YouTube/Yoplait Forget about the fall of cereal — another breakfast item is in trouble as health trends shift among young Americans.

Light yogurt sales fell 8.5% in the past year ending in late September, dropping $200 million from roughly $1.2 billion to $1 billion, according to Nielsen data. Wider yogurt industry sales declined 1.5%, the fourth consecutive year of falling sales.

The fall of light yogurt, once a diet staple, is bad news for companies selling the dairy item.

General Mills, the maker of Yoplait and three smaller yogurt brands, reported on Wednesday that net sales declined 7% to $3.9 billion in the first quarter. Much of that decline was attributed to plunging yogurt sales, which fell a whopping 15% in the quarter.

"We continue to see challenging trends on our US yogurt business, driven primarily by significant declines on our Yoplait Light and Greek 100 product lines," General Mills CEO Ken Powell said in an earnings call on Wednesday. "Consumers of traditional light yogurts are buy yogurts in this segments are pivoting away from this segment to products that provide more satiety, like Greek yogurts."

Greek 100 sales fell 33% in the first quarter compared to the same quarter last year. Yoplait

As a result, General Mills is renovating 60% of its yogurt portfolio this year. For example, the company is reworking Yoplait Greek 100, a light Greek yogurt product, to include more protein and less sugar. The new take on light Greek yogurt should hit the shelves by January — or, as Powell calls it, "weight management season."

General Mills is also doubling down on yogurt categories that are growing, like organic yogurt, which grew 12.6% in the last year, according to Nielsen data. The company is now rolling out organic Annie's and Liberté yogurts in the US.

"While light yogurt is on the decline, older households — particularly 65+ — are still buying more than their fair share of light yogurts," Jordan Rost, the vice president of consumer insights at Nielsen, told Business Insider. "On the other hand, organic yogurts tend to appeal to younger households, particularly families with young children."

The decline in light yogurt can be traced to a growing demand for natural, protein-rich foods that fill up health-conscious consumers, instead of simply low-calorie and low-fat options. That's been a huge help for Greek yogurt, which appeals to customers seeking a filling option packed with protein.

On the flip side of the rise of protein and organic options is the fall of sugar.

Low fat-diets were the norm in the US in the '80s and '90s. As food makers worked to cut fat from products, they began replacing it with another ingredient: sugar. As a result, "light" yogurts were often packed with sugar, yet advertised as low-fat therefore healthy choices.

Greek and organic yogurts fit into younger customers' conception of health better than light yogurt. mary_thompson/Flickr

Now, the pendulum is swinging in the other direction.

Health-conscious consumers are increasingly ditching the low-fat diets, thanks to the rise of "good fats" and research linking sugar to weight gain. Instead of fat being seen as public enemy No. 1, artificial ingredients are more likely to be demonized, sometimes without scientific proof.

Younger consumers especially are adopting this new understanding of health — and ditching light yogurts.

"This is a perfect example of the distinction between 20th and 21st century health and wellness appeals," says Rost. "While there's still an opportunity for 'light' products for some consumers, younger consumers are gravitating towards foods that are at least perceived to be more naturally healthy."

In other words, American millennials have a new understanding of healthy — and light yogurt isn't included.