You know the subtraction. Total carbohydrates, minus fiber, minus sugar alcohols — the number at the bottom is supposed to tell you which carbs actually count. You’ve done it so many times the math is automatic: scan the label, run the subtraction, sort the product into a ranking that decides what goes in the cart and what stays on the shelf.
One of those sugar alcohols is maltitol — subtracted identically to erythritol and fiber under a label the FDA has never defined: net carbs. Maltitol drives blood sugar past half the impact of table sugar. The formula calls that zero.
Are Net Carbs a Real Measurement?
The net carbs formula subtracts fiber and sugar alcohols from total carbohydrates, treating ingredients with glycemic indices ranging from 0 to 35 as biologically identical. The resulting number does not predict glycemic response or fat loss outcomes. Across 80+ controlled trials, carbohydrate type produced no significant difference in body weight when total intake was equated.
— Livesey 2003 · Nutrition Research Reviews · Systematic review of polyol GI; Gardner et al. 2018 · JAMA · n=609
A systematic review compiled the glycemic index of every major sugar alcohol. The range: erythritol at 0, maltitol at 35, polyglycitol at 39 — with table sugar sitting at 65. The net carbs formula subtracts all of them identically, as if a substance with zero glycemic impact and a substance delivering over half the blood sugar response of sugar are the same ingredient.
The number that came out of the subtraction was never measuring how your body actually processes those carbs. It was arithmetic performed on inputs the formula assumed were equal — inputs that range from biologically inert to carrying over half the glycemic punch of sugar.
ERYTHRITOL
Glycemic index: 0. No blood sugar impact. The net carbs formula subtracts it as zero. Correct.
MALTITOL
Glycemic index: 35. More than half the blood sugar impact of table sugar (65). The net carbs formula subtracts it as zero. Wrong.
Fix the inputs, and you fix the math. But fixing the math only matters if what the math is trying to measure — carb type’s effect on fat loss — is a real variable.
The largest controlled diet trial ever conducted assigned 609 adults to a low-carb diet or a low-fat one for twelve months. The weight difference at the end: 0.7 kilograms. Not statistically significant. After a year of eating fundamentally different ratios of carbohydrates, the bodies landed in essentially the same place.
The pattern didn’t weaken at scale. Across the field’s largest meta-analysis — 53 trials, roughly 68,000 participants — the gap widened to 1.15 kilograms, a difference the researchers who compiled the data published under one word: clinically insignificant.
Inside that same trial, individual results told the real story. Two people following the exact same diet diverged by roughly 40 kilograms — a within-group spread that dwarfed the between-diet difference by 57 to 1. What you subtract on the label matters fifty-seven times less than how consistently you eat.
Fiber still matters — for gut health, for satiety, for reasons entirely outside the number on a label. The failure isn’t the subtraction itself. It is the framework that turned a real nutritional concept into a single sorting number for a decision the evidence says was never about carb type.
Every label comparison, every bar swap, every minute spent reading sugar alcohol lines — the precision was real, but the target it was aimed at never predicted the outcome. The question of whether low-carb diets are actually required for fat loss was answered across 80 trials before the first “net carbs” label ever printed. What those trials found explains why the subtraction never matched your results.