Since before the times of Aristotle, humans have been making errors in reasoning when arguing issues from religion and politics to science and nutrition. Despite the millennia that separate us from ancient Greek times - and all the knowledge afforded to us by modern scientific research - fallacious reasoning continues to guide many of our dietary decisions. Often, pre-existing beliefs about food and nutrition can cloud the ability to assess dietary choices rationally and can lead to self-defeating - or downright dangerous - dietary practices.
Here are some examples I see regularly:
-- The fallacy of naturalism: "This food (or supplement) is natural. Natural foods are good for you. Therefore, this food (or supplement) is good for you."
Arsenic. Lead. Poison Ivy. Cyanide. Need I say more? There's a long list of natural substances that are toxic, and a natural provenance doesn't make a food, ingredient or supplement inherently healthy, beneficial or safe.
In other words, not everything natural is good for you. The natural sugar in fruit juice will make you as fat as the artificial sugar in soda if consumed in excess. Natural kava kava root supplements can cause liver damage as surely as lab-made acetaminophen. If your natural greens powder is contaminated with lead - as several brands have shown to be - it will be as toxic as if you'd licked industrial lead-based paint from a peeling wall. Judge each food or supplement on its individual merits, and don't let a "natural" claim confer a health or safety halo to a food or supplement that may be anything but.
-- The fallacy of mammalian solidarity: "No mammals other than humans drink milk into adulthood. Therefore, milk is an inappropriate food for adult humans - as mammals - to consume."
This argument is generally invoked to justify a pre-conceived opinion that milk is unhealthy. But on deeper reflection, one must ask: Do we really look to our fellow mammals as the final word in what's appropriate and healthy for us humans to eat? Dogs - our fellow mammals - eat their own poop, after all. And no mammals other than humans cook their food; are we to presume, then, that cooked food must be unhealthy?
The novelty of adult humans drinking milk among the mammals doesn't make it inherently right or inherently wrong, and for an explanation of why lactose tolerance into adulthood is actually a very normal evolutionary phenomenon for mammals, see my fellow U.S. News blogger David Katz's explanation of natural selection in action. We have research examples of positive health outcomes among humans on diets that both include and exclude milk. Drink it if you like it and tolerate it. Skip it if you don't - you can find those nutrients elsewhere. But just because lemmings don't drink milk into adulthood doesn't mean you have to follow their lead.
-- The fallacy of fat: "Only foods with fat can make you fat."
An overweight patient once explained to me that her daily fluffernutter sandwich was an appropriate choice because the marshmallow fluff it contained was just melted marshmallows, which are fat-free. This fallacious reasoning derives from a misunderstanding of basic metabolism: Excess calories (energy) from any source will be stored in the body as fat. This holds true for excess protein calories as surely as it does for excess carb or fat calories - and I've certainly seen my share of gym rats who plumped up on one too many protein shakes.
What's more, sugar and alcohol actually elevate levels of circulating fat in the blood - the triglycerides that are preferentially stored as body fat - more than dietary fat does! There's a reason why fat-free, high-sugar foods like marshmallow fluff, Coca-Cola, candy, juice and honey are not "free foods" on Weight Watchers: Eaten in excess, their calories will make you gain weight as surely as calories from fat-containing alternatives.
[Read: Soda, Calories and a Full Accounting.]
-- The fallacy of false cause: "I feel bloated every time I eat Fiber One bars. Fiber One bars contain gluten. Therefore, I must be gluten intolerant."
Perhaps. Or perhaps you are intolerant to any one of the half-dozen fermentable carbohydrates in that Fiber One bar rather than the wheat protein (gluten). Or perhaps you feel bloated because you're constipated, which is why you started eating Fiber One bars to begin with.
I also see this type of fallacious reasoning among patients who self-diagnose lactose intolerance, arguing that they feel awful the morning after having pizza or half a pint of Haagen Dazs. In fact, research data suggest that accurate self-diagnoses turn up in less than half of people who believe themselves to be lactose intolerant when they're given objective clinical tests. In the remaining folks, therefore, it's reasonable to assume that something else in that dairy food could be causing digestive distress - like a whopping amount of fat, perhaps?
People can be lousy diagnosticians when it comes to pinpointing their own food intolerances - though, to be fair, identifying food intolerances can be a tricky business. Few foods consist of single ingredients or compounds in isolation, and mixed diets ensure that multiple different foods are in various states of digestion at any given time when symptoms strike.
But objective tests are available for at least some food-related woes, such as celiac disease, lactose intolerance and fructose intolerance. And elimination-diet protocols, in which re-introduction of suspected trigger foods is systematic and controlled, can help separate fact from fiction.
[Read: Food Intolerance: Fact and Fiction.]
Convictions run deep when it comes to food and nutrition, and many of our eating practices are programmed at a young age as the result of family and cultural traditions. Challenging strongly-held beliefs can be difficult, but the effort may pay off in cases where these beliefs undermine our best efforts to enjoy optimal health.
Socrates is credited with having once said, "The unexamined life is not worth living." Borrowing his construction, I might suggest that the unexamined dietary dogma is not always worth swallowing.
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Tamara Duker Freuman, MS, RD, CDN, is a registered dietitian whose NYC-based clinical practice specializes in digestive disorders, Celiac Disease, and food intolerances. Her personal blog, www.tamaraduker.com, focuses on healthy eating and gluten-free living.
Please note that the author cannot offer individualized medical advice to readers who contact her via email.