Posts Tagged diet

Nutrition science doesn’t claim to have all the answers

May 26, 2012 at 5:30 am · Filed under In Defense of Food, Science ·Tagged , , , ,

In In Defense of Food, Michael Pollan explains what he sees as one of the problems with science-based dietary advice:

When Prout and Liebig nailed down the macronutrients, scientists figured that they now understood the nature of food and what the body needed from it. Then when the vitamins were isolated a few decades later, scientists thought, okay, now we really understand food and what the body needs for its health; and today it’s the polyphenols and carotenoids that seem to have completed the picture.

One has to wonder, if the scientists have long been so confident in the completeness of their understanding of human nutrition, why do they continue researching the subject?

It is true that scientists have sometimes expressed undue confidence in an idea. However, the suggestion that nutrition scientists see human nutrition as a solved problem is patently absurd. These scientists continue doing research precisely because they understand that there are many questions remaing about nutrition.

One sees this, for example, in the 1982 report Diet, Nutrition, and Cancer, which Pollan discusses at length. In introducing its interim guidelines, the report explained,

It is not now possible, and may never be possible, to specify a diet that would protect everyone against all forms of cancer. Nevertheless, the committee believes that it is possible on the basis of current evidence to formulate interim dietary guidelines that are both consistent with good nutritional practices and likely to reduce the risk of cancer.

The authors of the report thus readily acknowledged that they didn’t “really understand food.” They issued guidelines anyway on the belief that useful recommendations could be made based on what they did understand.

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Polyunsaturated fats and heart disease

May 17, 2012 at 5:03 am · Filed under In Defense of Food, Science ·Tagged , , , , , ,

Michael Pollan concludes his attempted takedown of the lipid hypothesis in In Defense of Food by addressing the supposed role of dietary changes in reducing heart disease:

Even if we accept the epidemic of obesity and diabetes as the unintended consequence of the war against dietary fat–collateral damage, you might say–what about the intended consequence of that campaign: the reduction of heart disease? Here is where the low-fat campaigners have chosen to make their last stand, pointing proudly to the fact that after peaking in the late sixties, deaths from heart disease fell dramatically in America, a 50 percent decline since 1969.  Cholesterol levels have also fallen. Epidemiologist Walter C. Willett of the Harvard School of Public Health…cites the increase in consumption of polyunsaturated fats “as a major factor, if not the most important factor, in the decline in heart disease” observed in the seventies and eighties… And so it would appear to be: We reduced our saturated fat intake, our cholesterol levels fell, and many fewer people dropped dead of heart attacks.

Whether the low-fat campaigners should take the credit for this achievement is doubtful, however. Reducing mortality from heart disease is no the same thing as reducing the incidence of heart disease, and there’s reason to question whether underlying rates of heart disease have greatly changed in the last thirty years, as they should have if changes in diet were so important. A ten-year study of heart disease mortality published in the New England Journal of Medicine in 1998 strongly suggests that most of the decline in deaths from heart disease is due not to changes in lifestyle, such as diet, but to improvements in medical care…For while during the period under analysis, heart attack deaths declined substantially, hospital admissions for heart attack did not. (60-61)

The referenced study is “Trends in the Incidence of Myocardial Infarction and in Mortality Due to Coronary Heart Disease, 1987 to 1994.” As the title suggests, this “ten-year study” is actually an eight-year study. That means that Pollan is attempting to debunk claims made about health trends over the course of a few decades by looking at a much shorter period. Moreover, Walter Willett’s claim applies specifically to the seventies and eighties, but a reader who did not bother to check the references would not realize that the period under consideration by Pollan’s “ten-year study” actually only had three years in common with this interval.

This becomes even more problematic when we look at what people were eating. Specifically, let’s look at the amount of polyunsaturated fats in the US food supply (page 65), which is most directly relevant to Willett’s claim.

Amount of polyunsaturated fat in the US food supply, per capita per day. The red lines indicate the beginning and end of the period considered by the NEJM study.

