What is a carb? Besides being delicious, carbs are a category of molecules in grains, produce, dairy and sweets that occur in three key forms.

Sugars

These are the easiest to break down into glucose, a major fuel source. They exist naturally in food (fructose in fruit; lactose in dairy); it’s added sugar you should worry about. Sweeteners such as table sugar, agave nectar and honey contribute calories without other good-for-you stuff, like fiber in fruit or protein in milk. Sugar  appears where you least expect it and leaves a trail of destruction in its wake.  “It tastes good, and the more that’s produced, the more we eat,”  Since 1970, the amount of sugar in processed foods has nearly doubled, largely because sugar is cheap and readily available. Today it’s pumped into pretty much everything to make food products more enticing—even ones that are already sweet, such as fruit juice. As a result, the average American gobbles up 43,800 more calories from added sugar (meaning sweeteners, like high fructose corn syrup and honey, that are added to packaged foods) per year than we did in 1977.

Fiber

This complex carb functions in almost the opposite way from sugars: You can’t digest it, and certain fibers move slowly through the digestive tract, helping you feel full, shuttling out some fat and slowing glucose absorption. Aim for 25 grams a day.

Starches

Also complex carbs, but not all starch is created equal. Take the starch in potatoes. It’s complex compared with sugars but still very easy to break down into glucose. Resistant starch—found in beans, lentils and whole grains such as barley and whole wheat, among others—is indigestible and functions like fiber. Processing is a factor, too. Refining whole grains strips their fibrous shell, so they act more like sugars in the body; cooking and cooling potatoes turns some of their starch resistant. Confusing? yes: Avoid refined carbs, seek whole grains, and don’t sweat the rest.