Image copyright Thinkstock Image caption Good fat, good fat, sugar

This week first lady Michelle Obama proposed new rules for the nutrition labels on packaged foods sold on US shelves. The new recommendations draw attention away from dietary fat while casting light on an even less nutritious ingredient - added sugar.

It's a big day for fat, which spent years as an enemy of health.

A growing body of research shows that fat can actually be good for you - and that when it comes to poor diets, another, more dangerous ingredient is hiding in plain sight.

Fat, carbohydrates and protein are macronutrients, the building blocks of all other foods. Carbohydrates and protein have four calories per gram, and fat has nine calories per gram.

As obesity rates rose toward the end of the 20th Century, health professionals looked for ways to slow the growth.

"The idea in the late 1980s was if you cut down on fat, you'd be cutting down on calories," says Marian Nestle, professor of nutrition, food studies, and public health at New York University

And so a new era of snacking began, with food companies promoting fat-free or low-fat items and consumers trying to cut down on fat in the name of healthful living.

The new label Image copyright Handout Highlights from the proposed updates to the US nutritional label: Calorie content given in bigger font

Serving size more realistic. A pint of ice cream goes from four servings to two

Items that could be consumed in one sitting, such as a 20oz (0.56l) soft drink, must give nutrition for both suggested serving sizes and total package size

Potassium and Vitamin D levels listed

Fat earned a slight reprieve at the turn of this century, when carbohydrates became the focus of many diets. But David Grotto, author of the Best Things You Can Eat, says the current conventional wisdom is moving away from the philosophy that an entire class of macronutrients can make or break your diet.

"We need carbohydrates, we need protein, we need fat," he says. "Fat as a category is not bad," even though certain types of fat, such as trans fatty acids, can be harmful.

In fact, research shows that dietary fat serves an important purpose: many vitamins are fat-soluble, so vegetables and other nutrient-rich foods are best consumed with a bit of fat to help the body better absorb the nutrients.

And a new study from Sweden suggests that consuming high dietary fat is associated with lower rates of obesity. Researchers suspect that full-fat foods make one feel full - and therefore less inclined to overeat.

"Most nutrition experts are on the same page now that it's not fat that's the villain, it's calories," says Grotto.

And one of the biggest culprits in boosting the calorie contents of food, say nutrition experts, is sugar.

"If you were on a desert island, eating sugar would be preferable to drinking seawater," says Walter Willet, dean of the nutrition department at the Harvard School of Public Health.

But that's about all he can say in favour of the sweet stuff. A type of carbohydrate, sugar only packs four calories per gram. But many products use a lot of it to boost taste. Consumption offers other problems, too.

"We know that high intakes of added sugars are associated with a number of risk factors for coronary heart disease," says Rachel Johnson, a professor of nutrition at the University of Vermont. Eating calories from sugar means you're not getting other essential nutrients that you might from a less sugary snack.

There is also increased risk of dangerous blood fat levels, inflammation and, according to a recent paper, heart disease-related death.

Troublingly, that same paper showed that sugar consumption was at a dangerously high level in the US. In fact, more than 70% of Americans exceed the recommendation that only one-tenth of the day's calories come from sugar.

Secret sugars Even though ingredients used the most in a packaged food usually sit at the top of the ingredient label, "food companies have gotten clever at breaking up the added sugar," says Walter Willet. You may see some of these listed instead: High fructose corn syrup

Cane crystal/cane juice

Isomalt

Sucrose

Maltose

Sorbitol

Dextrose

Why? While everyone was cutting back on fat in the 1980s and 1990s, sugar was slowly taking hold of the American diet. Those boxes of low-fat snack cakes? Nestle says that to replace the flavour benefits of fat, food manufacturers just upped the sugar, and therefore the calorie content.

The new nutritional labels announced by Mrs Obama aim to change that by giving new, prominent real estate to sugar, especially the added sugars which pose the biggest threats.

"Naturally occurring sugar comes along with vitamins, minerals, and fibre," says Nestle. "No one is worried about the amount of sugar in fruit or milk. It's the added sugars that dilute the vitamins, minerals, and fibre in food," and that contribute to high-calorie choices.

After all, says Grotto, it's total calories and quality of calories that make a meal choice good or bad - not necessarily what's in it.

"It's myopic to look at a single ingredient, but we always seem to love demonising something," says Grotto.

"Sugar has become the new fat."