Caffeine levels circulating in the bloodstream may play a measurable role in how much body fat a person carries and how likely they are to develop type 2 diabetes, according to a large genetic study published in 2023.
Rather than focusing on how much coffee or tea people report drinking, the research examined how long caffeine naturally remains in the body, a factor largely determined by genetics.
The findings suggest that caffeine metabolism itself, not just caffeine consumption, could influence long-term metabolic health.
The study was carried out by researchers from Karolinska Institute, University of Bristol, and Imperial College London. Their results were published in March 2023 in BMJ Medicine.
How Genetics Shape Caffeine Levels in the Body
People metabolize caffeine at very different rates. Some break it down quickly, while others process it more slowly, allowing caffeine to stay in the blood for longer periods.
These differences are strongly influenced by genetic variations, particularly in and around the CYP1A2 gene, which controls how caffeine is broken down in the liver, and the AHR gene, which regulates CYP1A2 activity.
Individuals with certain variants of these genes tend to metabolize caffeine more slowly. Interestingly, previous research has shown that these same individuals often consume less caffeine overall, likely because its effects last longer and feel stronger.
Despite drinking less coffee or tea, their blood caffeine levels remain higher for longer after consumption.
Using Genetics to Study Cause and Effect
To investigate whether caffeine levels actually influence body fat and disease risk, the researchers used a method called Mendelian randomization.
This approach uses genetic differences that are fixed at birth as a natural experiment, helping scientists separate cause-and-effect relationships from lifestyle or environmental influences.
The analysis included genetic data from just under 10,000 people, drawn from existing population databases.
By linking caffeine-related genetic variants with body mass index, total body fat, and disease outcomes, the researchers were able to estimate the likely impact of higher circulating caffeine levels over time.
Lower Body Fat and Reduced Diabetes Risk
The results showed a consistent association between genetically predicted higher blood caffeine levels and lower BMI as well as reduced total body fat. People whose genetics caused caffeine to remain longer in their bloodstream tended to carry less body fat on average.
This reduction in body fat appeared to be closely linked to diabetes risk. The study found that higher genetically predicted caffeine levels were associated with a lower likelihood of developing type 2 diabetes.
Statistical modeling suggested that about half of caffeine’s apparent protective effect against diabetes could be explained by its association with lower body weight and fat mass.
In other words, caffeine may not directly prevent diabetes on its own, but its influence on body fat could significantly reduce diabetes risk over time.
No Clear Link to Cardiovascular Disease
While the connections between caffeine, body fat, and diabetes were strong, the study did not find evidence that higher blood caffeine levels reduced the risk of cardiovascular diseases. Conditions such as atrial fibrillation, heart failure, and stroke showed no meaningful association with circulating caffeine in this genetic analysis.
This distinction is important, as earlier observational studies have sometimes suggested heart health benefits from moderate coffee consumption.
The new findings imply that caffeine’s most consistent metabolic effects may be related to weight regulation and glucose metabolism rather than direct cardiovascular protection.
Why Caffeine Might Affect Body Fat
The researchers point to several biological mechanisms that could explain the observed associations. Caffeine is known to increase thermogenesis, the process by which the body produces heat and burns energy. It has also been shown to increase fat oxidation, meaning the body becomes more efficient at using fat as a fuel source.
Short-term clinical trials have previously demonstrated modest reductions in body weight and fat mass following caffeine intake. However, those studies were limited in duration, making it unclear whether the effects persist over years or decades.
This genetic study adds evidence that even small metabolic effects, when sustained long-term, could have meaningful consequences at the population level.
Why the Findings Matter at a Global Scale
Caffeine is one of the most widely consumed psychoactive substances in the world. Coffee, tea, energy drinks, and caffeinated soft drinks are part of daily life for billions of people. Because of this widespread exposure, even relatively small effects on metabolism could translate into large public health implications.
The researchers note that calorie-free caffeinated beverages, in theory, could be explored as a tool to help reduce obesity and type 2 diabetes risk. However, they emphasize that caffeine is not without side effects, including sleep disruption, anxiety, and increased heart rate in some individuals.
Important Limitations and What Comes Next
Despite the strengths of Mendelian randomization, the authors caution that the method is not perfect. Genetic studies can still be influenced by unmeasured biological pathways, and they cannot fully replace controlled clinical trials. The findings do not prove that increasing caffeine intake will lead to weight loss or lower diabetes risk for everyone.
As University of Bristol genetic epidemiologist Benjamin Woolf explained, randomized controlled trials are needed to test whether non-caloric, caffeine-containing beverages can safely and effectively reduce obesity and type 2 diabetes risk in the long term.
Until then, the study provides strong evidence that how the body processes caffeine may be just as important as how much of it people consume.
The Takeaway
This research adds a new dimension to the understanding of caffeine and metabolic health. By showing that genetically higher blood caffeine levels are linked to lower body fat and reduced type 2 diabetes risk, it suggests that caffeine metabolism itself may influence long-term disease outcomes.
While caffeine is not a cure or preventive treatment on its own, its subtle metabolic effects, repeated daily across years, may help shape patterns of body weight and diabetes risk worldwide.
