Mothers that consumed high levels of dietary vitamin C during late pregnancy were more than twice as likely to report wheezing in their children two years on, shows a new study.
While the results could be explained by greater health awareness among the mothers - those with healthier diets may also be more likely to notice and report wheezing in their children - the researchers suggest that pregnant women should be cautious about taking vitamin C supplements.
The results were gathered from 1,300 women who answered a 145-item food frequency questionnaire on their diet during pregnancy and had blood antioxidant levels measured.
Researchers at the University of Aberdeen say that children of mothers in the top quintile of vitamin C intake were two to three times more likely to wheeze during their second year of life.
The authors also found that maternal vitamin C intake was related to parental reporting of eczema during at least one time period at follow-up.
"This is against the direction we were expecting," research fellow Geraldine McNeill told NutraIngredients.com. "A high fruit intake in children has been associated with less wheezing."
A number of epidemiological studies have also found an association between vitamin C consumption during pregnancy and reduced risk of allergy or asthma in children later on.
But recent US research found that infants who take multivitamins may be at a greater risk of developing asthma and food allergies.
"We think the mothers with the highest vitamin levels were probably more health-conscious and therefore may be more likely to notice wheezing in their children. But there are also one or two theories for how the vitamin could do some harm."
There is some evidence that vitamin C, an antioxidant, can become pro-oxidant, according to McNeill, although this has only been demonstrated in lab rather than human trials.
"It adds further grounds to evidence that women need to be careful with supplements during pregnancy. Diet is probably a more balanced means of achieving vitamin intake than supplements," said McNeill.
The researchers did however find that children of mothers with the highest vitamin E intake were half as likely to wheeze without a cold than those in the lowest intake.
The findings, published in the 15 January issue of the American Journal of Respiratory and Critical Care Medicine (vol 171, pp 121-128), confirm the views of another UK group, writing in an October 2004 issue of the same journal.
The researchers reported that in longitudinal and clinical studies only vitamin E has been shown to have a protective effect against asthma.
The Aberdeen team is about to analyse data from a five-year follow-up of the same children.
"We will be more confident then about whether women should take foods high in vitamin E," said McNeill. "We will probably not recommend greater intake of vitamin C however."
Researchers have been investigating diet in a bid to stem the rise in the numbers of children with asthma. One in eight children in the UK has asthma and this figure has increased six-fold in the last 25 years. In total, 8 million people in the country have the disease - or four times as many people as those with diabetes - and an average 1,400 people die from the disease each year, according to Asthma UK.