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Dr. Jones: Fish oil in pregnancy decreases the risk of childhood asthma? Really? This is Dr. Kirtly Jones from Obstetrics and Gynecology at 91Â鶹ÌìÃÀÖ±²¥ Care and this is a true fish story on The Scope.

Announcer: Covering all aspects of women's health, this is "The Seven Domains of Women's Health" with Dr. Kirtly Jones on The Scope.

Dr. Jones: Pregnant women have had mixed signals from science about how much fish to eat during pregnancy. The concern is that some fish have high levels of mercury, which is bad for the developing child. On the other hand, there are lots of studies that say fish in the diet, specifically the oils in cold-water fish, are good for the developing child-to-be.

The incidence of childhood asthma has doubled in the developing world in the last 20 years. There are lots of reasons proposed for this, maternal diet, decreased incidence of vaginal birth, not letting our kids play in the dirt and letting the dog lick their face to name a few, and we've discussed these on The Scope.

It's been known for some years that children who eat more fish, who have a diet enriched in Omega-3 fatty acids, have a lower risk of asthma. The proper term for what is lacking in the American diet in the fats direction is long-term polyunsaturated fatty acids, LCPUFAs, or specifically, eicosapentaenoic acid and docosahexaenoic acid, or EPA and DHA for short. And the EPA and the DHA are all that you non-biochemists need to remember.

So observational studies, who eats what and how do their babies turn out, suggest that moms who ate more fish had babies with less asthma. Okay. But association, fish and lower risk of asthma, doesn't mean causation, fish oil prevents asthma. Just published and making all the news is a pretty big, meaning 736 pregnant women, randomized trial starting at 24 weeks of pregnancy comparing fish oil and placebo, which was olive oil, in capsules during pregnancy and following the kids that were born for five years. Neither the moms who took the capsules during pregnancy nor the docs who followed the kids knew what capsule the pregnant moms got.

Now, as an occasional consumer of fish oil, I can tell you that you know by your fishy burps if you've taken fish oil, but they didn't even mention that in the New England Journal of Medicine publication. They should have asked me if you can really blind people to fish oil or olive oil.

Anyway, the study found a 30% reduction in the incidence of childhood wheezing, and that's a pretty big thing. Kids of moms who took fish oil had a 17% incidence of wheezing and asthma, and the kids of moms who took placebo, which was the olive oil, had a 24% incidence. The coolest part of this study is they actually measured the levels of EPA and DHA in the moms before they started the study and the biggest difference in the incidence of asthma was found in people who had the lowest levels of these molecules and then took the pills.

They also looked at some genetic markers in moms that were associated with lower levels of EPA and DHA. Some people have genes that make it easier to convert plant oils to EPA and DHA. The incident of asthma and wheezing was 34% in the kids whose moms had the lowest levels of EPA and DHA who took placebo. That means one-third of those kids had asthma and only 17% in the moms who took fish oil. That's a big reduction, a 50% reduction. Moms with the highest level of DHA and EPA before the study had just a little difference in the lower asthma, compared to those who took fish oil or those who took olive oil.

This was done in Copenhagen, by the way, so a lot of the moms probably ate more fish than we do in the US. So what is it? Is it the oil or some protein in fish oil that makes the difference? We don't exactly know how this works yet, but the effect of fish oil supplementation was pretty big. For moms with the lowest level of EPA and DHA in their blood, it would only take treating five women during pregnancy to prevent one case of wheezing or asthma in the children. That's a pretty big effect.

Now, what did the moms really take? They took 2.4 grams of fish oil each day. That's about 20 times more than the average American mom gets in her diet daily. I pulled my Trader Joe's Wild Salmon Oil off my shelf and it has 600 mg Omega-3 fatty acids in 1,200 mg of fish oil per capsule. I'd have to take four of them a day to get the same dose. Then I looked at my can of Wild Planet sustainably pole and line caught wild tuna, and one 5-ounce can has 2,000 mg of DHA and EPA. Now we're getting close, but I have to eat a can a day.

I would eat tuna fish sandwiches or casserole every day if my mother were still here to make them for me. And canned tuna was cheap when we were growing up so we ate a lot of it. My mom had a nutrition background and was always trying to get us to eat fish, but she only succeeded with tuna. She made a totally disgusting casserole with canned salmon called "Salmon Wiggle," that we could barely eat. You can check out the recipe for "Salmon Pea Wiggle" on Google and for whatever reason, I cannot imagine it gets four out of five stars.

So I wouldn't jump in and eat a can of tuna a day starting at 24 weeks, or eat salmon every day or buy a mega bottle of fish oil capsules. However, many pregnant women are already taking supplements of EPA and DHA in pregnancy for what is thought to be a benefit for fetal brains. Maybe there's more to it. In the meantime, some cold-water fish in your diet is good for you and your kids and stay tuned to The Scope.

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