Coffee is one of the most consumed beverages and psychoactive substances in the world.
Therefore, it’s important to explore its specific ingredients, their impact on health, and the overall association between consumption on health outcomes and if there are any potential drawbacks.
Coffee contains caffeine, with about ~80mg caffeine in each shot on average, however this varies significantly.
In the research, coffee is actually a large contributor to total antioxidant consumption in some individuals and reduces inflammation on average.
On top of this, it also contains a small amount of vitamins and minerals, the most predominant being B vitamins (mainly B2 & B5), manganese, potassium and magnesium.
Caffeine blocks the function of adenosine, which plays a role in muscle relaxation and sleepiness.
In addition, it can increase the production of neurotransmitters like norepinephrine and dopamine which is why it makes us feel more alert and may alter our mood.
Positives in the Research
Type II Diabetes
A 2018 systematic review involving almost 1.2 million participants, found that those who were in the highest category of coffee consumption (median of 5 cups/day) had a 29% lower risk of developing type II diabetes compared with those in the lowest category of consumption (median of 0 cups/day).
Most of the studies also adjusted for confounders such as age, sex, BMI, physical activity, smoking, alcohol and dietary factors.
It also highlighted that each ‘cup per day’ increase in caffeinated and decaffeinated coffee decreased T2D risk by 7% and 6% respectively up to about 8 cups per day.
This highlights that the potential benefits may also stem from the nutrients in the coffee, rather than just the caffeine.
A study that was a little bit different in nature looked at how a 4 year change in coffee consumption influenced the incidence of T2DM in the following 4 years.
It found that those who increased their coffee intake by >1 cup/day lowered their risk of T2DM by 11%, whereas those who decreased their coffee intake by >1 cup per day had a 17% increased risk.
One possible mechanism explaining this may be through the chlorogenic acid and trigonelline content in coffee. A randomized control trial showed that these significantly lowered glucose and insulin levels following an OGTT.
A systematic review looking at 11 studies with over 29,000 participants found that there was a 27% reduction in risk for AD for those in the highest category of coffee consumption compared with the lowest.
However, there was no association with the different measures of cognitive decline or dementia as a whole.
Another systematic review, which looked specifically at caffeine, found that those who consumed daily caffeine were 16% less likely to develop dementia.
Researchers speculate this could be through the improved cognitive outcomes seen through the consumption of polyphenols such as chlorogenic acid, or the association with an increase in insulin sensitivity, which is a strong risk factor for cognitive decline.
A meta-analysis found a non-linear relationship between coffee consumption and PD risk, where the greatest outcomes were seen at about 3 cups/day reducing the risk by 28%.
It found that caffeine consumption specifically had a linear relationship, where there was a 17% reduction in risk for every 200mg/day increment.
Another large study compared 0, 1-4, and >-5 cups of coffee consumed per day and found a 0%, 57%, and 60% reduction in risk of PD respectively.
On top of the antioxidant properties, another reason may be that the adenosine antagonistic effects of caffeine may improve motor deficiencies of PD and reduce dyskinesia (involuntary movements).
One study with almost 500,000 individuals with a median follow-up of 10.7 years found that coffee drinkers had a 21% reduced risk of developing chronic liver disease compared with non coffee drinkers.
The maximal protective effect was shown to be around 3-4 cups per day and was similar with all types of coffee, including decaf.
This is supported in a systematic review that looked at the association between coffee and hepatocellular carcinoma (HCC) and highlighted that every 2 extra cups of coffee per day that was consumed, was associated with a 35% reduction in risk.
This dose-response relationship occurred up to about 5 cups per day
When isolating the cohort studies specifically looking at the type of coffee used, an extra two cups of caffeinated and decaffeinated coffee were associated with reductions of 27% and 14% in the risk of HCC.
Other studies have looked at its effect on cirrhosis and found that the higher the coffee consumption, the greater the reduction in risk. However, this was only the case for alcoholic cirrhosis.
There are several mechanisms potentially exaplaining these outcomes.
