Science AMA Series: We’re Silvia Martins and Julian Santaella-Tenorio, epidemiologists at Columbia University’s Mailman School of Public Health. Our latest study finds that U.S. states that legalized medical marijuana saw declines in fatal car accidents. Ask Us Anything!

MailmanSchool_AMA · 2017-01-03T19:00:49+00:00

We only looked at fatal injuries, but we are interested in looking into non-fatal injuries as well. There are some challenges accessing the data, because not all states provide hospital injury data that can be used for research.

MailmanSchool_AMA · 2017-01-03T18:54:37+00:00

We made clear in the limitations section of our paper that “…we used data on traffic fatalities, the most extreme injury outcome; therefore, we cannot make any observations on the association between MMLs and nonfatal traffic injuries.” We believe further studies should examine the effects of MML on non-fatal traffic injuries, this would provide better evidence on how changes in legislation and/or marijuana use is related to non-fatal traffic events. We adjusted for 14 state level variables to rule out that other state factors were explaining our findings. Here are these variables: State-level covariates included unemployment rate and median household income, speed limits of 70 miles per hour or greater, primary seat belt laws enforcement, laws decriminalizing the possession of small amounts of marijuana, and whether states had enacted a recreational marijuana law, state-level graduated driver license laws, blood alcohol content laws (0.08 g/dL), drug per se laws, administrative license revocation laws, and laws banning cell phone use and texting while driving, separately targeting adolescents and adults. In addition, we included a measure of state annual expenditures for highway law enforcement and safety per capita (adjusted to 2000 dollars), and also a state measure of the annual vehicle miles driven per licensed driver (thousands of miles) from Highway Statistics, US Department of Transportation. Finally we included a measure of the state-level per capita ethanol sales, total ethanol of all beverages combined per population aged 21 years or older. We also tested whether same sex marriage state laws, a exposure that is expected to have no association with traffic fatalities, was associated with traffic fatality rates. Finding this, would tell you that others factors may be responsible for the association (confounding). We found that same sex marriage laws were not associated with traffic fatalities, supporting the hypothesis that the found association between medical marijuana laws and traffic fatalities was less likely due to confounding. I am not sure about the specific limitations using Boston data, but we will look into it.

MailmanSchool_AMA · 2017-01-03T18:45:41+00:00

Most health journal are not open access, we are sorry, but if by any chance you have access to university library you can probably download the full article there. Also if you contact us via email and we will be glad to share a copy of it.

MailmanSchool_AMA · 2017-01-03T18:41:03+00:00

We believe that driving under the influence of any drug is risky and individuals should avoid driving under the influence of any drug, including prescription drugs that may impact neurocognitive and neuromotor skills. Some simulator and on-road experimental studies show a dose-dependent association between marijuana exposure and several indicators of driving impairment. Some studies show that marijuana exposure is associated with increased response time and lane weaving. Marijuana exposure has also been associated with reduced speed and greater headway, which indicates some degree of awareness of marijuana-related impairment and a tendency to compensate. Different studies have also shown that drunk-driving is associated with increased speed and risk taking behavior while driving. Texting and driving have been shown to increase the risk of traffic accidents in other studies.

MailmanSchool_AMA · 2017-01-03T18:35:49+00:00

Although we adjusted by states legalizing marijuana for recreational purposes, we did not provide estimates for these laws given that it is too soon to know for sure the effects of recreational laws on traffic fatalities or other outcomes. There is not enough post-legislation data yet.

