did any other foods appear to have an impact on children's respiratory health when consumed by pregnant mothers?
The study did not find any protective effect against asthma or allergic diseases from a range of other foods: vegetables, fruit juice, citrus or kiwi fruit, whole grain products, fat from dairy products and margarine.
Are there foods consumed
by the mother that have been shown to increase a baby's chance of developing allergies and asthma?
Various studies have provided conflicting evidence about whether maternal avoidance of dairy during pregnancy and breastfeeding will reduce allergy development in at-risk infants.
Because the scientific jury is still out, current guidelines do not recommend excluding these essential foods, even in cases where the fetus is considered at higher than average risk of developing allergies (for example, children of parents with allergies, particularly if it's the mother).
Some experts suggest avoiding peanuts (a non-essential food) during pregnancy.
What foods should the baby avoid after it is born?
There is some evidence that avoiding so-called allergenic foods in the first year of life does have benefits in reducing the chance of developing dermatitis such as eczema and might even delay respiratory allergies. Avoidance of cow's milk from birth is recommended for siblings of children who are allergic to milk. Avoiding peanuts for the first few years seems wise, but it's still uncertain whether this approach doesn't just delay the inevitable (since kids don't often outgrow peanut allergy).
At-risk children should also avoid: 1) early bottle feeding; 2) early introduction of allergenic foods such as eggs and shellfish; 3) exposure to tobacco smoke; 4) allergenic environments.
Should mothers be concerned about pollutants, such as elevated mercury levels, found in some fish?
In March, Health Canada issued new recommendations for consumption of fish high in mercury, such as swordfish, king mackerel and canned albacore tuna. Pregnant women and nursing mothers should eat no more than 150 grams per month of such fish, and no more than 300 grams a week of canned albacore (white) tuna, which is higher in mercury than canned light tuna. Children one to four years old should eat no more than 75 grams a month of fish high in mercury, and no more than 75 grams a week of canned albacore tuna. Mercury exposure can adversely affect brain development.
What do you tell your patients?
I remind women of childbearing age of the importance of taking 400 micrograms of folic acid, which can help prevent certain birth defects such as spina bifida.
I point out a healthy diet (including apples and fish!) is important for their offspring. I discuss avoiding peanuts (since they are a non-essential food) during pregnancy and the first few years of life (particularly in high-risk offspring).
I also remind all mothers about the dangers of smoking and second-hand smoke and encourage them to breastfeed and introduce solids at six months, rather than four months.
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