Factors That Affect Caffeine Sensitivity

Caffeine sensitivity refers to how much effect caffeine has on our bodies. When people who have unusually high caffeine sensitivity ingest even a small amount of caffeine, they may experience significant symptoms, such as:

• Anxiety
• Tremors
• Headache
• Palpitations
• Insomnia

If you have a high caffeine sensitivity, you should generally try to avoid caffeine altogether.

How sensitive you are to caffeine is determined by several factors, including age and sex, but the biggest factor is genetic. Also, caffeine sensitivity is strongly modulated by the use of oral contraceptives (birth control pills) and by pregnancy.

How Caffeine Is Metabolized

Caffeine is rapidly absorbed from the intestines into the bloodstream, and it readily crosses the blood-brain barrier into the brain. During pregnancy, it freely crosses the placenta.

In your brain, caffeine binds to receptors for a neurotransmitter called adenosine, thus preventing adenosine from attaching to them. Because adenosine sends the brain messages that it's time to sleep, blocking adenosine receptors has the effect of producing wakefulness.

Caffeine is metabolized in the liver by an enzyme called CYP1A2, which breaks it down into chemicals called theophylline, paraxanthine, and theobromine. It's then excreted by the kidneys into the urine.

The half-life of caffeine is typically between 4 and 6 hours, which is typically how long caffeine affects people.

What Affects Caffeine Sensitivity?

Sensitivity to caffeine is largely determined by the activity of the CYP1A2 enzyme in the liver. The more active the CYP1A2, the less sensitive we are to caffeine. Several factors affect CYP1A2 activity.

• Age: CYP1A2 activity tends to decrease with age, so older people tend to be more sensitive to caffeine.
• Sex: Women tend to have lower CYP1A2 activity than men.
• Oral contraceptive use and pregnancy: Estrogens inhibit CYP1A2 activity and caffeine sensitivity increases.
• Genetic makeup: Several gene variants have now been identified that affect CYP1A2 activity.

Your Genes and Caffeine Sensitivity

Genetic testing has revealed three general categories of caffeine sensitivity related to our genetic makeup:

• Hypersensitivity to caffeine: These people have very low CYP1A2 activity. They have a strong response to even small amounts of caffeine, and will commonly experience jitters and insomnia.
• Normal sensitivity to caffeine: This is the vast majority of people. They can consume up to 400 mg of caffeine per day without adverse side effects.
• Low sensitivity to caffeine: These people have very high CYP1A2 activity, and often experience no effect at all from consuming caffeine. Smokers often have increased CYP1A2 activity due to epigenetic changes caused by tobacco use.

Pregnancy and Caffeine Sensitivity

Pregnancy reliably increases sensitivity to caffeine. The half-life of caffeine in pregnant women is often four times longer than it is for non-pregnant women—up to 16 hours. Furthermore, caffeine readily enters the bloodstream of the fetus, and the fetus has very, very little CYP1A2 activity.

While serious adverse effects on the fetus caused by the mother’s caffeine intake have not been documented, this issue has not been studied extensively. In general, pregnant women are advised to limit or avoid caffeine.