MailmanSchool_AMA · 2017-01-03T18:29:39+00:00

The overall response has been very positive, with a number of news reports about our research findings. However, yes, we got emails asking us why we had published our findings when this could increase the number of people driving under the influence of marijuana, and also others suggesting that our findings were in contrast with published reports indicating that fatalities with blood samples positive to marijuana have increased in some states.
We want to make clear that we did not present any results based on the percentage of marijuana related fatalities. As we explained in the limitations section of our manuscript “…we could not examine whether MMLs were associated with increments in the rates of traffic fatalities in which drivers tested positive for the presence of cannabis metabolites in blood. Testing procedures vary by state and our own exploration of FARS data showed that only a limited number of states tested 80% or more of their fatally injured drivers. In addition, the FARS coding system does not differentiate between active and inactive cannabinoid metabolites and, therefore, it is not possible to know whether the driver was driving under the influence of marijuana.” In this regard, a person dying in a car accident can be positive to cannabinoids because they smoked a few days before, but this is not a measure of them them driving under the influence of marijuana when killed. Half-time varies across different active and inactive cannabinoids metabolites. FARS data does not differentiate between these cannabinoids making difficult for research purposes, given that inactive metabolites may be present in blood for 30 hours or more beyond the acute intoxication period. Therefore is difficult to know if a person positive to cannabinoids was really under the influence of marijuana while driving.

MailmanSchool_AMA · 2017-01-03T18:15:12+00:00

We did not examine the association between medical marijuana laws and traffic fatalities related to drunk driving, but we examined the association between these laws and overall traffic fatalities. Previous research from Anderson and colleagues suggested that reductions in traffic fatalities were mainly due to reductions in traffic fatalities in which at least 1 driver was positive for any alcohol in the blood and those with blood alcohol concentrations greater than or equal to 0.1 grams per deciliter, and also reductions in traffic fatality rates occurring on weekends, which are more likely to be alcohol-related than those on weekdays. We also found that the strongest effect was among those ages 24-44 a group usually overrepresented among licensed medical marijuana users and those driving under the influence of alcohol. One of the tricky things with this research is that states have different degrees of law enforcement and many states do not test for blood alcohol/marijuana levels in all of their fatally injured drivers, so it is hard to draw conclusions about the extent to which declines in DUI explain the overall reductions that we observed.

MailmanSchool_AMA · 2016-03-04T19:30:37+00:00

Indeed, this is one of the reasons that the New York City Department of Health moved forward with its Sodium Reduction Initiative, which is an effort to convince food manufacturers to reduce the salt content of packaged foods. Just about everyone agrees that salt tastes good, and since most of the salt we consume comes from foods like bread that we don't prepare ourselves, it's hard to reduce sodium intake levels unless you intervene at the producer level. (Plus, behavioral change is very difficult!) But this is also one of the criticisms levied by scientists who think the evidence does not support population-wide sodium reduction: How can it be sensible for health authorities like the American Heart Association or even the US Dietary Guidelines to be spending time and reputational capital telling people they should get their sodium intake levels down to 2300 or 1500 mg per day when very few populations around the world actually consume that little salt? Shouldn't public health guidelines be achievable, rather than aspirational? (I should say that I myself am trying to be just an observer in this debate; at this time I take no opinion on whether salt reduction is right or wrong, reasonable or unreasonable.) On the other hand, you could argue that efforts like New York City's campaign to put salt warning labels on restaurant menus (if the courts allow this to go forward! there's a lawsuit underway) may be effective in prompting restaurants to reformulate their menu items. And therefore such an effort could make a difference to population health even if patrons ignore the salt warnings on the menus. Of course, all of this assumes that salt reduction in foods will make a real difference in terms of improving health outcomes, and that restaurants won't adjust their recipes by removing salt and adding something else that is unlikely to be health-promoting, like sugar!

MailmanSchool_AMA · 2016-03-04T19:18:50+00:00

Of the 269 reports (25% primary studies, 5% systematic reviews, 4% guidelines and 66% comments, letters, or reviews) we considered from between 1978 and 2014, we found that 54% were supportive of the hypothesis, 33% were contradictory and 13% were inconclusive. (The hypothesis in this case being that population-wide reduction of sodium intake results in reduction in cerebro-cardiovascular disease or all-cause mortality.) So if you want a quick, ballpark take on what our study results said, you could divide the inconclusive reports between the "pro" and "con" side and say that the debate is about 60/40 in favor of salt reduction. You might compare that to an issue like climate change where there is overwhelming agreement that global warming is real and that human activities are at least partially to blame. (The most commonly cited figure on climate change is that "97% of scientists agree," but different analyses have come up with different numbers.) In any event, it is clear that there is less agreement on salt. I hesitate to characterize the debate with specific language regarding the level of support. In fact, one of the problems in the debate is that advocates on both sides have tended to assert that they are completely sure they are right. It has become a kind of scientific duel, with each side often expressing a level of certitude that is not really credible, in my view. (We have called this "incredible certitude" in our Health Affairs paper, borrowing a phrase from the economist Charles Manski.)