Firstly, caffeine has been shown to reduce HCC proliferation.
Coffee also contains cafestol and kaweol, which increase the activity of phase 2 liver enzymes and therefore may improve metabolism and excretion of carcinogens.
It’s important to note that these mechanistic studies are in animal or cell models, so human trials are needed to state a definitive link.
On top of this, the polyphenols may also aid in reducing oxidative DNA damage through the anti-inflammatory properties.
A recent review highlighted that regular coffee consumption was associated with a decreased risk of developing hypertension, heart failure and atrial fibrillation.
There also seemed to be a J-shaped association with coronary artery disease, where moderate coffee consumption showed a decreased risk and high coffee consumption resulted in an increased risk.
An earlier systematic review involving almost 1.3 million participants, looked at the dose-response relationship with coffee consumption and overall cardiovascular disease risk.
It compared those with the lowest intake of coffee (median 0 cups/d), with the highest (>5 cups/d), second highest (3.5 cups/day), and third highest intake (1.5 cups/d) and found a 5%, 15%, and 11% risk reduction respectively.
The following graphs are extracted from the review and show the association with coffee consumption and the risk of CVD, coronary heart disease and stroke.
This supports the J shaped association, where the greatest reduction in risk seems to occur between 3-5 cups per day.
Although this is the case, there still seemed to be an overall relative risk reduction up to about 9 cups/d, which is a lot of coffee.
It’s important to keep in mind that this is just the average of the studies as some even showed an increased risk past 4 cups/day.
Interestingly, this analysis showed no significant association between decaf coffee consumption and CVD risk.
The authors speculate this may be because the overall decaf consumption was quite low, diminishing the power to detect any association.
There could also be reverse causation, where those with CVD related conditions may be more likely to have swapped to decaf coffee.
Overall, the positive outcomes of coffee and CVD risk may be explained through its positive influence on type 2 diabetes and reduction in inflammatory markers, or even a secondary effect of moving more throughout the day.
A study compared coffee intake from 0 to 4 or more cups per day and found that the higher the intake, the greater the reduction in risk of gallbladder cancer.
A systematic review also found a dose-response relationship with coffee intake and gallstone disease (up to 6 cups per day), with an overall reduced risk of 17% between coffee consumers vs non coffee consumers.
A meta-analysis found that there was about an 8% decrease in risk of depression for each cup per day increment in coffee consumption. They also specifically looked at caffeine and found similar results, between 68mg-509mg/day.
This was supported in another systematic review, highlighting the greatest risk reduction being at about 400ml per day.
A more recent study with a large cohort of university graduates also found that those who drank at least 4 cups of coffee per day showed a significantly lower risk of depression than participants who drank less than one cup of coffee per day.
A review found similar results when specifically looking at suicide and found when compared to drinking ≤ 1 cup/week, those who drank 2-3 cups/day or ≥ 4 cups/day had a 45% and 53% reduction in risk of suicide respectively.
The ‘mood-boosting effects’ through the modulation of adenosine, or a reduction in inflammation through the polyphenols may partly explain these outcomes.
However, mental health is a very complex topic with many variables influencing it.
For example, as coffee is often used in social situations, there could also likely be an influence in mental wellbeing through an increased amount of social activity. Additionally, those who consume more coffee may also do so in relationship to exercise, which would also influence outcomes.
A large prospective study examined the associations of the consumption of coffee with the risk of mortality.
Compared with non-drinkers, coffee consumption of 1 to 5 cups per day was associated with lower risk of mortality, whereas coffee consumption of more than 5 cups per day was not associated with risk of mortality.
Inverse associations were found for both caffeinated and decaffeinated coffee as well.
This is supported in other research where there was a dose-response relationship of up to about 5 cups per day. Decaffeinated coffee also showed similar results.
Although not specifically looking at coffee, caffeine consistently shows benefits in athletic performance.
If consistently used, this may result in greater cardiovascular fitness, muscle mass etc. over time.
Ultimately, you could then speculate that over time this may have a net benefit on health outcomes overall compared to if you had never consumed it at all.
