Do Coffee Drinkers Really Fall into 3 Groups?
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A new report divides coffee lovers into three groups depending on how their bodies respond to caffeine.

But as fun as it is for caffeine drinkers to figure out which group they fall into, not all experts are on board with the report's clear-cut conclusions.

According to the report, which was published June 6 by the Institute for Scientific Information on Coffee (ISIC), the answer lies in our genes. Specifically, caffeine sensitivity depends partly on a liver enzyme called CYP1A2, which is coded by the CYP1A2 gene. The enzyme is responsible for "inactivating 95 percent of all ingested caffeine," the report said. In other words, this liver enzyme breaks down caffeine in the body. [10 Interesting Facts About Caffeine]

ISIC members include several European coffee companies, including Nestlé.

The versions of the gene vary among people, according to the report, and these genetic variations split the population into two groups: "fast metabolizers" and "slow metabolizers." The fast group breaks down caffeine more quickly than the slow group, and thus the effects of caffeine don't last as long for this group, the report says.

J.W. Langer, a clinical pharmacology lecturer at the University of Copenhagen and the author of the report, claims that these fast metabolizers can drink "multiple cups of coffee a day" because their bodies can quickly clear caffeine from their systems. But for slow metabolizers, caffeine stays in the body longer, so "the physiological effects of caffeine last longer and are more pronounced," Langer said. Thus, the report recommends lower doses of caffeine for this group.

But where did the third group come from?

"You have the genes for the liver enzymes, but you also have to consider how coffee affects the brain," Langer told Live Science.

That’s where adenosine, a neurotransmitter, comes in. Adenosine binds to adenosine receptors, leading to the sensation of being tired. But caffeine can also bind to these receptors, blocking the adenosine from binding and, in turn, preventing tiredness and boosting alertness.

As with the liver enzyme responsible for breaking down caffeine, there are also genetic variations in the genes that produce the adenosine receptor, according to the report. These variations, along with variations in the enzymes, "combine to factor into the three caffeine sensitivity groups: high, regular and low," Langer said. (The low-sensitivity group includes the "fast metabolizers" while the high-sensitivity group includes the "slow metabolizers.")

The report says that, because individuals tend to consume "the amount of caffeine they feel comfortable with," their levels of consumption are based on "self-regulating mechanisms rooted in the individual's genetic make-up." In turn, the report advises those with high sensitivity to consume caffeine in small amounts and says those with regular sensitivity will be "safe and without problems" with a "moderate caffeine consumption of 5 cups a day." (It's unclear, however, if the report was referring specifically to 8 ounces of coffee as a cup, or a more colloquial definition; in Europe, coffee is typically served in smaller cups than it is in the U.S.)

But the new report has not been fully accepted by the scientific community. Nanci Guest, a dietitian and researcher at the University of Toronto who was not involved with the new report, said it's largely misleading. [10 Things You Need to Know About Coffee]

"The gist of this report is that you drink as much coffee as you feel comfortable with, and you'll be OK," Guest told Live Science. "That take-home message is not based in any real evidence, and this report freely promotes coffee intake without considering any of the risks."

According to Guest, the term "sensitivity" is not an accurate description because it assumes that individuals can "feel the effects of caffeine intake," including the possibility of "increased heart attack risk, high blood pressure and decreased endurance performance."

Langer, however, said that he defines sensitivity as "what you feel when you drink coffee," and hopes that this report will help people recognize that "everyone is a unique coffee drinker."

He also cautioned against drinking excessive amounts of coffee to achieve the effects of caffeine, stating that although "low sensitivity" individuals may need more caffeine to feel the effects, there are potential negative effects.

These negative effects include mainly "anxiety and panic attacks," particularly if you’re sensitive to caffeine, Langer said, but these sensitive individuals are in "the minority."

Guest emphasized that "the jitters" are just the tip of the iceberg when it comes to the negative effects of caffeine.

She also noted that there are inaccuracies in the report and that the advice given should be taken with a grain of salt. For example, while the report states that pregnant women should limit their caffeine intake to 200 milligrams (mg), Guest said zero caffeine intake is the safest. (The American College of Obstetricians and Gynecologists, however, supports the 200-mg. limit.)

Guest also refuted the "moderate" five cups of coffee a day, stating that such levels of intake should be considered carefully.

Moreover, while the report states that coffee consumption has possible preventive effects against Parkinson's disease, these reports have been disputed recently, Guest said.

Originally published on Live Science.