THURSDAY, April 23 (HealthDay News) -- Adults with a family history of asthma are up to six times more likely to develop the disease than an average person, say U.S. Centers for Disease Control and Prevention researchers.

They analyzed data on 1,500 adults, aged 20 and older, and classified them as being at average, moderate or high risk of asthma based on the number of closeness of relatives with asthma. The researchers found that 85 percent of people were at average risk, 13 percent at moderate risk, and 2 percent at high risk.

Asthma prevalence was 9.4 percent among those at average risk, 20.4 percent for those at moderate risk, and 37.6 for those at high risk. Asthma risk was 2.4 times higher for those at moderate risk and 4.8 times higher for those at high risk, compared to those at average risk.

After they adjusted for other factors, the researchers found that asthma risk was 2.4 times higher in the moderate-risk group and 4.8 times higher in the high-risk group. Other asthma risk factors included being black, obesity, low income, smoking or living with a smoker, and physical inactivity.

Average onset of asthma was 22 years in the average-risk group, 19 years in the moderate-risk group, and 17 years in the high-risk group.

"Our findings showed that a family history of asthma is an important risk factor for asthma, and that familial risk assessments for asthma can help identify people at highest risk for developing asthma," concluded lead author Tiebin Liu and colleagues.

The study is published in the May issue of Genetics in Medicine.

More information

The U.S. National Heart, Lung, and Blood Institute has more about asthma.

THURSDAY, April 16 (HealthDay News) -- People who don't get enough of the antioxidant vitamins A and C in their diet may be at increased risk for asthma, British researchers say.

The pooled results of 40 studies conducted between 1980 and 2007 showed that people with asthma had a significantly lower dietary intake of vitamin A than those without the disease. The average intake among those with asthma was 182 micrograms a day, which is between a quarter and a third of recommended daily intake.

The review authors also found that people with severe asthma had a significantly lower intake of vitamin C (about half the recommended daily intake) than those with mild asthma. In addition, low circulating levels of vitamin C in the blood and lower dietary intake of foods containing vitamin C were associated with a 12 percent increased risk of asthma.

There was no association between vitamin E intake and asthma risk, but blood levels of vitamin E were much lower among people with severe asthma than in those with mild asthma. Those with severe asthma had an average vitamin E intake of 2 milligrams/day, which is 20 percent lower than the daily recommended amount, the review authors said.

These findings don't prove cause and effect, but they do challenge a study published last year that found no association between antioxidants and asthma risk, said Dr. Jo Leonardi-Bee, of the division of epidemiology and public health at the University of Nottingham in the United Kingdom, and colleagues.

"Overall, our findings from [the current] systematic review and meta-analysis indicate that low levels of vitamin C intake, and to a lesser extent vitamin A, are consistently associated with asthma risk to a degree that, if causal, would be sufficient to be clinically relevant," they concluded.

Their findings for an association between dietary antioxidants and wheezing were less consistent. The report was published in the current issue of Thorax.

More information

The U.S. National Heart, Lung, and Blood Institute has more about asthma.