The pump you feel during a workout comes from increased blood flow flooding your muscles faster than it can leave, and the key ingredients responsible for this in pre-workout supplements are nitric oxide boosters. L-citrulline is the most common and well-supported, but several other ingredients work through different pathways to widen blood vessels, pull water into muscle cells, or both. Here’s how each one works and what actually matters for results.
How the Pump Happens
When you lift weights, your muscles contract repeatedly and demand more oxygen. Your body responds by releasing nitric oxide, a molecule that relaxes the smooth muscle lining your blood vessels, causing them to widen. This process, called vasodilation, increases the volume of blood flowing into working muscles. At the same time, the repeated contractions compress veins and slow blood from leaving. The result is a temporary buildup of blood and fluid in the muscle tissue, stretching the surrounding connective tissue and creating that tight, swollen feeling.
Most pump-focused pre-workout ingredients work by increasing nitric oxide production beyond what your body would generate on its own. More nitric oxide means wider blood vessels, more blood delivered per heartbeat, and a more pronounced pump. A few ingredients skip the nitric oxide pathway entirely and instead pull extra water directly into muscle cells, creating a different type of volume increase.
L-Citrulline: The Primary Pump Ingredient
About 70% of popular pre-workout supplements contain citrulline, making it the single most common pump ingredient on the market. It’s an amino acid your body converts into another amino acid, L-arginine, which then gets used to produce nitric oxide. The reason formulas use citrulline instead of arginine directly is a matter of absorption: roughly 70% of supplemental arginine gets broken down by your gut and liver before it ever reaches your bloodstream. Citrulline, by contrast, passes through nearly intact, with essentially 100% reaching circulation. In direct comparisons, citrulline raises blood arginine levels about 35% higher than the same amount of arginine supplementation.
Dosing matters significantly. Research on citrulline has used anywhere from 1 to 12 grams taken 40 to 120 minutes before exercise, but the most commonly studied effective dose is 6 to 8 grams. Many commercial pre-workouts contain an average of around 4 grams, which falls on the lower end. If your pre-workout lists citrulline malate (citrulline bonded to malic acid), keep in mind that roughly 40% of the weight is malic acid, so you need a higher total dose to get the same amount of actual citrulline.
Beetroot Extract and Dietary Nitrates
Beetroot juice and beetroot powder take a completely different route to boosting nitric oxide. Instead of supplying the raw material your body uses to make nitric oxide enzymatically, dietary nitrates rely on bacteria living in your mouth. These bacteria convert nitrates into nitrites, which are then absorbed in your gut and further converted into nitric oxide throughout your tissues and bloodstream. This is sometimes called the “non-canonical” pathway because it bypasses the normal enzymatic process entirely.
Under normal dietary conditions, this pathway contributes only a small fraction of total nitric oxide production. But when you ingest a concentrated dose of nitrates (anywhere from 2 to 20 times the typical daily intake of about 1.5 millimoles), plasma nitrate and nitrite levels rise substantially, meaningfully increasing nitric oxide availability. You’ll see beetroot listed on labels as beetroot extract, beetroot powder, or sometimes just “dietary nitrates” with a milligram count. It pairs well with citrulline because the two work through independent pathways.
Nitrosigine (Inositol-Stabilized Arginine Silicate)
Nitrosigine is a patented form of arginine that’s been chemically bonded to silicate and stabilized with inositol to solve arginine’s absorption problem. Unlike standard arginine, it has been shown to significantly increase blood arginine levels for up to six hours after a single dose. This extended window is part of its appeal: rather than a short spike, you get a sustained elevation that covers an entire training session and beyond. It shows up in many mid-range and premium pre-workouts, typically at doses of 1,500 milligrams.
Glycerol: The Water-Based Pump
Glycerol works through an entirely different mechanism than nitric oxide boosters. It’s an osmolyte, meaning it increases the concentration of dissolved particles in your body fluids. When you consume glycerol alongside a large volume of water (research has used 1,500 to 2,000 milliliters, or roughly 26 milliliters per kilogram of body weight), it raises your blood’s osmotic pressure. This creates a drive to retain fluid that your kidneys would normally filter out, expanding total body water and pulling extra fluid into tissues, including muscle cells.
The result is a pump that feels different from the blood-flow type. It’s more of a fullness or density in the muscle rather than the tight, throbbing sensation from vasodilation. Glycerol is often labeled as GlycerPump or HydroMax on ingredient panels, which are concentrated forms designed to avoid the clumping issues that raw glycerol causes in powder supplements. To get the effect, you genuinely need to drink a substantial amount of water alongside it.
Casein-Derived Peptides (VasoDrive-AP)
A newer category of pump ingredient involves short protein fragments derived from milk casein. The two specific peptides, known by their amino acid sequences VPP and IPP, work by inhibiting an enzyme called ACE that normally constricts blood vessels. By blocking this enzyme, these peptides allow blood vessels to stay more relaxed and open. In controlled testing, peak blood flow during reactive hyperemia increased from about 20.8 to 30.0 milliliters per minute per 100 milliliters of tissue, a roughly 44% improvement, without any change in resting blood pressure.
This ingredient appears on labels as VasoDrive-AP or “casein hydrolysate” and is typically dosed at 254 milligrams. It’s less common than citrulline or beetroot but increasingly shows up in formulas marketed specifically for pump enhancement.
Does the Pump Actually Build Muscle?
The pump isn’t purely cosmetic. Cell swelling from increased blood and fluid volume has been shown to stimulate muscle protein synthesis and reduce protein breakdown across multiple cell types, including muscle fibers. The leading theory is that pressure against the inside of the cell membrane is perceived as a threat to structural integrity, triggering signaling pathways that promote cellular reinforcement, essentially building the cell up to handle the stress.
There’s also a hypothesis that cell swelling may enhance muscle growth through increased activity of satellite cells, which are the stem cell-like precursors that donate material to growing muscle fibers. That said, direct long-term studies in humans measuring whether a bigger pump during training leads to more muscle growth over months of training are still limited. The basic science is compelling, but whether pump-focused supplementation adds meaningful hypertrophy beyond what heavy training alone provides remains an open question.
What to Look for on the Label
If your goal is maximizing the pump, the ingredients with the strongest evidence are L-citrulline at 6 to 8 grams, beetroot extract providing a meaningful nitrate dose, and glycerol paired with plenty of water. Nitrosigine at 1,500 milligrams is a solid alternative or complement to citrulline. VasoDrive-AP adds a third mechanism but is less well-studied for exercise-specific outcomes.
Watch for proprietary blends that list these ingredients without individual dosages. A blend that contains citrulline, beetroot, and glycerol sounds impressive, but if the total blend is only 3 grams, none of those ingredients are present at effective levels. The most useful labels list each ingredient’s dose individually so you can verify you’re getting enough to actually produce the effect.
One practical note: if you take blood pressure medication or have cardiovascular concerns, stacking multiple vasodilating ingredients can lower blood pressure more than expected. Clinical studies on nitric oxide supplements have specifically excluded participants taking antihypertensive medications or those with heart failure and coronary artery disease.