Medically reviewed by Mary Lucas, RN
Written by Our Editorial Team
Last updated 5/12/2020
Maybe it began when we cast it by the crateload off the side of ships in Boston Harbor, but American men aren’t big tea consumers. Americans, in general, lag behind much of the world when it comes to tea consumption. And L-theanine is just one piece of evidence we could be worse off for it.
Tea is said to help with everything from cancer prevention to better sleep, and it’s compounds like the amino acid L-theanine that are credited with these benefits. L-theanine is said to be the compound that gives traditional green tea it’s frothy texture and unique taste, and it may be responsible for some of its healthy reputation.
L-theanine is one of several compounds found in tea leaves and credited with the widely-touted benefits of tea.
The most promising research on L-theanine finds it may have anti-stress and anti-anxiety benefits, particularly in people prone to anxiousness.
Several studies were left off of our roundup, as they were poorly designed or funded by companies with potential bias in the results.
L-theanine is considered safe and its side effects mild.
Several studies have tied L-theanine to anti-stress and related anti-anxiety benefits. In animal studies, scientists have confirmed it actually increases feel good brain chemicals like serotonin, dopamine, and gamma-amino-butyric acid. L-theanine’s effects on brain waves is said to resemble the effects caused by meditation.
In the lab, a small study found L-theanine led to a reduced heart rate and the detection of stress indicators in saliva when participants were under stressful conditions — doing mental math exercises. A meta-analysis in 2014 looked at 11 different randomized human studies and found positive mental effects including alertness and attention on subjects who took L-theanine with caffeine.
Some studies have been specific to people who struggle with high anxiety. One found that group, in general, experienced decreased heart rate, improved visual attention, and improved reaction time. Those with minimal anxiety didn’t experience the same benefits. Another monitored brain waves and found dietary levels of L-theanine (like those found in a cup of tea) had positive effects on mental alertness and relaxation without drowsiness in people prone to anxiety.
The most well-known study on L-theanine and sleep was funded by a supplement company. However, because this study is widely-cited by other reputable sources, we include it here with a word of warning: research funded by those with a financial interest in the outcome has the potential to be biased. That being said, this particular study looked at the sleep effects of L-theanine on male children with ADHD. The researchers found that those who took 400mg of L-theanine daily saw improved sleep quality and quantity over a period of six weeks.
There is some indication that L-theanine may be beneficial to your immunity, or the ability of your body to fight infections and viruses. However, much of the research on this topic has been limited to animal tests rather than human, indicating more research is needed. One human study found that combining L-theanine and L-cystine (another amino acid) resulted in an improved immune response to the flu vaccine in elderly patients.
There is some evidence that L-theanine can reduce the negative side effects of cancer treatments including chemotherapy — this benefit is even cited by the Memorial Sloan Kettering Cancer Center, indicating its legitimacy.
Laboratory studies have indicated L-theanine may have antitumor benefits at the cellular level. But the consensus among cancer experts is that further research is needed before definitive statements can be made about the cancer-related benefits of this amino acid.
Drinking tea is often associated with feeling good — when you’re sick, when it’s cold outside, you’re relaxing with a good book, or after a spa day (a very masculine spa day, of course). And there is some evidence this feel-good association is physiological and science-based, not merely caused by some psychological expectation that tea makes you more chill.
However, more research is needed to support many of the claims that are made about L-theanine. Studies on the anti-anxiety and anti-stress effects of the compound are some of the more compelling.