The caffeine in coffee does raise metabolism slightly. This is why it is commonly used as a thermogenetic compound in many fat burners.
Studies show that it can increase resting metabolic rate anywhere from 3% to 11%, with larger doses having the greatest effects.
It’s important to note, that this is only a very small difference overall.
For example, one study that had participants consume 100mg of caffeine every 2 hours over 12 hours, only increased their daily energy expenditure by 79-150 total calories.
Another one showed an increase in energy expenditure by only 13 calories over a 3 hour period.
Although it is a factor, it doesn’t seem to make a large difference at all.
Negatives in the Research
Sleep / Insomnia
Most of the studies exploring this relationship look into the role of caffeine, rather than coffee specifically.
A 2023 systematic review highlighted that caffeine consumption reduced total sleep time by 45 minutes and sleep efficiency by 7%, whilst increasing sleep onset latency and wake after sleep onset by 9 minutes and 12 minutes respectively.
It also increased the proportion of light sleep and decreased the proportion of deep sleep.
It concluded that to mitigate these effects, 107mg of caffeine shouldn’t be consumed more than 8.8 hours prior to bed time.
This seems to be the most common drawback associated with coffee and can often result in a negative feedback loop of being tired and needing more caffeine throughout the day.
Ultimately, we know sleep is linked to a plethora of health outcomes. This also effects people to a different extent as there is such as large variability in the metabolism of caffeine.
Drinking decaf has been shown to have no effect on sleep disturbances. Therefore, choosing this type of coffee would prevent this particular drawback.
There are certain constituents in coffee that can inhibit the absorption of vitamins and minerals.
One study showed that the iron absorption of a meal decreased by 39% when consumed alongside one cup of coffee.
Long-term intakes have been associated with a reduction in iron stores, where each increase in cup of coffee per week decreased ferritin levels (a protein that shows iron storage) by 1%.
Although this is the case, the effects on outcomes are mixed.
For example, a 2022 meta-analysis found that high versus low coffee consumption (4 cups/day vs 1 cup/day) was actually associated with a lower risk of osteoporosis. However, ≥9 cups/day was associated with an increased risk.
Other studies have found that around 300mg or more of caffeine, increased bone loss in the spine of postmenopausal women and increased the risk of hip fractures in older populations.
Ultimately, to minimize these nutrient-blocking absorption effects and therefore mitigate risks of potential nutrient deficiencies, it’s advisable to have your coffee separate from food where possible.
Anxiety is a common side effect of caffeine, particularly when taken at higher doses.
A 2022 systematic review found that caffeine doses equivalent to about 5 cups per day, increased anxiety among healthy adults. This amount had a further effect on those with panic disorders, increasing the likelihood of panic attacks in this population group.
The degree to which this affects individuals seems to be influenced by specific polymorphisms (i.e.differences in genes) that determine how fast caffeine gets metabolzsed.
For example, studies have found that polymorphisms at the A(2A) receptor contribute to different degrees of caffeine-induced anxiety & arousal.
Although coffee is widely available, there may be times when it is not accessible or you are required to have an absence from caffeine.
This may result in caffeine withdrawal, which has been shown to occur from as little as 100mg per day, or when usual caffeine doses generally just decrease.
Some symptoms include impaired behavioral and cognitive performance, decreased or increased blood pressure, decreased motor activity, increased heart rate, hand tremors, increased diuresis, skin flushing, flu-like symptoms, nausea/vomiting, constipation, muscle stiffness, joint pains, and abdominal pain.
A double-blind study involving the sudden cessation of caffeine intake from a mean daily dose of 235 mg, resulted in moderate to severe headaches, increases in anxiety and depression, and reduced speed of simple motor tasks (finger tapping).
Typically the onset of symptoms occur 12-24 hours after caffeine cessation, peak at 20-51 hours and may last up to 9 days. The severity in symptoms also varies greatly between individuals.
However, habitual coffee consumption is not associated with the same impact on blood pressure and overall shows no negative outcomes.
