Flailing in the swell of bestselling diet books, infomercials for cleanses, and secret tips in glossy magazines, is the credibility of nutrition science. Watching thoroughly-credentialed medical experts tout the addition or subtraction of one nutrient as deliverance—only to change the channel and hear someone equally-thoroughly-credentialed touting the opposite—it can be tempting to write off nutrition advice altogether. This month we hear something is good, and next we almost expect to hear it’s bad. Why not assume the latest research will all eventually be nullified, and just close our eyes and eat whatever tastes best?
That notion is at once relatable and tragic, in that diet is inextricable from the amount of healthy time we spend on Earth. Improvements in diet are clearly associated with significant lengthening of lifespan and dramatic decreases in risk of most chronic diseases. Combining disease and longevity into the concept of healthspan, the number of healthy years of life—fundamentally more important but less readily quantifiable than lifespan—the data in favor of optimizing our diets are even more compelling. No one is arguing that diet is less than extremely important to health and well-being, but seemingly everyone is arguing as to what constitutes the best diet.
The voices that carry the farthest over the sea of diet recommendations are those of iconoclasts—those who promise the most for the least, and do so with certainty. Amid the clamor, Dr. David Katz is emerging as an iconoclast on the side of reason. At least, that’s how he describes himself. From his throne at Yale University's Prevention Research Center, where he is a practicing physician and researcher, said sea of popular diet media is the institution against which he rebels. It’s not that nutrition science is corrupt, just that the empty promises of memetic, of-the-moment diet crazes are themselves junk food. To Katz they are more than annoying and confusing; they are dangerous injustice.
Scientific publisher Annual Reviews asked Katz to compare the medical evidence for and against every mainstream diet. He says they came to him because of his penchant for dispassionate appraisals. "I don't have a dog in the fight," he told me. “I don’t care which diet is best. I care about the truth."
Katz and Yale colleague Stephanie Meller published their findings in the current issue of the journal in a paper titled, "Can We Say What Diet Is Best for Health?" In it, they compare the major diets of the day: Low carb, low fat, low glycemic, Mediterranean, mixed/balanced (DASH), Paleolithic, vegan, and elements of other diets. Despite the pervasiveness of these diets in culture and media, Katz and Meller write, "There have been no rigorous, long-term studies comparing contenders for best diet laurels using methodology that precludes bias and confounding. For many reasons, such studies are unlikely." They conclude that no diet is clearly best, but there are common elements across eating patterns that are proven to be beneficial to health. "A diet of minimally processed foods close to nature, predominantly plants, is decisively associated with health promotion and disease prevention."
Among the salient points of proven health benefits the researchers note, nutritionally-replete plant-based diets are supported by a wide array of favorable health outcomes, including fewer cancers and less heart disease. These diets ideally included not just fruits and vegetables, but whole grains, nuts, and seeds. Katz and Meller found "no decisive evidence" that low-fat diets are better than diets high in healthful fats, like the Mediterranean. Those fats include a lower ratio of omega-6 to omega-3 fatty acids than the typical American diet.
The Mediterranean diet, which is additionally defined by high intake of fiber, moderate alcohol and meat intake, antioxidants, and polyphenols, does have favorable effects on heart disease, cancer risk, obesity, metabolic syndrome, and "is potentially associated with defense against neurodegenerative disease and preservation of cognitive function, reduced inflammation, and defense against asthma."
They also found carbohydrate-selective diets to be better than categorically low-carbohydrate diets, in that incorporating whole grains is associated with lower risks for cancers and better control of body weight. Attention to glycemic load and index is "sensible at the least." Eating foods that have high glycemic loads (which Katz says is much more relevant to health outcomes than glycemic index—in that some quality foods like carrots have very high indices, which could be misleading) is associated with greater risk of heart disease.