by Marion Nestle
Mar 6 2011

The perils of food and nutrition research

I write a monthly (first Sunday) Food Matters column for the San Francisco Chronicle.  Today’s is about the difficulties of doing nutrition research.  The Chronicle headline writer titled it, “Be skeptical of food studies.”  Oops.

I’m not skeptical about Food Studies—the capitalized field of study—at all.  My NYU department started undergraduate, master’s, and doctoral programs in Food Studies in 1996 and they have flourished ever since.

The column is about small-letter food studies, meaning nutrition and food research studies:

Q: You were quoted saying that you didn’t believe newspaper reports linking diet sodas to an increased risk of stroke and heart disease. How do you decide whether research is good or bad?

A: This one was easy. I didn’t think it made sense. Mind you, I’m no fan of diet sodas. They violate my rule never to eat anything artificial. And I don’t like the taste of artificial sweeteners.

Whenever a study comes out claiming harm – or benefits – from eating a single food or ingredient, I get skeptical. That’s why I also questioned these recent study results: high-fructose corn syrup (HFCS) makes rats gain weight faster than sucrose (table sugar); zinc supplements prevent symptoms of the common cold; and pomegranate juice has greater health benefits than any other kind of fruit.

When I read about single-factor studies, I want to know three things: Is the result biologically plausible? Did the study control for other dietary, behavioral or lifestyle factors that could have influenced the result? And who sponsored it?

Plausibility: The diet soda study used a self-reported questionnaire to find out how often people reported drinking diet sodas. Nine years later, people who reported habitually drinking diet sodas had a 60 percent higher rate of stroke and heart attacks. The rate was somewhat lower when controlled for age, sex, race, smoking, exercise, alcohol, daily calories and metabolic syndrome.

Leaving aside the unreliability of self-reported dietary intake, the study raises a more important question. Was it designed to investigate the link between diet sodas and stroke or was this just an accidental finding? The questionnaire undoubtedly asked hundreds of questions about diet and other matters. Just by chance, some of them could be giving results that look meaningful. And the increase in stroke risk seems surprisingly high for something not previously known to be a stroke risk factor.

Mostly, I can’t think of a biological reason why diet sodas might lead to cardiovascular disease unless they are an indicator for some other stroke risk factor such as obesity, high blood pressure or binge drinking. It would take a study designed to test this idea specifically – and a good biological explanation – to convince me that diet sodas cause strokes.

The plausibility issue also rises in the HFCS study. Again, I’m not a fan of HFCS – we would all be healthier if we ate less sugar – but from a biochemical standpoint, HFCS and table sugar are pretty much the same. They have similar amounts of glucose and fructose, are digested as quickly and are metabolized the same way. Even the average amounts consumed are about the same. That soda companies are replacing HFCS with sucrose is strictly about marketing, not health.

Controls: The zinc-and-colds study was a comprehensive review (a “meta-analysis”) of previous studies done since the first one in 1984. Eleven studies have showed some benefit; seven have not. All of them were placebo controlled, double-blind. This meant that half the participants were given a dummy pill, and neither participants nor investigators were supposed to know who was taking what.

But in some studies, the zinc takers complained about the taste of the pills, hinting that they knew what they were.

I’ve heard this before. In the early 1970s, National Institutes of Health investigators did a study of vitamin C and the common cold. They got about 300 NIH employees to take either vitamin C or a placebo, double-blind. The tantalizing result: People taking vitamin C reported fewer colds and milder symptoms than people taking placebos.

Alas, many participants withdrew from the study before it ended. When asked why, they admitted tasting the pills. The investigators reanalyzed the results. Bingo! No matter what pill the participants actually took, those who thought they were taking vitamin C reported fewer colds and milder symptoms.

If the studies were not really blinded, the zinc results are questionable. I have no doubt that many people feel better when they take zinc supplements, but I’m not sure whether that’s because of something zinc really does or just its placebo effect.

Sponsorship: Vested interests influence the design and interpretation of studies. The best-designed studies control for factors that might influence results. Even so, their results require interpretation. Interpretation requires interpreters. Interpreters bring their own biases into the interpretation.

