Study Finds Gluten Contamination Common in "Naturally Gluten-Free" Flours

Celiac.com 01/20/2026 - Following a gluten-free diet is essential for people with celiac disease, but in many parts of the world, finding truly safe gluten-free ingredients can be difficult. The study published in Frontiers in Nutrition set out to understand how often “naturally gluten-free” flours are contaminated with gluten and whether products labeled as gluten-free are safer. This question matters because even small amounts of gluten can harm people with celiac disease by damaging their intestines and causing serious symptoms. The researchers wanted to compare different types of gluten-free flours from the market to see where risks were highest and what factors might contribute to contamination.

Types of Flours Tested

The study looked at 163 flour samples from a variety of grains that are often considered naturally gluten-free, such as oat, buckwheat, corn, and rice. These samples were collected from shops and markets and included three main categories: flours that were commercially produced and labeled gluten-free, naturally gluten-free flours that were packaged but not labeled gluten-free, and naturally gluten-free flours that were sold unpackaged and without any gluten-free label. By comparing these groups, the researchers could see whether labeling and packaging made a difference in the amount of gluten found in the flours. 

How Gluten Was Measured

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To determine whether the flours contained gluten, the researchers used a standard laboratory method that measures gliadin, a component of gluten that triggers celiac disease symptoms. In this approach, any flour with gluten levels above 20 parts per million was considered contaminated and potentially unsafe for people with celiac disease. This 20 parts per million threshold is a widely accepted limit used by international food safety standards. 

What the Study Found

The results showed that gluten contamination was quite common in flours that were not labeled gluten-free. Overall, more than half of all flour samples had gluten levels above the safe limit. Among the different categories:

• About one in six commercially produced flours labeled gluten-free showed gluten contamination above the safe threshold.
• Half of the naturally gluten-free flours that were packaged but unlabeled had unsafe levels of gluten.
• Nearly nine out of ten flours that were sold unpackaged and without a gluten-free label were contaminated. 

These findings suggest that flours marketed without gluten-free certification, especially when sold in bulk without packaging, are much more likely to contain gluten that could harm someone with celiac disease. 

Differences Among Grain Types

The risk of gluten contamination varied depending on the type of flour. Oat flours were the most frequently contaminated, with over 80 percent having unsafe gluten levels when they were not labeled gluten-free. Buckwheat flours were also commonly contaminated. In contrast, corn and rice flours had lower contamination rates, especially when they were commercially processed and labeled gluten-free. None of the rice flour samples labeled gluten-free showed unsafe gluten levels in this study. 

Role of Storage, Packaging, and Handling

Where and how flours were stored influenced contamination risk. Flours stored in areas with nearby gluten-containing products were more likely to be contaminated, especially when they were unpackaged and handled with shared utensils like spoons or scoops. Packaging alone helped reduce contamination, but even packaged products without gluten-free labeling could still be unsafe. The highest contamination rates occurred in flours sold unpackaged, where dust and contact with gluten-containing products could easily transfer gluten. 

Why This Matters

For people with celiac disease, maintaining a strict gluten-free diet is not optional; consuming gluten, even in very small amounts, can cause ongoing intestinal damage and lead to health complications. This study highlights that not all flours that might seem safe are actually free from gluten. Even products made from grains that are naturally gluten-free can become contaminated during harvesting, processing, storage, or sale, especially when they are not labeled and packaged carefully.

What This Means for People With Celiac Disease

This research underlines why gluten-free labeling and certification are so important for people with celiac disease. Certified gluten-free products have safeguards in place to minimize cross-contamination, but even some of these products can still contain gluten above safe levels. For consumers with medical needs, this means it is crucial to choose products that are both labeled and certified gluten-free whenever possible. 

It also suggests that people with celiac disease should exercise caution when buying naturally gluten-free flours in unpackaged or unlabeled forms, as these are more likely to pose a risk. Being aware of how products are handled, stored, and labeled can help individuals make safer choices and protect their health. 

Conclusion

The study makes clear that gluten contamination in flour products is a serious concern for anyone who must avoid gluten for health reasons. While flours that are commercially produced and properly labeled are generally safer, contamination can still occur, and naturally gluten-free flours without clear labeling are especially risky. This research reinforces the importance of strict standards for gluten-free certification and careful purchasing choices for people with celiac disease. By understanding where risks are highest and how gluten can enter the food supply, individuals and regulators can work toward safer gluten-free food options that better support health and well-being.

Read more at: frontiersin.org