Garlic and onions are among the best foods to eat for a healthier gut, according to a new study.
Leeks and Jerusalem artichokes are also beneficial, the findings suggest.
Scientists say there is growing evidence that consuming prebiotics – certain types of fiber often found in plants that stimulate beneficial bacteria in the gut – can help to maintain a healthy gut microbiome.
For the new study, researchers estimated the prebiotic content of thousands of food types by using pre-existing literature to find out which foods offer the highest prebiotic content.
The findings showed that the foods that pack the greatest prebiotic punch are dandelion greens, Jerusalem artichokes, garlic, leeks, and onions.
Researchers say that as well as supporting gut microbes, prebiotic-rich foods contain high amounts of fiber – something most people do not get enough of.
Adults are recommended to consume 30 grams of fiber a day, but estimates suggest they are only currently consuming about 20g a day on average.
Cassandra Boyd, a master’s student at San José State University, said: “Eating prebiotic-dense foods has been indicated by previous research to benefit health.
“Eating in a way to promote microbiome wellness while eating more fibre may be more attainable and accessible than you think.”
She explained that prebiotics, which can be thought of as food for the microbiome, are different from probiotics, which contain live microorganisms. Both can potentially benefit microbiome health, but they work in different ways.
Previous studies have linked higher prebiotic intake with improved blood glucose regulation, better absorption of minerals such as calcium, and markers of improved digestive and immune function.
Although most dietary guidelines do not currently specify a recommended daily allowance for prebiotics, the International Scientific Association for Probiotics and Prebiotics — a non-profit scientific organization that established the currently held definition of prebiotics — recommends an intake of five grams per day.
For the study, researchers used previously published scientific findings to analyze the prebiotic content of 8,690 foods.
Just over a third of the foods in the database (37 percent) were found to contain prebiotics.
Dandelion greens, Jerusalem artichoke, garlic, leeks, and onions had the greatest amounts, ranging from about 100 to 240 milligrams of prebiotics per gram of food (mg/g).
Other prebiotic-rich foods included onion rings, creamed onions, cowpeas, asparagus, and Kellogg’s All-Bran cereal, each containing around 50 to 60 mg/g.
Wheat-containing items rank lower on the list. Foods with little or no prebiotic content included dairy products, eggs, oils, and meats.
Boyd, who conducted the research with Doctor John Gieng, added: “The findings from our preliminary literature review suggest that onions and related foods contain multiple forms of prebiotics, leading to a larger total prebiotic content.
“Multiple forms of onions and related foods appear in a variety of dishes as both flavoring and main ingredients.
“These foods are commonly consumed and thus would be a feasible target for people to increase their prebiotic consumption.”
Based on the team’s findings, she said a person would need to consume around half of a small (4-ounce) onion to get five grams of prebiotics.
The research team hope the study will provide a basis to help other scientists assess the health impacts of prebiotics and inform future dietary guidelines.
Boyd presented the findings at NUTRITION 2023, the flagship annual meeting of the American Society for Nutrition in Boston, Massachusetts.