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PETA (People for the Ethical Treatment of Animals) has a history of (as the old saying goes) using science as a drunk uses a lamppost – for support rather than illumination. In that way they are typical of ideological groups. They have an agenda, they are very open about their beliefs, and they marshal whatever arguments they can in order to promote their point of view.

Favoring information that supports our current beliefs is a cognitive bias common to Homo sapiens, but ideology tends to take this simple bias to a new level. It can lead to the systematic distortion or denial of science, and render belief systems immune to logic and evidence.

PETA provides us with a nice example of how having an ideological agenda can motivate an individual or a group to embrace dubious science. In an article currently on their website, and making the rounds in social media (this is repeating a claim from at least 2008, but the current article is undated), the group warns: Got Autism? Learn About the Link Between Dairy Products and the Disease. They claim:

The reason why dairy foods may worsen or even cause autism is being debated. Some suspect that casein harms the brain, while others suggest that the gastrointestinal problems so often caused by dairy products cause distress and thus worsen behavior in autistic children.

Saying that “how” dairy harms the brain is being debated implies “that” dairy harms the brain is accepted and not being debated. This is misleading. It is not accepted that dairy harms the brain or is in any way linked to autism, and the evidence is largely against it.

Gluten-free and casein-free diets for autism have been around for decades. They are based largely on the anecdotal observation that children with autism can experience a worsening in behavior when they consume gluten (a protein in wheat and other grains) or casein (a protein in milk and other dairy products). Parents will sometimes try to place their children on a gluten-free or casein-free diet, and some report improvement in behavior.

Such observations are a reasonable basis for a hypothesis, but not a conclusion. Behavior in children, especially those with the challenge of autism, can be unpredictable. Unpredictable and variable symptoms lend themselves to confirmation bias, with a strong tendency to lead to the anecdotal experience that whatever is being looked for is real. For example, many parents believe that sugar makes their children hyperactive, when this is simply not true.

Such uncontrolled observations need to be confirmed by blinded observations. These studies have been done for both gluten-free and casein-free diets. A 2008 Cochrane review of these studies concluded:

Current evidence for efficacy of these diets is poor. Large scale, good quality randomised controlled trials are needed.

A more recent review from April 2014 came to a similar conclusion:

We observed that the evidence on this topic is currently limited and weak.

The pattern of evidence reveals that the methodologically poor studies, ones that are liable to confirmation bias, show some effect, but the properly blinded studies tend to show no effect. For example, a 2010 study (although small) observed children with autism on a gluten-free and casein-free diet, and then challenged them with either gluten, casein, or placebo in a blinded manner. There was no difference in behavior observed. A recent 2014 study also showed no association between dairy and behavior in autism.

As an aside, when such studies are pointed out, the emotional argument is sometimes made that we are “attacking mothers” or “criticizing parents.” This is a diversion, however. Our only point is that parents are humans and are subject to the same cognitive biases that we are all subject to. Being a parent does not magically render someone immune to bias. We need controlled and blinded observations so that we can differentiate between a real effect and self-deception. When an effect disappears under proper blinding, the most likely conclusion is that the phenomenon is not real but is an artifact of observational bias.

Given current evidence, that is the best conclusion we can come to regarding the effects of gluten and casein on autism. The evidence for any effect on behavior is weak and likely not real. There is also no credible evidence to suggest that casein plays a causal role in autism. The evidence is overwhelming that autism is a genetic disorder. Clinical signs are evident at least by 6 months of age, if not sooner, and there is evidence that the developmental processes leading to autism begin in the womb. Despite this, the PETA article concludes:

It isn’t surprising that dairy products may worsen this disease, considering that milk has already been strongly linked to cancer, Crohn’s disease, and other serious health problems. Anyone who wants to alleviate or avoid the devastating effects of autism should give cow’s milk the boot and switch to healthy vegan alternatives instead.

PETA claims that “milk has already been strongly linked to cancer.” That is their summary of the evidence. Now let’s read the conclusion of a 2011 systematic review of the published evidence:

For most cancers, associations between cancer risk and intake of milk and dairy products have been examined only in a small number of cohort studies, and data are inconsistent or lacking. Meta-analyses of cohort data available to date support an inverse association between milk intake and risk of colorectal and bladder cancer and a positive association between diets high in calcium and risk of prostate cancer.

(This May 2014 systematic review comes to the same conclusion.) So milk, if anything, has a protective effect for colon and bladder cancer, while high calcium diets (not necessarily from dairy, but milk is high in calcium) increase the risk of prostate cancer. For all other cancers the jury is still out. Milk therefore might reduce cancer risk in women, and probably also in men, as long as you limit your total calcium intake.

There is also not a “strong” association with Crohn’s. There is preliminary evidence at best. One study of the incidence of Crohn’s in Japan found an increase in incidence after the introduction of a more Western diet, including meat and dairy, and reduction in rice consumption. There are too many variables here to make any conclusions about dairy specifically. There is also preliminary evidence that a bacterium found in cow’s milk may be linked to Crohn’s. Crohn’s is still a poorly understood disease and there is no clear evidence that dairy is playing a causative role.

It’s not even clear if people with Crohn’s need to limit their dairy intake. There are no blanket recommendations for patients with Crohn’s. A recent study found no association between dairy intake and symptoms, suggesting that there is no need to restrict dairy. Some patients with Crohn’s may also have lactose intolerance, and this can be difficult to diagnose because of the overlap in symptoms, so a lactose-free trial period is often recommended. However, eliminating dairy can cause nutritional deficiencies, such as low calcium, and so this needs to be done with care.

Conclusion

There is no evidence that dairy products cause autism, and the current evidence (while weak) does not support the conclusion that dairy has an adverse effect on behavior in children with autism. Further, dairy has a complex relationship with cancer risk, and is as much a benefit as a potential risk factor. There is also not a strong link between dairy and Crohn’s, although some preliminary evidence warrants follow up.

PETA, however, has launched a campaign warning the public about the association between milk and autism, throwing in claims that it is “strongly” linked to cancer, Crohn’s, and other unnamed diseases.

This is clearly, in my opinion, a campaign of fear mongering based upon a gross distortion of the scientific evidence. The purpose is to advocate for a vegan diet, which fits their ideological agenda. They are likely aware that it is easier to spread fears than to reassure with a careful analysis of the scientific evidence.