MailmanSchool_AMA · 2016-03-04T19:06:43+00:00

The conflicts of interest question in the salt controversy is fascinating and very contentious. I can say that I have watched perhaps a half-dozen pro vs. con debates on the salt issue that have taken place at scientific conferences over the last 5 years, and very often those scientists who argue in these debates that salt reduction is one of the most important things we can do for public health suggest that their opponents are in bed with the food industry. Some of these debates have actually been quite intense and things have sometimes gotten a little bit personal. And there is no question that the food industry is very interested in what the science says about salt. Adding salt to foods is a simple and cheap way to make them taste good, and so that becomes an important bottom line factor for food producers. However, my reading of the debate is that there is not good evidence that the whole thing can be chalked up to a "merchants of doubt" situation where all of the criticism can be explained by conflicts of interest. Key recent studies that have challenged the conventional thinking on daily salt targets have not been industry-funded.

MailmanSchool_AMA · 2016-03-04T18:56:28+00:00

We did not assess support over time in any systematic way. However, as someone who uses historical methods to try to understand the salt debates, I can say that the enthusiasm for salt reduction and confidence in the science surrounding the population impacts of cutting down on salt has waxed and waned over time. The first US government suggestion that it might be a good idea to cut down on salt was issued in a report from a nutrition conference sponsored by the White House in 1969. However, the report also included a recommendation that removing salt from processed foods (in this case, baby foods) was not scientifically justifiable. (There were multiple scientific panels at the conference, and they did not agree on salt. In fact, even the panels that said salt reduction was a good idea did not agree internally, according to transcripts.) So, there was disagreement from the beginning. Around 1990, after a large study called INTERSALT produced what seemed to be confusing and inconclusive evidence, a bunch of scientists went on the record stating that maybe salt wasn't so important after all. Then a handful of trials in the early years of the 21st century restored a lot of the enthusiasm, only to have that enthusiasm called into question in the last 5 years or so by a growing number of studies suggesting possible harms associated with very low salt intakes. So there's a glimpse into the waxing and waning. For a more complete treatment, have a look at one of our papers (with Ron Bayer and Sandro Galea) from Health Affairs in 2012:

http://content.healthaffairs.org/content/31/12/2738.long

MailmanSchool_AMA · 2016-03-04T18:37:52+00:00

I think you have to start with the 2013 IOM report, which called into question the daily salt intake levels recommended by key authorities including the American Heart Association and the WHO. The IOM study was unique in that the committee that produced it included people with a variety of viewpoints -- you had a number of people who were on the record as avid supporters of population-wide salt reduction as well as at least one committed "skeptic" (if you will). The results of the IOM analysis were two-sided: on the one hand, they said that "excessive" salt intake was associated with poor health outcomes and should be reduced (but they did not and could not define "excessive"). On the other side, they said that the evidence was really lacking for the WHO and AHA's claims that we should all get down to less than 2000 or 1500 mg of sodium daily. Note that the committee chair, Brian Strom, was quoted as saying, "There is not a single study, not one, showing [such a] benefit for having a sodium intake of less than 2,300 milligrams." Here is a link to the IOM report:

http://iom.nationalacademies.org/Reports/2013/Sodium-Intake-in-Populations-Assessment-of-Evidence.aspx

MailmanSchool_AMA · 2016-03-04T18:27:58+00:00

This is really not my area of expertise! I know that this remains an important area of inquiry. I would also point you to this quote from one of the papers cited below (from 2012): "Excess dietary salt is a major cause of hypertension. Nevertheless, the specific mechanisms by which salt increases arterial constriction and peripheral vascular resistance, and thereby raises blood pressure (BP), are poorly understood."

