Some diets will let you subtract fiber from the total amount of carbs you consume each day (effectively letting you eat more carbs, because, as the diets say, “carbs from fiber don’t count”). Other diets say “no way” to this practice. So which one is right?
The answer is: It depends!
Here’s the Simple Version
Before getting into the chemistry — I’ll give you the short answer, in case you don’t care about the science…
If you’re doing an ultra-low-carb keto diet (atkins, or any other ultra-low-carb, high-fat diet that restricts your carbs to less than about 30g to 50g per day) then you CAN subtract both fiber, and sugar-alcohols from your carbs. In other words, if you’re following a keto-based diet, then yes, NET-CARBS are the accepted, and correct protocol!
If you’re doing any other diet that has more than about 50g carbs per day, you are doing a traditional approach to nutrition, and you SHOULD NOT use the net-carb concept. You should treat all carbs equally, regardless of whether those carbs come from fiber or not.
(Note that I talk about the key differences between a keto-based diet, and a traditional diet, here.. in case you want more info on how and why they work differently, and the pros and cons of each).
NOW.. if you care about WHY…. here’s the reason
Carbs are such a tricky thing in the weight-loss world. Some carbs are bad. Some carbs are good. Some will make you fat. Some won’t. It basically comes down to how quickly your body is able to process the carb. The simpler the carb, the faster your body can convert the carb to energy, the more likely the carb is to be a problem for weight loss.
Simple carbs (like sugar, white bread, etc.), as I’ve explained elsewhere, will cause your body to produce insulin in the blood-stream. (That’s the quick energy-burst that you feel after you eat a candy bar, or a banana).
In a traditional diet, that insulin spike isn’t necessarily a bad thing. In fact, it can be used as a tool to help fuel workouts, or help burn fat. In a traditional diet, carbs really aren’t that bad, as long as you eat them in moderation, and use them mainly as a tool to fuel your exercise and intense physical activity.
In a keto-based (ultra-low-carb, high-fat) diet, carbs are SEVERELY restricted because the premise of the diet is to change your body chemistry so that it gets all of the energy it needs from fat instead of from carbs. It takes 3-4 days of eating VERY few carbs to get the body into that mode (called ketosis). While in ketosis, if the body is fed any fast-burning carbs (like sugar, or anything else high enough on the glycemic index to cause an insulin spike) ketosis is shut off, and the body returns back to its normal state, where it prefers energy from carbs. Eating simple carbs in a keto-based diet will literally turn the fat-burn switch to the off position.
So why does this matter with relation to carbs from fiber?
Carbs from fiber are still carbs. There’s no getting around that. They have all the properties of other carbs — with one exception. They burn really, really slowly and generally can’t and won’t cause an insulin reaction. In other words, people on a keto-based diet can eat all the fiber-carbs they want and not risk damaging their state of ketosis. (Note that the calories from those carbs still count!)
On the other hand, people doing a more traditional diet still count fiber-based-carbs as carbs because… well… they’re still carbs, regardless of the glycemic potential.
So… keto people? You can subtract fiber from your carbs, because the whole point of restricting carbs is to prevent any sort of glycemic reaction, and fiber-carbs are safe, in that respect. Everyone else? A carb is a carb is a carb. They count!