Quantification of Self-Reported Caffeine Use - Caffeine: Chemistry, Analysis, Function and Effects - page 239

Table 13.4 Summary about caffeine metabolism. Information summarized

from the main article.

N Caffeine absorption peaks between 30 and 70 minutes following ingestion.
N In the human body, caffeine has a half-life of approximately 4-7 hours. The rate
of metabolism can be affected by pregnancy, oral steroid contraceptives, tobacco
use, and alcohol use.
N Caffeine concentrations can be assayed from blood, saliva, or urine.
N

d n 0 t 2 n g | 0

Caffeine concentrations are lower in saliva and urine than in blood, but
concentrations correlate strongly between samples.

N The strength of correlation between physiological caffeine concentrations and self-

reported caffeine use can depend on how much time has elapsed since caffeine
consumption and sample collection.

significant, correlations with [CAF] (r 5 0.31) and [PX] (r 5 0.42) (James et al
1989). The authors conclude that late afternoon [PX] is more representative of
caffeine consumption. Typically, caffeine consumption is greatest in the morning
and tapers off throughout the day, therefore most of the caffeine in coffee or tea
drank in the morning has been metabolized into PX by 5pm. PX has a similar
half-life as caffeine, but PX concentrations [PX] are less sensitive to recent
caffeine use and the concentrations are less variable throughout the day (Lelo
et al 1986). However, a third study compared self-reported caffeine intake among
moderate and high consumers across a 24-hour period with plasma [CAF]. Here,
a single plasma [CAF] sample drawn at 5pm correlated strongly with estimated
caffeine use from self-reports (r 5 0.97) and [CAF] was a better index of caffeine
ingestion than [PX] sampled at any time of day (Lelo et al 1986). However,
moderate and high consumers probably continued drinking caffeine into the
afternoon, in which case not all of the caffeine would have been metabolized by
5pm. If physiological concentrations are to be used to verify self-reported caffeine
use, these studies illustrate the importance of considering the time of sample
collection and the recency of caffeine use.

13.5 Conclusion
In conclusion, self-reported caffeine use is an effective measure of past and current
caffeine use. A thorough caffeine use history should query the source of caffeine
(i.e., coffee, tea, soda, etc.), the amount consumed per occasion, and the frequency
of use. When actual caffeine content information is not available, caffeine use can
be estimated using standardized caffeine content. Adjusting for the relative
amounts of caffeine contained in different beverages or foods will produce a more
accurate estimation of caffeine use. Self-reported caffeine use is measured with
questionnaires or interviews and may be either retrospective histories of past or
typical caffeine use, or prospective diaries in which caffeine use is recorded as it
occurs. Retrospective measures are fast and efficient, but can suffer from memory
or estimation errors. Prospective diaries may be more accurate, but could
inadvertently record atypical use. In general, studies have shown agreement