"Replace whole milk and full-fat milk products with fat-free or low-fat choices," says the Dietary Guidelines for Americans, the federal government's influential advice book, citing the role of dairy fat in heart disease.
Whether this massive shift in eating habits has made anyone healthier is an open question among scientists, however. In fact, research published in recent years indicates that the opposite may be true: Millions might have been better off had they stuck with whole milk.
Scientists who tallied diet and health records for several thousand patients over 10 years found, for example, that contrary to the government advice, people who consumed more milk fat had lower incidence of heart disease.
By warning people against full-fat dairy foods, the United States is "losing a huge opportunity for the prevention of disease," said Marcia Otto, an assistant professor of epidemiology at the University of Texas and the lead author of large studies published in 2012 and 2013, which were funded by government and academic institutions, not the dairy industry. "What we have learned over the last decade is that certain foods that are high in fat seem to be beneficial."
This year, as the Dietary Guidelines for Americans undergoes one of its periodic updates, the federal bureaucrats writing them must confront what may be the most controversial and weighty question in all of nutrition: Does the consumption of saturated fats - the ones characteristic of meat and dairy products - contribute to heart disease?
It is an important question. Heart disease is the leading cause of death in the United States, and the federal government has long blamed saturated fats.
But the idea that spurning saturated fat will, by itself, make people healthier, has never been fully proved, and in recent years repeated clinical trials and large-scale observational studies have produced evidence to the contrary.
After all the decades of research, the key lesson possibly is twofold. Cutting saturated fat from diets and replacing them with carbohydrates, as is often done, likely will not reduce the risk of heart disease. But cutting saturated fats and replacing them with unsaturated fats - the type of fats characteristic of fish, nuts and vegetable oils - might.
The shift in evidence has led to accusations that the Dietary Guidelines harmed those people who for years avoided fats - as instructed - and loaded up excessively on the carbohydrates in foods such as breads, cookies and cakes marketed as low-fat.
It also has raised questions about the scientific foundations of the government's diet advice: To what extent did the federal government, and the diet scientists upon which it relied, go wrong? When the evidence is incomplete on a dietary question, should the government refrain from making recommendations?
The dietary science has drawn skepticism on Capitol Hill. On Wednesday, a House committee will air concerns regarding the evidence behind the guidelines with Agriculture Secretary Tom Vilsack and Health and Human Services Secretary Sylvia Mathews Burwell.
The Dietary Guidelines have stepped back slightly from their blanket advice to reduce saturated fats, adding the caveat that saturated fats ought to be replaced with unsaturated fats. But Dariush Mozaffarian, a cardiologist, epidemiologist and dean of the Friedman School of Nutrition Science and Policy at Tufts University, said that the Dietary Guidelines have yet to retreat far enough from the idea that saturated fat is a dietary evil, and that the suspicion of whole milk is a good example. Judging a particular food solely on how much fat it contains, he said, can too easily blind people to its other benefits.
"If we are going to make recommendations to the public about what to eat, we should be pretty darn sure they're right and won't cause harm," Mozaffarian said. "There's no evidence that the reduction of saturated fats should be a priority."
Some, including representatives of the American Heart Association, disagree. In their view, the evidence for the dangers of saturated fats arises from these ideas: Consuming saturated fats raises levels of "bad" cholesterol in the blood, and higher levels of "bad" cholesterol, in turn, raise risks of heart disease.
They point to the trials of statin drugs, which show that the drugs lower "bad" cholesterol levels and lower risks of heart disease.
There is a "mountain of evidence" on how consumption of saturated fats raises the risk of heart disease, said Penny Kris-Etherton, a nutrition professor at Pennsylvania State University and a former member of the Dietary Guidelines advisory committee.
- - -
The history of the fat warnings is usually traced to the work of Ancel Keys, a scientist at the University of Minnesota, whose study of heart disease in the 1950s startled the medical world.
Keys examined fat consumption and rates of heart disease in various countries. In places where people eat lots of fat, he found higher rates of heart trouble.
To Keys, the data offered proof that the United States could improve people's health by reducing the fat in their diet.
"It is now abundantly clear that degenerative heart disease is not an inevitable consequence of ageing," he wrote in a paper presented at a medical symposium in 1953.
More evidence was coming. In the 1960s, several clinical trials - from Oslo, Los Angeles, Finland, London and Minnesota - put his suspicion to the test. Three of the five suggested that he was right.
The Oslo study, for example, studied 412 men who had previously had a heart attack. Half were given a special diet that was low in saturated fat; the other half were allowed to eat their usual diet, which was richer in saturated and trans fats. The special diet seemed to work: After five years, 64 subjects on the special diet had a relapse of heart disease; of those eating their regular diet, 90 people did.
Public health authorities, including those in the United States, were soon recommending that people reduce their consumption of saturated fats. The idea became a part of the U.S. official advice in 1977, when the Dietary Goals for the United States, a forerunner of the Dietary Guidelines, embraced the position.
- - -
But even as a Senate committee was developing the Dietary Goals, some experts were lamenting that the case against saturated fats was being made even though the evidence seemed thin.
"The vibrant certainty of scientists claiming to be authorities on these matters is disturbing," George V. Mann, a biochemist at Vanderbilt's medical school, wrote in the New England Journal of Medicine.
Ambitious scientists and food companies, he said, had "transformed [a] fragile hypothesis into treatment dogma."
Indeed, the subsequent 40 years of science have proved that, if nothing else, the warning against saturated fats was simplistic.
