Bitter Taste Perception

As a survival mechanism, humans developed an adaptation to taste toxic compounds as bitter. Plants produce many toxic bitter-tasting compounds, developed as pesticides, to disincentivize consumption. In response, herbivores developed more potent liver enzymes to neutralize these toxins, while omnivores (like humans) developed enhanced detection and screening mechanisms. It is estimated that 75% of humans have a relatively robust detection system, but some individuals have a diminished ability to sense bitter compounds. These individuals are less sensitive to the bitter taste of caffeine.

The type 2 taste receptors, or TAS2Rs, are responsible for tasting bitter compounds and one member in particular is responsible for tasting caffeine — Taste 2 Receptor Member 46 (TAS2R46). Genetic variants of this gene have been linked to reduced taste sensitivity. Since taste is one of the key drivers of food preferences and dietary habits, genetic variants that make individuals less sensitive to caffeine could drive dietary behavior. Individuals who are less sensitive to the bitter taste of caffeine may need less cream and sugar to mask the bitterness in their morning cup of joe. However, they may also be more likely to consume greater quantities — putting them at greater risk of sleep disturbances or anxiety if this trait is paired with other genetic variants.

Caffeine Sources

Caffeine comes in two varieties — natural and synthetic.

The primary sources of natural caffeine are coffee, tea, and dark chocolate. These are derived from coffee beans, tea leaves, and cocoa beans.

The synthetic version of caffeine is produced in the lab with petroleum-based chemicals. It is often referred to as “anhydrous caffeine” on food labels and it is the version most often found in soft drinks and energy drinks.

Frequent consumers of natural caffeine sources, like coffee and tea, are noted to have better health than regular consumers of synthetic caffeine sources, like sodas (diet or regular) and energy drinks. It is unclear if this is a response to the type of caffeine or if it is a result of the phytonutrients (healthy plant compounds) that accompany natural sources of caffeine, but this distinction is important to note. Therefore, it is advisable to choose natural sources of caffeine. Caffeine content can vary dramatically by coffee bean or tea leaf variety, as well as by preparation method, so it is wise to check the caffeine content of a beverage before you drink it.

Daily Recommended Intake

According to the USDA’s Dietary Guidelines, moderate coffee consumption can be part of a healthy diet. Moderate coffee consumption means 3-5 cups per day, providing approximately 200-400 mg of caffeine. For reference, one cup (8 oz) of brewed coffee contains 95 mg on average. A Starbucks Grande [16 oz] is a two-cup serving that provides 310 mg of caffeine. An identical serving of black tea will provide about 35 mg of caffeine, while green tea will give you 25 mg.

Adenosine: Your Battery Meter

Adenosine is used as an indicator of current energy stores, and when energy stores are depleted it signals to slow everything down so that energy stores can be replenished. Adenosine is made when you burn ATP, the body’s main energy currency. High levels of adenosine indicate that ATP is being burned faster than it is being made, sending a signal that an ongoing activity must be slowed. Adenosine acts as a sort of battery manager, as your cell phone’s battery is depleted, background apps are shut off in order to conserve battery power for only essential operations. High levels of adenosine are why you feel fatigue after an intense workout and they are also why you feel tired or sleepy as your bedtime nears.

While very high levels of adenosine produce potent feelings of fatigue, a baseline or basal level of adenosine in your cells allows for wakefulness. When adenosine levels drop below this baseline, you experience feelings of alertness. Just as high levels of adenosine signal a lack of fuel, low levels of adenosine signal an abundance of fuel. Fuel demanding to be used! This is how caffeine works its magic.

Caffeine Hacks Your Battery Meter

Caffeine, and the byproducts of its metabolism, bind to adenosine receptors, block adenosine from binding, and in so doing, trick the cell into thinking that adenosine levels are low. This is the signal that there is fuel to burn, so energy and activity are increased. There are several different adenosine receptors, but caffeine and its metabolites target two in particular — the A1 and A2A receptors. These receptors are just the first step in a long signaling chain, so their triggering produces a series of effects.

Caffeine-Induced Insomnia

Genetic variants in the adenosine A1 and A2A receptors have also been associated with poor sleep in response to caffeine. Poor sleep can mean either a reduction in sleep quality, lots of tossing and turning, or reduction in sleep duration due to trouble staying asleep. The disruption of sleep produced by caffeine is not related to a person’s ability to metabolize caffeine; instead, it is a function of the brain’s initiation and control of sleep. In studies of individuals with nearly identical caffeine metabolism, and identical amounts of caffeine in their blood, people with caffeine-sensitive adenosine receptors suffered from worse sleep.

Caffeine-Induced Anxiety

One of the downstream effects of caffeine binding involves the neurotransmitter dopamine and may play a role in producing feelings of anxiety. While dopamine has often been described as “the pleasure chemical,” the neurotransmitter is involved in both reward-seeking behaviors and pain-avoidance behaviors. Reward-seeking behaviors are encouraged by triggering feelings of pleasure when a task is accomplished or an item is obtained. On the other hand, pain-avoidance behavior can be promoted by triggering feelings of fear and anxiety.

For example, when a child commits to a lengthy search for the cookie jar and they find it, they enjoy pleasure sensations along with their cookie. Tenacity and commitment are encouraged. On the other hand, if a child were to accidentally come into contact with a hot stove during their cookie search, feelings of pain and fear would wash over them. By promoting anxiety when near the stove, the child would be protected from further injury.

Genetic variants in both the adenosine A2A receptor [ADORA2A] and the dopamine D2 receptor [DRD2] have been associated with feelings of anxiety and stress in response to caffeine.