The advocacy film Forks Over Knives focuses on the work of Drs. Caldwell Esselstyn and T. Colin Campbell. These protagonists argue that many common human ailments such as heart disease, cancer, and obesity are caused by the animal product-heavy Western diet. They contend that many of these issues would be greatly diminished or resolved if people ate a strict whole food, plant-based diet. The film promotes treating and preventing disease with a healthy diet rather than pharmaceutical drugs. Generally, these philosophies make sense; one hardly needs a Ph.D. in nutrition to understand that we would be better off eating vegetables rather than cheeseburgers. Unfortunately, a good deal of the “evidence” presented in the film for a whole food, plant-based diet is grossly misrepresented. Forks Over Knives fits into a common trend in science in the popular media: facts are twisted to fit certain motives, and they are dumbed down and misinterpreted to the point of no longer being true. In order to be more observant and get closer to 100% information, we need to be able to recognize common ways information is misrepresented. I will use examples from Forks Over Knives to illustrate some common misrepresentations employed by popular media so readers will be able to recognize them in other contexts in the future.
Forks Over Knives asserted that consuming animal-based products leads to an increased risk of cancer. Even such a simple statement implies several misrepresentations, and most clearly, a lack of distinction between correlation and causation. Billions of people globally consume animal products, and billions also get cancer, though this does not necessarily mean that consuming animal products causes cancer. In fact, humans are fundamentally prone to getting cancer due to some basic elements of biology. The human body contains trillions of cells, and millions are dividing at any given moment in order to keep us alive. However, every time a cell divides, it must copy its DNA for the new cell. While mistakes are very rare in this process, given the large scale, they do sometimes occur. If this mutation occurs in the right (or rather, wrong) spot, the cell can go out of control and divide more than it is supposed to. The body does have some defenses against these rogue cells, but unfortunately they do not always work. These rapidly dividing cells grow into cancer tumors and steal resources from the rest of the body. While the initial mutation can be caused by exposure to carcinogens, most mutations are the result of errors in DNA replication. Older people are more likely to get cancer because their cells have more time to make errors, they have more time to be exposed to carcinogens, and the body’s natural cancer-fighting defenses weaken with age. When considering the long life spans in the Western world, it makes sense that many people would die from cancer, and it is indeed the second leading cause of death in the United States. Cancer and animal product-heavy diets are both common in the Western world, but this does not necessarily mean that animal products cause cancer.
The concept of correlation vs. causation relates well to observational studies vs. experimental studies. In observational studies, researchers observe factors like the diet and health of a set group of subjects. They do not control what the subjects do; they simply observe their defined factors. It is very difficult to make meaningful conclusions based on observational studies because there is not always a clear, clean-cut relationship between a behavior and an outcome. This is especially prominent in health studies that attempt to define a relationship between two narrow factors, such as increased olive oil consumption and fewer heart attacks. People who eat more olive oil are more likely to display other healthy behaviors like exercise, eating vegetables, and going to the doctor regularly; these would be considered confounding factors. Therefore, it is hard to say which factor (or combination of factors) led to the lower incidence of heart attacks. In fact, its hypothetically possible that the olive oil negatively affected the subjects’ health, but their other healthy behaviors made up for it. Some statistical methods can be used to control for confounding factors, but errors and false causational relationships can still occur. Experimental studies are much better for establishing causational relationships. In these types of studies, greater control is exerted and confounding factors are minimized. In health studies, animal models are usually used; it is difficult and often unethical to completely control a human’s environment and behavior. Experimental studies usually contain a control group and an experimental group. For example, if a research group wanted to establish a possible relationship between consuming olive oil and heart disease, they might take two groups of rats with very similar genetics, ages, etc. They would keep both groups of rats in very similar environments but feed the experimental group food containing olive oil, and the other group food containing a different, and ideally neutral oil. If the researchers found a statistically significant difference in the rates of heart disease between the two groups, it would be easier to say this difference was due to the dietary olive oil compared to an observational study. Experimental studies are much more powerful than observational studies, but results in animal models do not always translate to humans.
Poor study design was also a huge issue in many of the studies presented in Forks Over Knives. For example, the film stated that breast cancer rates were 82 times lower in Kenya compared to the United States in 1978. While Forks Over Knives attributed this to Kenyans’ whole food, plant-based diet, this conclusion was more likely the product of poor study design. Kenya has a much lower GDP than the United States and far lower healthcare expenditures; women may not have the money to visit a doctor, and thus they might not be diagnosed with breast cancer even if it develops. Women in Kenya also have a shorter lifespan than women in the United States, so they might pass away before breast cancer has the chance to develop. This comparison would be more meaningful if the researchers studied women of similar socioeconomic status and similar healthcare utilization. Still, any differences would be questionable since it would be an observational vs. experimental study. The film also stated that only 18 Japanese men died of prostate cancer (as confirmed by an autopsy) in 1958, while 14,000 men died of the disease in the same year. While the socioeconomic gap between Japan and the U.S. is a bit smaller than that of Kenya and the U.S., Japan conducts far fewer autopsies than the U.S. Furthermore, prostatectomies are much more rare in Japan compared to the U.S., perhaps indicating that Japanese doctors are less concerned with prostate cancer than their American counterparts. So, it is much more likely that more men died of (or with) prostate cancer than what was reported; they simply weren’t diagnosed with the disease while the American men were.
