Genetics and stem cells

Four cups of coffee 'not bad for health' suggests review

"Drinking up to four cups of coffee a day carries no health risk, experts say. Scientists said those who stick to that limit have no need to worry," reports The Sun.

This was based on a review of studies that looked at the effects of caffeine on health. The researchers specifically investigated the effect of having more or less than 400mg of caffeine a day for adults (the equivalent of four cups of coffee), or 300mg/day (three cups) for pregnant women.

These amounts (400mg for adults and 300mg for pregnant women) were the recommended upper daily limits from a previous large-scale review of caffeine safety carried out in 2003.

Overall, the researchers found the available evidence suggests that consuming up to these amounts of caffeine doesn't have a negative effect on bone health, heart health, behaviour or on reproduction and development.

They found links with increased anxiety, higher blood pressure and headaches. Though these symptoms may not necessarily lead to adverse health outcomes in the long term, they need further research.

Currently the NHS recommends pregnant women have no more than 200mg of caffeine a day – less than the recommended upper limit in this study. Pregnant women should aim to stick to the 200mg a day limit as it will reduce any risks even further.

One important thing to remember is that caffeine is present in lots of products other than coffee, including tea, cola drinks, energy drinks and chocolate.

Where did the story come from?

The study was carried out by researchers from various institutions in the US, with most of the authors coming from ToxStrategies, a company that manufactures and processes drugs for human use.

The study was published in the peer-reviewed journal Food and Chemical Toxicology and is open-access, meaning it is free to read online.

The study was funded by the North American Branch of International Life Sciences Institute Caffeine Working Group. Grants were also received from the American Beverage Association and the National Coffee Association. The authors claim these funders had no input into the systematic review.

The Mail Online emphasised the positive side for coffee drinkers, "Coffee lovers rejoice – drinking up to four cups of your favourite beverage each day won't damage your health."

One problem with the media's emphasis on cups of coffee is that caffeine is found in many things besides coffee, including chocolate, coke, tea and energy drinks. Moreover, the claim that the research was based on 740 studies is wrong. Although the reviewers looked at a large number of papers, evidence on the main outcomes was only available from 381 studies.

What kind of research was this?

This systematic review aimed to look for studies published between 2001 and 2015, investigating the potential negative effects of caffeine. The researchers specifically aimed to look at effects in four healthy population groups – adults, pregnant women, adolescents and children.

It aimed to review the benchmark conclusions of a previous review in 2003 that had recommended caffeine intakes of:

  • ≤400mg/day in adults (about 4 cups of coffee per day)
  • ≤300mg/day in pregnant women
  • ≤2.5mg/kg per day in children and adolescents

A systematic review is useful for summarising all the relevant evidence in a specific health area but the strength and quality of the evidence is only as good as the gathered studies.

Often the difficulty with studies assessing food and drink exposures – such as the amount of caffeine – is that other health and lifestyle factors could influence the findings.

What did the research involve?

This systematic review was built around the question "For [population], is caffeine intake above [dose], compared to intakes [dose] or less, associated with adverse effects on [outcome]?" In other words: for a set population, is caffeine intake above a set amount, compared to the set amount or a lower intake, linked to health risks?

The researchers aimed to assess healthy adults, pregnant women, adolescents (aged 12 to 19 years), and healthy children (aged 3 to 12 years).

The levels of caffeine consumption compared for the different population groups were:

  • Healthy adults: 400mg/day compared with less
  • Healthy pregnant women: 300mg/day compared with less
  • Healthy adolescents and children: 2.5mg/kg per day compared with less

The outcomes of interest were:

  • Cardiovascular: assessed by looking at mortality, blood pressure, heart rate, heart rate variability and cholesterol.
  • Bone and calcium: assessed by looking at bone mineral density and osteoporosis and risk of fracture or fall. 
  • Human behaviour: assessed by looking at anxiety, anger, depression, headache, sleep, problematic or risk-taking behaviour.
  • Development and reproduction: assessed by looking at fertility, miscarriage, stillbirth, preterm birth, fetal growth and birth defects.

