Eat this. Don’t eat that. Five cups of coffee a day is fine. Like most retail nutrition advice, the new issue of Dietary Guidelines for Americans is presented as if there were scientific certainty about what we ought to eat.
Here is the way the Dietary Guidelines, which federal government published last week, touts its credentials: “A growing body of research has examined the relationship between overall eating patterns, health, and risk of chronic disease, and findings on these relationships are sufficiently well established to support dietary guidance.”
Yet what many experts in nutrition research will admit is that scientific certainty on these topics is often elusive, even on the health effects of some very common foods.
Now a new paper gives some insight into the unsettled state of the science. And although this may sound like an attack on the nutrition establishment by some marginal players, it is not. It comes from the mainstream: The paper appears in the journal Circulation, published by the American Heart Association. The author is Dariush Mozaffarian, the dean of the nutrition school at Tufts University. It was funded by the National Institutes of Health.
The paper lists key ideas in nutrition concerning the links between diet and heart disease, and for each idea, describes the level of scientific consensus that underlies it. There are four levels of scientific consensus, ranging from the highest -- “broad concordance and less controversy” -- to those suggesting turmoil -- “substantial controversy and/or uncertainty” and “insufficient evidence for meaningful conclusions.”
Take, for example, the idea, included in the new Dietary Guidelines, that Americans ought to limit their intake of saturated fats to 10 percent of all calories. Saturated fats are those characteristic of animals products. Although the authors of the Dietary Guidelines argue that this is based on established science, Mozaffarian describes the the alleged harms of saturated fats as uncertain. He lists the science on that topic as having “substantial controversy and/or uncertainty.”
Another example: Although the Dietary Guidelines encourage people to shift from butter to vegetable oil, Mozaffarian describes the science supporting the harms of butter as involving “substantial controversy and/or uncertainty,” while he describes the claim that vegetable oils have benefits as based on science that faces equal skepticism.
To be sure, Mozaffarian’s assessment of the evidence supports the Dietary Guidelines in places.
Like the Dietary Guidelines, his chart finds strong consensus on the benefits of fruits, vegetables, nuts, seeds and yogurt. It supports the idea that refined grains, sugar-sweetened beverages and more than moderate intakes of alcohol can do harm.
“There’s a lot of things in the guidelines that are pretty solid,” Mozaffarian said.
But it is important to remember, too, that his assessment is just one scientist’s take on the discussion. Others will differ on what nutritional guidelines are controversial and which are not.
“This is my subjective opinion,” Mozaffarian said. “It is based on my conversations with scientists and my reading of the literature, and my own research.”
Indeed, some scientists will dispute Mozaffarian’s assertion that there is general agreement on healthy amounts of sodium, most of which we consume through salt. In Mozaffarian's guide, there is general agreement that consuming even just “moderate” amounts of salt is harmful. (By moderate, Mozaffarian means between 2 and 3.5 grams of sodium per day.) Although most scientists agree that too much sodium is harmful, some scientists say “too much” begins at much more than that.
David Klurfeld, a USDA researcher responsible for the scientific direction of the human nutrition research conducted by agency laboratories, said he likes the attempt to grade the evidence but disagrees with some of his assigned evidence grades, “sometimes fairly strongly.”
“Anything in his table can fit into a healthy diet, but even the healthiest foods are not part of a good dietary pattern if they displace other useful and/or enjoyable foods,” he said.
These disagreements, however, only underscore a basic idea: Nutritional guidelines -- even those from authorities -- aren’t completely grounded in what might be called “settled science.”