This is similar to research looking specifically on older adults, where there were no significant changes in diastolic blood pressure or pulse pressure from coffee consumption.
The research on this is more related to caffeine rather than coffee specifically, however is still a consideration.
Toxic effects of caffeine are estimated to occur with intakes of 1.2 g or higher and doses on average of 8.8g were shown to be fatal.
This is a VERY large amount of caffeine though. To put this into perspective, 8.8g caffeine is the equivalent of around 100 shots of espresso.
However, the toxic dose stated at 1.2g is actually quite realistic to achieve if you are nearing 10+ cups of coffee per day.
Sublethal doses of ~7-10mg/kg may also cause chills, flushing, nausea, headache, palpitations and tremor.
It seems that the evidence of caffeine on fetal health is not conclusive.
The European Food and Safety Authority states that “habitual caffeine consumption up to 200 mg per day by pregnant women does not give rise to safety concerns for the fetus… or to the child when lactating”.
However, when looking at a meta-analysis it showed that higher caffeine intakes were associated with lower birth weights and pregnancy loss.
Each 100mg/day increment was found to be associated with a 13% and 7% increased risk respectively
On the other hand, a randomized controlled trial found that a reduction in caffeine did not significantly affect birth weight. However, this was only conducted in the second half of pregnancy, so results may have differed if it was throughout.
Overall the research is mixed and may be due to the large variation in how individuals metabolise and respond to the effects of caffeine.
Therefore, if you are more sensitive and are leaning on the side of caution, it may be a good idea to reduce or completely limit caffeine consumption during this period or drink decaf coffee as this shows no detriments in pregnancy outcomes.
Unfiltered coffee (i.e. coffee not filtered through a paper) contains a higher amount of diterpenes, which has been shown to raise cholesterol and triglycerides.
For example, a 12 week randomized control trial compared 4-6 cups of filtered coffee, 4-6 cups/day unfiltered coffee or no coffee. LDL increased by 0.48mmol/L in the unfiltered coffee and no signficant changes were seen in the other groups.
This is because it can inhibit bile acid synthesis and ultimately impacts lipid metabolism.
Although there is evidence that LDL cholesterol levels increase through unfiltered coffee, associations with coronary and cardiovascular-related disease seem to show the opposite effect, regardless of the type of coffee used.
Potential Confounding Variables
Something to keep in mind is that a lot of these studies don’t specify what is actually consumed with the coffee i.e. with or without milk, sugar, sweeteners etc.
For example, someone consuming 4 espresso shots per day, may see vastly different outcomes than someone consuming 4 cups of hazelnut mocha from Starbucks.
Similarly, if someone increased their overall coffee consumption which also contained milk, this would therefore increase their calcium and protein intake, potentially leading to many other positive outcomes.
Even if they increased their consumption of just coffee with water, this would contribute to more fluid intake which may also make a slight difference in health.
Additionally, those who drink coffee may also exercise more as caffeine is often taken as an ergogenic aid.
They also may attend more social events, leading to a greater sense of well-being and social connection.
They could also potentially be of higher socioeconomic status if they can afford more cups of coffee per day.
Another thing to consider is the large variation in caffeine content in coffee. Ultimately, 4 cups per day could yield completely different outcomes depending on where this is from.
All of these factors may significantly skew the associations we see in the research.
So, is coffee good or bad for your health?
Even if there are a lot of variables that may influence the associations, there are still some plausible mechanisms that may at least partly explain the positive outcomes such as the effect on the adenosine receptor and the anti-inflammatory properties it offers.
It is also clear that, in majority of the studies, there is a J shaped association between coffee intake and health outcomes, so more is not necessarily better.
And there are some pretty clear downsides, particularly if overconsuming caffeinated coffee or having it too late in the day.
It’s also important to keep in mind that there would be individuals who may not respond as positively, which is evident in the literature looking at differences in caffeine metabolism and anxiety.
Overall though, the research seems to be overwhelmingly positive in terms of coffee consumption on health outcomes, however this may be highly individualized – so find the ‘sweet spot’ that works well for you.