The data show that consumption of polyunsaturated fats increased from 26 grams per person per day in 1970 to 32 grams per person per day in 1985, with most of the increase coming from 1977 and 1985. However, in 1985 polyunsaturated fat intake more or less leveled out, and the net increase over the period under consideration in the NEJM study was only 1 gram. Thus, even if we were to suppose that Willett were right, it’s not clear that we should have expected to see much of a drop in the incidence of heart disease in the study.

Since dietary changes can have long-term effects on health, this doesn’t necessarily mean that Pollan’s argument is completely wrong. Indeed, the distinction he makes between incidence of heart disease and deaths due to heart disease is an important one. However, it’s just not clear how relevant the study he cites really is. The choice of intervals needs to be justified as a part of his argument, rather than hidden away in the references.

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Some corrections and clarifications

March 22, 2012 at 12:24 am · Filed under Corrections, Facts, In Defense of Food ·Tagged , , ,

Last Friday, while working on my last entry, I came to the realization that two sentences in my piece in the Berkeley Science Review in September were not quite right. These sentences contained a number of claims which were either subtly wrong, misleading, or which simply could have been expressed more clearly.

The sentences in question came in my response to Michael Pollan’s assertion that “the dietary advice enshrined not only in the McGovern ‘Goals’ but also in the National Academy of Sciences report, the dietary guidelines of the American Heart Association and the American Cancer Society and the U.S. food pyramid bears direct responsibility for creating the public health crisis that now confronts us.” I wrote,

However, McGovern’s Goals called for a substantial reduction in refined carbohydrate consumption and an increase in consumption of carbohydrates from fruits, vegetables, and whole grains. Americans collectively disregarded both of those recommendations, as well as a recommended decrease in daily energy intake.

These sentences have since been corrected. Here’s what I think deserves correction or further explanation (all data on historical consumption of foods are from USDA loss-adjusted food availability data):

  • George McGovern’s Dietary Goals for the United States didn’t directly call for a reduction in consumption of refined carbohydrates (which includes both refined grains and added sugars). The first edition (published in February 1977) called for a forty-percent reduction in sugar consumption. The second edition (published in December of the same year) called for a forty-percent reduction in consumption of refined and processed sugars. This means that it would have been consistent with the advice to replace added sugars with white flour. However, that isn’t what Americans did; they increased their consumption of both refined grains and added sugars. Consumption of added sugars did decrease somewhat as a percentage of total calories after 1977, but not by the 40% recommended by the Goals.
  • The Goals left some ambiguity over whether whole grains should be preferred to refined grains. The document listed among a set of “Changes in Food Selection and Preparation” the recommendation to “[i]ncrease consumption of fruits and vegetables and whole grains.” However, later, under the heading “Guide to increasing complex carbohydrate consumption,” the document noted that “there have been no studies that have found whole wheat flour to be superior nutritionally to white flour when consumed in a normal diet, and surprisingly few studies have even considered the question.”
  • The recommendation regarding total calorie consumption was found only in the second edition of the Goals. Moreover, that edition didn’t tell everybody to reduce calorie consumption. Instead, it made the recommendation to those who were overweight, and advised the general population to “decrease energy intake and increase energy expenditure.” Most likely, if this advice had been followed it would have resulted in a decrease in energy consumption per capita. However, if I were writing the piece today, I probably would have said that Americans “ignored a warning to avoid excessive caloric intake.”
  • Total consumption of fruits and vegetables actually increased slightly in the years after the publication of the McGovern report. However, the percentage of total calories from fruits and vegetables decreased. The Goals typically called for increases or decreases as a percentage of total calorie intake, so I think it is fair to say that the recommendation on fruits and vegetables was not followed. However, the word choice could have been more clear. Unfortunately, I was not able to find data on consumption of whole grains, but consumption of grains increased substantially. (Pollan claims in In Defense of Food that most of the increase was in the form of refined grains.)

While I should have been far more careful in writing these two sentences, I think that the point I was attempting to make was sound. Namely, considering that substantial parts of the dietary guidelines were not followed, it doesn’t make sense to blame the guidelines for health problems that have resulted. That said, it’s important to get the details right, and I should have been more careful. I’ll try harder to do both of those in the future.

