Given our seemingly insatiable obsession with foods like mac ‘n’ cheese, grilled cheese sandwiches, and mac ‘n’ cheese–stuffed grilled cheese sandwiches, it’s easy to forget that there’s a good chunk of people out there who can’t eat dairy products. In fact, those people are the norm: Roughly 75 percent of the world’s population has difficulty digesting lactose, the primary sugar in milk. If you’re lactose intolerant, many cheeses may leave you feeling bloated and uncomfortable—or worse.

But there are also certain cheeses that don’t seem to have that effect as much as others (and some have none at all). Which begs the question: If all cheeses are made from milk, why do some cause digestive havoc while others are fairly harmless?

One theory holds that aged cheeses have less lactose and therefore are less likely to induce symptoms. During the cheesemaking process, lactobacilli cultures are introduced to milk, converting the lactose into lactic acid, a substance that is easily digested by all humans. The longer the bacteria are left to do their thing, the lower the amount of lactose in the cheese. Eventually, the lactose levels dip low enough that the cheese can be eaten by someone with lactose intolerance with little or no adverse effects.

Going by this assumption, hard, extra-aged cheeses — your Parmesans, cheddars, and Swiss-style cheeses — are a safe bet if you’re lactose intolerant (and as a bonus, aging also brings delicious complexity). Younger, moister, softer cheeses that are aged for only a short amount of time (such as brie), fresh cheeses (like mozzarella and feta), and processed cheeses (hello, Velveeta) have higher levels of lactose, thus they are more likely to produce unpleasant effects.

There is another theory, however, that it’s not the lactose but the fat in cheese that makes certain varieties difficult to digest. The curdling process converts a good deal of milk’s lactose into lactic acid, while whey, the leftover liquid, washes much of the remaining lactose away. Even fresh cheeses contain only a fraction of the lactose that’s present in milk. But cow’s milk does have large, difficult-to-digest fat globules that remain in the cheese, which supposedly are the real source of stomach discomfort. This explains why goat and sheep milk cheeses, which have smaller fat particles, are often easier to digest, despite the fact that the fresh milk from both animals has roughly the same amount of lactose as that from cows.

The bottom line is that tolerance for lactose and different cheeses varies from person to person, and depends on the activity level of your stomach’s lactase (the enzyme responsible for lactose digestion). If you are lactose intolerant and want to get into cheese, try starting off with small amounts of hard, goat, or sheep cheeses and start exploring from there.