The fruits of a professional bureaucracy. Nina Teicholz points out at the Wall Street Journal that:
The Dietary Guidelines for Americans--jointly published by the U.S. Department of Agriculture (USDA) and the Department of Health and Human Services (HHS) every five years--have had a profound influence on the foods Americans produce and consume. Since 1980, they have urged us to cut back on fat, especially the saturated kind found mainly in animal foods such as red meat, butter and cheese. Instead, Americans were told that 60% of their calories should come from carbohydrate-rich foods like pasta, bread, fruit and potatoes. And on the whole, we have dutifully complied.
By the turn of the millennium, however, clinical trials funded by the National Institutes of Health (NIH) were showing that a low-fat regime neither improved our health nor slimmed our waistlines. Consequently, in 2000 the Dietary Guidelines committee started to tiptoe away from the low-fat diet, and by 2010 its members had backed off any mention of limits on total fat.
Yet most Americans are still actively trying to avoid fat, according to a recent Gallup poll. They are not aware of the USDA's crucial about-face because the agency hasn't publicized the changes. Perhaps it did not want to be held responsible for the consequences of a quarter-century of misguided advice, especially since many experts now believe the increase in carbohydrates that authorities recommended has contributed to our obesity and diabetes epidemics.(Source). I would note that the recommendations regarding salt intake also lack any scientific evidence to support them.