Leek & Lentil Soup with Kale

This savory soup combines umami-rich tomato paste and soy sauce with prebiotic-rich lentils, leek, garlic, and scallions.  It reheats easily for a quick lunch entrée, and with a side of crunchy vegetables and some fresh apple slices or orange segments, you're all set to eat a nutritious and delicious meal. 

Navigating the world of probiotics and prebiotics can be tricky, so this post will discuss the differences between these buzz words and the significant nutritional role of prebiotics.

Our gut is like a garden—probiotics (living organisms) require prebiotics (food) to grow.

Nutrition Highlight: Prebiotics

The modern American diet tends to be low in both probiotics, mainly found in fermented dairy like yogurt, and prebiotics, mainly found in vegetables, legumes, and whole grains. Unfortunately, the modern American diet tends to be low in all of these foods, so check out the MyPlate daily checklist for recommended daily amounts of the various food groups to help improve your intake.  The average adult should aim for at least 2.5 c of vegetables per day. To put that in perspective, eating 10 baby carrots is a 1/2 c serving.

Prebiotics are carbohydrates that do not get digested in the small intestine, thus qualifying them as dietary fiber.  Their indigestible nature allows them to be selectively fermented in the large intestine (i.e. gut) by probiotics, or good bacteria, such as Bifidobacteria and Lactobacillae. 

If a food is high in soluble fiber, that makes it a good candidate for being prebiotic or having prebiotic potential, since soluble fiber is highly fermentable (except for psyllium); see this 2013 Nutrients article for more information.

Why Prebiotics Matter

Consuming prebiotic rich food ensures a flourishing gut microbiota, therefore helping to enhance immune function, keep your large intestine healthy and free of cancer or infection, improve digestion, and improve uptake of zinc, iron, and calcium. 

Leek: high in fructans.

Getting Familiar with Prebiotic Names

Fructans are long chains of fructose molecules that can be subdivided into fructooligosaccharides (FOS) or inulin.  Galactans are long chains of galactose molecules and include galactooligosaccharides (GOS).  The raffinose family of oligosaccharides and resistant starch are also considered prebiotic. Xylans, pectins, and beta-glucans have prebiotic potential, meaning more research is needed.  

Sources of Prebiotics

The foods listed below contain a mix of prebiotic carbohydrates. See this 2014 article if you're interested in a breakdown of the prebiotic carbohydrates in the following foods and this table for comparison of galactan content in legumes.  Popular breakfast items, oats and flaxseed, are considered to have prebiotic potential. Within the Legume family, there are beans and pulses.  Due to differing starch structures, beans are higher in resistant starch than pulses (2016 analysis). Specific research on the prebiotic effect of resistant starch in beans is in the early stages.

Cereal and Legumes  

Barley, wheat, einkorn wheat, rye, oats, flaxseed, lentils & chickpeas, beans*

*Lentils and chickpeas are pulses and are high in prebiotic GOS and beans are high in prebiotic resistant starch.

Fruits

Best sources: jackfruit, dragon fruit, banana 

Other fruits have low amounts of FOS

Vegetables

Jerusalem artichoke, garlic, asparagus, onion, garlic, shallot, scallion, leek 

Leek and Lentil Soup with Kale

Printable Recipe

Packed with prebiotic-rich leek, garlic, lentils, & scallions.

2 T olive oil

1 medium leek, white and pale-green parts only, finely chopped  
1 garlic clove, minced

1 T tomato paste

1 cup brown lentils

2 T reduced-sodium soy sauce

1/2 tsp salt

Freshly ground pepper

Curly kale (1/2 leaf, roughly torn, per bowl) 

Thinly sliced scallions (optional; for serving)

Heat oil in a medium saucepan over medium heat. Add leek, garlic, and tomato paste and cook, stirring often, until fragrant and tomato paste begins to darken, about 5 minutes. Add lentils and 2 1/2 cups water. Bring to a boil; reduce heat, cover, and simmer, stirring occasionally, until lentils are tender, 55-60 minutes. Add just the amount of kale to be used when serving, and let wilt for about 2 minutes, then add soy sauce and season with salt and pepper. Serve lentils topped with scallions, if desired. For leftovers: top soup with raw kale and microwave 2-3 minutes. Adapted from Epicurious. Makes about 8 servings.