Highly branched cyclic dextrin (HBCD) is a type of engineered carbohydrate made from corn starch, designed to deliver energy during exercise while minimizing stomach discomfort. Sold under the brand name Cluster Dextrin, it has become a popular ingredient in intra-workout supplements because it leaves the stomach faster than glucose or traditional sports drink carbohydrates. That speed comes from its unusually large molecular structure, which keeps the concentration of dissolved particles in your drink extremely low.
How HBCD Is Made
HBCD starts as waxy corn starch, which is rich in a branching starch molecule called amylopectin. Two enzymes do the work of transforming it. First, an enzyme breaks the long starch chains into smaller branched clusters. Then a second enzyme, a branching enzyme derived from the bacterium Bacillus stearothermophilus, folds those clusters into a ring-shaped structure. The result is a large, globular carbohydrate with a cyclic (ring) core of about 50 glucose units and short linear chains averaging 16 glucose units each, branching outward. The whole molecule contains roughly 1,200 glucose units.
That size is the key to everything HBCD does differently. Maltodextrin, a common carbohydrate in sports drinks, tops out at about 17 glucose units per molecule. HBCD packs 60 to 70 times more glucose into a single molecule. Because osmotic pressure (the force that pulls water into your gut and makes you feel bloated) depends on the number of dissolved molecules rather than their size, HBCD can deliver far more total carbohydrate at the same molecular concentration.
Why It Leaves the Stomach Faster
A 10% HBCD solution has an osmotic pressure of just 9 mOsm. For comparison, 10% glucose sits at 646 mOsm, and a common dextrin used in sports drinks measures 117 mOsm. That enormous difference matters because the stomach empties low-osmolarity fluids more quickly. In a direct comparison, a 10% HBCD solution cleared the stomach in about 26.7 minutes, while a 10% glucose solution took 39.9 minutes.
Faster gastric emptying means the carbohydrate reaches your intestines sooner, where it can be absorbed and used for fuel. It also means less fluid sitting in your stomach during exercise, which reduces the bloating, sloshing, and cramping that derail many athletes mid-session. David Rowlands, an exercise metabolism researcher at Massey University in New Zealand, summarizes the consensus view: HBCD’s main practical benefit is that it empties quickly from the stomach, limiting the chance of GI distress.
Effects on Exercise Performance
The most striking performance data comes from a swimming study that tested athletes at 90% of their maximum effort. Swimmers who consumed HBCD lasted an average of 504 seconds before exhaustion, compared to 284 seconds with glucose and 309 seconds with no carbohydrate at all. That translates to roughly 70% longer time to fatigue, a statistically significant difference.
The mechanism likely involves both faster fuel delivery and a gentler stress response. In an exhaustive endurance trial, athletes drinking an HBCD-based sports drink showed no significant rise in noradrenaline (a stress hormone) during exercise, while those drinking glucose did. Post-exercise urine samples in the HBCD group also showed lower levels of several inflammatory signaling molecules, including IL-8, IL-10, and IL-12p40. These findings suggest HBCD may blunt the body’s stress and inflammatory response to hard effort, though the exact pathway isn’t fully understood.
Blood Sugar and Insulin Response
Despite its large molecular size, HBCD raises blood sugar quickly. Within 15 minutes of ingesting 15 grams, blood glucose levels climb noticeably. In one comparison, blood glucose rose by 5.1 mg/dL 30 minutes after an HBCD drink versus 2.0 mg/dL after a maltodextrin drink, with no difference in insulin levels between the two. That pattern (more glucose reaching the bloodstream without extra insulin) may help sustain energy delivery during exercise.
At smaller doses, the blood sugar picture is less dramatic. A CrossFit study using 30 grams of HBCD found no meaningful difference in blood glucose levels compared to a placebo after high-intensity workouts. This suggests the performance benefits depend partly on dose and exercise duration.
Potential Downsides
HBCD’s large molecular size is a double-edged sword. While it empties from the stomach quickly, some researchers have raised the concern that these bulky molecules may slow down once they reach the intestine. Because they need to be broken apart into individual glucose units before absorption, the breakdown process could create a bottleneck. One sports nutrition scientist described the molecule as potentially “gunking up” intestinal absorption. Whether this meaningfully limits fuel delivery during real-world exercise remains an open question, and the performance studies to date still show net benefits.
Cost is the other practical consideration. HBCD supplements are significantly more expensive per serving than maltodextrin or dextrose, which are effective carbohydrate sources in their own right. The premium buys you faster stomach emptying and less GI discomfort, benefits that matter most during prolonged or high-intensity sessions where gut issues are common.
How HBCD Compares to Other Sports Carbohydrates
- Glucose/dextrose: Simple, cheap, effective. But high osmolarity means more stomach discomfort at the concentrations needed for endurance fueling.
- Maltodextrin: A step up from glucose, with lower osmolarity because each molecule carries up to 17 glucose units. Still has roughly 13 times the osmotic pressure of HBCD at the same concentration.
- HBCD: The lowest osmolarity of common sports carbohydrates. Fastest gastric emptying. Each molecule delivers 60 to 70 glucose units. The tradeoff is higher cost and potentially slower intestinal breakdown.
The current standard in endurance nutrition uses a glucose-to-fructose blend (typically at a 1:0.8 ratio) to maximize absorption through two different intestinal transporters. HBCD doesn’t use that dual-transport approach. Some athletes combine HBCD with fructose to get both the gastric emptying advantage and the higher total absorption ceiling, though this combination hasn’t been rigorously studied.
Typical Dosing
Most studies have used between 15 and 30 grams of HBCD per serving, dissolved in water and consumed before or during exercise. The 15-gram dose produced measurable blood sugar elevations within 15 minutes. Performance benefits in the swimming study came from a solution consumed during the exercise session itself. For longer endurance efforts, repeated servings during exercise follow the same general carbohydrate guidelines used for any sports drink: 30 to 60 grams of carbohydrate per hour for sessions lasting over 60 minutes, adjusted based on intensity and tolerance.
HBCD dissolves easily and produces a thinner, less syrupy drink than an equivalent amount of maltodextrin, which makes it practical to mix at higher concentrations without the drink becoming unpleasantly thick.