It's just one little line added to those inscrutable nutritional labels, but the tiny change has sent tremors through the food industry — and may soon be felt in your pantry. Even if you've never heard of or cared about trans-fatty acids, packaged foods will now have to come clean about just how much of them they're packing, thanks to a government ruling issued last week.

To nutritionists, who know all fats are not created equal, that's good news. Unsaturated fats (those that are liquid at room temperature and contain fewer hydrogen atoms) are comparatively benign, but saturated fats (solid at room temperature with bonds filled, or saturated, with hydrogen) are notorious artery cloggers.

In the 1980s foodmakers began replacing "sat fats" with trans-fatty acids — unsaturated oils to which hydrogen has been added. This made the oils thick enough to use in baked products and margarine, gave packaged foods a longer shelf life and made all foods safer — or so it was thought. The new stuff was not listed as a fat, but if you saw a "hydrogenated" oil on an ingredient list, a trans fat is what you had.

That turned out to be a problem. Even small quantities of trans-fatty acids, we now know, raise bad cholesterol and other blood fats. They may also reduce levels of HDL — or good — cholesterol and increase the risk of diabetes. And because of antiquated labeling rules, products that were practically swimming in trans-fatty acids could be called "fat-free."

The industry has resisted adding trans fats to labels, in part because fats are what give snacks their flavor. But in the midst of an obesity epidemic, with lawyers eyeing foodmakers they way they once eyed tobacco companies, something had to give. Last week it did, when the Food and Drug Administration ruled that beginning Jan. 1, 2006, trans-fat grams will be listed right below the sat-fat line.

Manufacturers seemed braced for the move. Even before the ruling, food giant Kraft announced it was reducing trans-fat levels in many products. Last year McDonald's said it would do the same with its French fries, though it has yet to make the change. Frito-Lay already lists trans fats on its products; others are likely to follow.

As a rule, you shouldn't let your combined daily intake of sat and trans fats rise above 20 g. If you're at risk for heart disease, you should aim for 15 g. The math will be a lot easier once the numbers are marked on the labels.

• Jeffrey Kluger