The debate over the health benefits of coffee – which is the most consumed drink in the world after water – has been going on for decades. This discussion was revived on Tuesday thanks to new research in the journal BMJ Medicine, which found that high levels of caffeine in the blood may reduce a person’s body fat percentage and risk of type 2 diabetes.
The results of the research – conducted by an international team of researchers led by Susanna Larsson from the Karolinska Institute in Stockholm (Sweden) – are consistent with findings of decades-old observational studies that found a relationship between coffee consumption and a lower risk of diabetes. In a 2018 article published also by Larsson, it was estimated that with each cup of caffeinated coffee taken per day, the risk of developing diabetes falls by 7%. In the case of decaffeinated coffee, the drop was 6%. Another study, carried out by scientists from the World Health Organization (WHO) in 2017, calculated that three coffees a day reduced premature mortality by between 8% and 18%.
Despite the growing body of positive findings, these observational studies have not been able to confirm that the relationship is causal. In 1991, the WHO put coffee on the list of possible carcinogens. But years later, it reversed the decision when it was found that the higher incidence of cancer among coffee drinkers could be explained by other associated habits, such as smoking cigarettes. The authors of the BMJ Medicine paper state that they did not do randomized testing. This type of study – which is very expensive and difficult to carry out – are used to establish causality, i.e. how caffeine affects the development of diabetes or cardiovascular disease.
To at least partially overcome these limitations, the scientists used a novel method, known as Mendelian randomization, which uses genetic variations with a known function to assess the causal effect of a risk factor. In this case, the researchers analyzed the CYP1A2 and AHR genes, which determine the speed at which the body processes caffeine. The scientists studied these genes in 10,000 people. Individuals with genetic variants that cause caffeine to remain in the blood longer tend to drink less coffee, as they experience longer stimulation. Despite this, they have higher concentrations of caffeine in their blood plasma.
Following this method, which tries to separate the effect of caffeine in the blood on obesity or diabetes from other factors related to coffee consumption, the researchers observed that people with genetic variants that slow down the processing of caffeine and have greater presence of the substance in their blood had a lower body mass index (BMI) and a lower body fat percentage. The study found that 43% of the “protective effect of plasma caffeine on type 2 diabetes was mediated through BMI.”
Gemma Rojo, a researcher at the Regional University Hospital in Málaga, says the study is interesting, but believes “on an individual level, the observed effect is not so great as to think that drinking coffee will prevent diabetes, and for a person who already has diabetes, it won’t help you control it, because it won’t lower your blood sugar levels.” While she says the study may indicate that “the evidence for the benefits of coffee is greater than the evidence we have against it,” Rojo does not believe it is useful when it comes to making individual decisions about whether to drink coffee or not.
The authors of BMJ Medicine study conclude that “randomized controlled trials are warranted to assess whether non-caloric caffeine containing beverages might play a role in reducing the risk of obesity and type 2 diabetes.”
But Jorge Ferrer, from the Center of Genomic Regulation (CRG) in Barcelona, believes this is excessive given that the study “has not taken into account other variables, such as mental well-being or how caffeine affects quality of sleep.” However, he does consider this type of Mendelian randomization study useful to obtaining information on caffeine, as it is much more difficult to find results with the alternative, i.e. “following lots of people for several years, giving some caffeine and others a placebo” while controlling other variables.
And, as is the case with most findings related to diet, any positive effect caffeine may have is not a miracle cure to weight loss or preventing chronic diseases such as diabetes. Following a healthy diet and exercising are still the most effective ways to address these issues.
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