SAN FRANCISCO – Newborn lung function was significantly better in pregnant smokers who took vitamin C supplements, compared with smoking mothers on placebo, in a double-blind trial that randomized 179 women.
Among 159 infants who underwent pulmonary function tests at around 48 hours of age, results in the 76 newborns of smokers getting vitamin C were similar to results for 76 newborns of nonsmoking women in a nonrandomized comparison group. Both subgroups had better pulmonary function than did the 83 newborns of placebo-treated smokers, Dr. Cindy T. McEvoy and her associates reported at an international conference of the American Thoracic Society.
"We speculate that vitamin C supplementation in pregnant women who cannot quit smoking is helpful," said Dr. McEvoy of Oregon Health and Science University, Portland.
The current study randomized pregnant smokers aged 15 years and older, and prior to 22 weeks gestation of their singletons, to take 500 mg/day of vitamin C or placebo until delivery. The women were counseled throughout the trial to quit smoking but declined to do so.
Maternal plasma levels of ascorbic acid were significantly lower in the two smoking groups at randomization, compared with levels in nonsmokers. By mid-gestation, ascorbic acid levels in the vitamin C group were similar to levels in nonsmokers (59 and 58 micromol/L, respectively), but levels in the placebo group remained significantly lower (40 micromol/L).
Infant pulmonary flow volume, characterized as a ratio of the time to peak tidal expiratory flow to expiratory time, was significantly lower in the placebo group (0.345), compared with the vitamin C group (0.383) and the nonsmoking group (0.399), Dr. McEvoy said.
Investigators also measured the newborns’ passive respiratory mechanics, or compliance of the respiratory system (Crs/kg), and found significantly lower results in the placebo group (1.2 Crs/kg), compared with the vitamin C group (1.32 Crs/kg) or the nonsmoking group (1.36 Crs/kg).
Treatment did not significantly affect respiratory rate.
There were no significant differences between the randomized groups at the start of the study in characteristics including maternal age, insurance coverage, cotinine levels, medication adherence, history of asthma, or the proportion of women smoking 10 or more cigarettes per day.
Infant demographics were similar in the two groups of smokers and the reference group of nonsmokers, including birth weight, gestational age at delivery, sex, rate of delivery before 32 weeks’ gestation, and rate of vaginal delivery.
The investigators plan to perform infant pulmonary function tests again when the study babies are 1 year old and compare it with clinical outcomes such as episodes of wheezing. They have secured support from the National Heart, Lung, and Blood Institute to randomize a new cohort and measure newborn forced expiratory flows as the primary outcome, she said.
The investigators did not give vitamin C supplementation to infants, but that strategy may deserve study as well, Dr. McEvoy said.
The study excluded pregnancies with multiple gestation or fetal congenital anomalies and women who currently used illicit drugs or abused alcohol, had a history of kidney stones, had insulin-dependent diabetes, or who had been taking vitamin C daily since their last menstrual period. Before the treatment period began, participants were asked to take a daily placebo; those who complied with fewer than 75% of placebo doses were excluded from randomization.
Approximately 12% of U.S. women smoke during pregnancy and at least 500,000 newborns each year have been exposed to smoke in utero. Previous studies have shown that infants of smokers have worse lung function at birth and a higher risk of developing lung diseases, including asthma, bronchitis, and pneumonia, compared with infants of nonsmokers.
The current findings support evidence from nonhuman primates that daily vitamin C can block the in utero effects of nicotine on lung development and newborn pulmonary function (Am. J. Respir. Crit. Care Med. 2005;171:1032-9).
Dr. McEvoy reported having no financial disclosures. The National Heart, Lung, and Blood Institute funded the study.