It would make sense that if the Federal Dietary Guidelines were adhered to by most Americans and the Guidelines were sound scientifically that the country wouldn’t have seen a rise in both obesity and any of several other conditions which lead to diabetes and heart disease. But we do have these problems. In fact, FoodFacts.com thinks it’s important to note that those problems have been increasing in frequency for Americans almost since the Federal Guidelines have been issued. Sounds like weird science. But it’s coming to light that Federal Dietary Guidelines are based on a weak scientific framework.
The federal government’s dietary guidelines have changed little since first being issued in 1980. A revised set of recommendations released this month includes a new cap on added sugar, but this is unlikely to end the guidelines’ failure for 35 years to check the rise of obesity and diabetes. The problem, simply put, is a reliance on weak science.
But a serious course correction may finally be on the horizon. Congress, concerned about the continued toll taken by nutrition-related diseases, recently mandated the first-ever outside review of the evidence underlying the dietary guidelines and the process that produces them. The National Academy of Medicine will conduct the review this year. Yet this effort could do more harm than good if the academy endorses the weak science that has shaped the guidelines for decades.
The crux of the problem is that many of the dietary recommendations are not based on clinical trials, which can reliably demonstrate a cause-and-effect relationship. In a clinical trial, subjects are randomly assigned to one or more diets, whose health effects are then measured. Such studies are extremely challenging and expensive because subjects must be monitored closely or even provided food to ensure that they are adhering to the diet.
Instead, many of the dietary recommendations are justified by observational studies, using a scientific method known as prospective epidemiology. Researchers send out questionnaires to large numbers of people, asking about diet and lifestyle. They then follow up for years to record health outcomes.
This method cannot show causation, only associations. For instance, obesity might be associated with sitting in front of the television. But people who spend a lot of time watching TV might also eat more junk food. What’s making them fat: The TV-watching, the junk food, or something else entirely that no one thought to measure? Epidemiologists try to adjust for these variables, but there is always uncertainty.
It’s true that epidemiological science has had successes, most notably by linking smoking to cancer in the early 1950s. Yet heavy smokers had a risk of lung cancer 9 to 25 times greater than did nonsmokers, a big enough difference to give researchers confidence that the association was real. By contrast, studies that link nutrition with disease generally find differences in risk of 1 to 2 times.
Moreover, of the enormous number of associations generated by observational studies, only a small number are ultimately confirmed. In 2005 John Ioannidis of Stanford analyzed several dozen highly cited studies and concluded that subsequent clinical trials could reproduce only about 20% of observational findings. A 2011 paper published by the statistics journal Significance analyzed 52 claims made in nutritional studies, and none—0%—withstood the scrutiny of subsequent clinical trials. These are very poor odds on which to gamble public health. Yet policy makers have forged ahead anyway.
This has led to many flip-flops in dietary advice. At one point epidemiological data suggested that cholesterol might be linked to heart disease, and fat to cancer. For decades physicians told the public to avoid egg yolks and shellfish. Millions of Americans adopted low-fat diets and ate more carbohydrates. Yet these theorized links were later rejected. And a large body of evidence now suggests that eating excessive carbohydrates increases the risk of heart disease, obesity and diabetes.
Scientists should have known in 1980 that the recommendation to cut fat was unsound. Large clinical trials at the time did not support the theory, according to a systematic review published last year in the cardiology journal Open Heart. “It seems incomprehensible that dietary advice was introduced for 220 million Americans,” the authors wrote, “given the contrary results.”
What’s disturbing is how little this new evidence has been heeded. The guidelines continue to insist that Americans choose reduced-fat dairy products like skim milk. But even epidemiological evidence now contradicts this advice, and a randomized trialpublished last month in the American Journal of Clinical Nutrition found that people eating full-fat dairy, including whole milk, showed a number of better heart-disease outcomes.
The guidelines continue to place a cap on saturated fats—10% of total calories—based on what the authors consider “strong evidence.” But nearly a dozen meta-analyses or systematic reviews in recent years have found only a weak link between these fats and heart disease or cardiovascular mortality. So in many cases weak evidence supports the dietary guidelines, while strong evidence contradicts them.
Moreover, rates of obesity and diabetes remain stubbornly high, and this isn’t because dietary advice is ignored. Consider a 2008 report by the Agriculture Department that estimates changes in food consumption from 1970-2005: grains rose by 41%; vegetable oils by 91%; fish and shellfish by 37%; vegetables by 23%; and fruits by 13%. Eggs and red meat each fell by 17%, and whole milk by 73%. Yet during roughly the same period the incidence of diabetes doubled.
That’s why, as part of the budget bill that passed Congress in December, lawmakers appropriated $1 million for an independent review of the dietary guidelines. Congress wants to ensure that the next revision, due in 2020, will “better prevent chronic disease.” But we fear that the review, like the guidelines, will be dominated by epidemiology. Several members of the 2015 Dietary Guidelines Advisory Committee are also on the National Academy of Medicine, and Congress has asked them to recuse themselves.
The academy might go further by appointing a disinterested referee, someone from outside the field of nutrition, to lead the review. Ideally, this person would have a background in systematic methodology or evidence-based medicine, fields that focus on how to evaluate and prioritize varying results from scientific studies. This expertise would assure the public that the review is to be a serious, objective weighing of the evidence.
Diseases caused by poor eating habits destroy lives and cost the nation trillions in health care. When wrong nutritional advice is dispensed to the public, scientists lose credibility, opening the door to dietary cults. The current guidelines clearly aren’t working. This review offers a chance to steer them on a surer course.