Shifting Sands Of Nutritional Science
Jim Prevor’s Perishable Pundit, November 9, 2007
Our piece, An Observational Study Is Not An Experiment: Cautions In Research Interpretation, was just one of many pieces about the limited and confusing state of scientific knowledge on nutrition. Articles such as this led an industry friend to send along this note:
This stuff drives me crazy. You just never know what to do from an eating standpoint.
One study I read said eat only flax seed oil — it’s much better for you. So I was doing that. Then I read another study that said flax seed oil seems to have a strong link to accelerating the growth of prostate cancer. Chocolate’s good for you, no wait it’s bad for you…….on and on it goes.
You’ve also already seen stuff on soy — real good — or maybe not so good. Now another report: Men who eat a half a serving of soy each day have much lower sperm counts than men who do not, according to a study at Harvard Public School of Health in Boston. However, the soy industry claims the findings do not compare with its own studies that show soy to have no effect on sperm count, reported Fox News Channel.
From a consumer’s standpoint, I might as well eat what I want, when I want it, and in the quantities I desire. Woody Allen’s line about “health food” from the movie Sleeper comes to mind: “It’s got to be good for you; it tastes terrible”.
Well, I would suggest the science of nutrition is terrible.
To be fair, the science of nutrition suffers from three things:
First, laymen pay attention to it, which means that it gets covered in the media. Scientists have wrong ideas in all kinds of fields but rarely does the layman hear about some thesis that gets proved incorrect in particle physics.
Second, the stakes are so high — billions upon billions of dollars in medical costs, billions in sales of different foods — that there is a strong impulse to act on very imperfect and incomplete information. Normally that doesn’t apply to a lot of science.
When the pressure is on, much of science is uncertain. The recent death of Paul Tibbets, who flew the Enola Gay over Japan to drop the atomic bomb on Hiroshima, reminds us that one reason proposals to demonstrate to Japan a drop of the atomic bomb instead of actually using it on people were rejected is because, until very late in the process, nobody was sure it would work or how extensive the damage would be. As it turned out, the bomb was from twice to ten times as powerful as most estimates held it would be.
Third, it is so difficult to do legitimate experiments on human beings because of our ethical standards. For example, if there is a consensus that eating at least five servings of fruits and vegetables a day is required for optimal health, we can never do a test with a group being restricted to less than that consumption level for ethical reasons. We could give one group five a day and another 20 a day and see if that makes a difference, but we can’t actually do a controlled experiment where one group gets only one a day and we compare that to five a day.
Still, this all being understood, the news reports are bound to drive people crazy. If on November 7, 2007, you happen to be reading the Chicago Tribune, for example, you would have seen this headline:
No bones about it: Study firmly links obesity, cancer
If you, however, preferred to read The New York Times on the same day, you would have found a large graphic with this headline:
Fewer Deaths Among the Overweight
The New York Times graphic came with an article that explained that Causes of Death Are Linked to a Person’s Weight.
Now to some extent, the difference between these two articles is one resolved by careful reading. The Chicago Tribune piece is about cancer and only cancer:
One of the largest medical studies ever undertaken has confirmed what many public health officials already feared: Being overweight can give you cancer.
Whereas The New York Times piece is about death from any cause:
Linking, for the first time, causes of death to specific weights, they report that overweight people have a lower death rate because they are much less likely to die from a grab bag of diseases that includes Alzheimer’s and Parkinson’s, infections and lung disease. And that lower risk is not counteracted by increased risks of dying from any other disease, including cancer, diabetes or heart disease.
As a consequence, the group from the Centers for Disease Control and Prevention and the National Cancer Institute reports, there were more than 100,000 fewer deaths among the overweight in 2004, the most recent year for which data were available, than would have expected if those people had been of normal weight.
Another point is the use of specific terminology. The Chicago Tribune piece talks about “obesity,” whereas The New York Times piece talks about being “overweight.” These terms mean different things as The New York Times piece explains:
Researchers generally divide weight into four categories — normal, underweight, overweight and obese — based on the body mass index, which is a measure of body fat based on height and weight. A woman who is 5 foot 4, for instance, would be considered at normal weight at 130, underweight at 107 pounds, overweight at 150 pounds and obese at 180.
Still, the articles do seem to directly contradict each other at some points. For example, the Chicago Tribune piece claims:
It is already established that excess body fat is an important cause of diabetes, heart disease, stroke and early death.
However, The New York Times piece specifically refutes the notion that “excess body fat” — as opposed to obesity — is even correlated with, much less a cause of, early death:
“If we use the criteria of mortality, then the term ‘overweight’ is a misnomer,” said Daniel McGee, professor of statistics at Florida State University.
“I believe the data,” said Dr. Elizabeth Barrett-Connor, a professor of family and preventive medicine at the University of California, San Diego. A body mass index of 25 to 30, the so-called overweight range, “may be optimal,” she said.
And on the specific claim of the Chicago Tribune piece that obesity correlates with increased likelihood of cancer, The New York Times piece says no:
…contrary to expectations, the obese did not have an increased risk of dying from cancer. They were slightly more likely than people of normal weights to die of a handful of cancers that are thought to be related to excess weight — cancers of the colon, breast, esophagus, uterus, ovary, kidney and pancreas. Yet they had a lower risk of dying from other cancers, including lung cancer. In the end, the increases and decreases in cancer risks balanced out.
Ahh, this explains something. The Chicago Tribune piece deals with a study that actually found this:
…the study found that excess weight causes cancers of the esophagus, endometrium (uterine lining), ovary, kidney and pancreas, as well as leukemia, multiple myeloma and non-Hodgkin’s lymphoma.
It also causes breast cancer — but only in post-menopausal women.
The New York Times piece draws on research that doesn’t look at just certain cancers but at cancer in its totality, thus “….a lower risk of dying from other cancers, including lung cancer. In the end, the increases and decreases in cancer risks balanced out.”
Which to some extent just tells you that medical and nutritional research is just like any other kind of research: The answer you get depends on the question you ask.
By focusing on particular cancers, the research group that the Chicago Tribune piece draws on thinks that though “…scientists may continue to debate how much excess fat one has to have for his risk of cancer to go up … we have passed the point of debating whether the relationship exists. There is now consensus that obesity increases the risk of getting and dying from multiple types of cancer.”
Whereas the researchers that The New York Times piece draws on looks at the overall situation and “…concluded that, compared with people of normal weight, the overweight had a decreased death risk and the underweight and obese had increased risk.”
All in all, it is a cautionary tale for those anxious to use nutritional evidence to promote particular food items and even whole industries. We just don’t know an awful lot and what we think we know may yet be proved wrong.
Our letter-writer reminded us of the Woody Allen movie Sleeper. As far back as 1973 when that movie came out, it was clear that an awful lot of what we “knew” about nutrition wasn’t so. Unfortunately the interceding 34 years do not date this bit of dialog from the film, which takes place 200 years in the future after the Woody Allen character, who ran a health food store back in 1973, is revived from cryostasis. His doctors are discussing Woody Allen’s breakfast request:
Dr. Melik: This morning for breakfast he requested something called “wheat germ, organic honey and tiger’s milk.”
Dr. Aragon: [chuckling] Oh, yes. Those are the charmed substances that some years ago were thought to contain life-preserving properties.
Dr. Melik: You mean there was no deep fat? No steak or cream pies or… hot fudge?
Dr. Aragon: Those were thought to be unhealthy… precisely the opposite of what we now know to be true.
Dr. Melik: Incredible.
For the foreseeable future a focus on fresh, on flavor, on lifestyle — is likely to provide a sturdier ground for produce promotion than the shifting sands of nutritional science.