The net carbs formula
Net carbs are the carbohydrates your body actually digests and converts to blood glucose. The formula:
Dietary fibre passes through your digestive system without being absorbed or raising blood sugar. Sugar alcohols are only partially absorbed — hence the 0.5 multiplier for most types (erythritol is the exception, with essentially zero glycaemic impact).
Why this matters for keto
The ketogenic diet works by keeping net carb intake low enough (typically under 20–50g/day) to maintain ketosis — a metabolic state where your liver produces ketones from fat, which your brain and muscles use for fuel instead of glucose. Using total carbs rather than net carbs would incorrectly exclude many healthy high-fibre foods.
For example: 100g of broccoli contains 7g of total carbohydrates but 2.6g of fibre, giving just 4.4g of net carbs. On total carbs, broccoli looks expensive. On net carbs, it's one of the best keto foods available.
Not all sugar alcohols are equal
| Sugar Alcohol | Calories/g | Glycaemic Index | Net Carb Rule |
|---|---|---|---|
| Erythritol | 0.24 | 0 | Count as 0 (fully excluded) |
| Xylitol | 2.4 | 7 | Count as 0.5× (half) |
| Sorbitol | 2.6 | 9 | Count as 0.5× (half) |
| Maltitol | 2.1 | 35 | Count as full — high GI, spikes blood sugar |
| Isomalt | 2.0 | 9 | Count as 0.5× (half) |
Maltitol is the hidden danger in many "keto-friendly" products. Despite its low calorie count, its glycaemic index of 35 (compared to table sugar at 65) is high enough to significantly impact blood sugar and potentially disrupt ketosis. Always check which sugar alcohol a product uses.
Daily net carb targets by diet type
| Diet | Net Carb Target | Purpose |
|---|---|---|
| Strict / Therapeutic Keto | < 20g/day | Epilepsy, metabolic disease management |
| Standard Keto | 20–50g/day | Fat loss, metabolic health, ketosis maintenance |
| Liberal Low-Carb | 50–100g/day | Weight loss without strict ketosis |
| Moderate Low-Carb | 100–150g/day | General health, blood sugar management |