Ask the doctors 7/21

Dear Doctors: I had lunch with a co-worker, and she’s on a keto diet. She says she can have only 35 grams of carbohydrates per day. The label on the candy bar she was eating said it had 26 grams of carbs, which is almost her whole carb budget. But she said it only had 6 net carbs. What does that mean?

Dear Reader: For those who aren’t familiar, the aim of a keto, or ketogenic, diet is to severely limit the amount of carbohydrates you take in each day. Your body uses the carbs you consume as fuel. It converts them into glucose for immediate energy needs, and stores the excess in the liver as glycogen. The ease and speed of using carbs for energy makes them the body’s preferred energy source. But when deprived of adequate carbohydrates, the body will begin to burn fat. This is a biochemical process known as ketogenesis. It’s not as efficient as using carbs, so the body saves it as a plan B.

The threshold to achieve and maintain ketogenesis is somewhere in the neighborhood of 50 or fewer grams of carbohydrates per day. However, everyone’s metabolism behaves differently, and some people find they need to restrict further than that. As a point of comparison for how those limits affect your daily diet, a medium-sized apple has 25 carbs. So does one Oreo cookie. It’s not surprising, with the rise in popularity of the keto diet, that a new food industry has arisen. It is devoted to serving low-carb needs while still slaking high-carb cravings.

And that (finally) leads us to net carbs and the candy bar your friend was eating. Net carbs refers to the amount of total carbohydrates in a food, minus the fiber content. Take that medium-sized apple we mentioned earlier. It has about 25 grams of carbs, and about 4.5 grams of fiber. Subtract the fiber, and you’re left with 21.5 net carbs. The thinking is that because fiber doesn’t significantly affect blood-sugar levels, the grams of carbohydrates it represents can be ignored.

The other type of carbohydrate that gets a free pass with net carbs is something known as sugar alcohols. They are a type of carbohydrate that simulates sweetness. Because they don’t have a significant effect on blood sugar, they get deducted from total carbs. That’s how, once fiber content and sugar alcohols are accounted for, the 24 grams of carbs in your friend’s candy bar were magically reduced to 6 net carbs.

We think it’s important to note that the FDA isn’t on board with the concept of net carbs. And while counting net carbs can expand the food choices of someone who is restricting sugars or carbs, they can also be an excuse to add sweets and snacks to the diet. Net carbs isn’t an exact formula. Rather than embrace this somewhat fuzzy science, we urge carb-conscious people to instead eat foods that are naturally high in fiber and low in sugar.

Send your questions to askthedoctors@mednet.ucla.edu.