Coffee is widely known to be a stimulant. This is due to the presence of caffeine in the drink, reviewed by (Acheson et al., 1980). The properties of caffeine as a metabolic stimulant were originally discovered in the early twentieth century, in terms of changes in the rate of respiration of individuals that ingest caffeine through drinking coffee. (Higgins and Means, 1915). One cup of coffee contains about 100mg caffeine and is approximately equivalent to the ingestion of caffeine at 1.25mg/kg of body weight (Acheson et al., 1980).
A pioneering study of the effects of ingesting caffeine through drinking coffee, performed by Acheson and co-workers in 1980, determined that, in addition to increasing metabolic and respiratory rates, caffeine can increase the metabolic rate of subjects of normal or obese weight and actually stimulates the oxidation of fat in normal weight subjects (Acheson et al., 1980)
Caffeine functions within the human body by affecting biochemical pathways that in turn affect physiological processes, reviewed by (Magkos and Kavouras, 2005). Upon ingestion, caffeine is absorbed by the gastrointestinal tract (Magkos and Kavouras, 2005), One major biochemical function of caffeine is in the inhibition of adenosine receptors (Magkos and Kavouras, 2005) and another is in the role of caffeine as an antagonist of purines (Wager-Srdar et al., 1983).
Caffeine supplements are taken by athletes attempting to improve their performances because caffeine is known to increase metabolic rate during exercise and increases speed and endurance, but it has been shown that while improving performance, caffeine does not increase oxygen consumption (VO2) (Graham, 2001).
Based on this evidence, the hypothesis for this study is that caffeine ingestion will increase metabolic rate in the subject administered the coffee containing caffeine but will not increase the rate in the subject given the decaffeinated placebo.
Objectives of the study
The major objective of this study is to investigate the effects of the caffeine in one cup of coffee on the metabolic rate of human subjects. To fulfil this primary objective, it was also necessary to fulfil further requirements for the study: The initial objective of the study was to recruit two subjects for the experimentation to be performed upon. It was decided that one healthy male and one healthy female adult would be sufficient for this study. The normal resting metabolic rate or “baseline” of each subject in the fasting state was then determined and was followed by a randomized double-blind trial where one subject was given coffee containing caffeine and the other was administered decaffeinated coffee.
Materials & Methods
Two healthy subjects were recruited for the study, a 25 year-old male and a 24-year old female. Each subject was tested twice, in each case over a two hour period. The first experiment was conducted on 18th August 2008 and the second test was carried out on 2nd December 2008. In the first trial each subject was required to breathe into a Douglas bag in order for baseline data to be collected for each subject in the absence of ingested caffeine. This methods of measuring energy expenditure was deemed suitable for use in this study with reference to the review of (Levine, 2005), who claims that for short-term studies such as this, mask, hood or canopy systems are sufficiently accurate for the purposes of measuring energy expenditure. Both subjects were required to abstain from ingesting caffeine in their diet prior to the experiment and were required to refrain from ingesting food in order for them to be in the fasting state.
The second part or trial portion of the study was performed as a double-blind experiment to investigate the effect of coffee on metabolism when compared to the baseline levels obtained from subjects in the initial study. Again, subjects were required to have been in the fasting state and have not ingested caffeine for the previous week. One participant was administered coffee containing caffeine and the other received decaffeinated coffee as a placebo at time zero in a double-blind fashion. Data was again collected at time zero and at 30 minute intervals over a two hour period.
For both parts of the study, data on a wide range of variables related to metabolism were recorded. Atmospheric air pressure, temperature, volume of oxygen, nitrogen and carbon dioxide expired, and the time taken to record the gas content was noted. Data was sampled at the start of the test (time zero) and at subsequent 30 minute intervals over the 90 minute period of the study.
Calculations were performed based on the data collected in order to determine the volume of gas expired (VE) at standard temperature and pressure (STPD), known together as VE, STPD. The volume of oxygen consumed (VO2), volume of carbon dioxide consumed (VCO2), the respiratory quotient (RQ) and energy expenditure (EE) were also observed calculated based on the data obtained in the study.
The following calculations were performed in order to identify the VE, STPD, the VO2, VCO2, RQ and EE of each subject in the baseline part of the study and in the trial portion of the study following the administering of caffeine or placebo:
In terms of the metabolism and respiration of the female subject, the baseline and trial data for the subject follow similar patterns. The observed levels of VE (Figure 1A), VO2 (Figure 1B), VCO2 (Figure 1C), RQ (Figure 1D) and EE (Figure 1E) are very similar in both the baseline and trial experiments.
The most notable difference between baseline and trial data occurred with respect to respiration quotient (RQ) (Figure 1D) after thirty minutes in the trial experiment, where a large decrease in RQ is observed. The RQ returns to baseline levels at the 60 minute time point, suggesting that the observed result at 30 minutes may have been recorded erroneously. There is also an increase above expected levels in the value calculated for VE, STPD after 30 minutes in the baseline study (Figure 1A). Overall, there is no prominent evidence in these data to suggest the presence of caffeine in the female study during the trial experiment.
