Onions, Garlic, and Leeks Top the List of Prebiotic

Onions and garlic are among the high-fiber, prebiotic-packed foods experts recommend.

If you’re trying to eat a diet that can build and support a healthy gut microbiome, finding which foods contain the right microorganisms and nutrients (probiotics and prebiotics) and in the recommended amounts can be tricky.

Good news: Research presented at Nutrition 2023, the annual meeting of the American Society of Nutrition, performs a lot of the heavy lifting by revealing the foods with the highest amounts of prebiotics.

After reviewing the prebiotic content of thousands of foods, investigators revealed the foods that pack the greatest prebiotic punch:

As a bonus, in addition to supporting gut health, prebiotic-rich foods contain high amounts of fiber, which has been shown to support bowel health, keep you “regular” and help you feel fuller for a longer period of time, according to the U.S. Food and Drug Administration (FDA).

“Eating prebiotic-dense foods has been indicated by previous research to benefit health,” said a coauthor of the study, Cassandra Boyd, a master’s student at San José State University in California, in a press release. “Eating in a way to promote microbiome wellness while eating more fiber may be more attainable and accessible than you think,” she added.

Prebiotics are sometimes equated with dietary fibers, but only a subset of dietary fibers qualify as prebiotics, according to the International Scientific Association for Probiotics and Prebiotics (ISAPP).

“Prebiotics are a type of dietary fiber that we, the host, cannot digest, but select microbes of the gut microbiota can,” explains Gail Cresci, PhD, RD, a microbiome researcher with Cleveland Clinic Children’s in Ohio.

That means that prebiotics escape digestion and travel into the colon, where select members of the gut microbiota are able to digest them. Because of this, prebiotics are able to support an optimal composition of the gut microbiota, and this interaction can produce metabolites that have health benefits, says Dr. Cresci.

Prebiotics are different from and not to be confused with probiotics — the live microorganisms that are found in fermented foods such as cultured milk and yogurt, tempeh, miso, sauerkraut, and brine drinks that can help improve the diversity of the microbiome, according to the National Institutes of Health.

Think about it this way: Prebiotics are “food” for the microbiome, whereas probiotics contain live microorganisms. Both can potentially benefit microbiome health, but they work in different ways.

Despite its name, the Jerusalem artichoke (Helianthus tuberosus) has no connection to Jerusalem and isn’t even an artichoke, according to the University of Michigan; the plant is related to sunflowers. Its edible tuber, also known as a sunchoke, is a knobby root vegetable resembling ginger that tastes somewhat like a sweet, nutty potato.

“We are still learning, but current evidence suggests that the gut microbiome supports overall health and well-being through its role in digestion, production of beneficial metabolites, supporting immunity, pathogen exclusion, and maintaining gut barrier function,” says Cresci, referencing a review published in Signal Transduction and Targeted Therapy in April 2022.

There's also some evidence associating lack of microbiome diversity with obesity and obesity-related diseases such as type 2 diabetes, according to a paper published in the March 2022 Biomedicine & Pharmacotherapy.

For the study, researchers used previously published scientific findings to analyze the prebiotic content of more than 8,000 foods contained in the Food and Nutrient Database for Dietary Studies, a resource many scientists use to study nutrition and health.

Of those foods, more than one-third were found to contain prebiotics. Dandelion greens, sunchoke, 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 50–60 mg/g.

Wheat-containing items rank lower on the list. Foods with little or no prebiotic content include dairy products, eggs, oils, and meats.

The findings from the preliminary literature review suggest that onions and onion-related foods contain multiple forms of prebiotics, according to the authors.

“Multiple forms of onions and related foods appear in a variety of dishes as both flavoring and main ingredients. These foods are commonly consumed by Americans and thus would be a feasible target for people to increase their prebiotic consumption,” said Boyd.

Although most dietary guidelines do not currently specify a recommended daily allowance for prebiotics, ISAPP recommends an intake of 5 g per day. The recommended amount of fiber is 28 g a day based on a 2,000 calorie per day diet.

To get the minimum intake of 5 g of prebiotics, a person would need to eat approximately half of a small (4 oz) onion, 6 or 7 garlic cloves, one-quarter of a medium leek, ½ cup dandelion greens, or one-quarter of a sunchoke.

“One limitation of the study was that we made some assumptions about raw and cooked variations of certain food items containing the same prebiotic content,” says Boyd. That’s because there is limited research on how the prebiotic content of foods is affected by cooking, she explains.

Because prebiotics are a type of fiber, various cooking methods can reduce the fiber content of the food, says Cresci. “Eating the food raw or lightly cooking them would best preserve the prebiotic content,” she says.

“While the prebiotic content is decreased by cooking, there are still prebiotics present. This means that by consuming a larger serving size of these cooked foods, there will still likely be enough prebiotics present to provide a health benefit,” says Boyd.

If you suspect you may not be getting enough prebiotics, it’s best to start with small amounts and gradually increase portions and the number of servings per day, according to Monash University. By giving your gut and its bacteria more time to adapt, you won’t feel as gassy or bloated.

The Children's Hospital of Philadelphia recommends a mix of plant-based foods that have naturally occurring prebiotics, along with foods that have been fortified with them. Prebiotics are sometimes added to food items such as yogurts, cereals, breads, biscuits, desserts, or drinks, according to ISAPP. The word ‘prebiotic’ isn’t always used on the label; instead, look in the ingredients list for galacto-oligosaccharides (GOS), fructo-oligosaccharides (FOS), oligofructose (OF), chicory fiber, or inulin. Alternatively, prebiotics can be taken in supplement form.

What’s the difference between prebiotics and probiotics? Probiotics are beneficial microbes (such as bacteria and yeasts) that can help improve your gut health by adding new strains. In contrast, prebiotics are different types of fiber in foods that probiotics eat and thrive on. In other words, eating more foods that contain prebiotic fiber will help to further improve the health of your microbiome. This salad incorporates several prebiotic foods that are especially rich in these types of fiber. Examples include raw onion and garlic, dandelion greens, and sunchokes. Keep in mind that if you’re just starting to increase the amount of fiber in your diet, do so slowly and drink lots of water to help your body adjust to the healthy changes you’re making.

Evenly divide the salad ingredients between two serving bowls.

In a small covered jar, place all of the vinaigrette ingredients. Cover tightly and shake the dressing, then drizzle it over each salad. Serve immediately.

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