Researchers looked at studies involving almost 171,000 preschool children to understand why some kids catch colds and respiratory infections more frequently than others. They found that three main things increase a child’s risk: having asthma, being around cigarette smoke, and eating too many snacks instead of healthy foods. Interestingly, children born to older mothers seemed to get sick less often. The study also found that factors like family income and breastfeeding length didn’t have as much impact as previously thought. These findings can help parents and doctors focus on the most important ways to keep young children healthier.

The Quick Take

  • What they studied: What makes some preschool children more likely to get repeated respiratory infections (colds, ear infections, and similar illnesses)?
  • Who participated: The analysis combined data from 12 different studies that tracked almost 171,000 preschool children from around the world to identify common patterns in who gets sick more often.
  • Key finding: Three factors significantly increased infection risk: children with asthma were 3.3 times more likely to get repeated infections, kids exposed to secondhand smoke were 1.5 times more likely, and children who ate frequent snacks were 1.6 times more likely. Surprisingly, children born to older mothers had fewer infections.
  • What it means for you: If your child has asthma, avoid secondhand smoke, or eats lots of snacks, they may need extra attention to prevent repeated infections. However, this doesn’t mean these things will definitely cause problems—just that the risk is higher. Talk to your doctor about specific steps for your child.

The Research Details

This research is called a meta-analysis, which means scientists looked at 12 different studies that had already been completed and combined all their information together. This approach is powerful because it lets researchers see patterns across much larger groups of children than any single study could examine. The researchers searched multiple medical databases (including ones in English and Chinese) for studies published through September 2025 that looked at what causes repeated respiratory infections in young children. They only included observational studies, which means studies that watched what happened to children rather than randomly assigning them to different treatments. The researchers looked at specific factors that previous studies had suggested might matter: family income, mother’s age when the child was born, how long babies were breastfed, secondhand smoke exposure, asthma, vitamin A levels, and eating habits.

By combining information from many studies, researchers can see which risk factors are truly important and which ones might have seemed important by chance. This approach helps doctors and parents focus on the factors that really make a difference in keeping children healthy. The large number of children studied (almost 171,000) makes the findings more reliable than any single study could be.

This study is strong because it included a large number of children and combined results from multiple research teams. However, the researchers noted that they couldn’t check for publication bias (when studies with surprising results are more likely to be published), and some factors had fewer studies available, which made those results less certain. The findings for asthma, secondhand smoke, and snack intake were very consistent across studies, making them more trustworthy.

What the Results Show

The analysis clearly showed three main risk factors for repeated respiratory infections in preschool children. First, children with asthma had the highest risk—they were about 3.3 times more likely to experience repeated infections compared to children without asthma. This makes sense because asthma affects the airways and lungs, making them more vulnerable to infections. Second, children exposed to secondhand smoke had a 50% higher risk of repeated infections. Cigarette smoke damages the protective lining of airways, making it easier for germs to cause infections. Third, children who ate frequent snacks (more than 7 times per week) had a 61% higher risk. This likely relates to nutrition—frequent snacking often means less healthy meals and more sugary foods that don’t support immune system health. Interestingly, children born to older mothers showed a protective effect, meaning they had fewer infections. This might be because older mothers have more experience with childcare or different parenting practices.

The study also examined several factors that many people thought would be important but turned out not to have a clear effect based on current evidence. Family income level didn’t show a significant relationship with infection risk. Breastfeeding for less than 6 months didn’t clearly increase infection risk in the studies examined. Vitamin A deficiency also didn’t show a clear statistical connection to repeated infections, though this may be because vitamin A deficiency is less common in developed countries where most studies were conducted.

These findings largely confirm what doctors have suspected about asthma and secondhand smoke—both have long been recognized as harmful to respiratory health. The strong connection to diet (frequent snacking) adds newer evidence about the importance of nutrition for immune function. The lack of clear evidence for breastfeeding duration and vitamin A deficiency is somewhat surprising and suggests that either these factors matter less than previously thought, or that the way they were measured in different studies made it hard to see a clear pattern.

Several important limitations should be considered. First, the studies included were observational, meaning they watched what happened but couldn’t prove that one thing directly caused another—only that they were connected. Second, different studies measured things differently, which can make combining results tricky. Third, most studies came from developed countries, so results might not apply equally to children in other parts of the world. Fourth, the researchers couldn’t fully check whether studies with surprising results were more likely to be published (publication bias). Finally, some factors had fewer studies available, making those conclusions less certain than others.

The Bottom Line

Based on this research, parents and caregivers should: (1) Work with doctors to manage asthma effectively if a child has it—this is the strongest modifiable risk factor. (2) Protect children from secondhand smoke exposure as much as possible. (3) Encourage healthy eating patterns with regular meals and limit frequent snacking. These recommendations have strong evidence support. Other factors like breastfeeding and vitamin A supplementation should be discussed with your pediatrician based on your individual child’s needs, as the evidence for these is less clear.

These findings are most relevant for parents of preschool children (ages 2-5), pediatricians, and childcare providers. Children with asthma should receive special attention. Families where smoking occurs in the home should prioritize smoke-free environments. All families can benefit from understanding the connection between diet and infection risk. However, these findings don’t mean that children with asthma or those exposed to smoke will definitely get sick more often—just that their risk is higher.

Changes in infection frequency typically take several months to become noticeable. If you make changes like improving diet or reducing smoke exposure, you might see benefits within 2-3 months, though some children may take longer. Managing asthma effectively can show improvements more quickly, sometimes within weeks.

Want to Apply This Research?

  • Track respiratory infections monthly (colds, ear infections, sore throats) and note exposure to secondhand smoke and snacking frequency. This helps identify patterns and measure whether changes are working.
  • Set a goal to reduce frequent snacking by replacing it with scheduled healthy meals and snacks. Use the app to log meals and snacks, aiming for 3 meals and 1-2 planned snacks daily instead of frequent grazing throughout the day.
  • Monitor infection frequency every 3 months and track asthma symptoms if applicable. Also log secondhand smoke exposure and dietary patterns. Over time, this data shows whether lifestyle changes are reducing infection risk.

This research summary is for educational purposes only and should not replace professional medical advice. The findings show associations between certain factors and infection risk but do not prove direct causation. Every child is different, and individual circumstances vary. Parents should consult with their pediatrician about their specific child’s health, risk factors, and appropriate prevention strategies. This summary reflects research current as of the publication date and may not include all recent developments. If your child has asthma or other health conditions, work with your healthcare provider on a personalized management plan.