Pregnant women at risk from high street coffee
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Researchers found huge variations in the caffeine content of espresso bought on the high street, with one having six-times more caffeine than others.
The University of Glasgow team said customers were "completely unaware" of the amount of caffeine in the coffees sold.
High caffeine levels during pregnancy can increase the risk of having a baby with a lower birth weight, and may increase the risk of miscarriage.
The UK Food Standards Agency advises pregnant women to limit caffeine consumption to below 200mg a day - or four cups of strong coffee containing 50mg caffeine - throughout pregnancy.
This figure is also backed by the Royal College of Obstetricians and Gynaecologists (RCOG), but they also advise women to abstain from caffeine altogether during the first trimester of pregnancy.
The researchers analysed the caffeine content of 20 coffees, all bought in Glasgow and ranging in cup size from 23-70ml.
A Starbucks cup of coffee was found to contain 51mg of caffeine, but a Costa coffee had more than three times this amount (157mg).
A single espresso from a Glasgow café - Patisserie Francoise - contained 322mg of caffeine, the highest caffeine content found in the study.
In adults, caffeine has a half-life of around five hours, but this can be as much as 30 hours in women taking oral contraceptives, pregnant women, the developing foetus, young children and those with liver disease.
These groups are more susceptible to the effects of caffeine toxicity, the scientists said.
Study leader professor Alan Crozier, from Glasgow University's School of Medicine, said it was highly unlikely the findings were restricted to Glasgow coffee shops.
"This snap-shot of high street espresso coffees suggests the published assumption that a cup of strong coffee contains 50mg of caffeine may be misleading," he said.
"Our data show that one cup of high-caffeine content could cause as much difficulty to these susceptible consumers as six cups of coffee to another.
"At the low level, a pregnant woman and others with a need to restrict caffeine consumption might safely drink four cups per day without significantly exceeding the recommended caffeine intake.
"In marked contrast, at the higher end of the scale, drinking even one cup of espresso will be well in excess of the advised limit of 200mg a day.
"As many coffee houses prepare larger volume coffees, such as latte and cappuccino, by dilution of a single or double shot of espresso, further study on these products is warranted."
The findings are published in the latest edition of the Royal Society of Chemistry journal Food and Function.