Bamboo is notoriously poor in nutritional value. And yet, it is not only adequate sustenance for giant pandas, it is the one and only food they can eat. Were there no bamboo, there would be no giant pandas—at least not as we know them.
Similarly, eucalyptus leaves would make a very poor dietary choice for a human being marooned on a desert island. We couldn't hope to survive on them. But koalas do, and they couldn't survive on anything else.
Nor need we go nearly so far afield as the mountain forests of China or arid woodland of Australia to establish the remarkable links between food and those it feeds. I routinely marvel at the power of my horse, Troubadour, particularly when those magnificent muscles are engaged in full gallop or leaping fences with me on his back. I marvel all the more at the alchemy that turns a diet of grass, hay, oats, and very little else into a horse, and all that horse power.
There are creatures requiring sustenance on this planet only because there is sustenance to be had here. Were there no food on earth, there would be no life—at least not life as we know it. That is the general case.
The more specific case is that there is no animal on earth, now or ever, that requires any particular food not found on this planet. I trust that assertion is so self-evident as to require neither embellishment, nor defense. But consider then what follows: All creatures on earth are specifically suited to survive on the food found here.
Food came first. All of us—you, me, Troubadour; the koalas and pandas; anteaters, mackerel, and polar bears—followed. And we followed with adaptations to make use of the food we found in our particular ecological niches. Physiology adapted to burn some variation on the theme of available fuel, and no other kind of physiology ever existed. No machines of any kind run on power sources that don't exist.
This is the context in which all of our considerations, and fundamental conclusions, about diet and health should occur. The context is no substitute for the detailed content generated by modern science, of course, any more than it is for the culinary arts. All of the specific culinary talent practiced in the world's kitchens is bound by the food earth provides, but that talent has much to do with how delightfully those provisions are put to use. Similarly, scientific insight helps us know how best to apply the available foods to the promotion of health.
But whenever those insights seem to carry us outside the bounds of context, they inevitably prove wrong. Such are the follies of our nutritional history.
Some time back, we latched onto the notion that dietary fat was bad for us. In the fullness of time, this of course proved to be wrong—and had we hung on to the fundamentals of context, we would have predicted that from the start. Homo sapiens have always consumed fat as part of the dietary mix, and so do all of our close relatives in the animal kingdom. Some specific dietary fats are even known to be essential, namely the essential fatty acids, in just the way that vitamins and minerals are: Without them in our diets, we develop deficiency syndromes that cause illness and even death.
Some dietary fats are, indeed, bad for us—and they tend to be the fats either not native to our diets, or consumed now in quantities and proportions not native to our diets. This is true of some, but not all, saturated fatty acids. It is true of trans fat. And it is true of omega-6 polyunsaturated fatty acids, the most abundant of which in our diets is linoleic acid.
In our native diets, omega-6 fats, which tend to be pro-inflammatory, are consumed in balance with omega-3 fats, which are anti-inflammatory. (This is something of an over-simplification, but the gist is correct.) Our native ratio of omega-6 to omega-3 is thought to be between 1 to 1 and 4 to 1. In modern diets, it is generally from 11 to 1 to 20 to 1, representing an imbalance of as much as 20-fold from the standard to which we are adapted.
This, then, is the context for a recent study out of Australia, published in the British Medical Journal, demonstrating that the substitution of linoleic acid for saturated fat among coronary patients conferred no benefit, and potentially even increased mortality. This study does not indict polyunsaturated fats, or even omega-6s, although the investigators themselves may think it does. Rather, it adds to our incrementally advancing understanding of dietary fats and health and fits in perfectly with the notion that what's good for us is what has always been good for us. Omega-6 fats are part of that mix, but an excess of them is harmful. Saturated fats are part of that mix, but a small part (primarily, they are those saturated fats found in nuts, seeds, and meats—not those found in dairy.)
Even as we slowly and somewhat painfully rectify our misrepresentations of dietary fats, we seem inclined to repeat the follies of this history rather than learn from them. Of course carbohydrates are not bad for us! Everything from lentils to lollipops is a carbohydrate, and if anything, it makes even less sense to treat all carbohydrate as the same than ever it did for dietary fat. Some carbohydrate foods are good for us, and others are not.
This tendency to let misguided tales wag the dietary dogma reaches its fullest expression when we try to vilify or vindicate some particular nutrient. Fructose has been much in that spotlight of late, with the contention made that it is the single serious adverse exposure in the modern diet. But while it may be true that fructose can impose the harms of alcohol under theoretical extremes, under real-world conditions it appears to be just another sugar. So says a recent study in the Journal of Applied Physiology, Nutrition, and Metabolism—albeit one funded by the Corn Refiners Association.
Sugar is not poison, and fructose is not the only thing wrong with our diets. But we eat way too much added sugar, and that is bad for us. In fact, it is way too much because it is bad for us, and bad for us because it's so much more than the native intake to which we are adapted.
Attempts to vindicate dietary sodium are misguided for the same reasons. Of course excess salt is bad for us—that's what defines it as excessive.
Over and over again, the modern science of nutrition has wandered off the reservation by making exaggerated claims. Inexorably, the weight of evidence has accumulated to provide content consistent with the context of our native diet, and our adaptation to it. Nothing else would be plausible. We need modern science to generate the details—but the basic plot is all around us, written in the language of natural selection.
We can't live on bamboo, and pandas can't live on anything else. It's time to see the bamboo forest through the trees, learn the lessons of history, and stop getting bamboozled.
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David L. Katz, MD, MPH, FACPM, FACP, is a specialist in internal medicine and preventive medicine, with particular expertise in nutrition, weight management, and chronic-disease prevention. He is the founding director of Yale University's Prevention Research Center, and principal inventor of the NuVal nutrition guidance system. Katz was named editor-in-chief of Childhood Obesity in 2011, and is president-elect of the American College of Lifestyle Medicine.