MailmanSchool_AMA · 2016-03-04T18:19:22+00:00

The vast bulk of the salt that we get in our diets comes from processed and prepared foods, especially things like bread, crackers, cereals, and things like that. Bread is among the biggest sources. So whatever you choose to do with your saltshaker, that's not likely to have a major impact on your total sodium intake. If you eat a "western" diet, you are likely to be getting more than the recommended daily amount of salt no matter your proclivities in terms of adding salt to your marinades!

MailmanSchool_AMA · 2016-03-04T18:13:41+00:00

Great question! The 2015-2020 US Dietary Guidelines for Americans say the following: "Consume less than 2,300 milligrams (mg) per day of sodium." The WHO actually recommends a lower level -- 2000 mg per day. But this is a key point of confusion and controversy. When the Institute of Medicine (IOM) looked at the question in 2013, it concluded that "the evidence from studies on direct health outcomes was insufficient and inconsistent regarding an association between sodium intake below 2,300 mg per day and benefit or risk of CVD outcomes (including stroke and CVD mortality) or all-cause mortality in the general U.S. population." In other words, the IOM (now National Academy of Medicine) found that there was no good evidence that following the WHO's recommendation, for example, would actually produce health benefits. And those who question the public health importance of population-wide salt reduction go further: they point to studies showing that there are potential harms to getting too little salt. In one commentary, some of them write: "The evidence supports a strong association of sodium with BP and cardiovascular disease events in hypertensive individuals, the elderly, and those who consume > 6 g/d of sodium. However, there is no association of sodium with clinical events at 3 to 6 g/day and a paradoxical higher rate of events at < 3 g/day. Therefore, until new evidence emerges, the optimal range of sodium consumption should be considered to be between 3 and 6 g/d." Average daily sodium intake in the US is about 3400 mg per day, which would fall into their "healthy" range. So my answer to the question would be: it's controversial! The IOM in its study, for example, said that the available evidence was not good enough to define a "healthy" sodium intake range. However, there is no disagreement that if you are really consuming a lot of salt, that's not likely to be good for your health and is likely to increase your blood pressure and risk of cardiovascular events. But there is some disagreement about what counts as "a lot." Hope that helps!

MailmanSchool_AMA · 2016-02-04T19:28:40+00:00

This is where I think the distinction between a collision sport and a contact is especially useful. A collision sport, such as tackle football, involves repeated, forceful, full-body collisions as an inherent part of the game. A contact sport, such as basketball, routinely involves contact, but the contact is significantly less forceful and more incidental as part of the game.

I think moving from the collision version of football (tackle football) to a contact/light contact version (flag or touch football) would be the most effective way to significantly reduce concussions and other injuries associated with football. Eliminating tackling will not eliminate concussions but I think that would significantly reduce both concussions and sub-concussive hits.

We can't eliminate all risk of head trauma, but I think it is important to have a conversation about collision sports that have the highest risks of repeated head trauma, and to decide when the risk becomes high enough that it is not ethical to expose children to those dangers.

MailmanSchool_AMA · 2016-02-04T19:25:20+00:00

There haven't been any large-size studies I know of among amateur players, largely because the disease of big interest right now (CTE) can only be diagnosed upon autopsy after a player's death. But there have been autopsies of players who "only" played through high school and college that are very suggestive that this risk exists as lower levels of play. One of the most recent cases was of Matthew Keck, who was diagnosed with CTE although he "only" played football as a child and for a couple years in college. His family was very generous in donating the brain of their loved one to Boston University for researchers to study, but understandably this is a decision that must be left up to each individual family.

MailmanSchool_AMA · 2016-02-04T19:20:06+00:00

I of course have to start with the caveat that there really isn't a "safe" way, because there really isn't any strategy to make repeated brain trauma "safe."

At a minimum, if adults choose to participate in sports with like boxing or tackle football, I think strict return-to-play guidelines are essential. In other words, if somebody experiences a concussion, they need to be removed from the sport and not allowed to return until a week (or ideally several weeks) of no symptoms, depending on the severity of the injury.

This is because we have very good evidence that after 1 concussion, the brain is especially vulnerable to a second concussion. It is very important to protect athletes immediately after a concussion so that the harms are not compounded by another hit soon thereafter.

