The federal government's influential Dietary Guidelines for Americans, which were released Thursday, are updated every five years, and the debate over saturated fats, red meat, caffeine and salt was especially intense this time around.
The guidelines are the basis of everything from school lunch programs to the diets promoted in bestselling books, but in recent years some scientists have begun to question the one-size-fits-all approach.
A growing body of research supports the theory that a person's genetic makeup or microbiome (the organisms that live on or inside of you and help to make you who you are) plays a key role in how food affects the body - and that the impact can be different from one individual to another. That work supports a more personalized approach to diet, which some researchers have argued is the future of nutrition science.
Coffee
Old guideline: Not addressed.
2015 guideline: Up to 5 cups a day.
Earlier this year, the federal advisory committee that helps write the Dietary Guidelines for Americans weighed in on coffee for the first time and concluded that drinking up to five cups a day can be part of a "healthy lifestyle." The group wrote that "strong and consistent evidence shows that consumption of coffee within the moderate range . . . is not associated with increased risk of major chronic diseases."
And the committee didn't just stop there. It also said that consuming as many as five cups of coffee daily was associated with health benefits, such as reduced risk of heart disease and type 2 diabetes.
Those pronouncements are supported by dozens of studies showing that, on average, people who drink coffee are no worse off than those who don't. They may even be better off, in fact.
But the controversy continues.
Some of it has to do with genetics. Scientists have identified at least one part of the human genome that controls whether a person metabolizes caffeine slowly or quickly -- and those who are slow metabolizers may be at higher risk of hypertension and heart attacks the more coffee they drink.
Meat
Old guideline: Consume proteins such as lean meat, poultry and seafood as part of a balanced diet but replace foods high in solid fats with those that are lower in solid fats and calories and/or are a source of oils.
2015 guideline: Eat a variety of protein foods, including seafood, lean meats and poultry, eggs, legumes (beans and peas), soy products, and nuts and seeds. Get less than 10 percent of daily calories from saturated fats and meats that are high in saturated fat. Teen-aged boys and men should "reduce their overall intake of protein foods" such as meat.
In October, the research division of the World Health Organization caused a stir when it announced that bacon, sausage and other processed meats cause cancer and that red meat is "probably carcinogenic." The U.S. beef industry, as well as some scientists unaffiliated with that industry, argue that the body of research does not support this strong of a conclusion.
Scientists have looked at a possible link between red meat and colorectal cancer for decades, and several studies have found associations between the two, but since the studies were mostly observational, there was no way to definitively say that red meat causes cancer.
WHO defines red meat as beef, veal, pork, lamb, mutton and goat, while processed meat includes hot dogs, ham, sausages, corned beef and beef jerky, plus other meats that have been cured, smoked or salted.
Salt
Old guideline: Limit sodium to 2,300 milligrams a day - but 1,500 milligrams daily for anyone who is older than 50 or African-American.
2015 guideline: Adults and children ages 14 years and over should limit sodium to less than 2,300 mg daily, and children younger than 14 should consume even less. Use the Nutrition Facts label to check for sodium, especially in processed foods like pizza, pasta dishes, sauces and soups.
Most nutritionists agree that consuming too much salt can be dangerous to your health. The question is still just how much is too much.
The federal government is on the side urging that Americans aggressively limit salt intake. For two decades, it has warned that most people were eating dangerous amounts of salt that could increase their risk of high blood pressure and heart issues. The new guidelines maintain a daily limit of less than 2,300 milligrams of sodium - about a teaspoon of salt.
Some skeptics have argued that the typical American's consumption of about 3,500 milligrams per day does not raise significant health risks and that the upper limit should actually be closer to 6,000 milligrams. They say that consuming too little salt, which they define as below 3,000 milligrams daily, carries its own health risks - a directly contradictory claim.
For those seeking practical advice about whether to limit their salt intake, the controversy has been confusing.
In December, New York City became the first jurisdiction in the country to require warning labels on high-sodium dishes like sandwiches and salads served in chain restaurants. The symbol, a black-and-white salt shaker, appears on menu items with more than 2,300 milligrams of sodium. One example is the cheddar and bacon burger at TGI Friday's, which the company's nutritional information shows has 4,280 milligrams of sodium.