Forms of caffeine included coffee, tea, chocolate, cola drinks, energy drinks, energy shots, supplements, medicines, caffeinated chewing gum, caffeinated sports gel, and caffeinated sports bars.

Researchers searched three literature databases for English language studies meeting these criteria published between January 2001 and June 2015.

A total of 381 individual studies were included. Most of these (63%) were described as "controlled trials" – though these may not necessarily all have been randomised trials. The other studies were observational.

Most studies (79%) reviewed the adult population and 14% reviewed pregnant women. Very little evidence was available for adolescents (4% of studies) and children (2%).

What were the basic results?

The researchers were not able to pool the study results in a meta-analysis, and instead gave a narrative summary of the effects. Nearly all evidence relates to adults.


400mg/day of caffeine is not associated with significant concern for cardiovascular mortality. Six of nine observational studies assessing mortality found that higher intakes of up to 855mg/day did not affect mortality.

If blood pressure alone is considered, 400mg/day may be too high – studies generally suggested that intakes both above and below this level were associated with increased blood pressure (by a few mmHg). However, this small rise may not necessarily have an adverse effect on cardiovascular risk. There were no adverse effects on heart rate, heart rate variability or cholesterol.

Bone and calcium

Most of the data supports that 400mg/day in healthy adults is not harmful with respect to bone mineral density and osteoporosis. It also supports that an intake of 400mg/day is not associated with concern regarding risk of fall or fracture.

Human behaviour

400mg/day can lead to increases in measures of anxiety, sleep problems and may increase the chance of getting a headache. Headaches were observed in people who had previously been drinking levels of coffee just below the maximum recommended amount but no longer did.

Consumption of coffee led to delays in getting to sleep and decreases in quality of sleep. Caffeine intake did not influence anger, depression or confusion.

Development and reproduction

Up to 400mg/day does not seem to affect fertility. 300mg/day was not associated with increased risk of miscarriage, stillbirth, preterm birth, and the birth defects assessed in these studies. There was no conclusion on risk of 300mg/day or more on fetal growth.

How did the researchers interpret the results?

The researchers concluded that "evidence generally supports that consumption of up to 400mg caffeine/day in healthy adults is not associated with overt, adverse cardiovascular effects, behavioral effects, reproductive and developmental effects, acute effects, or bone status.

"Evidence also supports consumption of up to 300mg caffeine/day in healthy pregnant women as an intake that is generally not associated with adverse reproductive and developmental effects."


It appears that previous recommendations from the 2003 Health Canada review into the effects of caffeine on health remain suitable. Health Canada recommended a maximum daily coffee intake of 400mg/day for healthy adults and 300mg/day for pregnant women.

Although this review looked at a large number of studies and found that the evidence overall supports these recommendations, there are some limitations to the research:

  • The number of studies providing evidence for each health outcome varied.
  • Some results were based on a large number of studies, others on a much smaller number. This means the strength of evidence for different health outcomes varied.
  • Most evidence comes from observational studies. This means we can't be sure that other health and lifestyle factors aren't influencing the findings. Even if randomised controlled trials were possible, it is often difficult to make sure people stick to a given caffeine level and are assessed for long enough to measure adverse outcomes. 
  • The methods and quality of the individual studies will have varied. Studies will have differed in the way they measured caffeine intake or how they assessed health outcomes. This could have made the findings of individual studies inconsistent and difficult to directly compare.
  • We don't know that observations on measures such as blood pressure, anxiety and sleep would necessarily have led to negative overall health outcomes.
  • People may not accurately recall how much caffeine they consumed and state lower levels, particularly if they had a negative health outcome. We also don't know whether caffeine intake reflects recent or long-term intake. Studies may have differed in the period of assessment.
  • Lastly, there was very little evidence available for caffeine intake in children and adolescents.

Currently the NHS recommends having no more than 200mg of caffeine a day if you are pregnant, which is less than the upper limit for pregnant women in this research. Pregnant women should aim to stick to NHS advice.

Find out more in Should I limit caffeine in pregnancy?

NHS Attribution