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=Caffeine and Genetics=
There is some research indicating that there are genetic differences that change how caffeine effects people, but the research is a little unclear. However, it seems that having your genome tested could provide valuable information into how to use caffeine. The enzyme CYP1A2 metabolizes many drugs, and it's been shown to be responsible for over 95% of the metabolism of caffeine<ref name="BegasKouvaras2007"/>. The CYP1A2 enzyme is produced by the CYP1A2 gene, and a genetic change at rs762551 impacts how the enzyme is produced, with AA having greater activity than the AC or CC genotypes<ref name="snpe_rs76"/>. Sometimes the subjects with the AA variant are called "fast metabolizers". (I had my genome tested using 23andme.com which showed I have the AA variant of CYP1A2/rs762551.)
* A study of 101 recreationally competitive male athletes on a 10K cycling time trial with 2 or 4 mg/Kg caffeine, there was an improvement only in those with the AA variant of CYP1A2, with no effect on the AC and diminished performance for the CC<ref name="Guest-2018"/>.
* A study of 21 active subjects given 3 mg/Kg of caffeine and tested using a 30 second Wingate test showed performance improvements did not vary between AA and AC/CC variants, though AC/CC reported increased nervousness while the AA variants did not<ref name="Salinero-2017"/>.
* A study of 20 collage level tennis players showed that 6 mg/Kg caffeine improved intermittent treadmill performance with no differences between AA and C variants<ref name="KleinClawson2012"/>.
* Other factors beyond genetics impact CYP1A2 enzyme activity, with exercise, caffeine intake, broccoli all increasing it<ref name="Vistisen-1992"/>.
=Caffeine, Blood Pressure and Heart Rate=
Caffeine increases blood pressure at rest and under stress, including exercise stress. The effect of caffeine on heart rate is unclear, with both increases and decreases observed in studies. Generally caffeine decreases heart rate at rest and moderate intensity exercise, but increases it at maximal workloads.