Prenatal exposure to inflammatory molecules may pave the way for asthma, so researchers are investigating whether dietary and other changes during pregnancy might lower levels and spare children from getting asthma. One of the leading possibilities is increasing intake of the omega-3 fats in fish and some plant-based foods. Omega-3s have well-documented anti-inflammatory effects.

Australian researchers have enrolled about 600 mothers in a clinical trial to test several anti-asthma interventions, including diets high in omega-3s. In 2003, they reported interim results based on tests of their 18-month-old children. The findings were not showstopping, but children of mothers with diets rich in omega-3s during pregnancy were somewhat less likely to have wheezing problems.

Researchers at the University of Southern California presented more impressive data at the annual meeting of the American Thoracic Society in May 2004. They reported that mothers with a history of asthma who ate oily fish (salmon, trout, yellow fin tuna) at least once a month during their pregnancies cut the risk of their children developing asthma by about 70%. However, if the mother did not have a history of asthma, oily fish intake had no bearing on the asthma rates of the children.

Fish sticks turned out to be bad news. They increased a child's asthma risk if a mother ate them regularly regardless of her asthma history. The researchers believe that the increased risk may come from the trans fats in the batter used to coat fish sticks.

There is a big problem with telling pregnant women to eat oily fish: Oily species are more likely to be contaminated with mercury, which has been linked to neurological deficits in children. So pregnant women and young children are advised to avoid such fish. Salmon may be one way out: It has omega-3 fat and low levels of mercury. However, alas, recent research found high PCB levels in some farm-raised salmon, depending on the source. Salmon from Chile and Washington state was less contaminated than salmon from Scotland and the Faroe Islands. Fish oil capsules are another possibility, because they also supply omega-3 fats and have little or no mercury.