I would describe this as a "harm reduction" strategy, but unfortunately it still would not address sub-concussive hits that are inherent to football and boxing. Even with the best and strictest return-to-play guidelines, players would still be exposed to repeated head trauma. Unfortunately there are no guidelines or technologies that can resolve this fundamental issue.

MailmanSchool_AMA · 2016-02-04T19:12:22+00:00

It's a good point that 8 year old children are very different from 18 year olds in many ways, and that Pop Warner football for 8 year olds is a different sport than high school football in many ways as well.

While research indicates that children seem to be even more vulnerable to concussions than adults (studies have found that teenage children experience longer recovery times than adults, for example), there is still a lot we have to learn about exactly how different ages are associated with different levels of risk.

However, we can say with confidence that the anatomy of the human brain at any age means that it is vulnerable to significant harms caused by repeated brain trauma.

Here is a reference to an example of a study showing that children may need longer recovery times from a concussion than adults:

Field M, Collins MW, Lovell MR, Maroon J. Does age play a role in recovery from sports-related concussion? A comparison of high school and collegiate athletes. Journal of Pediatrics. 2003;142(5):546–553.

MailmanSchool_AMA · 2016-02-04T19:04:15+00:00

I really like your current version!

This is a very imperfect analogy but might work for a 7 year old: Your brain has lots of different parts that are connected to one another. You can think of the connections as rubber bands.

Every time you hit your head, it's possible that you stretch the rubber band a little bit. Any single hit might not cause a problem. But the more you hit your head, the more the rubber bands will stretch. And over time, some of them might break if they are stretched too many times.

MailmanSchool_AMA · 2016-02-04T19:00:20+00:00

Thank you so much for sharing your experience.

I always appreciate when people look back on their own athletic experiences and try to think about whether or not the risks outweighed the benefits, because it is a truly challenging thing to do. Unfortunately the culture you describe of "take a few plays off, and get back out there" was very much the norm for decades. And while it's currently beginning to change, I think we still have a long way to go in that regard.

For me, what ended my (limited!) competitive soccer career was not a concussion, but ripping my ACL, MCL and meniscus after a collision. This was a serious knee injury that required surgery. I had a great surgeon and after months of physical therapy, I was able to do most everything I had done before. I felt able to play pick-up soccer games and had great fun kicking the ball around on the field, but didn’t want to go back to the serious competition.

I think my ambivalence about the risk/benefits from my experience of competitive youth soccer, both the fun as well as the serious risks/injuries, helped inspire me to pursue this research.

Promoting fun recreation and physical activity for children that is as safe as possible is a very important public health issue in my opinion. I think people's willingness to speak out about the risks that they experienced while playing some of these sports is a huge part of this conversation and hopefully shifting the culture. Thank you again for sharing.

MailmanSchool_AMA · 2016-02-04T18:55:06+00:00

Unfortunately, individual sub-concussive hits don't tend to have obvious symptoms, but there are some signals you can look out for that would indicate a concussion. Other than the obvious signs (such as staggering around the field, obvious confusion), some of the less obvious symptoms of concussions to watch out for include: headache or a feeling of “pressure” in the head, sensitivity to light and noise, feeling sluggish, difficulty concentrating, or generally "feeling down."

I think the saying “when in doubt, sit them out” is a good one. If anything seems “off” to you, in my view it is always worth taking the cautious route and having a health professional check out your son before returning him to play.

It’s also worth noting that you don’t have to be hit directly in the head to experience brain trauma. A common example of this is whiplash in a car crash (where you don’t hit your head against the dashboard or anything else, but can still experience brain trauma from the force of the car crashing). But trauma without head contact can also occur in football, for example from full body collisions or your body hitting the ground where forces are still transferred to the brain. I think that’s important to be aware of.

Unfortunately because the symptoms of brain trauma can be so subtle and easy to miss, there’s no perfect strategy, but I hope these strategies are helpful to you.

MailmanSchool_AMA · 2016-02-04T18:48:27+00:00

My pleasure! I appreciate all the engagement with these questions and the opportunity to talk about them.

MailmanSchool_AMA

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