I mention pomegranate juice because one of its major producers sponsors studies to hype its benefits. Yes, pomegranates are delicious, but antioxidant powerhouses? So are most fruits. Pomegranates may have high antioxidant activity, but compared with what? Its maker does not say. Sponsored research is also about marketing, not health.

Nutrition research is hard to do, which makes study design and interpretation particularly challenging. Nutrition is a thinking person’s field, requiring careful analysis at every step. When you hear a result that sounds too good to be true, it usually is.

  • Holly

    There is some evidence that sweet flavors are sensed by taste receptors in the gut, increasing expression of glucose transporters in enterocyte membranes. Whether the increased glucose uptake balances the lack of sugar ingested when you eat a non-caloric sweetener, I cannot say. I’m also curious whether the receptors have any effect on metabolic hormones.

    A. Moran, M. Al-Rammahi, D. Arora, D. Batchelor, E. Coulter, K. Daly, C. Ionescu, D. Bravo, and S. Shirazi-Beechey. Expression of Na+/glucose co-transporter 1 (SGLT1) is enhanced by supplementation of the diet of weaning piglets with artificial sweeteners. British Journal of Nutrition, 104(05):637–646, 2010.

    R. Margolskee, J. Dyer, Z. Kokrashvili, K. Salmon, E. Ilegems, K. Daly, E. Maillet, Y. Ninomiya, B. Mosinger, and S. Shirazi-Beechey. From the Cover: T1R3 and gustducin in gut sense sugars to regulate expression of Na+-glucose cotransporter 1. Science’s STKE, 104(38):15075, 2007.

    O. Mace, J. Affleck, N. Patel, and G. Kellett. Sweet taste receptors in rat small intestine stimulate glucose absorption through apical GLUT2. The Journal of physiology, 582(1):379, 2007.

  • I just found this via twitter and I cannot tell you how excited I am. I can’t wait to read more more more.


  • Nick

    “Statistics are like a bikini: what they reveal is suggestive, but what they conceal is vital.”

    Aaron Levenstein

  • Anthro

    I’ve been waiting for this column. I often rebut the off hand comments made here by food faddists who assume that you agree with them because you are in the “nutrition” field, which has a very broad interpretation in the minds of the general public. I try to find examples in the posts listed at the right of the blog, but today’s post will now do that job nicely.

    Thanks for clarifying the basic scientific principles that so often get overlooked by food writers even in otherwise respectable publications. The HFCS vs. sugar is such a good example; one I’ve tried to rebut repeatedly elsewhere. Now I’ll have a good link to offer.

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  • Beth

    Hey–here’s something I was wondering about your assessment of: the letter from Don Huber’s research that was making the rounds recently.

    How much weight did you give that?

  • Kate Leiva

    Brava! Your column is a fine complement to the Chronicle’s Sunday food section, the best addition in years; I was thrilled when your column was added to the mix. Your overall approach to food and nutrition–honoring science, reason and taste–is a breath of fresh air, and we can depend on you to be honest even when it’s painful–sort of like the ACLU of food. I’m so grateful that your contributions have made it west.

  • JE

    What scientists think is biologically plausible can change fairly quickly. Twenty years ago who would have imagined that polyphenols in various kinds of berries could favorably impact brain function? Yet recently conducted genetic research shows that natural defense systems in the brain are activated by berry polyphenols. Regarding the commercial biases of sponsored research, who could be more impartial than the U.S. Department of Agriculture?

    Berries for Brain Protection:

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  • I only wish I was back in the classroom right now teaching my statistic classes! It is for these common assumptions people make whenever they hear the word “study” that I argue that every student graduating high school should have exposure to at least one basic course in statistics. This way they can be educated consumers and recognize a well-designed study versus a biased one.

    This post reminds me that, in my blog, perhaps I should devote some time this issue as well. I dismissed the diet soda study, but I did not explain WHY… my readers deserve that clarification.

    Thank you, as always, Dr. Nestle!

  • maxie

    @je: you’re being facetious, right?


  • JE

    @Maxie: Did you read the article I linked to? It summarizes several studies sponsored by the USDA – all the studies are referenced. It turns out berries (especially blueberries) are serious brain food. This is what I meant by the evolving nature of biological plausibility. Epigenetic science is showing us how diet and nutrition can directly influence the behavior of genes.

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