By itself, cutting saturated fats appears to do little to reduce heart disease. Several evidence reviews, summing up years of research and appearing in leading academic publications - the American Journal of Clinical Nutrition, the Annals of Internal Medicine, the British Medical Journal - found no significant evidence of a link between heart disease and saturated fats.
To many critics, the trouble with the fat warning was not merely academic.
The "campaign to reduce fat in the diet has had some pretty disastrous consequences," Walter Willett, dean of the nutrition department at the Harvard School of Public Health, has said. "With more fat-free products than ever, Americans got fatter."
Bestsellers such as "Good Calories, Bad Calories" by Gary Taubes and "Big Fat Surprise" by Nina Teicholz launched critiques of the government position.
"There's a large body of scientific literature to show that a high-carb diet, as recommended by the Dietary Guidelines for Americans, provokes a number of heart-disease risk factors," said Teicholz, whose critique of the guidelines appears in a recent issue of the British Medical Journal.
- - -
For the bureaucrats writing the forthcoming Dietary Guidelines, the shifting evidence against saturated fats may be a lesson, experts said: There were weaknesses in each of the three lines of evidence used.
First, there were those studies by Keys showing that a country's fat consumption was linked to its rate of heart disease. After the Keys paper appeared, scientists began adding other countries to his analysis, and when they did, the pattern suggesting a link between fat consumption and heart disease became less distinct.
More important, by the very nature of his research, Keys data could only show that saturated fat consumption was associated with heart disease, not that consuming saturated fat caused heart disease. That's because his study was observational - that is, it was based on observing subjects rather than randomly assigning them to high-fat and low-fat diets. It was possible, in other words, that some unaccounted factor caused the varying rates of heart disease.
The second line of evidence in the case against saturated fats came from those experiments in the 1960s. These suggested that subjects who consumed less saturated fat suffered less from heart disease.
As further scientific review showed, none of the experiments was perfectly designed to assess the danger of saturated fats, and the results in some cases were modest. Moreover, the diets that seemed to show a benefit were not just low in saturated fats, they were also high in unsaturated fats - the fats common in fish, nuts and vegetable oil.
"We have strong evidence that replacing saturated fats with carbohydrates has no effect on cardiovascular disease," said Alice Lichtenstein, a Tufts University nutritionist who served this year on the Dietary Guidelines advisory panel.
- - -
Even so, the advisory panel has continued to tout the benefits of limiting saturated fat to 10 percent of the diet, and of swapping whole milk for fat-free.
In doing so, the panel is relying on the third piece of the argument against saturated fats, which is that two-step chain of logic: first, that saturated fats raise the levels of "bad" cholesterol in the blood, and second, that higher levels of "bad" cholesterol in turn raise the risks of heart disease.
Scientists generally agree on the premises of that argument.
The trouble, according to critics, is that connecting the two and drawing the conclusion that saturated fats lead to heart disease is a vast oversimplification, for a handful of reasons.
First, while consumption of saturated fats tends to raise levels of "bad" cholesterol in the blood, they also tend to raise the levels of "good" cholesterol levels, and that may have compensating effects.
Second, saturated fatty acids come in chains of carbon of varying lengths, and each one differs in its effects on heart-disease risks. Some molecules appear to raise the amount of "bad" cholesterol in the bloodstream, while other longer chains appear to have no appreciable effect.
It gets even more complicated. "Bad" cholesterol comes in two forms. One consists of particles that are smaller and denser, and these appear to be strongly linked to heart disease; the other type consists of lighter, fluffier particles that appear to have lesser effects on heart disease. Saturated fats do raise the levels of "bad" cholesterol but seem to produce mainly the lighter, fluffier and less dangerous particles.
As a result of such complexity, as well as the ways in which food sources vary in their health effects, "blanket recommendations to reduce total saturated fats may not be appropriate," according to the most recent Annual Review of Nutrition, an academic publication that provides research summaries.
So, what about whole milk?
Although nutrition advice is often presented in terms of "macronutrients" - fats, proteins, carbohydrates - foods may be more than the sum of their scientific parts.
Milk is a good example.
Repeated research on milk, funded not by the industry but by public institutions, has provided evidence that the fats in milk are different.
In 2013, New Zealand researchers led by Jocelyne R. Benatar, collected the results of nine randomized controlled trials on dairy products. In tallying the tests on 702 subjects, researchers could detect no significant connection between consuming more dairy fat and levels of "bad" cholesterol. (Four of the nine studies were funded by the industry. Those results were consistent with those of the trials funded by government entities.)
The same year, Otto and Mozaffarian, then both at the Harvard School of Public Health, conducted another study on the effects of milk. Their study sought to address a key weakness in the previous research, which had yielded contrary results on the link between milk and heart disease.
One of the flaws of nutrition studies is that they rely on people to accurately recall what they have eaten over the course of a year. Those recollections are vulnerable to inaccuracy, especially for dairy fats, which can be found in small amounts in many foods.
To improve estimates, Otto and Mozaffarian used a blood sample for each of more than 2,800 U.S. adults. Using the blood sample, they could detect how much dairy fats each had consumed. And over the eight-year follow-up period, those who had consumed the most dairy fat were far less likely to get heart disease, compared with those who had consumed the least.
The advocates of whole milk allow that it has more calories than its low-fat cousins, and for some, that might be reason to avoid it. But the traditional case against whole milk - based on the risk of heart disease - has frayed enough now that many argue that it is time for the Dietary Guidelines to yield to the new findings.
"There is no scientific basis for current dietary advice regarding dairy," Benatar said. "Fears [about whole milk] are not supported by evidence. The message that it is okay to have whole-fat food, including whole-fat milk, is slowly seeping into consciousness. But there is always a lag between evidence and changes in attitude."
