Metabolic rates and intake of caffeine during pregnancy – Tips from Other Journals
Anne D. Walling
Since elimination of caffeine from the blood slows during pregnancy and fetal concentrations of caffeine are believed to be in equilibrium with those of maternal blood, high caffeine intake during pregnancy could adversely affect the fetus. Some evidence links high caffeine intake to poor fetal growth, but the effect of caffeine intake during pregnancy is difficult to evaluate because of the unreliability of self-reported caffeine use and potential confounding by other factors that could cause intrauterine growth retardation, particularly cigarette smoking. To clarify these issues, Cook and colleagues analyzed maternal serum levels of caffeine and cotinine in relation to pregnancy outcome.
Caffeine and cotinine levels were measured from stored blood samples of women who received obstetric care at a London teaching hospital. Patients were excluded from the study if they had multiple gestations, insulin-dependent diabetes or late entry to antenatal care, or if they did not speak English. The women were interviewed at 14, 28 and 36 weeks of gestation using a structured questionnaire that included an estimate of caffeine and alcohol intake. Data were collected on obstetric and fetal outcomes of each pregnancy. At least one blood sample was obtained from 1,500 women, and complete data were available for 640 women.
Although each level of caffeine intake ranged widely, reported caffeine intake correlated with blood level. Smokers showed higher intakes of caffeine but lower blood levels. The authors note that this finding is likely a result of the increased metabolism of caffeine in smokers. Overall, caffeine consumption was associated with an estimated 1.3 percent fall in birth weight for each 1,000 mg per week increase in intake. In smokers, the effect was a fall of an estimated 1.6 percent after adjustment for cotinine, and the negative effect persisted after controlling for social class and alcohol intake.
The authors conclude that caffeine intake is negatively associated with birth weight in smokers despite the protective effect of increased caffeine metabolism. Since caffeine levels will increase in any smoker who quits, women who reduce or stop smoking during pregnancy must be counseled to reduce caffeine intake also.
Cook DG, et al. Relation of caffeine intake and blood caffeine concentrations during pregnancy to fetal growth: prospective population based study. BMJ 1996;313: 1358-62.
COPYRIGHT 1997 American Academy of Family Physicians
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