United States law mandates that Dietary Guidelines for Americans, containing "nutritional information and guidelines for the general public," be issued every five years. The 2015-2020 edition of Dietary Guidelines for Americans was recently released, the eighth such set of guidelines published since this process began in 1980. Here's hoping these are the last such governmental guidelines issued.
Does this mean I reject a connection between diet and health? No. Is it because I think these guidelines provide bad dietary advice? Not really -- at least, not if the guidelines are judged based on the answer to this question: If the guidelines as written were followed by everyone in the U.S., would this lead to an overall improvement in health? Despite reasonable differences in how people interpret the science behind the guidelines, the answer to that question is most likely "Yes."
Unfortunately, however, this is the wrong question. The real question we need to ask of the guidelines is this: Will the promulgation of these guidelines (including how they are interpreted and responded to by the public, legislators and regulators, the food industry, lobbyists and other stakeholders) lead to an overall improvement in health? The answer to this question is probably "No." Worse still, there is a reasonably high likelihood that the totality of effects will lead to a net harm in population health, as past guidelines arguably have.
It isn't immediately obvious why these two questions have different answers. After all, if it would be beneficial for individuals to follow the guidelines, how could the guidelines lead to harm? As per the quote famously (and, as it happens, inaccurately) attributed to Yogi Berra, "In theory, there is no difference between practice and theory. In practice, there is." The difference between the theoretical effect on individuals and the practical impact on the population arises because there are powerful groups with huge interests--financial and otherwise--at stake. Whether it's the food industry formulating and marketing products to align with the guidelines, or policymakers crafting regulations that comport with the guidelines, or journalists and nutritionists and writers of diet books and educators responding to the guidelines, or members of the public who receive these messages while choosing to eat what tastes and feels good to them--these are the folks who ultimately determine the directions in which these guidelines take the public's health.
To see how this has played out in practice, look at the years and years of dietary guidelines that focused on a reduction in dietary fat intake. On its face, this made sense; higher levels of fat (especially saturated fat) in the diet led to higher levels of cholesterol in the blood, which in turn increased the risk for heart disease, so eating less fat (especially saturated fat) made great sense. But we've all seen the unintended consequences of this well-meaning advice. One of the ways saturated-fat intake was reduced was by increasing trans-fat intake (for example, replacing butter with margarine), with its attendant consequences. Health educators developed the Food Pyramid--a logical approach that unfortunately led to the belief that people could safely eat as much bread, cereal, rice and pasta (the "base" of a "healthy" diet) as they wanted. The food industry capitalized on this belief, marketing such high-carb, low-fat treats as SnackWell's cookies--no doubt exacerbating the public's misconception that food that is fat-free can't make you fat. We know this was a misconception, because as the population ate less fat (i.e., dietary lipids) it became fatter (i.e., more overweight or obese).
As predictable as this five-year cycle of guideline revision is the Sturm und Drang that accompanies its release, with responses ranging from skepticism to advocacy to outrage. A dispassionate look at the history of dietary advice highlights the uncertainty behind the flip-flops on recommendations. The British Medical Journal published a careful look at the science behind the guidelines that raised questions; predictably, others defended the scientific basis and attacked the critical article. This squabbling and inconsistency leave the public confused and dismissive, opening up huge holes for the food industry to drive its trucks through; no matter what the guidelines say, the food industry will come out ahead, as it always does.
All of this must make it sound as if I feel hopeless, but I'm actually optimistic, because there's something new this time: in the process of passing this year's budget, the U.S. Congress has mandated an authoritative review of the scientific basis for the Dietary Guidelines for Americans. The National Academy of Medicine will put together a panel of experts, who perhaps will understand that we need more of the right sort of science--the science of public health rather than biochemical/mechanistic/nutrition--to answer the real question: do we know that issuing these guidelines does more good than harm? If the answer to this question is "No," as I think it is, here's hoping the 2015-2020 version will be the last such set of guidelines issued.