Archive for May, 2012

Nutrition science doesn’t claim to have all the answers

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

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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Should communication between pea plants raise tough issues for vegetarians?

I was just about ready to get back into my review of In Defense of Food this week. That is, until yesterday morning, when Michael Pollan tweeted,

Cool piece on how pea plants communicate with one another, possibly raising some tough issues for vegetarians.

The basic science in the blog post, I have to say, is genuinely interesting. The idea is this:

[A] team of scientists from the Blaustein Institute for Desert Research at Ben-Gurion University in Israel published the results of its peer-reviewed research, revealing that a pea plant subjected to drought conditions communicated its stress to other such plants, with which it shared its soil. In other words, through the roots, it relayed to its neighbors the biochemical message about the onset of drought, prompting them to react as though they, too, were in a similar predicament.

Curiously, having received the signal, plants not directly affected by this particular environmental stress factor were better able to withstand adverse conditions when they actually occurred. This means that the recipients of biochemical communication could draw on their “memories” — information stored at the cellular level — to activate appropriate defenses and adaptive responses when the need arose.

Stuff like this is fascinating to me, and I’d love to know more about it. However, I probably won’t look to the New York Times to further enrich myself because the Times seems to have a rule that requires all discussions of plant responses to external stimuli to include a discussion of the ethics of eating plants and its implications for vegetarians (see also this article from December 2009 and this one from March 2011).

I often hear vegetarians dismiss this argument as a disingenuous display of concern for plants, but I think there’s a bit more to it than that, at least when the argument is made well. Some animal rightists argue that killing animals is incompatible with generally accepted ethical principles. The “right” way to make the “plants like to live” argument is to argue that the very same line of reasoning amounts to an argument against eating plants. If the argument could be made soundly, it would present a problem for the argument against eating animals–unless the person making it were also willing to give up eating plants. The point is that an argument based on a need to be logically consistent doesn’t deserve to be taken seriously if it isn’t itself logically consistent. This an instance of the reductio ad absurdum, which I’ve written about in another context. Such an argument, it should be noted, has nothing to do with whether the person making the argument cares about plants or animals, and everything to do with proving that an argument fails to meet its own standards of consistency.

That said, I believe there are good reasons to give a pig more consideration than a pea plant. More than anything, I see this as an argument that arises from imprecision.

The New York Times piece asks,

Is it morally permissible to submit to total instrumentalization living beings that, though they do not have a central nervous system, are capable of basic learning and communication? Should their swift response to stress leave us coldly indifferent, while animal suffering provokes intense feelings of pity and compassion?

I find myself unmoved by this argument because my “intense feelings of pity and compassion” for animals do not arise from the simple fact of the animals’ capacity for “basic learning and communication” nor from their “swift response to stress.” Such an argument would be even more foolish than the New York Times‘s Michael Marder implies. Indeed one could easily envision computers or robots that had similar traits, but most people’s reasons for not eating these entities are entirely selfish. They probably don’t taste very good, they contain toxic substances, they’re hard to chew, they’re likely to scratch our throats and mouths, and they tend to be a lot more expensive than most foods. I doubt any vegetarian would argue that a computer deserves better than to be eaten.

More broadly, I would reject the idea that the existence of plant survival mechanisms is evidence that plants take an interest in living. Indeed, if one considers the process of natural selection, it shouldn’t be surprising that an organism that’s alive today has mechanisms that have increased its chance of survival. According to the theory of natural selection, those organisms with survival mechanisms should be more likely to have survived. That’s exactly how natural selection works.

The question, then, is what makes animals different? I would argue that the difference lies in the fact that animals tend to respond to stimuli in ways that we can relate to. When we watch a video of a pig writhing at slaughter, it’s easy to believe that we’d react similarly if we were exposed to similar stimuli. Because we associate both the stimulus and the reaction with pain, it’s not unreasonable to guess that the pig is experiencing something similar to what what we know as pain. But that’s not quite enough to draw that conclusion; one could envision a robot programmed to react similarly. The most important piece of information is that science tells us that the pig–unlike a robot–has the capacity to experience pain and suffering similarly to humans. The pig’s suffering is similar in all of those ways to an experience that we want to avoid, but it’s much harder to relate to the pea plant in dry soil. In that respect, I think it’s perfectly reasonable to give the pig more consideration than the pea.

I want to emphasize here that I’m not arguing that an organism should be considered based simply on its overall similarity to humans. Instead, I argue for consideration based on sentience and the interests that arise from it but that sentience can only be meaningfully understood by comparing certain traits (for example, nervous systems and responses to stimuli) to their human counterparts. This means that some traits (e.g. intelligence) are not directly relevant. (If anybody wants to propose an alternative, I’m interested to hear about it in the comments.)

Might this line of reasoning lead to a stronger argument for sparing a pig than, say, a chicken? Perhaps, but the issue is complicated by the fact that it takes many chickens to provide the same amount of meat as a single pig. In any case, I think there’s a good case for giving either more consideration than a pea plant. I’d feel less comfortable saying the same of an ant or an oyster, and that doesn’t particularly bother me.

Inevitably, some will say that this line of reasoning is anthropocentric. Perhaps so, but I don’t see that as much of a criticism at all. Specifically, insofar as it gives precedence to sensations similar to those that we know as humans, I don’t think it’s any more anthropocentric than making decisions based on what we know (and can know) as people. And I much prefer that to the alternative.

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