Forks Over Knives placed a good deal of emphasis on one study to support the “animal products cause cancer” hypothesis. The 1968 study, entitled “The Effect of Dietary Protein on Carcinogenesis of Aflatoxin” involved two groups of rats. Both groups were exposed to a large amount of a carcinogen, aflatoxin, and then one group was fed a diet containing 5% casein (an animal protein often found in milk), and the other 20%. The rats fed the high protein diet quickly developed liver cancer, while liver cancer was rare in the low protein group. However, Forks Over Knives neglected to mention that nearly half of the low protein rats died within a year, while none of the high protein rats suffered the same fate, even with cancer. The liver plays a key role in neutralizing toxins in the body, and when the low protein rats were not receiving enough protein, the enzyme activity in the liver dropped. This caused the aflatoxin to build up in the liver, killing healthy cells. The liver lost its filtration capabilities, and the supposedly healthy low protein rats began dying off. Forks Over Knives extrapolated these results, which considered one animal protein in isolation and a non-human mammal, to all animal proteins and humans. Later, a more representative study was conducted with rhesus monkeys. The monkeys were given a small but consistent dose of aflatoxin over time, rather than an enormous dose at one time like the rats. Like the rats, the monkeys were divided into two groups and fed a diet containing either 5% or 20% casein. However, in this case, the monkeys on the low protein diet quickly developed liver cancer, while the high protein monkeys did not. These results still do not necessarily directly translate to humans in the real world, but the conditions are a bit more applicable than those in the rat study. The rats and casein study highlights how poor study design and oversimplification can produce false conclusions.
Forks Over Knives also used wartime Norway as example of how people can achieve improved health by eliminating their consumption of animal products. Several errors and misrepresentations were present in their “evidence.” The film states that when the Germans invaded Norway, they confiscated the livestock and farm animals to feed their own troops. This supposedly resulted in a decrease in heart disease in the Norwegians, as displayed in this graph:
Once again, this narrative is oversimplified to the point of being false. This graph in particular is quite misleading; when focusing on just the yellow line, it looks like there was a huge drop in heart disease during the German occupation of 1939-1945. This interpretation is echoed by the narration of the film. However, like the other death rate statistics in the film, this graph only represents the mortality from circulatory diseases, i.e. what was written on the death certificate. It does not reflect how many people actually had heart disease during this time, or how many died with heart disease but had their deaths noted under a different and more obvious cause (notably, tuberculosis deaths increased during the war, and many tuberculosis victims also had signs of heart disease). This graph also shows the death rate per 10,000 people, with the drop reflecting a difference of only 6 in 10,000. When considering this scale, the drop seems quite minor, and perhaps within the range of random variation. Furthermore, the film neglects to mention that the Norwegians largely replaced their livestock consumption with fish, which is still an animal. There was also a wartime black market that included non-fish animal products, so their consumption did not necessary drop as much as the film implies. In all, nutrition patterns changed considerably during this time period in Norway, so it is difficult to attribute the drop in heart disease to reduced animal product consumption. Again, this supposed drop in heart disease could also be an issue of diagnosis. Though Norway had a rather small Jewish population prior to the war, the country passed laws in 1939 that placed restrictions on Jews practicing medicine. This may have left fewer doctors to diagnose heart disease or declare it the cause of death. The German occupation also had negative effects on the Norwegian economy, likely leaving the Norwegians with fewer resources to see a doctor. The misleading graph shown does little to support the film’s hypothesis, and the film also ignores key information that could have led to a reduced incidence of deaths due to heart disease during the German occupation of Norway.
Unfortunately, there are many more misrepresentations in the film than what I have discussed here. I encourage readers to view Forks Over Knives and other representations of science in the media with a skeptical eye. The diet industry in the United States is extremely profitable, and when combined with Americans’ generally poor understanding of science, many people lose money to ineffective and sometimes harmful diet plans. These consequences can also extend to dietary supplements and other “health-promoting” treatments and products (e.g. anti-aging treatments, detoxifying treatments, and essential oils). Particularly with regards to health, peer-reviewed scientific papers are typically more reliable and trustworthy than celebrity endorsements, daytime TV shows, and advocacy films with associated diet plans for sale. However, even peer-reviewed scientific papers contain misrepresentations from time to time. We can become more observant and accelerate the cycle of learning by learning to recognize “evidence” that is supported by poor study design, sampling bias, correlational relationships, and other misrepresentations. In turn, this allows us to reach a higher level and create more value for others.
Highly recommended for further reading; a key source for this paper and a good example for how to evaluate science as represented in the popular media
Causes of death in the United States and other health-related information over time
Top causes of death in the United States in 2013
Cell division rate
Healthcare patterns in Japan
Information on Japan’s low autopsy rate
Restrictions on Jewish doctors in wartime Norway
Information on the conditions in wartime Norway