There are a number of ways to administer caffeine such as liquid (e.g. coffee or energy drink), capsule, gum, nasal spray, and mouth rinse. Each type of caffeine possesses slightly different absorption characteristics that are described below and summarized in Figure 1.
Gum
The markedly faster rate of absorption with the gum is seen when examining the 200-mg dose, as a large increase in plasma caffeine concentration occurred between 5 and 15 minutes.
Capsules
The largest increases in caffeine concentration with the capsules were seen between 25-45 minutes.
Mouth Rinsing
Caffeine mouth rinsing is a relatively new form of caffeine supplementation that is administered as a “swish and spit” fashion over 5-20 seconds. Unfortunately, data looking at blood levels of caffeine levels reported no increase in blood caffeine concentration.
Energy Drinks
A number of studies have investigated the effects of commercially available energy drinks, e.g. Red Bull, that contains 160 mg caffeine, 2000 mg taurine, and 10.5 g glucose in 500cc. That said, the current data reports equivocal results for performance improvements in aerobic capacity, handgrip strength, jump performance or cognitive tests when compared to either placebo or caffeine-only. Interestingly, blood concentrations of caffeine appear to take rise more slowly and are overall lower when caffeine is obtained through energy drinks rather than warm coffee.
Nasal Spray
Caffeine administered twice via nasal spray for 20 seconds had no effect on a cognitive performance test (The Stroop task), Wingate sprint cycling performance, 30-minute cycling time trial, or blood concentrations of caffeine.
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Figure 1: *Caffeine content can vary by substance concentration, brewing technique, and volume.
Overall, oral caffeine is rapidly and completely absorbed , with 99 percent being absorbed within 45 minutes of ingestion provided it’s not taken via mouthwash or nasal rinse. Caffeine metabolism is increased by smoking, fasting, and vigorous exercise, whereas its metabolism is inhibited to a small (perhaps clinically insignificant) degree by grapefruit.