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Diet, Nutrition, Cancer, and Sloppy Journalism

February 29, 2012 at 7:52 am · Filed under Facts, In Defense of Food, Misrepresentation of sources, Science ·Tagged , , , , , ,

Michael Pollan argues in In Defense of Food that the Dietary Goals for the United States, compiled by Senator George McGovern’s Senate Select Committee on Nutrition and Human Needs, ushered in an era in which dietary guidelines focused on nutrients to the exclusion of whole foods. He explains that McGovern’s committee initially recommended a reduction in red meat and dairy consumption before the interested lobbies complained. The result was a compromise, urging Americans to “choose meats, poultry, and fish that will reduce saturated fat intake.”

Pollan explains the significance of the change,

First, notice that the stark message to “eat less” of a particular food–in this case meat–had been deep-sixed; don’t look for it ever again in any official U.S. government dietary pronouncement. (24)

As evidence of this last point, Pollan trots out the National Academy of Sciences’ 1982 report Diet, Nutrition, and Cancer. He tells us,

The lesson of the McGovern fiasco was quickly absorbed by all who would pronounce on the American diet. When a few years later the National Academy of Sciences looked into the question of diet and cancer, it was careful to frame its recommendations nutrient by nutrient rather than food by food, to avoid offending any powerful interests. (25)

He further notes, “With each of its chapters focused on a single nutrient, the final draft of the National Academy of Sciences report, Diet, Nutrition and Cancer framed its recommendations in terms of saturated fats and antioxidants rather than beef and broccoli” (26).

After a few minutes of skimming the report, I couldn’t help but wonder whether Pollan and I had looked at the same document. He is correct to point out that the recommendations referenced fat, rather than meat. He’s not quite correct to say that each chapter focuses on a single nutrient. Some of the chapters discuss multiple nutrients (for instance, Chapter 10, titled “Minerals,” discusses selenium, zinc, iron, copper, iodine, molybdenum, cadmium, arsenic and lead), but on that point he’s close enough for me.

Where Pollan misses the mark badly is in failing to note that the report’s recommendations weren’t found in these nutrient-focused chapters. These chapters summarized the existing science on the various nutrients, without saying what we should do about it. The report did include a section called “Interim Dietary Guidelines,” but this was found in the report’s Executive Summary. Summarized, the guidelines were as follows:

  1. “The Committee recommends that the consumption of both saturated and unsaturated fats be reduced in the average U.S. diet.”
  2. “The committee emphasizes the importance of including fruits, vegetables, and whole grain cereal products in the daily diet…Results of laboratory experiments have supported these findings in tests of individual nutritive and nonnutritive constituents of fruits (especially citrus fruits) and vegetables (especially carotene-rich and cruciferous vegetables).”
  3. “[T]he committee recommends that the consumption of food preserved by salt-curing (including salt-pickling) or smoking be minimized.”
  4. “The committee recommends that efforts continue to be made to minimize contamination of foods with carcinogens from any source. Where such contaminants are unavoidable, permissible levels should continue to be established and food supply monitored to assure that such levels are not exceeded. Furthermore, intentional additives (direct and indirect) should continue to be evaluated for carcinogenic activity before they are approved for use in the food supply.”
  5. “The committee suggests that further efforts be made to identify mutagens in food and to expedite testing for their carcinogenicity. Where feasible and prudent, mutagens should be removed or their concentration minimized when this can be accomplished without jeopardizing the nutritive value of foods or introducing other potentially hazardous substances into the diet.”
  6. “[T]he committee recommends that if alcoholic beverages are consumed, it be done in moderation.”

Thus, of the six interim guidelines, only one was expressed in terms of nutrients, two phrased their recommendations in terms of foods (one of those even recommended eating less of something!), two were seemingly commonsense encouragements to keep dangerous substances out of the food supply, and one was a reminder not to drink too much.

Although the recommendation pertaining to vegetables did briefly mention carotenes, the committee included a clarification between the second and third guidelines:

These recommendations apply only to foods as sources of nutrients–not to dietary supplements of individual nutrients. The vast literature examined in this report focuses on the relationship between the consumption of foods and the incidence of cancer in human populations. In contrast there, is very little information on the effects of various levels of individual nutrients on the risk of cancer in humans. Therefore, the committee is unable to predict the health effects of high and potentially toxic doses of isolated nutrients consumed in the form of supplements.

