Pregnant women, don't stop eating fish!


Tuna, shark, king mackerel, tilefish, swordfish. If you’ve ever been pregnant, or known someone who has been pregnant, this list of seemingly random aquatic vertebrates is all too familiar to you. It’s the “avoid while pregnant” list of seafoods, and it’s just one of the confusing set of messages surrounding pregnancy and fish consumption.

(For the video version of this post, click here).

Because aren’t we supposed to be eating more fish? Fish are the main dietary source for omega-3 fatty acids, which can cross the placenta, and may promote healthy brain development. Of course, some of these fish contain mercury which, as Jeremy Piven taught us all, may be detrimental to cognitive development.

Thankfully not while pregnant

These contradictory facts led the US FDA, in 2014, to recommend that pregnant women consume more fish, but not more than 3 times a week.  You have to love the government sometimes.

A study appearing in JAMA pediatrics is making some waves with its claim that high levels of fish consumption, more than 3 times per week during pregnancy, is associated with more rapid neonatal growth as well as higher BMIs throughout a child’s young life. Now, contrary to what your mother-in-law has been telling you, more rapid infant growth is not necessarily a good thing, as rapid infant growth is associated with overweight and obesity in childhood and adulthood.

But fish as the culprit here? That strikes me as a bit odd. Indeed, prior studies of antenatal fish consumption have shown beneficial or null effects on childhood weight gain.  What is going on here?

The authors combined data from 15 pregnancy cohort studies across Europe and the US, leading to a final dataset including over 25,000 individuals. This is the studies greatest strength, but also its Achilles heel, as we’ll see in a moment.

But first the basic results. Fish consumption was based on a food frequency questionnaire, a survey instrument that I, and others, have a lot of concerns about. Women who reported eating less than or equal to 3 servings of fish a week had no increased risk of rapid infant growth or overweight kids.  But among those eating more than 3 servings, there was around a 22% increased risk of rapid growth from birth to 2 and overweight at age 6.

These effects were pretty small, and, more importantly, ephemeral. The authors looked not only at the percentage of obese and overweight children, but the raw differences in weight. At 6 years, though the percent of overweight and obese kids was statistically higher, there was no significant weight difference between children of mothers who ate a lot of fish and those who didn’t. When statistics are weird like this, it usually suggests that the effect isn’t very robust.

In fact, this line from the stats section caught my eye, take a look:


That means the authors used numbers predicted by a statistical model to get the weight of the children rather than the actual weight of the children. I asked the study’s lead author, Dr. Leda Chatzi, about this unusual approach and she wrote “Not all cohorts had available data on child measurement at the specific time points of interest… in an effort to increase sample size and…power in our analyses, we…estimated predicted values of weight and height”.

So we have a statistical model that contains as a covariate, another statistical model. This compounds error into the final estimate, and in a study like this, where the effect size is razor thin, that can easily bias you into the realm of significance.

Pimp My Ride bias

And, at this point it probably goes without saying, but studies looking at diet are always confounded. Always. While the authors adjusted for some things like maternal age, education, smoking, BMI and birth weight, there was no adjustment for things like socio-economic status, sunlight exposure, diabetes, race, or other dietary intake.

What have we learned? Certainly not, as the authors suggest, that

no. just no.

That they wrote this in a study with no measurement of said pollutants is what we call a reach.

Look, you probably don’t want to be eating fish with high levels of mercury when you are pregnant. But if my patients were choosing between a nice bit of salmon and a cheeseburger, well, this study doesn’t exactly tip the scales.


"Well, son, you'll either be a violent criminal or a triathlete. Only time will tell".


For the video version of this post, click here. Predicting future crime is a cool idea one that has seen play from dystopian novels to Hollywood movies, but the results never seem to work out that well. Thats the case in a study appearing in JAMA psychiatry that well be discussing in the next 150 seconds. This study examines the relationship between resting heart rate and future crime. It is 100 times larger than all prior studies of this phenomenon combined. And what it says is, yes, your heart rate is associated with your risk of future violence. But, Im going to argue, that it doesnt matter. Here are the details.

Swedish researchers looked at a cohort of around 750,000 Swedish men reporting for mandatory military conscription evaluation at the age of 18. Using Swedens robust national reporting system for health and crime, they were able to follow these individuals for as many as 35 years. They collected data on violent crime, non-violent crime, being a victim of violence, and even unintentional injury.

What they found was that those with lower heart rates were more likely to experience all of these outcomes. If your heart rate was less than 60 beats per minute, your risk of committing a violent crime was 25% higher than someone whose heart rate was above 83. Thats before adjusting for things like cardiorespiratory fitness and socioeconomic factors, but accounting for those confounders actually increased the risk - to about a 50% increased likelihood of violent crime.

The authors offer two explanations. One, that low resting heart rate is a marker of chronically low physiological arousal - those with low resting heart rate might engage in risky behaviors to bring themselves up to a more normal level. Or two, its a marker of fearlessness - in the stressful situation of a conscription exam, low resting heart rate suggests youre not easily frightened. If thats the case, maybe you dont fear the consequences of your actions as much in the future. These theories cant be teased out in the context of the study, but they are certainly intriguing.

Where things go a bit off the rails though is in the introduction, discussion, and accompanying editorial, where the prognostic value of resting heart rate is seriously considered. The authors imply that, perhaps, we should be paying special attention to those with low RHR. Aside from the fact that this rankles my libertarian sensibilities, I dont believe this is at all supported by the data.

The authors dont give us enough data to assess how good a test low resting heart rate is, but I made some rough estimates and heres what I found.

If we had a million 18-year olds, according to this study roughly 58,000 would commit a violent crime, and 200,000 would have a resting heart rate less than 60. If we targeted that group, wed capture roughly 10,100 future criminals, and 190,000 future innocents. Wed be right about 5% of the time. Interestingly, if we just picked a random 200,000 people and labeled them aspotential criminals, wed be right 5.8% of the time. A test that works better when you dont do it is not a very good test. So no, we should not identify these adolescents as being at risk, as the authors suggest, or consider resting heart rate as a mitigating factor in criminal trials as the editorialists suggest. Doing that would, well, really get my heart rate up.

Does coffee stave off melanoma? The answer won't surprise you.

Coffe_time A recent study appearing in the Journal of the National Cancer Institute claims that coffee might prevent melanoma.  Watch the following video to see what other hoops this poor data set has had to jump through.

Click below to view the video:

Analysis: Can Coffee Prevent Melanoma? | Medpage Today.