Why 20g? The Physiological Threshold for Ketosis

In the ketogenic community, the "20 grams of net carbs" rule is often treated as dogma. However, in 2026, metabolic science has provided a clearer understanding of why this specific number acts as a universal "fail-safe" for entering nutritional ketosis. It isn't a random figure; it is tied directly to the storage capacity of the human liver.

Hepatic Glycogen and the Ketone Switch

The human liver stores approximately 80 to 100 grams of glycogen. This glycogen is the body's primary reserve for maintaining blood glucose levels during short-term fasting. Hepatic ketone production (ketogenesis) is only initiated when liver glycogen levels are significantly depleted. Consuming 20g of net carbs or fewer per day ensures that the glucose influx is low enough to prevent the replenishment of these liver stores, thereby keeping the "ketone switch" in the ON position.

Muscle Glycogen vs. Liver Glycogen

A common misconception is that all carbohydrates impact ketosis equally. Muscle glycogen, which can total 300-500 grams, does not directly inhibit ketosis because muscles lack the enzyme (glucose-6-phosphatase) to release glucose back into the bloodstream. Ketosis is governed by the Liver. Therefore, the 20g limit is specifically designed to target hepatic glucose concentrations, regardless of how much energy is stored in your bicep or quadricep muscles.

Individual Thresholds and GLUT4 Activity

While 20g is the standard induction limit, some individuals—particularly those with high insulin sensitivity and high GLUT4 transporter activity (common in athletes)—can maintain ketosis at 50g or even 70g of carbs. However, for the majority of the population struggling with varying degrees of insulin resistance, the 20g threshold is the clinical baseline required to guarantee the metabolic shift within 48 hours.

The Net Carb Calculation in 2026

To stay within this 20g boundary, precision tracking of "Net Carbs" is required. This involves subtracting non-impact fibers and specific polyols (like Erythritol) from the total carbohydrate count. Because fiber does not trigger a significant insulin response from the pancreas, it does not contribute to hepatic glycogen replenishment, allowing for the consumption of nutrient-dense, fibrous vegetables while remaining in a therapeutic ketogenic state.