The mostnotable difference between baseline and trial levels of recorded and calculated data from the male subject has arisen with respect to the respiratory quotient of the subject (Figure 2D). The trial levels of RQ are constant throughout the time course of the experiment but are lower than the constant levels observed during the baseline study. This suggests a factor has been introduced in the trial study that has suppressed the RQ levels in the male subject.
Another notable difference is in the energy expenditure (EE) of the male subject (Figure 2E). In the trial experiment, the levels of EE remain constant throughout the time period of the experiment, but the baseline levels decrease by approximately 30% between 30 and 60 minutes and remain at this low level at 90 minutes. In similar fashion to the female subject (Figure 1), the male subject has similar levels in both the baseline and trial experiments for the VE (Figure 2A), but levels of VO2 (Figure 2B) and VCO2 (Figure 2C) remain at a higher level in the trial experiment while baseline levels decrease. Overall, it appears that the most notable observation in terms of the potential effects of caffeine on the male subject are with regard to the higher levels of energy expenditure in the trial portion of the study (Figure 2E).
From the data collected in this study and from the subsequent calculations performed in this double-blind study suggests that the male subject was administered caffeine while the female subject received the decaffeinated coffee placebo. This conclusion can be arrived at by noting the differences between levels of certain factors between the baseline and trial parts of the study, but most notably, by the increased level of energy expenditure of the male in the trial study when compared to the baseline study (Figure 2E) and when compared to the energy expenditure of the female in both the baseline and the trial portions of the study (Figure 1E).
The observation that energy expenditure in the male (Figure 2E) remains at a similar level throughout the trial portion of the study, while levels decrease as time proceeds in the baseline experiment suggests that energy is continuing to be expended by the male because of an increase in the metabolic rate of the subject, likely to be due to the presence of caffeine in the male. This observation is consistent with previous studies in which the effects of caffeine on energy expenditure have been investigated and are reviewed by (Graham, 2001; Graham et al., 1998; Magkos and Kavouras, 2004; Magkos and Kavouras, 2005).
As expected, the observed levels of VO2 and VCO2 and therefore RQ are not affected by caffeine. It has been previously shown that this is the case at rest (Graham, 2001) or during intensive exercise (Bell et al., 1999). RQ, a measure of the balance of oxygen consumption and carbon dioxide output, should therefore also not be affected by caffeine.
In order that the data from this study be accepted by the scientific community, the experimental design of any further study should be more rigorous. A further study should include a larger number of test subjects in order to more accurately determine the effects of caffeine on metabolic rate. A sample size of fifty to one hundred subjects would be more suitable for a study such as this. Comparing only one individual to another does not allow sufficient controls in order to account for many variables related to each individual involved.
One such variable may be gender. Investigating and comparing one male and one female subject in this study raises the question as to whether there are gender specific differences in the effects of caffeine on metabolism. A study designed such as this is not sufficient to answer this question and therefore the outcome of this study cannot be considered accurate. There are a number of differences in baseline values between the two subjects, which may be related to gender (Figures 1 and 2). The physiology of the subjects could also have changed in the intervening time period of four months between the baseline and trial experiments. Subjects could have increased or decreased in weight, have increased or decreased their level of fitness and the female subject may have been pregnant or be menstruating in only one of the portions of the study. For future studies, it is recommended that both the baseline and trial studies be performed on the same day.
Many genetic and physiological factors may be involved in the effect of caffeine on respiration rates. Caffeine itself functions in a number of different biochemical pathways in the body, reviewed by (Magkos and Kavouras, 2005) and a genetic difference or polymorphism in any of these pathways could alter the outcome. The presence or level of obesity of the subjects will affect the observed metabolic levels recorded in a study such as this, due to the differences in observed effects of caffeine on subjects of normal and obese weight (Acheson et al., 1980).
A recent study investigated the effect of non-exercise activity thermogenesis and identified the possibility that small movements in individuals, such as fidgeting, directly affects the total energy expenditure of the individual subject (Levine et al., 2000). For this reason, during a study such as that conducted here, the subjects involved should be closely monitored and prevented from such fidgeting activities. Alternatively, subjects should be divided into two groups (in the proposed future study with a larger number of experimental subjects for investigation), one that exhibit no non-exercise activity thermogenesis and one that does. Data from each group can be compared to identify the potential effects of caffeine on non-exercise activity thermogenesis versus the effects of caffeine on metabolism without such fidgeting activities affecting the outcome.
Both subjects were observed to exhibit movements of their bodies that may have resulted in non-exercise activity thermogenesis in this study and therefore such activity is likely to have affected the outcome of the study in terms of the total energy expenditure observed here. This observation provides a further caveat to the conclusions provided by the data calculated from this study.
Overall, this study has provided the expected outcome that caffeine increases the energy expenditure of an individual while not affecting their gas exchange or respiratory quotient, but because of the number of potential variables in the study, a further, more rigorous investigation is required to confirm that the data presented here are viable and accurate.