Dear Doctors: I had lunch with a colleague and she is on a keto diet. She says she can only eat 35 grams of carbs a day. The label on the candy bar she ate said it had 26 grams of carbs, which is almost her entire carb budget. But she said there were only 6 net carbs. What does that mean?

Dear reader: For those unfamiliar, the goal of a keto or ketogenic diet is to dramatically limit the amount of carbs you eat each day. Your body uses the carbohydrates you eat as fuel. It converts them into glucose for immediate energy needs and stores the excess in the liver as glycogen. The ease and speed with which carbohydrates can be used for energy make them the body’s preferred energy source. But when deprived of adequate carbohydrates, the body will start burning fat. This is a biochemical process known as ketogenesis. It’s not as efficient as using carbs, so the body registers it as plan B.

The threshold for reaching and maintaining ketogenesis is around 50 grams or less of carbohydrates per day. However, everyone’s metabolism behaves differently, and some people find they need to restrict themselves further. For comparison of how these limits affect your daily diet, a medium sized apple contains 25 carbs. The same goes for an Oreo cookie. It’s no surprise, with the growing popularity of the keto diet, that a new food industry has emerged. It is dedicated to meeting low carb needs while quenching high carb cravings.

And that brings us (finally) to the net carbs and candy bar your friend was eating. Net carbs refer to the total amount of carbohydrates in a food minus the fiber content. Take that medium sized apple we talked about earlier. It contains about 25 grams of carbohydrates and about 4.5 grams of fiber. Subtract the fiber and you’re left with 21.5 net carbs. The idea is that since fiber does not significantly affect blood sugar, the grams of carbs it represents can be ignored.

The other type of carbs that gets a free pass with net carbs are so-called sugar alcohols. Although parts of their structures resemble sugar and alcohol, they are neither. Rather, they are a type of carbohydrate that simulates sweetness. Because they have no significant effect on blood sugar, they are subtracted from total carbs. That’s how once fiber content and sugar alcohols were taken into account, the 24 grams of carbs in your friend’s candy bar was magically reduced to 6 net carbs.

We think it’s important to note that the FDA disagrees with the concept of net carbs. And while counting net carbs can expand the food choices of someone who is limiting sugars or carbs, they can also be an excuse to add sweets and snacks to their diet.

Net carbs is not an exact formula. Rather than embrace this somewhat hazy science, we urge carb-conscious people to fill their plates with whole foods that are naturally high in fiber and low in sugar.