They were serious about this point, too. In his critique of reductionism (which Pollan quotes), T. Colin Campbell (a co-author of the report) wrote that the Federal Trade Commission sued General Nutrition Centers for using the report’s guidelines to advertise supplements.

Although Pollan has argued that nutrition science “knows much less than it cares to admit” (14), the report shows that the science admits its limits more than Pollan cares to admit. The lack of recommendations on so many of the nutrients considered is an implicit acknowledgment of the lack of evidence. Reading the chapters focusing on the various nutrients, one sees the point made more explicitly. For instance, the researchers wrote in the conclusion of their section on selenium, “firm conclusions cannot be drawn on the basis of the present limited evidence.”

As for those chapters on nutrients, one finds plenty of references to whole foods there, too. For instance, though Pollan writes, “The language of the final report highlighted the benefits of the antioxidants in vegetables rather than the vegetables themselves” (26), the report’s summary of the epidemiological evidence on vitamin A reads,

A growing accumulation of epidemiological evidence indicates that there is an inverse relationship between the risk of cancer and consumption of foods containing vitamin A (e.g., liver) or its precursors (e.g., some carotenoids in dark green and deep yellow vegetables). Most of the data, however, do not show whether the effects are due to carotenoids, to vitamin A itself, or to some other constituents of these foods.

Of course, none of this is directly relevant to the question of how much we can learn about human nutrition by studying nutrients (instead of foods), and it’s on that question that Pollan seems to make his main argument. However, what we can observe here is a pattern of misrepresenting the claims that science has made. This, of course, is helpful to somebody trying to discredit a scientific discipline. But it should be much less helpful if Pollan wants to convince the reader that he is a careful researcher and a credible source of information.

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Our national eating disorder

May 30, 2010 at 11:40 am · Filed under Facts, The Omnivore's Dilemma ·Tagged , , , , , , ,

Michael Pollan begins The Omnivore’s Dilemma by lamenting the role of experts in our food choices. He writes,

For me the absurdity of the situation became inescapable in the fall of 2002, when one of the most ancient and venerable staples of human life abruptly disappeared from the American dinner table. I’m talking of course about bread. Virtually overnight, Americans changed the way they eat. A collective spasm of what can only be described as carbophobia seized the country, supplanting an era of national lipophobia dating to the Carter administration. (1)

This didn’t exactly match my recollection of how the low-carb craze went down. I knew that a substantial minority of the country went on the diet, but in my own life, I’d most often heard the Atkins diet discussed as a subject of ridicule.

Of course, my own experience need not be representative of the nation, so I decided to look into it. Pollan doesn’t cite any sources or give data to back up his claims, and the numbers I could find tell a far less dramatic story. NPD Group reported that low-carb diets peaked in February of 2004 when 9.1% of Americans identified as followers of these programs. ACNielsen reported that sales of white bread dropped by 4.7% in 2003. Thus, while the low-carb fad had a noticeable impact on our collective eating habits, it was not as far-reaching as Pollan would have us believe.

To Pollan, the “violent” (2) change in our cultural eating habits is evidence of “a national eating disorder” (2),  which he posits would not have been possible in a culture having “deeply rooted traditions surrounding food and eating.” He tells us that such a culture “would probably not…feed fully a third of its children at a fast-food outlet every day” (3).

Pollan points out that Italy and France “decide their dinner questions on the basis of such quaint and unscientific criteria as pleasure and tradition, eat all manner of ‘unhealthy’ foods, and lo and behold, wind up actually healthier and happier in their eating than we are.” We are, he says, “a notably unhealthy people obsessed by the idea of eating healthily.”

But would a people obsessed with healthy eating really feed a third of their children fast food every day? Would such a people consume high-fructose corn syrup in huge quantities, as Pollan will soon tell us we do? Indeed, the USDA reports that in a 2000 survey of a nationally representative sample of 2,000 adults aged 18 and older, 70% reported eating “pretty much whatever they want.”

It’s hard not to wonder if our national eating disorder is one of thinking too little about what to eat rather than too much.

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