Scientists have revealed how genetic and environmental factors determine if a person needs one, or several cups of coffee to get through the day, a finding that may lead to better interventions for those attempting to abstain from the widely consumed beverage.
The study, published in the journal Behavior Genetics, noted that coffee addiction is affected by a positive feedback loop between genetics and the environment via a phenomenon known as quantile-specific heritability.
"It appears that environmental factors sort of set the groundwork in which your genes start to have an effect," said Paul Williams, study co-author from the Lawrence Berkeley National Laboratory (Berkeley Lab) in the US.
"So, if your surroundings predispose you to drinking more coffee—like your coworkers or spouse drink a lot, or you live in an area with a lot of cafes -- then the genes you possess that predispose you to like coffee will have a bigger impact," Williams said.
According to the Berkeley Lab scientist, these two effects are synergistic.
He said the phenomenon is also associated with cholesterol levels and body weight, and may play a role in other human physiological and behavioural traits like alcohol addiction.
As part of the study, Williams assessed 4,788 child-parent pairs and 2,380 siblings from the Framingham Study—an ongoing research launched by the National Institutes of Health (NIH) in the US in 1948 to investigate how lifestyle and genetics affect rates of cardiovascular disease.
In the Framingham Study, participants submitted detailed information about diet, exercise, medication use, and medical history every three to five years.
Based on this data, Williams used statistical methods to calculate what proportion of the participants' coffee drinking could be explained by genetics, and what must be influenced by external factors.
Earlier studies had shown that the most significant environmental factors influencing coffee drinking were culture and geographic location, age, sex, and whether or not one smokes tobacco.
The current assessment showed that between 36 and 58 per cent of coffee intake could be genetically determined—although the exact set of causative genes remain unknown.
However, the study noted that the association between a parent's coffee drinking and an offspring's habit got increasingly stronger for each of the offspring's coffee consumption quantile, or bracket -- zero cups per day, one to two cups, two to four cups, or five or more cups.
"When we started to decode the human genome, we thought we'd be able to read the DNA and understand how genes translate into behaviour, medical conditions, and such. But that is not the way it's worked out," Williams said.
"For many traits, like coffee drinking, we know that they have a strong genetic component -- we've known coffee drinking runs in families since the 1960s. But, when we actually start looking at the DNA itself, we usually find a very small percentage of the traits' variation can be attributed to genes alone," he added.
In genetics research, Williams said, traditionally it is assumed that one's surroundings and lifestyle alter gene expression levels in consistent and measurable ways, creating the outward manifestation of a trait.
The current study noted that the situation is more complex, adding that the methods used to arrive at the findings may help explain the diversity of traits we see in the real world.
"This is a whole new area of exploration that is just now opening up. I think it will change, in a very fundamental way, how we think genes influence a person's traits," Williams said.
